langmuir films of ionic liquids · ], medidas microscópicas, microscopia de força atómica (afm),...

Langmuir Films of Ionic Liquids

Integrated course on Chemical Engineering Master’s Thesis

Tomás Mello do Rego

Examination Comitee:

Chairperson: Professor António Luís Vieira de Andrade Maçanita

Members of the committee: Jorge Morilla Filipe; Doctor Pedro Morgado

Supervisors:

-Professor Michel Goldmann (INSP, University 6 Pierre et Marie Curie)

-Professor Jorge Morilla Filipe (Instituto Superior Técino)

November 2017

1

Contents

Acknowledgments 3

Abstract 4

Resumo 5

Figure Index 6

Table Index 9

Acronyms and abbreviations 10

Roman letters 10

Greek letters 11

Molecules 11

I. Introduction 12

II. Experimental Techniques 16

II.1.Langmuir Films 16

II.1.1. Amphiphilic Molecules 16

II.1.2. Langmuir through and spreading technique 17

II.2.Macroscopic Measures 18

II.2.1. (π-A) Isotherms 18

II.3.Microscopic Measures 21

II.3.1. Brewster angle microscopy (BAM) 21

II.3.2. Atomic force microscopy (AFM) 22

II.4.X-ray experiments 24

II.4.1. X-ray Reflectivity (XRR) 24

II.4.2. X-ray Fluorescence (XRF) and Total Reflection X-ray Fluorescence (TRXF) 25

III. Results and Discussion 28

III.1.Langmuir Isotherms 28

III.1.1. [C18mim][NTF2] 28

III.1.2. [C12C12im][NTF2} 35

III.1.3. [C20mim][NTf2] 39

III.2.Microscopic experiments 42

III.2.1. Brewster Angle Microscopy 42

III.2.2. Atomic Force Microscopy 47

III.3.X-ray experiments 52

III.3.1. X-ray Reflectivity (XRR) 52

III.3.2. Total Reflection X-ray Fluorescence (TRXF) and Grazing Incidence X-ray Fluorescence (GIXRF) 62

2

IV. Conclusions and future work 66

V. Bibliography 68

VI. Appendixes 70

VI.1. Overlap of the isotherm on salted water and pure water + area shift. 70

VI.2. Comparison between pure water and salted sub-phase of C20mimNTf2. 71

VI.3. BAM results on C20 with salted sub-phase 72

VI.4. BAM results on C20 after a compression cycle. 74

VI.5. C18 AFM with salted sub-phase. 75

3

Acknowledgments

From Portugal to France, there are numerous people I want to thank for all the support and help

given during this project.

First, I want to thank the institute of Nanoscience’s of Paris (INSP) and Professor Michel

Goldman for accepting me as an intern. A special thanks to Professor Michel for being my advisor and

guiding me during this project. Michel was always happy to lend a helping hand and to explain all the

technical and theoretical knowledge needed. His help extended above the professional level, helping

me to accustom to the new country I was in. It was a pleasure to work under his mentorship.

I want to thank Professor Eduardo for giving me and trusting me with this opportunity. From my

previous projects with him to this, he has always been someone whose I admired as a professional.

Eduardo taught me most the prerequisite knowledge necessary to this work and was always in par with

the experiments and results.

All the interns on the laboratory that were responsible for a great environment and specially to

Nathalie Bonatout, who I bombarded day after day with questions and doubts about the experiments

and the equipment. Although Nathalie was always hands full with her doctor’s degree she would always

find the extra time to help me analyze and understand all the results I obtained.

Last, I want to give thanks to all my family and friends, who supported and cheered for me during

all this journey.

4

Abstract

Ionic Liquids (ILs), also known as “molten salts”, are considered a new very promising

environment friendly class of material, they are organic salts that due to their crystalline structure display

a melting temperature close to room temperature. ILs have seen a rocket-like increase on popularity in

the last years and, although their interfacial proprieties are poorly studied, have already countless

applications, with new ones being proposed every day.

This project focuses on understanding the behavior and structure of one Ionic Liquid from the

imidazolium family ([C18mim][NTf2]) through Langmuir monolayer techniques and the study of similar

ionic liquids from the same family ([C12C12im][NTf2] and [C20mim][NTf2]). This study includes

macroscopic measures, (π-A) isotherms on pure water and salted water [Na][NTf2] sub phase,

microscopic ones, Atomic Force microscopy (AFM), Brewster angle microscopy (BAM) and X-ray

experiments, carried out at SOLEIL and ESRF synchrotrons.

For each molecule, the distinct phases present in each isotherm were characterized. Two of the

molecules ([C18mim][NTf2] and [C20mim][NTf2]) exhibited the formation of three-dimensional crystals.

These crystals were observed and measured with BAM and AFM. The instability of the film formed by

these molecules, identified by previous works, was confirmed and resorting to X-ray experiments,

plausible explanations were formulated. A better understanding of this ionic family was achieved,

although further studies are necessary to confirm some of the hypothesis proposed.

Keywords

Ionic Liquids, [C18mim][NTf2], Langmuir Isotherms, X-ray, Brewster Angle microscopy, Atomic Force

Microscopy.

5

Resumo

Líquidos iónicos (LIs), também conhecidos por “sais fundidos”, são considerados uma nova

classe de materiais não nociva para o meio ambiente, são sais orgânicos que devido à sua estrutura

cristalina apresentam temperaturas de fusão próximas da temperatura ambiente. Nos últimos anos, a

popularidade dos LIs tem aumentado exponencialmente, com inúmeras aplicações já propostas.

Este projeto foca-se em compreender o comportamento e a estrutura de um liquido iónico da

família Imidazolium ([C18mim][NTf2]) através de filmes de Langmuir e o estudo de líquidos iónicos da

mesma família (([C12 C12im][NTf2] e ([C20mim][NTf2]). Este estudo inclui medidas macroscópicas,

isotérmicas (π-A) em água e água com o sal [Na][NTf2], medidas microscópicas, microscopia de força

atómica (AFM), microscopia de angulo de Brewster e experiências com raio-x, realizadas nos

sincrotrões SOLEIL e ESRF.

Para cada molécula, as diferentes fases presentes em cada isotérmica foram identificadas e

caracterizadas. Duas das moléculas ([C18mim][NTf2] e [C20mim][NTf2]) exibiram a formação de cristais

tridimensionais. Estas estruturas foram observadas e dimensionadas com as técnicas BAM e AFM. A

instabilidade do filme formado por estas moléculas, identificada em obras prévias, foi confirmada e

recorrendo às experiencias raio-X, explicações plausíveis foram formuladas. Um melhor conhecimento

desta família de líquidos iónicos foi obtido, apesar de ainda ser necessário um maior número de

experiências para confirmar algumas das hipóteses propostas.

Palavras chave

Líquidos iónicos, [C18mim][NTf2], Isotérmicas de Langmuir, Raio-X, Microscopia de Ângulo de Brewster,

Microscopia de Força Atómica.

6

Figure Index

Figure 1- # of published papers about ILs from the year 2000 to 2009. Columns describe the amount of

papers published per year.[1] ............................................................................................................................ 12

Figure 2- Table with applications of ionic liquids. Six types of applications are commented: electrochemistry,

biological, analytical, solvents and catalysis, engineering and physical chemistry. ........................................ 13

Figure 3- Cations chosen for this project: a) head- imidazolium group; the groups R1 and R2 were replaced

with three diferent combinations. When R1 is replaced with b) the cation is 1-octadecyl 3-

methylimidazolium (C18mimNTf2), with c) 1-icosyl 3-methylimidazolium (C20mimNTf2), with d) 1-dodecyl 3-

dodecylimidazolium (C12C12imNTf2) ................................................................................................................. 15

Figure 4- Anion used for all molecules bis(trifluoromethylsulfonyl)imide ([NTf2]-) ......................................... 15

Figure 5- Illustration of an Amphiphilic molecule. A hydrophobic long tail (black) and a hydrophilic head

(red) are observed. ........................................................................................................................................... 17

Figure 6a) and b)- Respectively, photograph and illustration of a Langmuir through. The various parts of the

through are identified on fig. a): barriers (1), shallow top (2), surface pressure sensor (3), a wilhelmy plate

(4) and temperature regulator (5). On figure b) we can observe how the amphiphilic molecules arrange at

the surface of water. ........................................................................................................................................ 17

Figure 7- Force induced on the Wilhelmy plate. I is the length of the plate and y the surface tension. ......... 19

Figure 8- Schematic π (surface pressure) vs A (area per molecule) isotherm, includes all possible phases:

gaseous, liquid expanded, liquid condensed, solid-like and collapse. ............................................................. 20

Figure 9- Principle of Brewster angle microscopy technique between an interface air-water. With no film

on the surface, when the incident angle is equal to the critical angle, no reflection occurs. Adding molecules

to the surface and creating a film leads to a reflection dependent on the films structure and thickness. ..... 21

Figure 10- Schematic diagram of the microscope at the Brewster angle. Ob1,0b2,0b3: microscope objectives;

L1,L2,L3,L4,L5: lens; P: polarizer (glan); A: analyzer (dichroic sheet); Q: quarter wave plate; 𝜃𝐵: brewster angle

.......................................................................................................................................................................... 22

Figure 11-Diagram of AFM work principle. A laser diode emits a beam which is reflected by the cantilever,

who is contact with the sample, and measured by a position-sensitive photodetector. ................................ 23

Figure 12- Tapping mode principle and force vs distance curve example. The tip approaches and retracts

from the sample creating a force vs distance curve. ....................................................................................... 23

Figure 13- Scheme of the XRR mechanism on a molecular monolayer with a liquid sub-phase. The incident

beam is reflected by each interface depending on its electron density and thickness. The reflected beam is

then measured by a detector. .......................................................................................................................... 25

Figure 14a) and b)- Simplistic schemes of XRF (left) and TRXF (right). In traditional x-ray fluorescence the

angle used is above the critical angle, while in total reflection x-ray fluorescence the angle used is below the

critical angle. In the second case, only a very small part of the incident beam enters the bulk, nullifying the

backscatter into the detector........................................................................................................................... 26

Figure 15-Sketch of theoretical conditions for GIXRF and TRXF between two mediums. Depending on the

incident angle (θ1) the beam is reflected at a same angle (θ3) and a intensity depending on θ1 and the

refracting mediums (θ3) and refracted on angle (θ2) and intensity given by the Snell-Descartes Law. .......... 27

Figure 16- Base (π-A) Isotherm of C18mimNTf at 20ºC, a compression rate of 30 cm2/min and 10 minutes of

solvent evaporating time. Blue curve corresponds to the compression curve and orange to the expansion.

.......................................................................................................................................................................... 29

Figure 17- Three consecutive compression/expansion cycles (π-A) isotherms of C18mimNTf2 on pure water

sub phase at 20ºC, a compression rate of 30 cm2/min and 10 minutes of solvent evaporating time.

Respectively blue, orange and gray curves are 1st, 2nd and 3rd cycle. .......................................................... 31

7

Figure 18- Two compression/expansion cycles (π-A) isotherms of C18mimNTf2 on pure water sub phase with

different relaxation times. Both done with a compression rate of 30 cm2/min and 10 minutes of solvent

evaporating time. Orange curve was done with relaxation time of 15 minutes and blue with a relaxation

time of 66 hours. .............................................................................................................................................. 32

Figure 19-Four compression/expansion cycles (π-A) isotherms of C18mimNTf2, one on pure water sub

phase(grey) and three on salted water ([Na][NTf2]) sub phase. Both done at 20ºC, a compression rate of 30

cm2/min and 10 minutes of solvent evaporating time. For the salted water sub phase three concentrations

were used: 0.03 (yellow), 0.01 (blue) and 0.002 (orange) mM. ...................................................................... 33

Figure 20- Isotherm for consecutive cycles of a C18 film with salted water (0.01mM) as sub-phase at 20ºC, a

compression rate of 30 cm2/min and 10 minutes of solvent evaporating time. Yellow curve corresponds to

the first cycle, blue to the second, orange to the third and grey to the fourth and last. ................................ 35

Figure 21- Two consecutive compression/expansion cycles isotherms of C12C12mimNTf2 on pure water sub

phase at 20ºC, a compression rate of 45 cm2/min and 10 minutes of solvent evaporating time. Respectively

blue and orange curves are 1st and 2nd cycle.. ................................................................................................. 36

Figure 22- C20 Isotherms for pure water (grey) and salted sub-phases at 20ºC, a compression rate of 45

cm2/min and 10 minutes of solvent evaporating time. Two different concentrations of salt were used

0.03mM (orange) and 0.01mM (blue). ............................................................................................................ 37

Figure 23- C12C12 Compression cycles with salted water (0.01mM) as sub phase at 20ºC, a compression rate

of 45 cm2/min and 10 minutes of solvent evaporating time. Blue curve corresponds to the first cycle,

orange to the second, grey to the third and yellow to the fourth and last cycle. ........................................... 38

Figure 24- C20 two consecutive compression cycles with pure water) as sub phase at 20ºC, a compression

rate of 30 cm2/min and 10 minutes of solvent evaporating time. Blue curve corresponds to the first cycle,

orange to the second and last cycle. The expansion of the orange curve was started midway of the second

plateau for demonstration purposes. .............................................................................................................. 39

Figure 25- Five consecutive compression/expansion cycles isotherms of C20mimNTf2 on pure water sub

phase at 20ºC, a compression rate of 30 cm2/min and 10 minutes of solvent evaporating time. Respectively

blue, orange, gray, yellow and blue curves are 1st, 2nd, 3rd, 4th and 5th cycle. .................................................. 41

Figure 26- BAM experiment isotherm at 20ºC, a compression rate of 30 cm2/min and 10 minutes of solvent

evaporating time. The numbers correspond to the area and pressure where the corresponding image was

taken. ................................................................................................................................................................ 43

Figure 27- Six images taken during the BAM experiment on C19 with pure water sub-phase......................... 43

Figure 28- Two images of the C12C12 BAM experiment. Left screenshot was taken at 26.34 mN/m &

0.03nm2/molecule and right screenshot at 25.15mN/m & 0.28 nm2/molecule. ............................................ 44

Figure 29- BAM experiment isotherm of C20mimNTf2, salted sub phase (0.01 mM) ....................................... 45

Figure 30- Six images taken during the BAM experiment on C20 with salted water sub-phase (0.01mM). .... 46

Figure 31- Compression/expansion isotherm of C18mimNTf2with pure water as sub-phase at 20ºC, a

compression rate of 30 cm2/min and 10 minutes of solvent evaporating time. The numbers identify the area

and pressure where the substrate was transferred. ....................................................................................... 48

Figure 32- AFM scans of C18mimNTf2. Scan 1 and 2 substrate were transferred during compression and

respectively at 18 mN/m and 25mN/m. Scan 3 substrate was transferred during expansion and at 15 mN/m.

