introduction to surf act ants and self-assembly

8/3/2019 Introduction to Surf Act Ants and Self-Assembly

http://slidepdf.com/reader/full/introduction-to-surf-act-ants-and-self-assembly 1/30

1

Introduction to surfactants andself-assembly



2

Self-assembled nanostructures fromamphiphilic molecules

Micelle Bilayer Cubic phase

1 nm

Surfactant science is nano-science!



3

Amphiphilic (cf. amphi (gr.) = both)

Hydrophobic hydrophilic+

OIL WATER SOLUBLE STUFF

Surfactants, polymers, proteins,……



4

SDS: CHSDS: CH33(CH(CH22))1111SOSO44-- NaNa++

CTAB: CHCTAB: CH33(CH(CH22))1515NN++(CH(CH33))33 ClCl--

CC

1212EE

88: CH: CH

33(CH(CH

22))1111(OCH(OCH

22CHCH

22))88OHOH

E

xamp

les



5

In general:

CH3-N+-CH3 Cl-

CH3

CH3

NaNa22SOSO44

H(OCH2CH2)8OH

CH3(CH2)10CH3

CH3(CH2)14CH3

CH3(CH2)10CH3

Water solubleOil insoluble

Water insolubleOil soluble



6

Also, other water soluble groups can be used:

O

OH

O

OHOH

CH2

OH

O

O

OHOH

CH2

OH

O

OH

O

OHOH

CH2

OH

Sugars



7

When amphihilic molecules are put ina system with an interface: water/oil orwater/air:

oil

water

they enrich at the surface: they are surface activewe call them surfactants (in biology often lipids)



8

oil

water

The water – waterinteraction is more favorable thenthe water – oil interaction!

In other words, it is the strongcohesion between water moleculesthat imparts low solubility ofhydrocarbons in water.

Hydrophobic effect!

Like dissolves like!



9

Proteinfolding

Immiscibility

Wetting

J. Phys. Chem., 2002,106,521.



10

Let us put SDS in water!

+



11

Equivalent conductivity

At low concentrations, SDS behaves asa “simple” salt. At a well-defined concentration

many physical parameters

change. Here micellesare formed, hence theconcentration is calledcritical micelle concentrationcmc.

(micelle = small bit)

cmc



12

An SDS – micelle! Approx. 60 SDS moleculesper micelle.



13

Interior is liquid-like, similar to hydrocarbon droplet.The micelle can solubilize non-polar substances.

Surface is highly charged with

a high concentration of counter-ions.



14

Micellization is a start-stop process:Start: hydrophobic effect (phase separation!).Stop: electrostatic repulsion among headgroups.

-

- --

-

Repulsion!

Micelle has finite size!

Size distribution



15

The micellization is a co-operative process!

Unfavorable!

16 14

12 8



16

Consider the aggregation of N surfactantsinto a micelle of aggregation number N

( )lno

f f f kT C μ μ = +

lno mic

mic mic

C kT

N N μ μ

⎛ ⎞= +

⎜ ⎟⎝ ⎠( )

( )1 /

o omic f

N

mic f

C C e

N

μ μ −⎛ ⎞= ⎜ ⎟

⎝ ⎠

NS SN

Chemical potential monomer

Chemical potential micelle

-

Solve for Cf and Cmic as functions of Ctot

If N large, Cf constant.



17

N = 60

N→∞

For N→∞, theconcentration offree SDSconstant above cmc:phase separation model.

For finite N,concentration of freeSDS decreases abovecmc

(experimental result)

This is due to electrostatic effects, not capturedin a description based on an equilibrium constant!



18

Kinetics:The micelle is held together by physical forces;it is a “living” entity!

We can identify two kinetic time-scales:

1. Life-time of one surfactant in a micelle:

2. Life time of one micelle,

considerably longer: for SDSat 25 C around 1 ms.

1 1

off onk k cmc

τ = =1 N N A A A ++

kon

koff1 25s for SDS at C τ μ ≈



19

What happens if[SDS] is increased

further?

First, aggregate growth, then crystallization intolyotropic liquid crystals.

cpp = critical packing parameter.



20

The critical packing parameter, cpp(surfactant number).

A

v

-

cpp =v

A x

For sphere: 1/3

For cylinder: 1/2For bilayer 1

From volume to area ratios:



21

Reversed

cpp ≤ 0.5

cpp > 1

cpp ≈ 1

Normal

Bilayer

Reversed



22

cpp can be changed by:

Changing concentration, adding a secondsurfactant, changing temperature, addingsalt etc.

-- -

-

-

+ + + +

Screening

+

+

-- -

-

+ +

Adding a co-surfactant, non-charged

- T -



23

Microemulsions:

“Are stable solutions containing “oil”, water anda surfactant”

Fish-cut:

Sample with equal volumes

of oil and water, addsurfactant!



24

Two examples:

1. Washing (practical)

2. Fat digestion (biological)



25

1. WashingDirt on fibers

Surfactants adsorb andreplace dirt

Dirt is solubilized inaggregates

Aggregates and dirtrinsed off



26

Shampoo, a complex product:



27

2: Fat digestion.

Bile salt



28

Fat digestive enzyme, lipas

Bile salt emulsifiesthe fat, so the enzymecan brake it down!



29

Schematic illustrationof steps in solubilization

and digestion of fats!



30

Read more:

introduction to surf act ants and self-assembly

Documents