mixed adsorbed films at oil-water interfaces

M I X E D A D S O R B E D FILMS AT OIL-WATER I N T E R F A C E S

Eric H u t c h i n s o n 1

From the Department of Chemistry, Stanford University, Calif. Received July ~1, 19]~8

INTRODUCTION

In a recent paper on the effects of interfacial adsorbed films on the rate of diffusion of-molecules across an oil-water interface (6) it was postulated that the role of the film was not merely that of a sieve, but rather that it imposed an energy barrier on the diffusing molecule due to t he interaction of the diffusing molecule with the film molecules. This appeared to be borne out by the observed large effect of the shape and nature of the diffusing molecule on the rate of diffusion through the film. At that time no information was available as to the number of molecules, either of the film material or of the diffusing molecules in the film at any time.

Previous work (7,8) has shown that the number of molecules in the film may be calculated in the case of only one solute in the solution. The present paper describes the determination of the number of molecules in the interracial film composed of more than one solute component, i.e., a case essentially similar to that encountered in the diffusion experiments.

The experiments described here consisted of placing a solution of n-octyl alcohol in benzene in contact with a solution of sodium dodecyl sulfate in water and studying the interfacial tension of the system as a function of the concentrations of the two solutes. In the previous work (6) sodium cetyl sulfate was used to form the interracial film, but there were some doubts as to its purity. For quantitative investigation, it was clearly desirable to use a sample of sodium dodecyl sulfate, the high purity of which was fairly well established by experiment, n-Octyl alcohol was used since its behavior at a benzene-water interface, in the absence of a solute in the aqueous phase, was known, and freezing point data of its solutions in benzene were available (7).

EXPERIMENTAL

Measurements of the interracial tensions were made by means of the sessile "bubble" method (2,3,7) described in earlier papers and no further

1 Bristol Myers Postdoctorate Fellow and Research Associate in Chemistry. Present address: Chemistry Department, Fordham University, New York 58, N. Y.

521

5 2 ~ ERIC HUTCHINSON

description will be given here. The apparatus was not thermostated, but the laboratory temperature during the experiments remained in the region of 23 ° ± I°C. The densities of the solutions of alcohol in benzene were known from earlier unpublished work, and the densities of the dilute solutions of sodium dodecyl sulfate were t akenas being the same as that of pure water. Experiments were carried out using both distilled water and conductivity water to prepare the aqueous solutions, and no significant difference could be observed. Attempts to check the results by means of the drop-volume method (5) failed, owing to the poor re- producibility of the method for this system. This may be ascribed to the aging which occurs during the first 20-30 min. from the beginning of the experiment.

RESULTS

Interfacial tensions are recorded in Table I for solutions of n-oetyl alcohol in benzene against a number of aqueous solutions of sodium dodecyl sulfate. In Fig. 1 the inteffacial tension is shown as a function of the freezing point depression 0 of the benzene solutions, and in Fig. 2 as a function of the mean mole fraction of the sodium dodecyl sulfate.

3O ,j

t . ) ~ o

~ ~ oCTYL AL.C,C)I.-)CR_ IN BENZENE

FREEZING POINT DEPRESSION

I I I'0 2.0 3.0

FIG. 1. Interfacial tension of solutions of octyl alcohol ill benze*m against aqueous solutions of sodium dodecyl sulfate as a function of tlm freezing point depression of the solution. Curve 1. Octyl a l c o h o l v s , water. Curve 2. Octyl alcohol vs. 1.0 X 10:4 M sodium dodecyl sulfate. Curve 3. Octyl alcohol vs. 2.5 X 10 -4 M sodium dodecyl sulfate. Curve 3. Oetyl alcohol vs. water 5.0 × 10 -4 M sodium dodccyl sulfate. Curve 5. Octyl alcohol vs.

7.5 × 10 -4 M sodium dodecyl sulfate. Curve 6. Octyl alcohol vs. 10.0 × l0 -4 M sodium dodecyl sulfate.

MIXED &DSORBED FILMS 5 2 3

~E ~ k ~'~ N~ SODIUM DODECYL. SULFATE IN WATER

3o(\

~ fo

! I I I 0"5 I '0 1.5

MOLE FRACTION OF DETEROENT N~I0 ~

Fro. 2. Interfacial tension of solutions of sodium dodecyl sulfate against solutions of octyl alcohol in benzene as a function of the mean mole fraction of the detergent N , .