.......................................................................................................................................................................... 49

Figure 33a) and b)- Respectively, profile 1 (collapse scan) and profile 2 (ESP scan). Profiles show the size of

the crystals of the AFM scans........................................................................................................................... 50

Figure 34- Compression/expansion isotherm of C20mimNTf2 at 20ºC with pure water as sub-phase.

Compression/expansion rate used was 30 cm2/min. The numbers identify the area and pressure where the

substrate was transferred. ............................................................................................................................... 51

Figure 35- AFM scans of C20mimNTf2. Scan 1 substrate was transfered at 17mN/m and scan 2 at 26mN/m. 51

8

Figure 36- Profile 3, obtained from collapse scan of C20mimNTf2. Profiles show the size of the crystals of the

AFM scans. ....................................................................................................................................................... 52

Figure 37a and b- Fig. a) shows the normalized reflectivity intensity in function of normalized wave-vector

and the fit for the two boxes model obtained for the scan at 0mN/m. Fig. b) shows the electron density

obtained from the fit in function of depth (z) as a box and as a curve fit. Depth (z) starts at the interface

between the superphase (air) and the 1st layer progressing to the sub-phase, which is reached about 50 Å.

.......................................................................................................................................................................... 54

Figure 38a and b- shows the normalized reflectivity intensity in function of normalized wave-vector and the

fit for the three-layer model obtained for the scan at 0mN/m. Fig. b) shows the electron density obtained

from the fit in function of depth (z) as a box and as a curve fit. Depth (z) starts at the interface between the

superphase (air) and the 1st layer progressing to the sub-phase, which is reached about 50 Å. .................... 55

Figure 39- Fig. a) shows the normalized reflectivity intensity in function of normalized wave-vector and the

fit for the two-box model obtained for the scan at 11.5 mN/m. Fig. b) shows the electron density obtained

from the fit in function of depth (z) as a box and as a curve fit. Depth (z) starts at the interface between the

superphase (air) and the 1st layer progressing to the sub-phase, which is reached about 50 Å. .................... 58

Figure 40a) and b)- Fig. a) shows the normalized reflectivity intensity in function of normalized wave-vector

and the fit for the three-box model obtained for the scan at 11.5 mN/m. Fig. b) shows the electron density

obtained from the fit in function of depth (z) as a box and as a curve fit. Depth (z) starts at the interface


.......................................................................................................................................................................... 59

Figure 41- Fig. a) shows the normalized reflectivity intensity in function of normalized wave-vector and the

fit obtained for the scan at 24 mN/m. Fig. b) shows the electron density obtained from the fit in function of

depth (z) as a box and as a curve fit. Depth (z) starts at the interface between the superphase (air) and the

1st layer progressing to the sub-phase, which is reached about 50 Å. ............................................................ 61

Figure 42- Example of a sulfur, Compton and elastic peak. ............................................................................. 63

Figure 43- TRXF experiment on [C18mim][NTf2] with pure water as sub phase. At blue normalized Sulphur

intensity as a function of area per molecule in nm2/molecule and at purple corresponding isotherm, surface

pressure (π) in mN/m in function of the area per molecule. ........................................................................... 64

Figure 44a) and b)- GIRXF experiment on [C18mim][NTf2] with pure water as sub phase. Left graph displays

the reflectivity intensity for pure water sub phase at 24mN/m and for salted water sub-phase (0.01mM) at

20mN/m. Four peaks are shown, Silica, Sulphur, Argon and the elastic plus inelastic peak. Right graph

display the normalized Sulphur fluorescence vs penetration depth for pure water sub-phase (full lines) at 0

and 24mN/m and for salted water sub-phase (dotted lines) at 0 and 20mN/m. ............................................ 65

Figure 45-Two compression/expansion cycles isotherms on C18. One was done with salted water

([Na][NTf2]) with a concentration of 0.01mM (blue curve) and the other with pure water. The area shift

between both was added to the isotherm done in pure water and resulted on the orange curve. ............... 70

Figure 46- Two compression/expansion cycles isotherms on C18. One was done with pure water (blue curve)

and the other salted water ([Na][NTf2] with a concentration of 0.01mM (orange curve). ............................. 71

Figure 47- BAM experiment isotherm on C18 with salted sub-phase at 20ºC, a compression rate of 30

cm2/min and 10 minutes of solvent evaporating time. The numbers correspond to the area and pressure

where the corresponding image was taken. .................................................................................................... 72

Figure 48- Four images taken during the BAM experiment on C19 with pure water sub-phase. ..................... 73

Figure 49- Four images of C20 after a full compression time. Image 1 taken after waiting 2 minutes, 2 after

27, 3 after 56 and 4 after 1 hour. ..................................................................................................................... 74

Figure 50- Figure a) and b) scan taken at the plateau (25mN/m) of a C18 film at 20ºC and a compression rate

of 30 cm2/min. Figure b is the corresponding profile obtained from the scan. .............................................. 75

9

Table Index

Table 1- IUPAC names, formulas, origin and purity of the three molecules used, the solvent and the salt

used in some sub-phases. 16

Table 2- Compressibility of C18 on pure water sub-phase for three intervals of the compression and two of

the expansion curve. Compressibility was calculated through equations 2 and 3. 30

Table 3- Area shifts and compressibility of the isotherms with salted water as sub-phase. Compressibility

was calculated through equations 2 and 3. 34

Table 4- Surface area loss for the different concentrations of salt on the sub-phase. 34

Table 5- Pressure intervals and corresponding compressibilities for the three phases shown in the

compression and for the expansion below the ESP. Compressibilities were calculated through equation 2

and 3. 40

Table 6- Area, pressure and curve of each image taken during C18 pure water BAM experiment. 42

Table 7- Area, pressure and curve of each image taken during C20 salted water BAM experiment. 45

Table 8- C18mimNTf2 AFM scans and corresponding size of the scan, pressure at which the substrate was

transferred and if the was after compression or expansion. 48

Table 9- C20mimNTf2 AFM scans and corresponding size of the scan, pressure at which the substrate was

transferred and if the was after compression or expansion. 50

Table 10- Results of the fits for the two models for the scan at 0mN/m. Two-layer model corresponds to the

model where the anion is between the cation and the three-layer where the anion is beneath the cation.

Experimental electron density (𝑛𝑒) corresponds to the normalized density obtained through the fit

multiplied by the water 𝑛𝑒 and calculated, where the layer length is used to calculate the corresponding

correct electron density. 56

Table 11- Results of the fits for the two models for the scan at 11.5mN/m. Two-layer model corresponds to

the model where the anion is between the cation and the three-layer where the anion is beneath the

cation. Experimental electron density (𝑛𝑒) corresponds to the normalized density obtained through the fit

multiplied by the water 𝑛𝑒 and calculated, where the layer length is used to calculate the corresponding

correct electron density. 60

Table 12-Results of the XRR fit on C18 at 24 mN/m based on a tri-layer model. 1st box corresponds to the top

layer of alkyl chains and corresponding ion pairs and 2nd box to the two groups of alkyl chains and ion pairs

facing opposite directions. Experimental electron density (𝑛𝑒) corresponds to the normalized density

obtained through the fit multiplied by the water 𝑛𝑒 and calculated, where the layer length is used to

calculate the corresponding correct electron density. 62

Table 13-Waiting time for each image displayed below 74

10

Acronyms and abbreviations

ILs- Ionic Liquids

ESP - Equilibrium Surface Pressure

BAM- Brewster Angle Microscopy

AFM- Atomic Force Microscopy

STP- Standard Temperature and Pressure

TRXF- Total Reflection X-ray Fluorescence

GIXR- Grazing Incidence X-ray Fluorescence

LE- Liquid expanded

LC- Liquid condensed

C18- [C18mim][NTf2]

C12C12- [C12C12im][NTf2]

C20- [C20mim][NTf2]

Roman letters

A - Area per molecule

𝑒 - Electrons

𝑙 - Length

NA - Avogrado’s number

𝑛 - Refractive index

𝑛𝑒- Electron density

M – Molar mass

𝑦 – Surface tension

I- Intensity

z- depth

11

Greek letters

𝛽 - Compressibility

π - surface pressure

𝜃 – angle

ρ - density

ρnum – numerical density distributions

Molecules

[C18mim][NTf2] = 1-octadecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

[C20mim][NTf2]= 1-eicosyl 3-methylimidazolium bis(trifluoromethylsulfonyl)imide

[C12C12im][NTf2]= 1-dodecyl 3-dodecylimidazolium bis(trifluoromethylsulfonyl)imid

[Na][NTf2]= Sodium bis(trifluoromethylsulfonyl)imide

12

I. Introduction

One hundred years ago, the discovery of the first ionic liquid changed the basic concept of a

salt. Ethylammonium nitrate is believed to be the first example of an ionic liquid and was discovered by

the German Chemist Walden. Until then, it was never suspected that ions could form a liquid at room

temperature unless diluted in a molecular solvent. [2]

The understanding and study of ionic liquids has progressed slowly after Walden, but in the last

three decades we are observing a rocket-like increase in interest and publications, shown by figure 1.

This is greatly due to the green chemistry movement. The continuous look for improvement in both the

efficiency and environmental step of processes led to the study and consequently to the discovery of

numerous new interesting proprieties and applications.

d

Ionic Liquids are acknowledged as a new and promising type of solvent, capable of solving both

polar and apolar compounds. They are purely ionic, salt-like materials that are liquid at unusually low

temperatures. In some contexts, the term has been restricted to salts whose melting point is below some

arbitrary temperature, typically 100 °C. Thus, it is important to keep in mind the terms such as room

temperature ionic liquid, molten salt, liquid organic salt, fused salt, liquid electrolytes, ionic melts, ionic

fluids, fused salts, and liquid salts have all been used to describe these salts in the liquid phase. [3]

Due to their non-flammability, non-volatility and recyclability, they are a greener alternative to

the common solvents used in industry.

The main difference between typical ILs and the typical inorganic salts with melting points above

300ºC is their chemical structure. Typical ILs display a generous size difference between the two ions,

resulting in an asymmetric shape and a less stable crystalline stable. Therefore, the solidification

216 361

561729

916

1126

16021739

2042

2365

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

# o

f p

ub

lish

ed

pap

ers

on

ILs

Year

Figure 1- # of published papers about ILs from the year 2000 to 2009. Columns describe the amount of papers published per year.[1]

13

happens at lower temperatures. Unlike classic salts, ILs can form a liquid entirely composed of ions,

without the need of a solvent at room temperature.

These particularities give these materials quite unique and economical interesting

characteristics, such as outstanding solvating potential, thermal stability and the most important of all,

the ability to tune their proprieties through the combination of thousands possible cations and anions.

Many applications in multiple fields of Science are studied, worked on and developed. In the

figure 2 it’s displayed a succinct infographic on the possible ionic liquid applications:

One of the most promising uses is as a solvent. ILs are an environmental friendly substitute of

volatile organic compounds (VOCs) solvents. As the name suggest these compounds display a very

high volatility and while the environmental impact depends on the chemical, many of them display a

danger to health in short and long terms. In contrary, ILs display very low volatility due to their strong

ionic (coulomb interactions), and other important “green” characteristics talked before (non-flammability,

non-volatility and recyclability).[4] [5]

ILs can also be used as a method of increasing the efficiency of a process, biological or chemical

as a solvent, a catalyzer or as both. IFP is known to be the first company to operate an ionic liquid pilot

plant, where they use ionic liquids as solvents in the dimersol process (dimerization of alkenes). The

utilization of the IL not only enhances the extraction of the products but also the activity and selectivity

of the catalyst.[6] For these reasons, the medical industry has taken quite an interest on ILs, using them

as solvents for proteins, in bio catalysis as the reaction media and to modify certain proteins to generate

immunotoxins.[7]

Lately they have been studied extensively as a potential electrolyte in electrochemical devices,

more specifically in Lithium batteries. Room-temperature ionic liquids, which have low vapor pressure,

high thermal stability, and low flammability, are considered as a potential alternative due to remarkable

safety advantages over conventional organic electrolytes. Many studies investigating potential

application of room-temperature ionic liquid electrolytes in lithium−ion and lithium−sulfur batteries have

demonstrated cyclability and capacity improvement over the batteries operating with conventional

electrolytes.[8][9]

Elec

tro

chem

istr

y • Electrolyte in batteries

• Metal plating

• Solar Panels

• Fuel Cells

• Electro-optics

• Ion propulsion

Bio

logi

cal

• Biomass processing

• Drug delivery

• Biocides

• Personal care

• Embalming

An

alyt

ics • Matrices for mass

spectrometry

• Gas Chromatography columns

• Stationary phase for HPLC

Solv

ents

an

d c

atal

ysis • Synthesis

• Catalysis

• Microwave Chemistry

• Nanochemistry

• Multiphase reactions and extractions

Engi

nee

rin

g • Coatings

• Lubricants

• Plasaticisers

• Dispersing agents

• Compatibilisers

Ph

ysic

al C

hem

istr

y • Refractive Index

• Thermodynamics

• Binary and ternary systems

Figure 1- Applications of Ionic Liquids Figure 2- Table with applications of ionic liquids. Six types of applications are commented: electrochemistry, biological, analytical, solvents and catalysis, engineering and physical chemistry.

14

For many of these applications, used or being studied, ILs are used at their own interface

air/liquid or adsorbed at surface of solids, even though their interfacial proprieties are poorly

investigated. Therefore, the study of their behavior and interfacial proprieties is of utmost importance

and interest. A better understanding of these proprieties can lead to an increase of efficiency and

development of numerous new applications.

ILs with long alkyl chains form molecular thick layers at the surface of aqueous solutions,

denominated by Langmuir monolayers and can be studied under Langmuir techniques. This is possible

due to their amphiphilic characteristic, having a polar “head”, which is soluble in water and a non-polar

“tail”, which is insoluble. Due to this trait they show a similar behavior to ionic surfactants forming

organized structures such as micelles.

One of the most promising amphiphilic ionic families is based on an imidazolium molecule and

has been a target of studies in recent years by both INSP and IST institutions, culminating in an article

and a master thesis:

Filipe et al. reported the formation of Langmuir films of [C18mim][NTf2] at air/water

interface. Experimental results were supported by molecular dynamics simulation.[10]

Valente studied the behavior of multiple molecules of the imidazolium-based family with

a broad range of Langmuir techniques.[11]

Both these papers resulted in multiple hypothesis and questions. The main objective of this

thesis being in finding explanations or further prove that confirms this theories through an exhaustive

study on the ionic liquid [C18mim][NTf2] at both the interfaces of air/water and air/salted water.

