Application of the Gibbs Absorption Isotherm to Mixed Films at Air-Water Interfaces

157 Pages·05.657 MB·English

by  Conan, Robert J

FORDHAM UNIVERSITY
G raduate School of A rts and Sciences
February 1, _ 1950..
This dissertation prepared under my direction by
Robert <J« Conan
entitled Application of the Gibbs Absorption Isotherm to Mixed
Films at Air-Water Interfaces*
has been accepted in partial fulfilment of the requirements for the
Degree of. Doctor of Philosophy.
Eric Hutchinson
{Faculty Adviser)

application of the gibbs adsorption isotherm to
MIXED FILMS AT AIR-WATER INTERFACES
by
Robert J. Gonan, Jr.
B. Sc. Syracuse University, Syracuse, New York
M. S. Syracuse University, Syracuse, New York
THESIS
Submitted in p a rtia l fulfillm ent of the requirements
for the degree of Doctor of Philosophy in Chemistry
in the Graduate School of Fordham
University September,
1949
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ACKNOWLEDGMENT
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by Dr. E. Hutchinson in the course
of th is work.
TABLE OF CONTENTS
Page
Introduction
Theory 13
Method 31
M aterials 37
Experimental Results 40
Discussion and Conclusion 108
References 11?
I. INTRODUCTION
Adsorption may be defined as an accumulation of matter
at or near an interface. In the absence of an interface,
and a consequent change of force field , adsorption is not
known to occur. Adsorption phenomena are recognized at a ll
types of interfaces which may be obtained by a combination
of the three states of matter taken either in pairs or in
m-
trip lic a te .
In the case of liquid-gas and liq u id -liq u id interfaces,
the thermodynamic basis underlying adsorption has been
elegantly outlined by G-ibbs (8); which predicts that the
surface concentration or surface excess of a given component
in a solution is a function of the rate of change of the
surface tension with the activ ity of that component, but in­
dependent of the absolute value of the surface tension of
the solution. As w ill be demonstrated, later, the ordinary
surface tension-concentration curves are easily explained
on the basis of th is theory.
Three general types of surface ten slon-activity curves
are distinguished at. the present time (see Fig. I), In
Type I there is a progressive lowering of the surface tension
as the concentration of the s.olute is increased. Type II
exhibits a progressive and slight rise of surface tension
0
with increasing concentration. Neither present* complications
-1 n_int_erpr et a t-io n—f r-om-a-bhe or etic-al—viewpoint-___However.,_
FIGURE" I
M
O
i
S
N
F
T

e
C
A
F
W
S
COWCF/VTRATiON
TYPES OF SURFACE TFNSIOW-CONCENTRATION CORVES
2
the Type III curves, where the surface tension of the solution
f ir s t decreases rapidly, passes through a minimum, and then
rises again as the concentration of the solute is increased,
are at f ir s t sight incompatible with the theory as it is
commonly applied. For th is theory, applied to the horizontal
portion of the curve at the position of the minimum, would
suggest that there is no adsorption at a ll at th is point,
That this interpretation is false is obvious from the fact
that the surface tension of the solution, at th is concentration
is far below the surface tension of the pure solvent. Such a
marked lowering of surface tension must imply the presence of
matter other than pure solvent in the surface. The object
of th is thesis is a quantitative study of a numbbi* of these
“Type III curves“ and the application of a thermodynamic
interpretation that w ill account for th is unusual behavior.
It is the above mentioned contradiction between theory and
experimental observation which is to be resolved.
A great many examples of Type III surface tension-
concentration curves have been obtained by numerous in v esti­
gators using every experimental method known for the measure­
ment of surface tension of solutions. A minimum was firs t
noted by Raleigh (33) in aqueous solutions of sodium oleate
using the capillary rise method. Later w riters (11) verified
his findings, reporting that the Mdip“ had a depth of about
2 dynes/cm. Other examples of this same effect are shown in
3
aqueous solutions of soaps, paraffin chain salts, some dye-
stu ffs, many synthetic detergents and even inorganic salts (25)
This minimum generally occurs in very d ilute solutions, and
has also been observed at oil~water interfaces. Another
effect which is frequently noticed with solutions of these
compounds is that of surface ageing, ..e. time effects in the
establishment of the equilibrium surface tension of solutions.
Salts normally exhibiting both of these effects of special
in terest here are the sodium alkyl sulfates, such as sodium
octyl, decyl, dodecyl sulfates, which have been studied" by
a number of authors (6, 19, 32).
Numerous attempts have been made to explain these
Type III curves. MeBain and Miles (25) made a special study
of the surface tension-concentration curve of lauryl sulfonic
acid at the air-w ater interface, and found a minimum in the
A dOVCPVT1tAT/<*/ ad
curve a t A0 .006 molal with an observed depression of about
10 dynes/cm. Calculations made on the basis of the Gibbs
Adsorption Theorem indicated no adsorption at the minimum.
However, direct measurements by the microtome method (23)
showed positive adsorption of the lauryl sulfonic acid over
the entire concentration range studied, including that of
the minimum. (Other experiments with the microtome on
substances showing Type I and II surface tension curves
indicated at least qu alitativ e agreement (28) with the
values obtained by theoretical calculations), Moreover, the