Post on 09-Apr-2018
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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.
http://www.acmite.com/market-reports/chemicals/world-surfactant-market.html 3
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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.
http://www.acmite.com/market-reports/chemicals/world-surfactant-market.html 4
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
The authors abstract
(on the front page of the paper).
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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
The authors abstract
(on the front page of the paper).
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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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Graph of Interfacial Tension versus log ofSurfactant Concentration
Graphical
Analysis(On page 3 of
the Paper)
Values obtained at 45 and 65oC are of relatively high correlation
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Graph of Interfacial Tension versus log ofSurfactant Concentration
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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Graph of Interfacial Tension versus log ofSurfactant Concentration
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