During the research, it was decided two more molecules of the same family (imidazolium-based)

would be studied, [C12C12im][NTf2] and [C20mim][NTf2] with the objective of understanding their behavior

and consequently finding similarities with [C18mim][NTf2].

The three different cations are all, as said, imidazolium based (figure 3). All the same molecules

have the same anion, [(CF3SO2)2N=NTf2]- most commonly known as [NTf2]-. (Figure 4) This anion is

widely used in ionic liquids and is known as a non-coordinating (weak interaction with cations) and non-

spherical anion.

[C18mim][NTf2] will also be referred as C18, [C12 C12im][NTf2] as C12C12 and [C20mim][NTf2] as

C20 for simplification purposes. Figure 3 displays the cations used and figure 4 the anion.

15

Figure 3- Cations chosen for this project: a) head- imidazolium group; the groups R1 and R2 were replaced with three diferent combinations. When R1 is replaced with b) the cation is 1-octadecyl 3-methylimidazolium (C18mimNTf2), with c) 1-icosyl 3-methylimidazolium (C20mimNTf2), with d) 1-dodecyl 3-dodecylimidazolium (C12C12imNTf2)

R1 R2

CH2

CH3

R1-

R2- CH3

R1-

CH3

CH3

R2-

CH2

R1- CH2

CH3

R2- CH2

CH3

Figure 4- Anion used for all molecules bis(trifluoromethylsulfonyl)imide ([NTf2]-)

16

II. Experimental Techniques

II.1.Materials and Solvents

The three molecules studied in this work (C18, C12C12 and C20) have the same origin: lolitec. The

degree of purity of C18 and C12C12 is > 98% and of C20 is > 96%.

The same Solvent was used to make all the solutions used in the film spreading: Chloroform

from VWC Chemicals with a purity degree of > 99.9%.

The salt used in some sub-phases [Na][NTf2] is from lolitec and has a purity degree of > 98%.

Table 1 contains all the information about the molecules, solvent and salt used.

Table 1- IUPAC names, formulas, origin and purity of the three molecules used, the solvent and the salt used in some sub-phases.

Molecule IUPAC nomenclature Formula Origin and Purity

[C18mim][NTf2] 1-octadecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

C24H43F6N3O4S2 lolitec: > 98%

[C12C12im][NTf2] 1-eicosyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide C29H53F6N3O4S2 lolitec: > 98%

[C20mim][NTf2] 1-dodecyl-3-dodecylimidazolium bis(trifluoromethylsulfonyl)imid

C26H47F6N3O4S2 lolitec: > 96%

Chloroform Trichloromethane CHCl3 VWR Chemicals >99.9%

[Na][NTf2] Sodium

bis(trifluoromethylsulfonyl)imide NaF6N1O4S2 lolitec: > 98%

II.2.Langmuir Films

II.2.1. Amphiphilic Molecules

To understand the basics of a Langmuir isotherm it’s imperative to understand what an

amphiphilic molecule is.

These molecules contain both a hydrophobic part (non-polar, lipophilic) and a hydrophilic part

(polar, lipophobic). The non-polar part is typically referred to as “tail” and the polar as “head”, which can

be charged or uncharged (Fig. 5). Ionic Liquids are a type of amphiphilic molecules containing a charged

head, as is demonstrated by figure 3 in chapter I. Introduction.[12]

17

II.2.2. Langmuir through and spreading technique

The hydrophilic and hydrophobic characteristics make it possible to create a molecular thick

layer at the surface of the air/water interface, in which the hydrophilic group in the water anchors the

hydrophobic group up and above the surface. The molecules stay at the surface and don’t dissolve. This

layer is denominated by Langmuir film.

The study of Langmuir monolayers at air/aqueous solutions interface is conducted in Langmuir

through (Fig. 6a) and 6b)). Most of these throughs, nowadays, are made of Teflon

(polytetrafluoroethylene), a material which reduces sub-phase containment, due to its hydrophobic,

lipophobic and chemically inert proprieties. They include one or two barriers (1), a shallow top where

the sub-phase is poured (2), a surface pressure sensor (3), a wilhelmy plate (4) and a temperature

regulator through water circulation (5).[13]

Polar head

(water loving)

Non-polar tail

(water hating)

Figure 5- Illustration of an Amphiphilic molecule. A hydrophobic long tail (black) and a hydrophilic head (red)

are observed.

1 1

2

3

4

Wilhelmy plate Langmuir film

Sub-phase, mostly aqueous

5

Figure 6a) and b)- Respectively, photograph and illustration of a Langmuir through. The various parts of the through are identified on fig. a): barriers (1), shallow top (2), surface

pressure sensor (3), a wilhelmy plate (4) and temperature regulator (5). On figure b) we can observe how the amphiphilic molecules arrange at the surface of water.

18

This technique starts with an exhausting cleaning and rinsing of the through, followed by filling

the shallow top with an aqueous solution. Before the spread of the solution, it’s necessary to verify the

cleanliness of the solution through compression and expansion and if at lowest area the surface

pressure is higher than 0.5 mN/m than the one at the largest area, it’s necessary to repeat the first step.

It’s also necessary to calibrate the surface pressure sensor, when it’s measuring the air and the bare

sub-phase, for the air the surface pressure value must be the surface pressure of water at the

experiment temperature and for the bare sub-phase as close to null as possible.

After this is verified, the spreading can occur. This step usually happens with the aid of a solvent.

The deposition is done with a syringe and the solution spreads rapidly to cover all the available area. As

the solvent evaporates, the monolayer is formed at the air-water interface and a Langmuir film is created.

A movable barrier is then used to compress or to expand, varying the available area and thus having an

impact on the surface tension.[13]

If used, the solvent must satisfy some requirements: it must fully dissolve the molecules to be

studied and be chemically inert with the through material; should have low water solubility, preventing

carriage of the solution to the bulk; should fully evaporate on a reasonable amount of time, however it’s

recommended to avoid very volatile solvents, since with one it’s very hard to maintain the concentration

on a solution.

The spreading can be done without a solvent, by placing a crystalline sample of the solid

material in contact with a pure water surface. This results in a spontaneously generated monolayer

formed by molecules detaching from the crystal surface and spreading over the sub phase that are,

therefore, in equilibrium with the crystals. This is defined as the equilibrium surface pressure (ESP).[14]

This method is used to measure the ESP’s of different compounds.

II.3.Macroscopic Measures

II.3.1. (π-A) Isotherms

The main method of characterizing a Langmuir monolayer is through a surface pressure (π) vs

Area per molecule (A) isotherm, this displays the relationship between the surface pressure and the

area occupied on the sub-phase liquid by the molecules of the film.[15]

The surface pressure is measured through the relation 1:

𝜋 (𝐴) = 𝑦0 − 𝑦 (1)

π is defined as the difference between the surface tension of the bare sub-phase 𝑦0 and the

surface tension 𝑦 of the sub-phase covered by the amphiphiles.

19

If we compress a surfactant film on water we observe that the surface tension decreases and

the surface pressure increases. What is the reason for this decrease in surface tension? On

compression, the surface excess increases and hence the surface tension must decrease. This,

however, is relatively abstract. A more illustrative explanation is that the surface tension decreases

because the highly polar water surface (high surface tension) is more and more converted into a

nonpolar hydrocarbon surface (low surface tension) and hence the surface pressure of the film (π)

increases during compression.[16]

This is measured through the wilhelmy-plate method (Fig.7). A thin plate of glass, platinum, or

filter paper is vertically placed halfway into the liquid. In fact, the specific material is not important, as

long as it is wetted by the liquid. Thus, the surface tension can exert a downward force equal to 2lγ,

where l is the length of the plate. One measures the force required to prevent the plate from being drawn

into the liquid. In honor of Ludwig Wilhelmy, who studied the force on a plate in detail, the method was

named after him. The Wilhelmy-plate method is simple and no correction factors are required. It’s

necessary to always perverse the cleanliness of the plates and prevent contamination.[16]

Monomolecular amphiphilic films show ordered phases like a three-dimensional system, a

normal Langmuir isotherm describes the various phases of the molecule, as shown by the generic

isotherm in figure 8. All the phases can occur, but most amphiphiles do not show all phases or they may

look different, an example are the ionic liquids, which isotherms will be described à posteriori.

Figure 7- Force induced on the Wilhelmy plate. I is the length of the plate and y the surface tension.

20

In the Gaseous phase, the molecules are not interacting one with each other. In liquid expanded

phase the molecules are interacting each other but there is no order and organization. As the

compression occurs space is limited, so the film becomes stiff reaching to a more condensed phase,

the liquid condensed state, which present usually a chain ordered (orientation and/or positional).

These phases, identified by the compressibility coefficient (see below), are usually separated

by a constant surface pressure region which indicates a plateau of phase coexistence.

When the area reaches the minimum limit of area per molecule (and maximum compaction),

further compression does not allowed to maintain all the molecules on the surface. One then observes

the collapse which correspond to the transfer of molecules above the monolayer or dissolution in the

liquid sub-phase.

There are some notable differences between real and schematic isotherms, phase transitions

are usually accompanied by small pressure increases, mostly due to contaminations and the

compression of the newly formed phase.

The best way of identifying a phase from an isotherm is through the compressibility coefficient,

which is a measurement of the compressional elasticity of the film [17]. Compressibility (𝛽) is calculated

through equations 2 and 3:

𝛽 = −1

𝐴(

𝑑𝐴

𝑑𝜋)

𝑇 (2)

𝛽 = ∆𝜋

ln (∆𝐴) (3)

Where 𝛽 is the compressibility, A the area per molecule (nm2/molecule) and π is the surface

pressure (mN/m). Using a linear regression, the compressibility of the interval is calculated, this was

done using ExcelTM linear regression tool.[18]

Figure 8- Schematic π (surface pressure) vs A (area per molecule) isotherm, includes all possible phases: gaseous, liquid expanded, liquid condensed, solid-like and collapse.

21

II.4.Microscopic Measures

II.4.1. Brewster angle microscopy (BAM)

This technique was first introduced in 1991 by Hönig and Hénon and enables the visualization

of Langmuir monolayers and adsorbate films at the air-water interface (Fig. 9). The images aid in

understanding both the structure and flow behavior of monolayer materials.[19]

The reflectivity of a plane interface between two media of refractive index n1 and n2 depends on

the polarization α of the incident light and the angle of incidence θ. For a Fresnel interface (an interface

where the refractive index changes steeply from n1 to n2 for z = 0) and a polarization parallel to the

interface, the reflectivity vanishes at the Brewster angle. For a real interface, the reflected light intensity

has a minimum at the Brewster angle, but does not vanish and is strongly dependent on the interfacial

properties.[20]

Knowing the refractive indexes of the two surfaces, in this case air/water interface, which

indexes are respectively 1.00029 at STP and 1,33 at 20ºC (for visible light) the Brewster angle is

calculated through Snell-Descartes law:

𝑛1 sin( 𝜃𝑖) = 𝑛2 sin( 𝜃𝑡) (4)

𝜃𝑖 + 𝜃𝑡 = 90º𝐶 (5)

𝑛1 sin( 𝜃𝑖) = 𝑛2 sin( 90ª − 𝜃𝑡) = 𝑛2 sin( 𝜃𝑖) (6)

tan(𝜃𝐵) =𝑛2

𝑛1 (7)

Whereas specified before, 𝑛1 and 𝑛2 are respectively the refractive indexes of the incident and

the transmitting mediums; 𝜃𝑖 and 𝜃𝑡 the angles of incidence and reflection and 𝜃𝐵 the Brewster angle.

In this project only two different sub phases were used, pure water and salted water and while

the addition of salt alters the refractive index it’s negligible. The value used as the Brewster angle was

53º.

θ θ

Figure 9- Principle of Brewster angle microscopy technique between an interface air-water. With no film on the surface, when the incident angle is equal to the critical angle, no reflection occurs. Adding molecules to the surface and creating a film leads

to a reflection dependent on the films structure and thickness.

22

Figure 10 shows a complete schematic diagram of a BAM microscope:

Figure 10- Schematic diagram of the microscope at the Brewster angle. Ob1,0b2,0b3: microscope objectives; L1,L2,L3,L4,L5:

lens; P: polarizer (glan); A: analyzer (dichroic sheet); Q: quarter wave plate; 𝜃𝐵: brewster angle

With a constant angle of incidence, the formation of a monolayer on the water surface modifies

the Brewster angle condition and light reflection is observed. Molecular density and structure have direct

application on the brightness of the reflection, thus enabling the detection of monolayer inhomogeneities

and solid-liquid reversible or irreversible transformations.[21]

II.4.2. Atomic force microscopy (AFM)

Atomic force microscope (AFM) is a scanning near-field tool invented in 1986 for nanoscale

investigation.[22] It is a versatile and powerful method that images, measures and manipulates matter at

surfaces with atomic resolution.[23]

The basic idea of AFM is to use a sharp tip scanning over the surface of a sample while sensing

the interaction between the tip and the sample. The tip is on a flexible cantilever and is sensible to

electronic, size and density changes in the surface. During the test, a laser diode emits a laser beam

onto the back of the cantilever over the tip. As the cantilever moves it deflects the beam and the angular

deflection of the reflected laser beam is detected with a position-sensitive photodiode.

23

Figure 11 display a scheme on how an atomic force microscope works:

The AFM can be operated with three different modes: contact, non-contact and tapping. In this

project, the microscope was only operated in tapping mode.

In this mode, the cantilever deflection (force signal) is recorded as a function of its vertical

displacement (distance signal) as the tip approaches toward and retracts from the sample to obtain a

force vs distance curve, as shown in figure 12. Moreover, spatial resolution can be achieved, generating

a two-dimensional image, through the acquisition of multiple force vs distance curves.[24]

The scanned samples are obtained through the Langmuir-Blodgett method. This method

consists on the insertion of a substrate on a specially prepared Langmuir through, (small hole thus

enabling a full submersion) before deposition of the film. With the aid of the surface pressure sensor

automatic module the film is compressed to the desired pressure or area per molecule and then the

Figure 11-Diagram of AFM work principle. A laser diode emits a beam which is reflected by the cantilever, who is contact

with the sample, and measured by a position-sensitive photodetector.

Figure 12- Tapping mode principle and force vs distance curve example. The tip approaches and retracts from the sample creating a force vs

distance curve.

24

substrate is extracted, this is called transfer. During the extraction, the module will try to maintain the

surface pressure that decreases due to the loss of molecules and instability of the film.