Curve 1. Sodium dodecyl sulfate vs. benzene. Curve 2. Sodium dodecyl sulfate vs. 0.075 M oetyl alcohol solution. Curve 3. Sodium dodeeyl sulfate vs. O. 142 M octyl alcohol solutiom Curve 4. Sodium dodecyl sulfate vs. 0.315 M octyl alcohol solution. Curve 5. Sodium dodecyl sulfate vs. 0.63 M octyl alcohol solution. Curve 6. Sodium dodecyl sulfate Vs. 1.26 M octyl alcohol solution.

i

30 o I

°

z

L,. 2 0 - -

M

20 40 60 80 I O0 I I I I I AREA PER MOLECULE [ANGSTROMS] ~

FIG. 3. Force-area curves of films adsorbed at a benzene water interface. Curve 1. Sodium dodecyl sulfate alone. Curve 2. A mixed film containing both octyl alcohol and sodium dodecyl sulfate. Data taken from section D of Table I. Curve 3. Octyl alcohol alone.

524 ERIC HUTCHINSON

Included in Table I are the respective surface excesses of the alcohol and ,the detergent calculated as described below.

DISCUSSION

The surface excesses of the various components of the system may be calculated as follows? The system benzene Jr oetyl alcohol II water -[- sodium dodecyl sulfate may be regarded as approximating to a par- ticular example of the generalized system.

Phase" (K" components) II Phase~ (~ components), in which a and are mutually insoluble phases, and all the K", K~ are volume components, there being no components which are purely surface components. (In

TABLE I

Interracial Tension of Solutions of Octyl Alcohol in Benzene against Aqueous Solutions of Sodium Dodecyl Sulfate

Moles ale . / Moles NaLSa/ liter liter

A 1.26 0.63 0.315 0.142 0.073 0

B 1.26 1 X 10 -4 0.63 0.315 0.142 0.073 0

C 1.26 2.5 X 10 -4 0 . 6 3 0.315 0.142 0.073 0

D 1.26 5.0 ×10 -4 0.63 0.315 0.142 0.073 0

Dynes/era. G.-raol. X 10-1°/era~.

F

15.1 20.0 18.0 17.1 21.1 14.0 26.5 8.6 29.5 5.6 35.1

14.4 20.7 16.4 i8.7 18.0 17.1 23.4 11.7 23.5 11.6 2~.1 10.O

12.5 22.6 14.9 20.2 15.9 19.2 18.1 17.0 20.9 14:2 24.4 10.7

11.1 24.0 i3.8 21.3 15.1 20.0 18.1 17.0 20.1 15.0 23.0 12.1

F2 ab r~ fla F

z~2 Area

0 7.52 7.52 21.5 0 6.24 6.24 26.4 0 5.30 5.30 31.i 0 3.04 3.04 55.0 0 2.12 2.12 78.0 0 0 - - - -

0.23 4,58 4.81 34.2 0:27 4,71 4.98 33.2 0.35 3.28 3.63 45.5 0.47 1.80 2.27 72.8 0.53 1.15 1.68 98.5 0.66 0 0.66 250.0

0.32 4.35 4.67 35.3 0.32 3.91 I 4.23 39.0 0.45 3.02 [ 3.47 47:6 0.65 1.38 2.03 81.2 0.67 1.13 1.80 91.8 0.75 0 i 0.75 220.0

0.37 4.19 4.56 36.0 0.56 3.49 4.05 40.8 0.66 2.99 3.65 45.3 0.77 1.75 2.52 65.5 0.87 1.08 1.95 84.5 0.99 0 0.99 167.0

The author is greatly indebted to Dr. F. O. Koenig for this theoretical analysis.

M I X E D A D S O R B E D F I L M S 5 2 5

TABLE I--Continued

Moles ale. / liter

1.26 0..63 0.315 0.142 0.073 0

~' 1.26 0.63 0.315 0.142 0.073 0

0 0 0 0 0 0 0 0

Dynes/cm. G.-mol. X 10 -10/em3 Moles NaLSa/ ~2

liter Area

7 . 5 X 1 0 -4

1.0 X 10 -s

1" F F2ab p~6c p

10.8 24.3 0 .55 4.18 4.68 35.2 11.9 23.2 0.84 3.26 4.10 40.2 13.8 21.3 0.99 2.70 3.69 44.8 16.4 18.7 1.15 1.43 2.58 63.8 17.5 17.6 1.30 0.94 2.24 73.8 19.4 15.7 1.49 0 1.49 111.0

9.65 25.4 0.74 3.46 4.14 39.9 10.1 25.0 1.11 2.89 4.00 41.2 11.0 24.1 i.32 2.37 3.69 44.8 14.1 21.0 1.53 1.38 2.91 56.9 15.2 19.9 ,1.73 1.00 2.73 60.5 17.9 17.2 1.98 0 1.98 83.2

