presentation olaleye

8/8/2019 Presentation Olaleye

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Critical Analysis of a Scientific Paper

Interfacial Thermodynamics of Surfactants at the CO2-

Water Interface Sandro R. P. da Rocha and Keith P.Johnston, Langmuir 16, 3690-3695 (2000)

Olaleye Oladiran,CHEN 568A

9/28/2010

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Motivation

The principle of interfacial tension,

surfactant adsorption, and the

thermodynamics of the formation

of microemulsion based on the

structure of surfactants is useful in

the design of surfactants for

processes involving water-CO2

interface.

Examples of such processes are

cleaning, wetting, fabric softener,

paints, adhesives and cosmetics.

http://www.acmite.com/market-reports/chemicals/world-surfactant-market.html 2


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The principle of Interfacial Tension

The property of the surface of a liquid that allows it to resist an external

force is termed surface tension. It is revealed, for example, in floating

of some objects on the surface of water. When this tension is observed

between dissimilar liquids e.g. oil and water, it is called interfacial

tension.



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Surfactant adsorption

Surfactants are compounds that lower the surface tension of a liquid, allowing

easier spreading, and lowering of the interfacial tension between two liquids,or between a liquid and a solid.

Surfactants are made up of two parts: the hydrophilic and the hydrophobic

part. They reduce the surface tension of water by adsorbing at the liquid-gas

interface.


Surfactant used: PerfluoropolyetherCOO-NH4


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Interfacial Tension Measurement

Figure 1:Schematic diagram of the tandem high-pressure

variable-volume pendant drop tensiometer.

Laplace Equation:

gzR

P VK

(!(0

2

Where,

P = Interfacial Pressure drop

R0

= Radius of curvature at the

apex of the drop,

z = Vertical distance from apex.

= Surface Tension

No detailed

experimentalprocedure

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Interfacial ThermodynamicsDetermination of Surface excess concentration ():

Where, x = Surfactant mole fraction

xdRTd ln+!K -----(1) Gibbs Adsorption Equation

Determination of the Surface Pressure versus Area Isotherms:

Where,

= Limiting adsorption at the saturated interface,

a = Surfactant concentration required to reach half, = 0 ; 0 = Binary CO2-water interfacial tension.

)/1ln( aCRT S+!4 g -----(2) Surface Equation of State

Determination of the Maximum Saturation of Surfactant

Monolayer(m) and the Critical Microemulsion Concentration

(cc) by Graphical Analysis:

The above parameters were determined from the curve of interfacial

tension (mN/m) versus log [Cs(M)]6


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Main Points

1. Based on experimental results and molecular surface equation of state,the area per surfactant at the critical microemulsion concentration is much

larger at the water-CO2 vs water-oil interface for two primary reasons:

a. The first reason is that 0 (without surfactant) is much smaller for the

water-CO2 interface; thus, less surfactant is required to lower to a

typical value for microemulsion, 1 mN/m.

b. The second reason is the larger entropic contribution to the freeenergy of the monolayer, due to greater penetration of the small CO2

molecules in the tail region relative to larger oils.

2. The value of the critical microemulsion concentration, using the the

surfactant perfluoropolyetherCOO-NH4+, varies from 0.26 to 1.5mM for a

temperature range of 25-65 C.

3. The enthalpy of microemulsion formation is -37.6 kJ/mol.

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Abstract

The authors abstract

(on the front page of the paper).

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Abstract



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Ambiguous statement

(At 20oC, 0, water= 72.8 mN/m while 0, hydrocarbon oil = 20-25 mN/m.


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Abstract



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Ambiguous statement

(At 20oC, 0, water= 72.8 mN/m while 0, hydrocarbon oil = 20-25 mN/m.

No experimental values for the water-oil interface.


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Graph of Interfacial Tension versus log ofSurfactant Concentration

Graphical

Analysis(On page 3 of

the Paper)

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Graphical

Analysis(On page 3 of

the Paper)

Values obtained at 45 and 65oC are of relatively high correlation

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Graphical

Analysis(On page 3 ofthe Paper)

Values obtained at 45 and 65oC are of relatively high correlation.

Values obtained at 25oC have low correlation.

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Graphical

Analysis(On page 3 ofthe Paper)

Values obtained at 45 and 65oC are of relatively high correlation.Values obtained at 25oC have low correlation.

The post-cc line is generally horizontal.

It does not slope towards the x-axis.14


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Results

Calculated

values(Page 3)

Insufficient temperature range for surfactantcharacterisation

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Results

Calculated

values(Page 3)

Values cannot be trusted especially at 25o

C

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Results

Calculated

values(Page 3)

Determination of area is not explicit enough in the paper

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Results

Calculated

values(Page 3)

Values cannot be trusted due to low correlation

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Results

Calculated

values(Page 3)

Values cannot be trusted due to low correlation

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Results

Calculated

values(Page 3)

No mention of how enthalpy change was determined

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Critical Analysis Summary

STRENGTHSAddressed an important subject

matter in the chemical industry

Applied a good number of

chemical engineering principles todeal with the subject matter

Explicit historical background

Adopted both experimental,

theoretical and graphical analysis

Worked with an ideal range of

temperature values.

WEAKNESSESAbsence of theory background

Absence of required mathematical

model

No explicit experimental procedure

Unjustified experimental values

Ambiguity of statement

Inadequate assumptions

No new model

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Citation Report

The paper has been cited 61 time since 2000.

It was cited 13 times in 2003.

Average citations per year is 5.55

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Classification of Surfactants


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Theoretical Background

chemweb.calpoly.edu/dgragson/Teaching/.../SurfTens_New.pdf

-- (1)

--- (2)

--- (3)

--- (4)

--- (5)

--- (7)

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Surface tension vs Concentration

chemweb.calpoly.edu/dgragson/Teaching/.../SurfTens_New.pdf 27


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Surface tension vs Natural Logarithm ofConcentration

chemweb.calpoly.edu/dgragson/Teaching/.../SurfTens_New.pdf 28


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Methods ofMeasuring Surface Tension

Du Noy Ring method: measures maximum pull exerted on the ring by the

surface.

Wilhelmy plate method: Measures force due to wetting

Spinning drop method: The diameter of a drop within a heavy phase is

measured

Pendant drop method: Geometry of a drop is analyzed optically.

Bubble pressure method: Maximum pressure of each bubble is measured.

Drop volume method: Measures time between drops produced

Capillary rise method: Measures capillary height

Stalagmometric method: A method of weighting and reading a drop of liquid.

Sessile drop method: Measures the contact angle

Test ink method: A method for measuring surface tension of substrates using

test ink

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