The substrate used were silicon wafers. They must be cut, since they are bought as circles with

15-20 cm diameter, this was done with a diamond-pointed cutter. The final preparation step involves the

cleaning of all organic residues. This was done with a piranha solution, a mixture of sulfuric acid (H2SO4)

and hydrogen peroxide (H2O2), volume 2:1 respectively.

II.5.X-ray experiments

II.5.1. X-ray Reflectivity (XRR)

X-ray reflectivity (XRR) is a technique for studying the detailed surface properties of materials.

Specifically, x-rays are used to probe the electron density perpendicular to the surface and thereby

obtain information about the surface roughness, thin film thickness and density.

Starting at a grazing angle, where total reflection occurs, reflection intensity is measured as a

function of the incident angle with respect to the film surface.

The reflectivity is measured as the ratio between the total reflected beams and the incident

beam (equation 8). The reflectivity intensity can be decreased with respect to an ideal interface (Fresnel

reflectivity) due to the roughness of the interfaces .[25]

0

IR

I

(8)

Where 𝐼0(𝜃) is the incident intensity and 𝐼(𝜃) the specular intensity, the sum of all reflected

beams.

25

The fundamental mechanics of this experiments are shown by Figure 15:

Each layer present in the film creates an interface, according to its layer thickness and electron

density. The interference resulting from the reflection on each interfaces impacts the reflectivity. This

impact is shown as oscillations that can be used to infer the characteristics of each layer.

As said before, the experiment starts at total reflection and at zero penetration depth. As the

angle is increased, so does the penetration depth. The incident beam is reflected at each interface,

resulting in multiple beams. The detector is responsible for measuring the total intensity off all these

beams.

II.5.2. X-ray Fluorescence (XRF) and Total Reflection X-ray

Fluorescence (TRXF)

Total reflection x-ray fluorescence (TXRF) and Grazing Incidence X-ray fluorescence (GIXRF)

and analysis are powerful analytical tools with respect to detectable elemental range, simplicity of

quantification and detection limits, making them versatile techniques. They have the capacity to detect

almost all elements of the periodic system, namely from boron to uranium. These techniques are the

usual choice for chemical analysis of Langmuir films since they are especially suited for applications in

which only a very small amount of sample is available, as only a few micrograms are required for the

analysis.[26]

The analysis of major and trace elements in a sample by TXRF and GIRXF is made possible by

the behavior of atoms when they interact with radiation. When materials are excited with high-energy,

short wavelength radiation (e.g. X-rays), they can become ionized. If the energy of the radiation is

sufficient to dislodge a tightly-held inner electron, the atom becomes unstable and jumps to an excited

state. When the atom relaxes to its ground state, the energy is released due to the decreased binding

energy of the inner electron orbital compared with an outer one. Because the energy of the emitted

Detector

Film

Sub-

phaseeee

Incident beam Reflected beams

Figure 13- Scheme of the XRR mechanism on a molecular monolayer with a liquid sub-phase. The incident beam is reflected by each interface depending on its electron density and thickness. The reflected beam is then measured by

a detector.

26

photon is characteristic of a transition between two specific electron orbitals of a particular element, the

resulting fluorescent X-rays can be used to detect and identify the elements present in the sample.[27]

TRXF is special energy-dispersive x-ray analytical technique that extends x-ray fluorescence

(XRF) down to the ultra-trace element range and is based on the phenomenon that below the critical

angle the penetration of the w-rays is limited (Fig. 14). The beam gets reflected from a flat polished

surface of any material at the same angle as the incident one and has almost the same intensity as the

primary beam except for a small portion that is refracted and penetrates the reflecting medium. This

evanescent wave loses intensity exponentially as it penetrates deeper into the medium, exciting the

atoms it encounters, resulting in them emitting their characteristic radiation in all directions. Since there

is virtually no backscatter into the detector, extraordinary detections limits can be achieved, up to a

femtogram (fg =10-15 g).[28]

Figure 14a) and 14b) display simplistic schematics of, respectively, XRF and TRXF

experimental techniques and where it is possible to observe the differences between each technique.

The GIXRF technique is an extension of TXRF in which the X-ray fluorescence is measured as

a function of the angle between the sample surface and the incident beam. The angle is varied

continuously from 0° to far above the critical angle of total external reflection. Since the penetration

depth (depth at which the intensity inside the medium is reduced to 1/e) is proportional to the angle, this

technique permits the continuous analyses of the film from 0 nanometers till the pretended depth.

The result is displayed as a fluorescence spectrum that includes all the characteristic of

wavelengths of the atoms present in the sample.

Figure 15 serves as a good example to explain the theoretical conditions of TXRF and GIXRF.

Three angles can be observed, the incident angle θ1, the refracted angle θ2 and reflected angle θ3. The

difference between these two x-ray techniques comes from the incident angle θ1, while TXRF the angle

Figure 14a) and b)- Simplistic schemes of XRF (left) and TRXF (right). In traditional x-ray fluorescence the angle used is above the critical angle, while in total reflection x-ray fluorescence the angle used is below the critical angle. In the second case, only a very small part of the incident beam enters the bulk, nullifying the backscatter into the detector.

(x-ray tube wasn’t used on the experiments- just for demonstration purposes).

27

is always below the critical angle and is the same during all the experiment, in GIXRF the angle varies

from 0º to the pretended maximum one.

Total reflection x-ray fluorescence (TRXF) was performed at SOLEIL synchrotron for various

points of a compression cycle at a fixed penetration depth, while Grazing Incidence X-ray Fluorescence

(XRF) was done at the ESRF synchrotron with two sub phases, pure water and salted water at the same

pressure and a penetration depth range from 4 to 12 nanometers.

In this project, these techniques were indispensable of the formulation of certain theories and

were used detect the concentration and position of the anion [NTf2]- through the Sulphur atom

fluorescence.

Figure 15-Sketch of theoretical conditions for GIXRF and TRXF between two mediums. Depending

on the incident angle (θ1) the beam is reflected at a same angle (θ3) and a intensity depending

on θ1 and the refracting mediums (θ3) and refracted on angle (θ2) and intensity given by the Snell-Descartes Law.

3

28

III. Results and Discussion

All the results obtained during this project will be displayed and discussed in this chapter. The

sub-chapters represent each different experiment: (π-A) Langmuir isotherms, AFM, BAM and X-ray

experiments (XRR, GIXRF and TXRF).

III.1.Langmuir Isotherms

Langmuir isotherms were used to measure the surface pressure under cycles of compression

and relaxation. In this chapter varied experimental conditions were tried as an attempt to understand as

much as possible of the behavior of this imidazolium ionic liquid family.

Three Langmuir throughs (different sizes) were used for these studies having a direct implication

on the rates of compression and relaxation. The throughs were used based on necessity and availability.

For the measurements with normal water, big throughs were used due to a high compression area and

for the measurements with salted water, smaller throughs were used.

Experiments with different conditions were done to characterize each molecule, including,

compression cycles, weekend relaxation times and addition of salt ([Na][NTf2]) to the sub phase.

All these tests were done for [C18mim][NTf2], while for the other two molecules this was not the

case. If the section doesn’t have the experiment and there is no indication of the appendix containing

the results the experiment wasn’t realized.

The results are organized by molecule and each section contains all the relevant Langmuir

isotherm experiment results and the corresponding commentaries and conclusions.

III.1.1. [C18mim][NTF2]

III.1.1.1. Base isotherm and compression cycles

As a comparison target and a method to introduce the terms used to characterize each isotherm,

a C18 isotherm is displayed and analyzed at standard conditions: pure water sub phase; 20ºC and default

compression rate (30cm2/min).

Figure 16 describes the base isotherm of [C18mim][NTf2]. A cycle of compression and expansion

at 20ºC with 10 minutes of solvent evaporating time and a compression rate of 30 cm2/min. At blue we

observe the compression curve and at orange the expansion.

29

As one of the most used ionic liquids, C18, the graphic displayed on figure 16 is a very typical

ionic liquid isotherm. As said in the introduction normally we observe four phases, gas, liquid expanded,

liquid condensed, solid and the corresponding phase transactions. In this ionic liquid, as in most, this is

not the case. It displays only one singular phase and a phase transition.

Compressing the film, one observe a lift-off (area when the surface pressure can be measured)

of about 1.2 nm2/molecule, indicating a homogeneous layer. The film can be compressed displaying a

steady pressure increase over an extensive range of area to 0.27nm2/molecule, where one observes

the start of a plateau at 25 mN/m. Considering the molecules architecture these low areas per molecule

observed in this region are not compatible with a monolayer. The plateau can be identified as the

collapse of the monolayer, since it’s the molecules transition to an area above the interface and

formation a multilayer.

Expanding the film after this collapse, one observes an immediate decrease of the surface-

pressure up to 15 mN/m, where a new plateau is observed. This plateau ended (on expansion) at an

area of 0.3 nm2/molecule, similar to the value where the collapse plateau starts upon compression. This

surface-pressure value of 15 mN/m has been identified in previous works [11] as the one of the ESP for

this compound. It is then coherent to observe such a plateau when expanding the film from the collapse

state, . It is also coherent that the end of this plateau corresponds to an area where all the deposited

molecules can have a monolayer arrangement. These results indicate, that upon compression, above

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1 1.2 1.4

π (

mN

/m)

Área per molecule (nm2/molecule)

Lift-off Start of collapse

Collapse Plateau

ESP Plateau

Figure 16- Base (π-A) Isotherm of C18mimNTf at 20ºC, a compression rate of 30 cm2/min and 10 minutes of solvent evaporating time. Blue curve corresponds to the compression curve and orange to the expansion.

30

the value of the ESP (15 mN/m) and the beginning of the collapse (25mN/m) the film is in a metastable

state.

As said in the introduction, the compressibility can be used to find changes in structures during

compression/expansion and characterize compression regimes. Five separate compressibilities were

calculated, three for the compression curve, above and below the ESP and full curve before the collapse

and two for the expansion, above and below the ESP. The results are displayed on table 1, as well as

the intervals used and the curve it corresponds to. The minimum limit of 8 mN/m was used for the

compression curve, since below it the slope isn’t constant, the same reasoning was used for the value

of 4 mN/m for the expansion.

Table 2- Compressibility of C18 on pure water sub-phase for three intervals of the compression and two of the expansion curve. Compressibility was calculated through equations 2 and 3.

Curve Pressure interval (mN/m) Compressibility (m/mN)

Compression [8;24] 43



Expansion [4;14] 62

Expansion [16;24] 42

The compressibilties measured on compression are 46 before the ESP, 42 below and 43 for the

curve before the collapse. A compressibility value above 20 indicates a liquid expanded (LE) phase. A

slight change is seen when surpassing the ESP in the compression curve. This means the existence of

two different compression regimes, a small structural change of the monolayer and a thicker film. Filipe

et al.[10] also reported and commented on this phenomenon through the compression modulus, the

inverse of compressibility. The results obtained are in agreement with his article, since they report an

increase to higher values of compressibility modulus (1

𝛽). This means a transition to a different

compression regime.

The compressibilties measured on expansion are 62 before the ESP and 42 below. The first

one cannot be interpreted since the system is out of equilibrium and in a phase coexistence (multilayer

in equilibrium with the monolayer). For surface pressures below the ESP the film is in the monolayer

state and the compressibility indicates a LE type of phase, same one observed in the compression.

Due to the repulsion between the ionized heads only the LE phase is observed and a more

organized liquid phase (LC) is never observed and entirely skipped, heading straight to a phase

transition to a 3D multilayered phase. During this transition, previous works (Eduardo Filipe et al.[10] and

Raquel Valente[11]) reported the appearance of ramified crystals. Another characteristic displayed by C18

and common to most ionic liquids is the broad range of area of compression of the liquid phase.

To study the stability of the film, successive cycles were performed.

31

Figure 17 shows three compression/expansions cycles on pure water sub phase with 10

minutes of solvent evaporation time and a compression rate of 30 cm2/min. Blue curve corresponds to

the 1st cycle, the orange the 2nd cycle and grey the 3rd .

With successive cycles the isotherm shifts to smaller areas and plateau shifts to higher

pressures: in the 2nd cycle the plateau happens at 26 mN/m and the 3rd at 27 mN/m. While in the 2nd

cycle the collapse plateau is still existent, in the 3rd it’s barely visible. Further compressions would tend

to an equilibrium isotherm (next cycle would be the same as the previous) and to the disappearance of

both plateaus, ESP and the collapse.[11] The compression curve seems to follow the expansion curve of

the previous cycle and the hysteresis is bigger in the first cycle and gets lower progressively.

The reason for this mechanism is currently unknown. The low areas of the 2nd and 3rd cycle

aren’t compatible with a monolayer, this can indicates that during each cycle some part of the 3D

structure do not respreads to form a monolayer.

This shift also happens with time as shown by figure 18.

III.1.1.2. Relaxation time

Relaxation time is the time difference between the deposition and the compression. This type

of test was realized with the objective of testing the stability of the film. Two compression isotherms were

0

5

10

15

20

25

30

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

π(m

N/m

)

Area per molecule (nm2/molecule)

1st cycle 2nd cycle 3rd cycle

Figure 17- Three consecutive compression/expansion cycles (π-A) isotherms of C18mimNTf2 on pure water sub phase at 20ºC, a compression rate of 30 cm2/min and 10 minutes of solvent evaporating

time. Respectively blue, orange and gray curves are 1st, 2nd and 3rd cycle.

32

made for this test: One with 15 minutes and another whose film was only compressed after waiting for

one weekend (≈66h): deposition at 6 pm of Friday and compression at 10 am of Monday.

Figure 18 displays two compression curves on pure water sub phase with a compression rate

of 30 cm2/min. Orange curve was obtained with a relaxation time of zero and blue of 66 hours.

Figure 18- Two compression/expansion cycles (π-A) isotherms of C18mimNTf2 on pure water sub phase with different relaxation times. Both done with a compression rate of 30 cm2/min and 10

minutes of solvent evaporating time. Orange curve was done with relaxation time of 15 minutes and blue with a relaxation time of 66 hours.

It’s observable that with a large relaxation time, the plateau region disappears and the isotherm

shifts to very low areas, very similar to what was observed with successive cycles. This means that the

monolayer state is unstable, without any outside stimulation the film is evolving.

These results lead to the conclusion that the mechanism which happens with successive cycles

is the same that happens with time. While the compression and expansion seems to fasten the process,

this shows they aren’t the main responsible for the film instability.

Valente in her thesis [11] performed a more extensive study on this case and, as with successive

compressions, concluded upon a certain number of cycles or time evolution stops and the monolayers

remains stable, reaching an equilibrium isotherm. This means that the film keeps evolving till it reaches

a certain number of cycles or hours. At this point the mechanism responsible for the instability of the film

and the disappearance of molecules from the monolayer is now in equilibrium with the monolayer.