5.0 X 10 -8 5.00 30.4 4.50 - - 4.50 36.5 2.5 X 10 -3 9.98 25.1 2.78 - - 2.78 59.5 1.5X10 -3 16.20 18.9 (3.18) - - 3.18 (57.5) 1.0 X 10 -3 17.9 17.2 1.98 - - 1198 83.3 7.5X10 -4 19.7 15.4 1.49 - - 1.49 111 5,0 X 10 -4 23.0 12.1 0.99 - - 0.99 166 2.5 X 10 -4 24.4 10.7 0.75 - - , 0.75 219 1.0X10 -4 25.7 9.4 0.66 - - 0.66 249

a NaLS = sodium dodecyl sulfate. b = surface excess of sodium dodeeyl sulfate. c = surface excess of octyl alcohol

t h e p a r t i c u l a r s y s t e m u n d e r i n v e s t i g a t i o n t h e t w o p h a s e s a r e n o t s t r i c t l y

m u t u a l l y i n so lub l e , b u t t h e a p p r o x i m a t i o n is good : )

ONe"± F l a Jr" ~ I ' 2 a = - - (1)

a~,~ a~ ( a v ) (2)

w h e r e c o m p o n e n t 1 ~ = w a t e r ,

w h e r e c o m p o n e n t 2 " = s o d i u m d o d e c y l s u l f a t e ,

w h e r e c o m p o n e n t 1~ = b e n z e n e ,

w h e r e c o m p o n e n t 2~ = n - o c t y l a l coho l .

£1", £2", Yl~, £2 ~ a r e t h e s u r f a c e exces se s ( g . / m o l e cm. 2) of c o m p o n e n t s

1% 2K, 1~, 2~, r e s p e c t i v e l y . N1% N~a±, N1 a, N2# a r e t h e m o l e f r a c t i o n s of

c o m p o n e n t s 1", 2% la, 2~, r e s p e c t i v e l y , (N2a± b e i n g t h e mean mole fraction

526 I~RIC HUTCHINSON

of the sodium dodecyl sulfate), ~i ~, ~2 ~, ~1 ~, ~2 B are the chemical poten- tials of components 1 ~, 2 ~, 1~, 2B, respectively, and ~, = interfacial ten- sion. The complete solution for all 4 unknowns requires a normal conven- tion, to fix the position of Gibbs' surface. The equation used in this case is

A2~F.o "(~) ~- Al~Fi B(~) + A~F2~(") = l, (3)

where As" = area/g, tool. of component 2% A1 a = area/g, mol. of component 1 a, A2 a = area/g, tool. of component 2~.

This equation, which is closely analogous to that of Guggenheim and Adam (4), is the expression of a hypothesis as to the physical nature of the adsorbed film. It implies that the film is monomolecular and contains no water molecules. This latter condition is speculative, while the assump- tion that the film is monomolecular seems justified by previous work (7).

The choice of an equation such as the above is arbitrary, and, provided the equations are dimensionally correct and contain only observable intensive variables, an infinite variety of such equations may be written to determine uniquely the position of the Gibbs surface. However, while such a convention is arbitrary, it may embody a hypothesis as to the physical nature of the fihn. The value of the results arising from the use of such a convention must be judged entirely from the reasonableness of the derived data and the correspondence with data obtained by other methods. For example, the close agreement between the force-area curves for fat ty acids obtained from a study of insoluble films and from soluble adsorbed films lends sound physical support to the reusonableness of the common "Gibbs" convention of writing l~w~t~r = 0 in dilute solutions of fat ty acids.

Previous work (7) led to the conclusion that the hydrocarbon chains of the film molecules are entirely in the benzene so that the values to be assigned to the areas A~ ~, Ai ~, A2 a, are unambiguous, viz., the areas ob- tained from close packed insoluble films. Since the convention defines Yl ~(~) = 0 we have

0~2" F2,(.) = ( 0~[ ) (4) 0N2a4 - ~ T.P,

and, since the work of McBain and Johnston (10) shows that, at the concentrations used, the sodium dodecyl sulfate behaves approximately as an ideal uni-univalent electrolyte, we may write

2RT F~(~) = _ ~0~,

where N9.%. -- mean mole fraction of the sodium dodecyl sulfate. I'~ ~ is readily calculated from the slopes of the "1' "~ N2%. curve. To be quite

MIXED ADSORBED FILMS 527

exact, since the convention places the Gibbs surface in the physical boundary layer the value of F~ "(~) calculated in this way should be cor- rected by a small factor

~F2 "(~) = ~x-C2%_,

where C~%~ = concentration of the detergent in moles/ml. ~ 10 -8 moles/ml, aUd 5 x - - l e n g t h of the detergent molecule ~30A. Hence 5F~,(~) ~ 3 × 10 -13 g.-mol./cm. ~ which is negligible.