0

5

10

15

20

25

30

35

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4

π(m

N/m

)


0 hours 66 hours

33

III.1.1.3. Addition of Salt

The salt used was Sodium bis(trifluoromethylsulfonyl)imide [Na][NTf2], the anion being the same

as the one in the ionic liquids. It was hoped the addition of this salt in small concentrations to the sub

phase would increase the stability of the ionic liquid film.

Langmuir isotherm cycles were performed for three different sub phase concentrations (0.03;

0.01; 0.002 mM) and compared to an isotherm done with pure water.

Figure 19 displays four compression/expansion cycles at 20ºC and a compression rate of 30

cm2/min. Respectively, yellow, blue and orange correspond to the salted water cycles with

concentrations of 0.03mM, 0.01mM and 0.002mM and the grey curve to the pure water.

The observation isotherm leads to the conclusion adding salt to the sub phase shifts the curve

to higher areas per molecule. Compressibilities were calculated as method of comparison and the region

above the ESP of the compression curve was chosen, due to stable slope. In table 2 are listed the area

shift between the salted sub phase cycles and the pure water cycle and the compressibility of the upper

compression curve (between 16 and 24 mN/m). Compressibility of the pure water cycle was calculated

previously and is 42m/mN for the upper part of the curve (between [16, 24] mN/m).

0

5

10

15

20

25

30

0.05 0.25 0.45 0.65 0.85 1.05 1.25

π(m

N/m

)


0,03 mM 0,01 mM 0,002 mM No salt

Figure 19-Four compression/expansion cycles (π-A) isotherms of C18mimNTf2, one on pure water sub phase(grey) and three on salted water ([Na][NTf2]) sub phase. Both done at 20ºC, a compression rate of 30 cm2/min and 10 minutes of solvent evaporating time. For the salted water sub phase three concentrations

were used: 0.03 (yellow), 0.01 (blue) and 0.002 (orange) mM.

34

Table 3- Area shifts and compressibility of the isotherms with salted water as sub-phase. Compressibility was calculated through equations 2 and 3.

Concentration (mM) Area shift (nm2/molecule) Compressibility (m/mN)

0,03 0,19 30

0,01 0,18 35

0,002 0,13 29

The change on compressibility can be due to the area shift, since the compressibility directly

depends on the inverse of the area and the derivative of it. This means a shift to higher areas also

decreases the compressibility. To work around this issue, the area shift was added to the isotherm done

with pure water sub phase. The curves overlapped each other and had the same compressibility, the

graph is on Appendix 1. The ratio − 𝛥𝐴

𝛥𝜋 was also calculated for both the isotherms with pure water and

0.01mM salt concentration and the obtained result was equal: 2.3*10-2.

Multiplying the area shift by the number of molecules and dividing by the total it’s possible to

calculate the percentage of surface area lost (table 3). This gives us the approximate area the salt is

occupying on the surface and the decrease of the available area of the monolayer.

Table 4- Surface area loss for the different concentrations of salt on the sub-phase.

Concentration (mM) Surface area loss (%)

0.03 14

0.01 13

0.002 9.5

To test the impact of adding [Na][NTf2] on the stability of the film, four cycles of

compression/expansion were performed on a salted sub phase of concentration 0.01mM, 20ºC and

compression rate of 30 cm2/min. The results of this test are displayed on figure 20.

35

The results show that the salted sub phase stabilizes the film. The hysteresis between the

compression and expansion curve is significantly smaller and the shift to smaller areas with successive

cycles also decreases. While there is considerable evolution between the first and second cycle,

between the 2nd, the 3rd and the 4th the shift is small, this didn’t happen for the pure water sub phase,

where between cycles there was always a significant decrease in the area. For these reasons it’s

possible to conclude that the addition of [Na][NTf2] increases the stability of the film.

This also indicates that the area variation upon time/cycles on pure water sub-phase is probably

due to a variation of the amount of salt adsorbed at the interface. The collapse plateau can then be

either a collapse of the C18 molecules, of the [Na][NTf2]one, or more probably of both. In this last case,

one can consider that only one specie is able to respreads when the film is expanded.

III.1.2. [C12C12im][NTF2]

III.1.2.1. Compression cycles

As a way to characterize C12C12 isotherm and to find similarities between its behavior at the

interface and C18’s behavior, two consecutive compression/expansion cycles isotherm are displayed

and analyzed at standard conditions: pure water sub phase; 20ºC and default compression rate.

(30cm2/min).

-1

4

9

14

19

24

29

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

π(m

N/m

)


1st cycle 2nd cycle 3rd cycle 4th cycle

Figure 20- Isotherm for consecutive cycles of a C18 film with salted water (0.01mM) as sub-phase at 20ºC, a compression rate of 30 cm2/min and 10 minutes of solvent evaporating time. Yellow curve corresponds to the first

cycle, blue to the second, orange to the third and grey to the fourth and last.

36

Figure 21 displays two cycles of compression and expansion of [C12C12im][NTf2]. The

experiment was done at 20ºC with 10 minutes of solvent evaporating time and a compression rate of 45

cm2/min, the 2nd compression followed immediately after the 1st. The blue isotherm refers to the 1st

cycle and the orange to the 2nd cycle.

Compressing the film, one observes a lift-off (area when the surface pressure can be measured)

of about 1.2 nm2/molecule, indicating a homogeneous layer. The film can be compressed displaying a

steady pressure increase over an extensive range of area up to 0.63nm2/molecule, where one observes

the start of a plateau at 25 mN/m. Considering the molecules architecture, of roughly double the width

of a singular chain IL, this indicates the low areas per molecule observed in this region are not

compatible with a monolayer. The plateau can be identified as the collapse of the monolayer, since it’s

the molecules transition to an area above the interface and form a multilayer.

In the expansion curve, the pressure quickly decreases to 14 mN/m and we observe another

plateau. This plateau corresponds to the measured ESP value of the compound. In the 2nd cycle the

ESP plateau shifts to a value of 17mN/m.

The plateau-region is not smooth and has a small slope, which is due to the high compression

speed used. This compression/expansion rate was used since lower speeds results in the appearance

of small bumps above 15mN/m on the compression curve.

Since C12C12 is a larger molecule, due to having two carbon chains, it was expected to observe

a shift compared to C18, since each molecule occupies a larger area. As with C18, a large hysteresis is

0

5

10

15

20

25

30

35

0 0.2 0.4 0.6 0.8 1 1.2 1.4

π(m

N/m

)


1st cycle 2nd cycle

Figure 21- Two consecutive compression/expansion cycles isotherms of C12C12mimNTf2 on pure water sub phase at 20ºC, a compression rate of 45 cm2/min and 10 minutes of solvent evaporating time.

Respectively blue and orange curves are 1st and 2nd cycle..

37

observable from the 1st cycle to 2nd one and the pressure of the pressure of the plateau doesn’t change

between cycles. With multiple compressions the phase transition region tends to disappear and the

isotherm will evolve reaching the equilibrium.

The compressibility of the compression curve, between 4 and 24mN/m (constant slope) of the

1st cycle was calculated and the value of 14 m/m was obtained. This value indicates this phase is LC

and that C12C12 doesn’t display the LE phase. The compressibility of C12C12 is lower than half then of

C18, meaning C12C12 is less elastic. This can be due to the area shift, but also that these molecules form

a thicker film.

III.1.2.2. Addition of salt

One should underline that the [Na][NTf2] surface density should be divided by a factor of about

two in this case, due to the double carbon chains present in the molecule.

Two different salted water ([Na][NTf2]) concentrations were used 0.01 and 0.03 mM and then

the results were compared to an isotherm done with pure water.

Figure 22 displays three compression/expansion cycles at 20ºC and a compression rate of 30

cm2/min. Respectively, orange and blue correspond to the salted water cycles with concentrations of

0.03mM, 0.01mM and the grey curve for the pure water.

0

5

10

15

20

25

30

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

π(m

N/m

)


0,01 mM Salt 0,03 mM Salt No salt

Figure 22- C20 Isotherms for pure water (grey) and salted sub-phases at 20ºC, a compression rate of 45 cm2/min and 10 minutes of solvent evaporating time. Two different concentrations of salt

were used 0.03mM (orange) and 0.01mM (blue).

38

Using a salted sub phase has a very different impact on C12C12 instead of shifting the curve to

larger areas, we observe a shift to lower pressures, 1 mN/m and 3 mN/m, respectively, for 0.01mM and

0.03mM.

These results lead to the conclusion that with a C12C12 film the salt [Na][NTf2] doesn’t occupy

the surface neither reduces the available area for the monolayer, but instead has impact on the structure

formed at the plateau. The salt has no impact on the compressibility of the film.

To test the impact of adding [Na][NTf2] on the stability of the film, four cycles of

compression/expansion were performed on a salted sub phase of concentration 0,01mM, 20ºC and

compression rate of 30 cm2/min. The results of this test are displayed on figure 23.

The salt seems to have higher stabilizing impact on C12C12 then on C18, since the 2nd, 3rd and

4th cycle are almost identical, although a shift still exists from the 1st and 2nd cycle it’s significantly smaller

then with a pure water sub phase. The effect happens only on the first compression.

Due to the two alkyl chains present on C12C12 and their impact on the size of molecule, the same

concentration of [Na][NTf2] should have a bigger impact on stability then with C18, since its necessary

less salt per area to equilibrate the charges. Due to the width of the double chains for the same occupied

area a monolayer of C12C12 displays less molecules, and thus less ions then a single chain IL.

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1 1.2 1.4

π(m

N/m

)


1st cycle 2nd cycle 3rd cycle 4th cycle

Figure 23- C12C12 Compression cycles with salted water (0.01mM) as sub phase at 20ºC, a compression rate of 45 cm2/min and 10 minutes of solvent evaporating time. Blue curve

corresponds to the first cycle, orange to the second, grey to the third and yellow to the fourth and last cycle.

39

III.1.3. [C20mim][NTf2]

III.1.3.1. Compression cycles

As a way to characterize C20 (π-A) isotherm and to find similarities between its behavior at the

interface and C18’s behavior, two consecutive compression/expansion cycles isotherms are displayed

and analyzed at standard conditions: pure water sub phase; 20ºC and default compression rate.

(30cm2/min).

Figure 24 displays two cycles of compression/expansion of [C20mim][NTf2] with pure water sub-

phase. The experiment was done at 20ºC with 10 minutes of solvent evaporating time and a

compression rate of 30 cm2/min. For demonstration purposes, during the 2nd cycle the film was

expanded at 25mN/m and not at the end of the through. The blue isotherm refers to the 1st cycle and

the orange to the 2nd cycle.

Three distinct phases and two different phase transitions are observable. The isotherm indicates

first a LE phase (monolayer) up to 17 mN/m and 0.55 nm2 per molecule, at 17mN/m we observe a

plateau of a phase transition which extends over a broad range of area between 0.55 and 0.35

nm2/molecule. From the analysis of the isotherm, there are two possibilities for the phase 2nd phase

shown: a LC phase or a tri-layer (multilayer). Since the ratio of the areas of the phase transition 0.55

0.22=

Figure 24- C20 two consecutive compression cycles with pure water) as sub phase at 20ºC, a compression rate of 30 cm2/min and 10 minutes of solvent evaporating time. Blue curve

corresponds to the first cycle, orange to the second and last cycle. The expansion of the orange curve was started midway of the second plateau for demonstration purposes.

0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1

π(m

N/m

)


1st compression 2nd compression-stopped at 30mN

40

2.5 is close to 3 (0.55

0.22= 2.5), this could indicate a collapse a transition to a two-layer, tri-layer (multilayer).

A two-layer system is improbable since, for ILs film it’s unstable with usual amphiphilic molecules due

to the fact that you cannot have a charged surface. Although the collapse should happen at similar

surface pressure then for C18, it’s possible due to the increase of the chain length that it happens at

lowers pressures. Compressibility (see below), BAM and AFM results should answer what’s the correct

hypothesis.

The pressure rises up again at 33mN/m and below 0.15 nm2/molecule reaching high values of

surface pressure up to 55mN/m.

The expansion isotherm changes depending on when the expansion starts, if the film is

expanded during or before the 2nd phase transition, two different phases equilibriums are observed, one

at 10mN/m and the other at 15mN/m, if the film is expanded after the end of the 2nd phase transition

only one equilibrium phase happens.

If we admit the 2nd phase transition is the collapse and the formation of a multi-layer, the

equilibrium phase that happens on both expansion isotherms corresponds to the equilibrium surface

pressure, since it’s the equilibrium between the multilayer and the monolayer. The other phase transition

corresponds to the equilibrium between the LC or tri-layer and the multi-layered crystal.

The compressibility was calculated for the compression (1st cycle) of the two phases shown in

the isotherm and for the expansion curve only below the ESP, since in the metastable region above the

ESP the slope is close to ∞. Table 4 contains the compressibility for these two phases, as well as the

pressure interval used for the regression.

Table 5- Pressure intervals and corresponding compressibilities for the three phases shown in the compression and for the expansion below the ESP. Compressibilities were calculated through equation 2 and 3.

Curve Pressure interval (mN/m) Compressibility (m/mN)

Compression [7;13] 39.0



Expansion [7;4] 39.3

Both C18 and C20 present similar LE compression regimes, since the compressibility of the liquid

phase (first interval) is similar to the one of C18 LE phase. This means they have a similar structure in

the liquid phase. For the second interval the compressibility of C20 is significantly lower (~19m/mN),

meaning a denser structure. This value (close to 20) is in agreement with the LC hypothesis. The

compressibility of the third phase is roughly the same as the second one, this is unusual since these low

areas are only compatible with a multilayer, and thus a denser film. In the expansion below the ESP we

observe the same compressibility as the first interval, meaning the film is in the LE phase.

41

While the compression of the liquid phase displays hysteresis, like the C18, the compression of

the thin crystal (2nd phase) and phase transition to the multilayer doesn’t. The phase transition to the

multilayer happens at very similar areas, unlike C18 and C12C12. A more exhaustive test on this case is

figure 25.

Figure 25- Five consecutive compression/expansion cycles isotherms of C20mimNTf2 on pure water sub phase at 20ºC, a compression rate of 30 cm2/min and 10 minutes of solvent evaporating time.

Respectively blue, orange, gray, yellow and blue curves are 1st, 2nd, 3rd, 4th and 5th cycle.

For the test displayed above, 1 hour was waited between the 1st and 2nd cycle as an attempt

to reach the equilibrium isotherm. As before a large hysteresis is observed for the compression of the

1st phase. The 2nd phase transitions happens at similar areas and lower surface pressure. The lower

surface pressure can be due to the evaporation of the sub phase during the waiting time, since it affects

the calibration of the sensor.

While C18 and C12C12 progress to the equilibrium isotherm with no collapse plateau-like region[11]

with multiple cycles, C20 isotherms stops evolving with a small number of cycles/waiting time. Even

though this happens, the first phase transition to the thin crystals stops to be visible and only the phase

transition to the multilayer seems to occur.

III.1.3.2. Addition of salt

The addition of salt to the sub phase has a small impact on C20 behavior and as the previous

molecules shifts the curve to the right. The results are in Appendix 2 and were obtained with a [Na][NTf2]

concentration of 0.01mM, 10 minutes of solvent evaporation and a compression/expansion rate of 30

cm2/min.

-5

0

5

10

15

20

25

30

35

40

0 0.2 0.4 0.6 0.8 1

π (

mN

/m)


1st compression 2nd compression3rd compression 4th compression5th compression

42

With this salt concentration the isotherm is shifted to the right approximately 0.25 nm2/molecule.

III.2.Microscopic experiments

III.2.1. Brewster Angle Microscopy

During a BAM experiment, a (π-A) isotherm is measured and during this cycle a film is recorded.

This film was then analyzed and from it, images of the most relevant changes were saved.

These images are displayed and commented in this section and are accompanied by the

corresponding isotherm (except on C12C12), as a graphical help to where each screenshot corresponds.

The structures observed display different brightness depending on their density, brighter means

higher surface density and darker the opposite. The Brewster Angle Microscope can only focus one line

on the screen and this line must be adjusted regularly during the experiment. It’s possible the line

placement is not perfectly adjusted for all the images taken. The further way the objects are from the

middle of screen the more unfocused they are.

III.2.1.1. [C18mim][NTf2]

Figure 26 and figure 27 displays, respectively, two cycles of compression/expansion on C18 on

pure water sub phase and the corresponding images (table 5). The experiments were realized at 20ºC,

a compression and expansion rates of 9.5 cm2/min and a solvent evaporation time of 10 minutes.

The arrow points to the corresponding area/pressure where the image was taken.

Table 6- Area, pressure and curve of each image taken during C18 pure water BAM experiment.

Image Pressure (mN/m) Area per molecule

(nm2/molecule) Curve

1 20.62 0.51 Compression




5 13.63 0.30 Expansion


43

Figure 27- Six images taken during the BAM experiment on C19 with pure water sub-phase.

1 2

3 4

5 6

0

5

10

15

20

25

30

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

π(m

N)

Area por molecule (nm2/molecule)

1

2 3 4

5 6

Figure 26- BAM experiment isotherm at 20ºC, a compression rate of 30 cm2/min and 10 minutes of solvent evaporating time. The numbers correspond to the area and pressure where the

corresponding image was taken.

44

Due to a low compression/expansion rate the collapse appears at lower pressures and is not

steady. During the compression, at about 18mN/m (figure 27 image 1) we start to observe the

appearance of circular domains, and upon reaching the collapse the film transitions to large ramified

crystals (figure 27 image 2;3;4) with different brightness, higher density in the center and lower in the

peripheral regions. During the expansion, we observe the “melting”, first, of the darker areas of the

crystals and then the rest of them. (Figure 27 image 5;6)

The appearance of the circular domains can be the reason to why we observe a small

compressibility change during the compression above 15 mN/m. As displayed by the (π-A) isotherms,

C18 undergoes a phase transition to a 3D phase 25mN/m but the ESP is estimated at 14 mN/m. These

domains are probably the first appearance of the collapse process which is not detectable by the

isotherm.

The same type of crystals were observed with salted water. No difference in size or structure

was observed. The resulting isotherm and images taken are on appendix 3.

III.2.1.2. [C12C12im][NTf2]

For this molecule, the BAM results showed unsatisfying. During the compression no structures

are observed, only when reached the plateau small domains start to be detectable. The number of

domains start to increase during the plateau compression. Figure 28a) and b) are both images from the

end of the plateau, taken, respectively at 26.34 mN/m & 0.3 nm2/molecule and 25.15 mN/m & 0.28

nm2/molecule.

Figure 28- Two images of the C12C12 BAM experiment. Left screenshot was taken at 26.34 mN/m &

0.03nm2/molecule and right screenshot at 25.15mN/m & 0.28 nm2/molecule.

These domains are similar to the ones formed by C18 above 15 mN/m, and are probably

collapsed domains as suggested by the isotherm. However, the isotherm indicates that the collapse

start at about 0.7nm2/mol. One then expect a larger surface occupied by the 3D domains at such low

area (0.3nm2/molecule) except if the domains are very thick which does not seems to be the case,

considering their intensity.

45


Figure 29 and 30 are, respectively, a cycle of compression/expansion on C20 on salted sub

phase (0.01mM) and the corresponding screens (table 6). The experiments were realized at 20ºC, a

compression and expansion rates of 9.5 cm2/min and a solvent evaporation time of 10 minutes.

The numbers correspond to the area/pressure where the image was taken.

The experiment showed is on salted water sub phase ([Na][NTf2]), since there was noticeable

difference between pure water and salted sub phase and the images were clearer for the experiment

with salted water.

Table 7- Area, pressure and curve of each image taken during C20 salted water BAM experiment.

Screen Pressure (mN/m) Area per molecule


1 16,90 0,58 Compression




5 16,25 0,23 Expansion




5

Figure 29- BAM experiment isotherm of C20mimNTf2, salted sub phase (0.01 mM) at 20ºC, 30 cm2/min compression rate.

0

5

10

15

20

25

30

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

π(m

N/m

)

Area per molecule (nm2/mol)

1 2

3 4

6 7

8

46

Figure 30- Six images taken during the BAM experiment on C20 with salted water sub-phase (0.01mM).

BAM results confirm the three phases observed in (π-A) isotherms. The first and second phase

being, respectively, liquid expanded and condensed and the other crystalline multilayer. The 2nd phase

observed at about 17mN/m and 0.65 nm2/molecule is a liquid condensed phase that slowly covers the

entire surface before the next phase transition (figure 30 image 1;2;3). The last phase corresponding to

a non-ramified high-density crystal due to their white bright color (figure 30 image 4;5). In the expansion

isotherm, we observe first the melting of the LC domains and then the 3D crystals (figure 30 image 6;7).

Reaching a full expanded through, a net-like of circular domains are still observable in the film (figure

30 image 8), even after one hours of waiting, further images showing this are shown at Appendix 4.

1

1

1

1

1

2

3 4

5 6

7 8

47

While the last phase crystals display similar brightness to the ones formed by C18, the size and

shape are quite different, C18 transitions to a very branched crystal with higher density on the center and

low-density dendrites, while C20 cylindrical-shaped crystals.

The domains that remain after full expansion differ from the ones C18 form above 15mN/m during

compression. The ones formed by C20 are bigger and group, while the C18 are normally seen apart from

each other.

The domains remaining after a full cycle can be one of the reasons of the hysteresis observed

in the three ionic liquids, but why are they only observed in C20? They can simply not be detectable due

to lower sizes of the molecules or simply the mechanism is not the same and the reason has no

correlation to these domains.

III.2.2. Atomic Force Microscopy

The scans obtained were visualized and analyzed with the freeware program GwyddionTM. [29]

Scanned areas range from 10 and 20 μm. This project will focus on the height measurements, even

though the microscope measures other surface parameters, like peak force, stiffness and adhesion.

All the results display small bulk circles, those circles correspond to impurities. This was proven

due to their existence on a sample with no film transferred. This could be due to a bad cleaning (incorrect

volume or impurity filled acid or base) or due to a faulty substrate cutting.


Figure 31 and figure 31 display, respectively, one (π-A) isotherm of C18 and three scans. The

scans were transferred at these pressures: during the compression (18mN/m), at the collapse (25mN/m)

and at ESP during expansion (15mN/m) (table 7). The isotherm it is used as the example isotherm to

point out at which pressures the samples were transferred and is the same as figure 16.

48

Table 8- C18mimNTf2 AFM scans and corresponding size of the scan, pressure at which the substrate was transferred and if the was after compression or expansion.

Scan Size (μm) Pressure (mN/m) Curve

1 10x10 18 Compression 2 20x20 25 Compression 3 10x10 15 Expansion

Figure 31- Compression/expansion isotherm of C18mimNTf2with pure water as sub-phase at 20ºC, a compression rate of 30 cm2/min and 10 minutes of solvent evaporating time. The numbers identify the area

and pressure where the substrate was transferred.

0

5

10

15

20

25

30

0 0.2 0.4 0.6 0.8 1 1.2 1.4

π(m

N/m

)


1

2

0

3

0

49

In the compression, below the collapse, nothing is observable (figure 32 image 1), while in the

collapse copious quantities of dense crystals (figure 32 image 2) are observed and in the ESP in the

expansion, crystals are also observed but in less quantity (figure 32 image 3). In the collapse crystals,

it’s also observed a higher height and thus a higher density in the center. Nothing being observable in

image 1 means it’s either a homogeneous layer or no monolayer was transferred or that the monolayer

respreads during the drying of the substrate, the domains that appeared in the BAM experiments above

15mN/m weren’t transferable.

Figure 33a) and b), respectively, display the profiles (difference between heights) obtained from

the collapse and ESP scan. The profile corresponds to the line drawn in each scan.

1

Figure 32- AFM scans of C18mimNTf2. Scan 1 and 2 substrate were transferred during compression and respectively at 18 mN/m and 25mN/m. Scan 3 substrate was transferred during expansion and at 15 mN/m.

2

1 2

3

50

Both crystals display a height between 40 and 50 nanometers, even though the crystal obtained

from a collapse transfer is wider, about 0,5 μm (collapse- 1.5 μm, ESP- 1.0 μm) . This information leads

to the conclusion that the crystal is melting, starting with the less density areas, the peripheral ones.

The information obtained from these results complements the ones obtained with the BAM and

(π-A) isotherms, upon reaching the plateau-like region we observe a transition from LE to 3D (collapse)

with the formation of crystals. During the expansion, we observe the melting of the crystals and at

15mN/m we observe the equilibrium between said crystals and the liquid phase (melting of the less

dense phases first).

Non-salted and salted sub phase samples present no noticeable difference. (Appendix 5)


Figure 34 and 35 display, respectively, one isotherm and two scans. The scans were transferred

at these pressures: at 15mN/m, the 1st crystalline phase and at 25mN/m the collapse (25mN/m), the 3D

crystalline phase (table 8). The isotherm corresponds to the 1st compression/expansion cycle of figure

24 and is used as the example isotherm to point out at which pressures the samples were transferred.

Table 9- C20mimNTf2 AFM scans and corresponding size of the scan, pressure at which the substrate was transferred and if the was after compression or expansion.

Scan Size (μm) Pressure (mN/m) Curve

1 10x10 17 Compression 2 10x10 26 Compression

Profile 1 Profile 2

Figure 33a) and b)- Respectively, profile 1 (collapse scan) and profile 2 (ESP scan). Profiles show the size of the crystals of the AFM scans.

51

In the 1st crystalline layer (figure 35 a)) we observe no transferred liquid condensed domains or

if we do, they are all uniform. Due to film instability and the crystal low thickness it’s possible they aren’t

transferable to the substrates, this is supported by previous works that reported transfer limitations [31]

ranging from reduced mobility to size and pH. The 2nd hypothesis is less possible since from the BAM

experiments the LC domains only cover the entire surface at the very end of the phase transition and

the transfer was done in the middle. In the collapse scan we observe a copious amounts of 3D crystals

with similar height (figure 35 b)).

The profile obtained from the collapse scan is displayed on figure 36.

3

0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1

π(m

N/m

)

Area per molecule (nm2/mol)

Figure 35a) and b)- AFM scans of C20mimNTf2. Scan 1 substrate was transfered at 17mN/m and scan 2 at 26mN/m.

Figure 34- Compression/expansion isotherm of C20mimNTf2 at 20ºC with pure water as sub-phase. Compression/expansion rate used was 30 cm2/min. The numbers identify the area and pressure

where the substrate was transferred.

1 2

1 1

2

52

This profile shows a crystal size of 40 nanometers and 1.5 μm length. This crystal is almost

identical to the one formed by C18 on the collapse. Together with the BAM results, this leads to the

conclusion that the crystals formed at the collapse with C18 and C20 are very similar, although in BAM

they seemed to have different shapes.

No images were taken above 25mN/m, since the trough used didn’t have enough compressibility

range.

III.3.X-ray experiments

X-ray experiments were only done for [C18mim][NTf2].

III.3.1. X-ray Reflectivity (XRR)

The XRR technique served as a method of measuring the number and characterizing the layers

present on a C18 film at various pressures. The experiment started with a grazing angle, where total

reflection occurs, and the angle was progressively increased till the reflected intensity was infimal.

This technique measures total reflectivity intensity and this value is influenced by the films’

thickness, density, surface and interface roughness.

The results were normalized so the maximum intensity (total reflection) is equal to 1. The

reflected intensity is displayed as a function of the wave-vector perpendicular to the surface (z axis).

This vector is dependent on the incident beam wavelength and angle (eq. 8):

𝑄 = 4𝜋

𝜆sin(𝜃𝑖) (8)

Profile 3

Figure 36- Profile 3, obtained from collapse scan of C20mimNTf2. Profiles show the size of the crystals of the AFM scans.

53

Where Q is the wave-vector on axis Z, λ and 𝜃𝑖, respectively, the incident beam wavelength and

angle.

The curve obtained was fitted using the program StochFitTM. In StochFitTM, both model

independent and model dependent fitting routines depend on the calculation of the reflectivity based on

a recursive method, first proposed by Parratt.[31] The electron density profile is divided into very small

layers (< 1 Å), and the reflectivity at each layer is calculated considering the layers beneath it, starting

from the sub-phase and ending with the superphase. The final reflectivity at the interface is thus based

on the contributions of each layer beneath it.[32]

The fit uses three parameters for each layer: electron density, lengths and roughness. Electron

density corresponds to the electrons per volume; Length corresponds to the thickness and the size of

the layer in the z axis; roughness to the quantifications of the deviations in the direction of the normal

vector of a real surface from its ideal form.

The program uses boxes to differentiate each interface and the number used for the fit is

dependent on the model used by the user.

This procedure was performed at three different points of the (π-A) compression isotherm: at

0mN/m in the equilibrium between liquid phase and vapor, at 11.5 mN in the liquid compression region

and at 24 mN/m in the plateau. For the two first pressures, two monolayers models were fitted: one

consisted of two boxes, one the alkyl chain and the other both ions, while the other model consisted of

three boxes, one the alkyl chains and the other two for each ion (imidazolium head and [NTf2]-). For the

scan at the plateau pressure a model based on a tri-layer was tried.

The first model (two boxes) represents a structure where the anions are between the cations,

forming a checkerboard distribution, while the second model (three boxes) a structure where the anion

is below the cation. The first model and the tri-layer model, as well as the starting values for the fits were

based on the simulations, done by Filipe et al.[10]

The program needed some parameters as inputs: scattering length density (SLD) of

[C18mim][NTf2] and of the sub-phase, the x-ray wavelength and the sub-phase roughness. The SLD of

C18 is 10.61, while of pure water is 9.41 (Both were calculated using the SLD calculator of the program),

the wave-length is set by the user and was used 1.55Å and pure water roughness is 3.2Å.[33] The

program normalizes the wave-vector with the wave-vector value corresponding to the maximum

Intensity measured and normalizes the electron density with the value of the sub-phase.

Figure 37a) and b) show, respectively, the fit for the two boxes model and the electron density

obtained for 0 mN/m. In figure a) we observe the normalized reflectivity in function of the normalized

wave-vector and the fit obtained, while in figure b) the resulting electron density in function of depth (z).

0 depth (z) is defined as the interface between the superphase (air) and the first layer and progress in

the direction of the sub-phase.

https://en.wikipedia.org/wiki/Normal_(geometry)

https://en.wikipedia.org/wiki/Normal_(geometry)

54

Figure 38a) and b) show, respectively, the fit for the three-layer model and the electron density

obtained for 0 mN/m and 0.9 nm2/molecule. In figure a) we observe the normalized reflectivity in function

of the normalized wave-vector and the fit obtained, while in figure b) the resulting normalized electron

density in function of depth (z). Table 9 displays the results for each model and the comparation between

them.

Figure 37a and b- Fig. a) shows the normalized reflectivity intensity in function of normalized wave-vector and the fit for the two boxes model obtained for the

scan at 0mN/m. Fig. b) shows the electron density obtained from the fit in function of depth (z) as a box and as a curve fit. Depth (z) starts at the interface


55

Figure 38a and b- shows the normalized reflectivity intensity in function of normalized wave-vector and the fit for the three-layer model obtained for the scan at 0mN/m. Fig. b) shows the electron density obtained from the fit in function of depth (z) as a box and as a curve fit. Depth (z) starts at the interface between the superphase (air)

and the 1st layer progressing to the sub-phase, which is reached about 50 Å.

56

Table 10- Results of the fits for the two models for the scan at 0mN/m. Two-layer model corresponds to the model where the anion is between the cation and the three-layer where the anion is beneath the cation. Experimental electron density (𝑛𝑒) corresponds to the normalized density obtained through the fit multiplied by the water 𝑛𝑒 and calculated, where the layer length is used to calculate the corresponding correct electron density.

Layer Experimental 𝒏𝒆 (electrons/Å-3)

Calculated 𝒏𝒆 (electrons/Å-3)

Length (Å) Roughness (Å)

Two-layer model

Aklyl Chains 0.99 0.35 4.60 4.50

Imidazolium head + [NTf2]-

0.40 0.18 11.14 3.63

Error 1811

Three-layer model

Aklyl Chains 0.76 0.20 7.86 5.09

Imidazolium head 0.37 0.08 6.22 2.92

[NTf2]- 0.41 0.24 6.45 3.84

Error 1805

The error is calculated by the sum of the squares of the deviations between the experimental

result and fit. Two electron densities (𝑛𝑒) are shown, the experimental, where the normalized electron

density obtained by the fit is multiplied by 0.33 electrons/Å-3 (the electron density of water) and

calculated, where the layer length is used to calculate the corresponding correct electron density,

considering the molecules scatter over all available surface, this electronic density is used to compare

the veracity of the fit. Equation 9 was used to calculate the calculated electron density of each layer and

Equation 10 of the water:

𝑛𝑒 =𝑒

102𝐴𝑙 (9)

𝑛𝑒 =𝑒𝑁𝐴𝜌

𝑀10−30 (10)

Where 𝑛𝑒 is the electron density in electrons/Å3, 𝑒 the number of electrons per molecule, A the

area per molecule in nm2/molecule, 𝑙 the length in Å, N the number of Avogadro, M the molar mass in

g/mol and ρ the density in kg/m3.

It’s not possible to confirm which models fits better the film at 0 mN/m, since both models

displayed similar errors.

For both models the experimental electron density differs highly from the calculated electron

density. Usually the experimental value is lower than the calculated, due to the loss of molecules from

the surface. One of the reasons that can influence this value is the non-homogeneity of the layers,

molecules clustering or leaving fully spreading on the surface, can increase the experimental value. A

ratio between experimental and calculated of roughly 3 is observed in almost all layers.

The alkyl chains in both models have high roughness, for the two-layer model being almost the

same as the layer length. At this point the alkyl chains present no organization and are scattered over

the z axis, some completely lying down over the surface, others presenting some verticality, this

difference between the arrangements of the chains can lead to these high values of roughness.

57

The length obtained for the alkyl chains differ significantly between each model, while the length

and roughness obtained for the ion pairs is similar, summing the length of each pair for the three-layer

model results in 12.67 Å and 11.14 Å was obtained for the two-layer model.

The electronic density profiles show a peak in density for the alkyl chains and then a decrease

corresponding to the ion pairs, for the three-layer model first the cation and then the anion. For the three-

layer model the anion displays higher electron density, this is reasonable since [NTf2]- has more

electrons then the imidazolium head.

Figure 39a) and b) show, respectively, the fit for the two-layer model and the electron density

obtained for 11,5 mN/m and 0.4 nm2/molecule. In figure a) we observe the normalized reflectivity in

function of the normalized wave-vector and the fit obtained, while in figure b) the resulting electron

density in function of depth (z).

58


obtained for 11.5 mN/m and 0.4 nm2/molecule. In figure a) we observe the normalized reflectivity in

function of the normalized wave-vector and the fit obtained, while in figure b) the resulting normalized

electron density in function of depth (z). Table 10 displays the results for each model and the comparison

between them.

Figure 39- Fig. a) shows the normalized reflectivity intensity in function of normalized wave-vector and the fit for the two-box model obtained for the scan at 11.5 mN/m. Fig. b) shows the electron density obtained from the fit in function of depth (z) as a box and as a curve fit.

Depth (z) starts at the interface between the superphase (air) and the 1st layer progressing to the sub-phase, which is reached about 50 Å.

59

Figure 40a) and b)- Fig. a) shows the normalized reflectivity intensity in function of normalized wave-vector and the fit for the three-box model obtained for the scan at 11.5

mN/m. Fig. b) shows the electron density obtained from the fit in function of depth (z) as a box and as a curve fit. Depth (z) starts at the interface between the superphase (air) and the

1st layer progressing to the sub-phase, which is reached about 50 Å.

60

Table 11- Results of the fits for the two models for the scan at 11.5mN/m. Two-layer model corresponds to the model where the anion is between the cation and the three-layer where the anion is beneath the cation. Experimental electron density (𝑛𝑒) corresponds to the normalized density obtained through the fit multiplied by the water 𝑛𝑒 and calculated, where the layer length is used to calculate the corresponding correct electron density.




Two-layer model

Aklyl Chains 0.92 0.40 8.40 6.34

Imidazolium head + [NTf2]-

0.39 0.23 18.52 4.11

Error 1513

Three-layer model

Aklyl Chains 1.06 0.44 7.59 7.02

Imidazolium head 0.38 0.09 11.95 4.38

[NTf2]- 0.39 0.46 6.86 1.14

Error 1557

As before, for both models the experimental electron density calculated differ significantly and

the alkyl chains’ roughness are very high compared to the chains’ length. At this point the surface is still

not fully covered and the alkyl chains still don’t display significant organization on the z axis, so the same

reasons can apply at this pressure and area.

For the two-layer model the experimental electron density is similar for 0 and 11.5 mN/m,

although it expands over a greater length. Alkyl chains length increase is explained by the increase of

verticality of the chains. The ions pair length increases, although we should have observed a decrease

due to the increase of compaction, this can be due to a faulty differentiation of the layers in the fit.

The three-layer model resulted in the same electron density for the ion pairs and a greater length

for the cation. The results are not possible, since the anion electron density is 10x higher than the

imidazolium head electron density. The impossibility of correct differentiation of the three-layers favors

the two-layer hypothesis.

While the difference is small, the fit using the two-layer model has a lower error.

The tri-layer used to fit the plateau result consisted of two boxes, where the first consists of one

alkyl chain and the corresponding ion pairs and the other box, a group of two alkyl chains in contact with

each other and their corresponding ion pairs, facing opposite directions on the z axis. For this model,

some limitations were imposed, the electron density and thickness of the 2nd box must be higher.


obtained for 24 mN/m and 0.3 nm2/molecule. In figure a) we observe the normalized reflectivity in

function of the normalized wave-vector and the fit obtained, while in figure b) the resulting normalized

electron density in function of depth (z). Table 11 shows the results for each model and the comparasion

between them. Table 11 displays the results for the fit.

61

Figure 41- Fig. a) shows the normalized reflectivity intensity in function of normalized wave-vector and the fit obtained for the scan at 24 mN/m. Fig. b) shows the electron density

obtained from the fit in function of depth (z) as a box and as a curve fit. Depth (z) starts at the interface between the superphase (air) and the 1st layer progressing to the sub-phase,

which is reached about 50 Å.

62

Table 12-Results of the XRR fit on C18 at 24 mN/m based on a tri-layer model. 1st box corresponds to the top layer of alkyl chains and corresponding ion pairs and 2nd box to the two groups of alkyl chains and ion pairs facing opposite directions. Experimental electron density (𝑛𝑒) corresponds to the normalized density obtained through the fit multiplied by the water 𝑛𝑒 and calculated, where the layer length is used to calculate the corresponding correct electron density.




1st box 0.54 1.38 7.85 3.70

2nd box 0.62 3.03 7.16 3.60

Error 41870

First, a big error is noticeable, this is normal since the reflectivity curve shows more complexity.

A significant difference between experimental and calculated electron density is also observable. Both

the calculated electron density and the model were based in the surface being fully covered by a tri-

layer and this is not experimentally true, as shown by the BAM experiments, this can be one of reasons

of the discrepancy between the electronic densities. Thickness of the 2nd box is expected to be higher

than observed, due to the impact of the extra ion pair.

For these reasons, the model used wasn’t considered a good approach for the correct fit of this

reflectivity curse.

III.3.2. Total Reflection X-ray Fluorescence (TRXF) and Grazing

Incidence X-ray Fluorescence (GIXRF)

TRXF and GIXRF were used to quantify the Sulphur (present on [NTf2]-) on the [C18mim][NTf2]

film at the interface. The results are obtained as a fluorescence spectrum which provides information

about the excitation source and the sample. On the spectrum multiple peaks are observable, each

corresponding to a specific energy wavelength. These peaks include the elastic (Rayleigh) and inelastic

(Compton) scattering of the incident beam and all the photons emitted in the x-ray range by the atoms

in the sample, for these peaks the intensity is proportional to the number of atoms in the analyzed

volume.

The elastic and inelastic peaks are result of the scattering of the incident light by electrons.

Elastic scattering, i.e. without loss of energy of the scattered x-ray but with direction of propagation

modified, has the same wavelength as the incident x-ray; whereas inelastic scattering has an increased

wavelength (decrease of the energy), which happens when an x-ray photon hits an electron, transferring

its energy.[34] Both elastic and inelastic peaks depend on the incident x-ray.

All peaks are fitted with Gaussian line shapes and, although the Compton peaks appear at

slightly lower energy than the Rayleigh peak on the fluorescence spectrum, often it’s very hard to

separate each other. For these reasons, the peak are often fitted together.

63

In both of these experiments the Sulphur content, present on the anion [NTf2]-, of a

[C18mim][NTf2] film was analyzed by its characteristic fluorescence energy emission: 2.3 KeV. The

Sulphur content was then normalized with the elastic plus inelastic peak. Figure 42 is an example of the

Sulphur, the elastic and the inelastic peak.

As said in the introductory chapter, TXRF was done at the SOLEIL synchrotron with pure water

as sub phase. The scans were done at a fixed penetration depth and continuously during a compression

curve. As a way to obtain more information about the instability of the film, scanning was kept for a few

minutes after reaching full compression. Figure 43 displays the intensity vs area per molecule graph

obtained (blue) and the corresponding isotherm (purple). Due to a problem with the x-ray shutter it was

not possible to scan between 0.35 and 0.45 nm2/molecule. The surface pressure sensor had a problem

during the measurements and we observed higher values of area per molecule.

Figure 42- Example of a sulfur, Compton and elastic peak.

No

rmal

ized

Su

lph

ur

Flu

ore

scen

ce

Π (

mN

/m)

Area per molecule, nm2/molecule

64

As the film is

compressed and as the surface pressure rises, so does the Sulphur quantity at the surface reaching a

peak at 0.28 nm2/molecule at the end of the compression. As the film is left to stabilize, the surface

pressure decreases, as well as the Sulphur fluorescence.

A steady increase of the fluorescence is observed due to the increase of the surface density of

the film. C18 molecules have less space available and the space between them reduces, thus increasing

the number of molecules irradiated in the scanned volume. To verify this the ratios between fluorescence

intensity and areas are compared (𝐼𝑓

𝐼𝑖 𝑎𝑛𝑑

𝐴𝑓

𝐴𝑖) for the points: (0.7 nm2/molecule; 3.0 intensity and 0.5

nm2/molecule; 5.5 intensity). A ratio of areas of 1.4 and a ratio of fluorescence of 1.8 was obtained. This

values are close, demonstrating there is no change or only a very small variation on the film and the

fluorescence increase is mainly due to the increase of surface density.

The observed peak corresponds to a jump in the surface density of the film and leads to believe

that it’s the transition to the multilayer, the formation of the crystals observed in AFM and BAM.

As expected, upon leaving it fully compressed, the film tends do the equilibrium state, where the

crystals are in equilibrium with the liquid phase. Here, the Sulphur Fluorescence follows the decrease

of surface pressure, this could be due to a decrease in surface density or as Raquel Valente reported

in her thesis[11] the anion leaving the surface. In her work, Raquel performed successive

compression/expansion cycles and scanned the surface. The results that were obtained led to the

conclusion that the anion was leaving the surface.

GIXRF was performed at the ESRF synchrotron and with two different sub phases, pure water

and salted water ([Na][NTf2]) at a concentration of 0.01mM. For the pure water sub phase fluorescence

scans were done at 0 and 24 mN/m and for the salted at 0 and 20 mN/m. For both scans, scans were

performed from the smallest angle possible till the critical angle, since surpassing would make the

intensity plummet. This corresponds to a penetration depth range from 0 to 12 nm. Fig. 44a) shows two

fluorescence spectrums at maximum depth, one obtained at 24 mN/m for pure water sub phase (line)

and the other with salted water at 20 mN/m (dotted). Figure 44b) displays a graph of fluorescence

intensity vs penetration depth for the four scans performed.

Figure 43- TRXF experiment on [C18mim][NTf2] with pure water as sub phase. At blue normalized Sulphur intensity as a function of area per molecule in nm2/molecule and

at purple corresponding isotherm, surface pressure (π) in mN/m in function of the area per molecule.

65

Analysis of figure 44a) spectrum show that the film contained traces of Silicon and Argon, the

first one is due to the detector and the second due to a faulty atmospheric sterilization. For approximately

the same pressure the scan done with salted water has more anions at the surface then the one done

with pure water, this is most likely due to the stabilization proprieties of the salt.

Although below 5nm of depth there is no detection of Sulphur fluorescence this doesn’t

necessarily mean there is none, since at these very low angles the evanescent wave may not have

enough intensity to excite the Sulphur electrons.

As did the TRXF results, figure 44b) show that an increase of pressure implies an increase of

the number of anions at the surface. For the scans at 0 mN/m the results show that the ion [NTf2]- on

the salted water sub phase is mostly concentrated at greater depths then on pure water sub phase,

since the fluorescence intensity on pure water is higher between the range of 5 to 8 nm and reaches the

same value for both sub phases above 8 nm penetration.

A significant difference on intensity is observed at plateau pressures between the pure water

and salted water sub phases. This was already shown by the spectrum on figure 44a). If this is due to

the stabilization properties of the salt, this means that the instability mechanism, we observe upon

multiple compressions and with time, removes molecules from the surface

Figure 44a) and b)- GIRXF experiment on [C18mim][NTf2] with pure water as sub phase. Left graph displays the reflectivity intensity for pure water sub phase at 24mN/m and for salted water sub-phase (0.01mM) at 20mN/m. Four peaks are shown, Silica, Sulphur, Argon and the elastic plus inelastic peak. Right graph display the normalized Sulphur fluorescence vs penetration depth for pure water sub-phase (full

lines) at 0 and 24mN/m and for salted water sub-phase (dotted lines) at 0 and 20mN/m.

water NaNTf2

Penetration depth, nm Energy, KeV

Inte

nsi

ty

No

rmal

ized

Su

lph

ur

Flu

ore

sce

nce

Compton and

elastic peak

66

IV. Conclusions and future work

Through the macroscopic measures the distinct phases of each molecule were

characterized. C18 and C12C12 both show a plateau-like phase transition at 25mN/m, whereas C20 shows

two transitions, the first at 16mN/m and the second at 25mN/m. C18 phase transition and C20 2nd phase

transition happen at very similar areas, these areas aren’t compatible with a monolayer, leading to the

conclusion that it’s a transition to a 3D structure. All molecules showed instability relatively to multiple

compression/expansion cycles and with time tending to an equilibrium isotherm. C18 and C12C12 both

tend to an equilibrium isotherm where the phase transition is not observed, while for C20 the 2nd transition

remains at same pressures/areas even after multiple cycles. Adding ([Na][NTf2] salt to the sub-phase

has the same impact on C18 and C20, the salt seems to rise to the interface occupying a significant

amount of surface area, while for the molecule C12C12 a decrease on surface pressure is observed. This

salt also has a stabilizing impact on all molecules.

BAM and AFM experiments identified and characterized the organized structures formed by

each molecule. Above its ESP (15mN/m), C18 organized as circular domains and when the film reaches

the plateau region, highly ramified crystals start to be observed. With AFM these crystals were measured

and a height between 40 and 50 nanometers and a width of 1.5 μm was obtained. C12C12 molecule didn’t

form any 3D structures, only a large number of circular collapsed domains were observed at the plateau.

C20 shows three phases and two phase transitions. At 17 mN/m, the first phase transition (to LC) for C20

occurs, this is observed in BAM as liquid condensed domains start to be seen and slowly cover the

entire surface, at 25mN/m the 2nd plateau is reached and bright (dense) cylindrical crystals start to be

observed. The liquid condensed domains were not observed in AFM, probably because they weren’t

transferable to the substrate. The denser crystals were measured and had the same height as the ones

formed by C18. A small network of domains was observed after a full compression and expansion cycle.

X-ray experiments (XRR; GIXRF; TRXRF) were only done for C18 and served as a way to

test models. Three XRR scans during compression were performed and analyzed: at the start (0mN/m),

half of the compression (11.5mN/m) and the plateau (24mN/m). With XRR, two models were tested for

the conformation of the C18 monolayer: anions between the cations or anions below the cations and a

tri-layer model. For 0 mN/m both models achieved equivalent results, while for 11.5 mN/m the model

with the anions between cations achieved better results. For the plateau a tri-layer model was tested

with unsatisfactory results. GIXRF and TIXRF provided extra information about the film instability. The

fluorescence spectrums showed a decrease of anion molecules with time and multiple cycles, meaning

entire molecules or just anions are descending to the bulk of the sub-phase.

Through these experiments a better understanding of the interfacial characteristics of this

imidazolium family and mainly of [C18mim][NTf2] was obtained. During compression C18 displays, at the

beginning, a liquid and gas coexistence then to a liquid expanded (compressibility > 20) phase and

finally a collapse at 25 mN/m, a phase transition to a multilayer. During expansion a plateau is observed

at 15 mN/m that corresponds to the ESP (equilibrium surface pressure). Through the BAM, AFM and

67

fluorescence experiments the multilayer formed at the collapsed were identified and characterized:

Highly ramified crystals with a height between 40 and 50 nanometers and a width of 1.5 μm. With BAM,

small circular domains that seem the beginning of the collapse mechanism were observed during

compression above the ESP. C12C12 also displays the liquid and gas coexistence and the two plateaus

(collapse and ESP) at similar pressure values but double the area, due to its two alkyl chains, although

the liquid phase was identified as liquid condensed (compressibility < 20). No crystals are formed at the

collapse, although a large number of circular collapsed domains are observed in the BAM. C20, although

having a very similar molecular structure as C18, two plateaus corresponding to phase transitions (17

and 25mN/m) and the two liquid phases are observed (LE and LC), the ESP plateau is also observable

but happens at lower pressures (10mN/m). The collapse plateau happens at the same areas and

pressures then C18 and in this phase transition, crystals with the same size and width of the ones formed

by C18 are observed, although in BAM they seem less ramified and more cylindrical.

The addition of the [Na][NTf2] salt, even at a small concentration, seems to increase the

stability of all films and for C18 and C20 it seems to occupy a part of the surface, where for C12C12 it shifts

the isotherm to lower surface pressures. The effect of the salt on the stability seems to be enhanced for

C12C12: less salt is needed to stabilize the film at similar areas per molecule, comparing to C18 and C20.

This is compatible with the larger area occupied by the two chained C12C12. Fluorescence experiments

of this and previous works seem to indicate a decrease of anions at the interface with time and cycles,

suggesting that ion pairs or just anions are leaving the interface to the bulk sub-phase.

Some behaviors still need further study such as the mechanism of the film instability and the

structure of the crystals formed by C18 and C20. Although the study of C12C12 didn’t help in understanding

the behavior of C18, due to the lack of similarities between them, the results obtained were interesting

and should be a target of further study.C20, in the other hand, helped in understanding the results of C18,

since it formed similar crystals, size-wise, at the same area/pressure and still showed the same type of

instability for the liquid phase and its 1st phase transition.

The further study of [C20mim][NTf2] should be a priority for future works, particularly through

x-rays experiments: reflectivity (XRR); fluorescence (XRF) and diffraction (XRD). The use of the salt

[Na][NTf2] can be used to avoid instability problems. With reflectivity we could test different models for

molecular organization and verify if they coincide with the ones used for C18 through fluorescence we

could verify if the instability mechanism is similar to the one of C18. A thorough diffraction study should

be done for both molecules with the objective of comparing the different structures formed and finding

similarities or differences. The study of ILs of the same family with longer chains ([C22mim][NTf2],..,)

would also be interesting as a way of enhancing the knowledge of the interfacial behavior of C18 and

this imidazolium IL family.

68

V. Bibliography

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3. Amarasekara, A.S., Acidic Ionic Liquids. Chem Rev, 2016. 116(10): p. 6133-83. 4. Welton, T., Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chem

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framework for the environmental assessment of solvents. 2007.

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7. Greaves, T.L. and C.J. Drummond, Protic ionic liquids: properties and applications. Chem Rev, 2008. 108(1): p. 206-37.

8. Kazemiabnavi, S., et al., Electrochemical Stability Window of Imidazolium-Based Ionic Liquids as Electrolytes for Lithium Batteries. J Phys Chem B, 2016. 120(25): p. 5691-702.

9. Swiderska-Mocek;, A.L.A., Ionic liquids as electrolytes for Li-ion batteries—An overview of electrochemical studies. Journal of Power Sources, 2009. 601–609.

10. Filipe, E.J., et al., Crystalline-like structures and multilayering in Langmuir films of ionic liquids at the air-water interface. Chem Commun (Camb), 2016. 52(32): p. 5585-8.

11. Valente, R.M.M., Ionic Liquids at Interfaces, in Chemical Engineering. 2016. 12. Salomé dos Santos, B.M., Tiago dos Santos, and Filipe E. Antunes,, Amphiphilic Molecules

in Drug Delivery Systems. 2013.

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14. Martin, D.K., Nanobiotechnology of biomimetic membranes. Fundamental biomedical technologies. 2007, New York: Springer. xi, 173 p.

15. Gaines, G.L., Insoluble monolayers at liquid-gas interfaces. Interscience monographs on physical chemistry. 1966, New York,: Interscience Publishers. xiv, 386 p.

16. Hans-Jürgen Butt, K.G., Michael Kappl, Physics and Chemistry of Interfaces. 2003. 17. R. S. Bhande, Y.A.L., P. A. Giri, Study the Behavior of π-A Isotherm with Increasing Amount

of Octadecanoic acid at Air–Water Interface Journal of Chemical and Pharmaceutical Research, 2012.

18. Yu, Z.W., J. Jin, and Y. Cao, Characterization of the liquid-expanded to liquid-condensed phase transition of monolayers by means of compressibility. Langmuir, 2002. 18(11): p. 4530-4531.

19. Biolin Scientific KSV NIMA, Imaging the Structure of Thin Films: Brewster Angle Microscopy, in Application note 2017. p. 4.

20. Henon, S. and J. Meunier, Microscope at the Brewster-Angle - Direct Observation of 1st-Order Phase-Transitions in Monolayers. Review of Scientific Instruments, 1991. 62(4): p. 936-939.

21. Honig, D. and D. Mobius, Direct Visualization of Monolayers at the Air-Water-Interface by Brewster-Angle Microscopy. Journal of Physical Chemistry, 1991. 95(12): p. 4590-4592.

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69

22. Binnig, G., C.F. Quate, and C. Gerber, Atomic Force Microscope. Physical Review Letters, 1986. 56(9): p. 930-933.

23. Sugimoto, Y., et al., Chemical identification of individual surface atoms by atomic force microscopy. Nature, 2007. 446(7131): p. 64-67.

24. Liu, S.Y. and Y.F. Wang, Application of AFM in Microbiology: A Review. Scanning, 2010. 32(2): p. 61-73.

25. Als-Nielsen, J. and D. McMorrow, Elements of modern X-ray physics. 2nd ed. 2011, Hoboken: Wiley. xii, 419 p.

26. Wobrauschek, P., Total reflection x-ray fluorescence analysis - a review. X-Ray Spectrometry, 2007. 36(5): p. 289-300.

27. Philip J. Potts, A.T.E., Peter Kregsamer, Christina Streli, Margaret West, Peter Wobrauschek, X-ray fluorescence spectrometry. Journal of Analytical Atomic Spectrometry, 1999.

28. S.H. Nowak , D.B., W. Błchucki, W. Cao , J.-Cl. Dousse, P. Hönicke, J. Hoszowska , Ł. Jabłoński, Y. Kayser , A. Kubala-Kukuś, M. Pajek , F. Reinhardt , A.V. Savu , J. Szlachetko, Grazing angle X-ray fluorescence from periodic structures on silicon and silica surfaces. Spectrochimica Acta Part B: Atomic Spectroscopy, 2014. 98: p. 65-75.

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70

VI. Appendixes

VI.1. Overlap of the isotherm on salted water and

pure water + area shift.

Figure 45-Two compression/expansion cycles isotherms on C18. One was done with salted water ([Na][NTf2]) with a concentration of 0.01mM (blue curve) and the other with pure water. The area shift between both was added to the

isotherm done in pure water and resulted on the orange curve.

0

5

10

15

20

25

30

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

π(m

N/m

)


0.01mM

Pure water + shift

71

VI.2. Comparison between pure water and salted

sub-phase of C20mimNTf2.

0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1 1.2

π(m

N/m

)


No salt

salt

Figure 46- Two compression/expansion cycles isotherms on C18. One was done with pure water (blue curve) and the other salted water ([Na][NTf2] with a concentration of 0.01mM (orange curve).

72

VI.3. BAM results on C20 with salted sub-phase

Table 13- Area, pressure and curve of each image taken during C18 salted water BAM experiment.

Image Pressure (mN/m) Area per molecule






0

5

10

15

20

25

30

0 0.2 0.4 0.6 0.8 1 1.2 1.4

π(m

N/m

)

Area per molecule ( nm2/molecule)

2

3 4

Figure 47- BAM experiment isotherm on C18 with salted sub-phase at 20ºC, a compression rate of 30 cm2/min and 10 minutes of solvent evaporating time. The numbers correspond to the area and

pressure where the corresponding image was taken.

1

73

Figure 48- Four images taken during the BAM experiment on C19 with pure water sub-phase.

1 2

3 4

74

VI.4. BAM results on C20 after a compression cycle.

Table 13-Waiting time for each image displayed below

Figure 49- Four images of C20 after a full compression time. Image 1 taken after waiting 2 minutes, 2 after 27, 3 after 56 and 4 after 1 hour.

IMAGES WAITING TIME (MINUTES)

1 2 2 27 3 56 4 60

75

VI.5. C18 AFM with salted sub-phase.

1

Figure 50- Figure a) and b) scan taken at the plateau (25mN/m) of a C18 film at 20ºC and a compression rate of 30 cm2/min. Figure b is the corresponding profile obtained from the scan.

langmuir films of ionic liquids · ], medidas microscópicas, microscopia de força atómica (afm),...

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