With the known values of F2"("), and employing Eqs. 2 and 3, values for F1 a(~) and F~(~) may be obtained, but the following approximation was made to facilitate the calculations. Inspection showed that F2 "(~ is relatively small, being of the order 0.3 ~ 1.5 X 10 -1° g.-mol./cm 2. Hence we may write approximately

AI~F1 e(~) ~ - A 2 ~ F 2 ~(u) = 1, (6)

instead of AI~F1 ~(~) -~- A2~F2 ~(u) = 1 - - A2"F~"(~). (7)

Since A2" ~ 0.12 × 10 l° emY/g.-mol, the right hand side of Eq. 7 will have values ranging from 0.96 to 0.82 instead of unity, and the error involved will vary from 4% to 18%. Large errors are only involved in the case of very dilute solutions of octyl alcohol, in which case F: ~(~) ~ 1.5 X 10 -1° g.-mol./cm. 2

Using the approximate solution, the calculation of FI~ and F2~ follows exactly the methods described in the earlier paper (7,8), viz.,

N2~ T02 0~/ F2~(~) = N1 ~'L. T" 0-O'

1 + N~.~/NI~'

where L = latent heat of fusion of benzene-ergs/mole, To = freezing point of benzene,

O = freezing point depression of the solution, T = temperature of experiment.

A~N~ + A2~N~ (8)

Values obtained in this way are given in Table I, and in most cases the error introduced by approximation is less than 10%.

An exact solution to the problem involves the knowledge of the differ-

( 0~, ) (9). While this information is at present lacking, it seems ential 0-P T.N.

reasonable to assume that it would be small, and as this present work is primarily exploratory only the simplest approximation has been used

528 ERIC HUTCHINSON

throughout. The area per molecule in the mixed film is given by

1 1

As in the case of mixed adsorbed films at an air-water surface, the purity of materials is a critical factor in these experiments. The sodium dodecyl sulfate used is known to be of a high degree of purity. Aging at an air-water surface was almost entirely absent, and the results were in fairly good agreement with those of Miles and Shedlowsky (11). At oil-water interfaces against pure oils no aging beyond the first 2 or 3 rain. was ever observed. In contact with solutions of octyl alcohol in benzene, which themselves showed no signs of aging against pure water, solutions of sodium dodecyl sulfate aged slightly, ca. 1-2 dynes/cm, during the first 20-30 rain., and thereafter remained at constant interracial tension for periods up to 2 days. It therefore seems probable that orientation and rearrangement at a benzene-water interface takes appreciably longer than a t an air-water surface.

Examination of Table I shows that, as in the case at an air-water sur- face, the octy| alcohol generally forms the major portion of the inter- facial film, but that, in order to achieve this, the concentration of alcohol

in the benzene phase must be very much higher \ N~a± ~ 50-1000 than

that of the detergent in the water. Hence, it should follow that, if the probable impurity in sodium dodecyl sulfate is dodecyl alcohol, the effect of the impurity, in the equilibrium interfacial film, should be very much less than is the case at air-water surfaces, since experiment has shown that the chain length of the solute in the benzene has practically no effect on adsorption (7). However, it is very doubtful whether true equilibrium is attained in normal practice with impure solutions of sodium dodecyl sulfate. Since minima are reported for interracial tension, as well as for surface tension (12), it seems probable that the free alcohol goes to the interface along with the sodium dodecyl sulfate, but that it interacts strongly with the detergent molecules in the film and does not pass through the interface to its equilibrium position in the bulk benzene phase at any measurable rate.

Experiments were carried out in which 40 ml. benzene were shaken vigorously with 40 ml. of sodium dodecyl sulfate solution containing an added 0.1% of dodecyl alcohol. The initial interracial tension was com- pared with that obtained after the system had been shaken, and the resultant emulsion broken in a centrifuge. Even for periods of shaking extended to 24 hr., there was no such increase in interracial tension as should have resulted if the free alcohol had attained equilibrium distri- bution between the benzene and water.

M I X E D ADSORBED FILMS 529

As in the case of mixed films at an air-water surface, the mixed films at the benzene-water interface appear to have properties intermediate between those of the two pure solute components. In all cases, the area at high compression is between those for the pure materials and neither in the force-area curves nor in the slopes of the respective interfacial tension-concentration curves is there any evidence indicative of com- pound or complex formation.

ACKNOWLEDGMENT

The author wishes to express his thanks to Prof. J. W. McBain, F.R.S., for kind encouragement and advice.

SUMMARY

Measurements have been made of the interfacial tensions of aqueous solutions of sodium dodecyl sulfate against solutions of octyl alcohol in benzene. By the application of the Gibbs adsorption theorem approximate calculations have been made of the surface excess of alcohol molecules and detergent molecules in the mixed adsorbed interracial film on the assumption that this film contains no water molecules. The suggestion is made that, for systems in true equilibrium, the effect of oil-soluble~im - purities in the sodium dodecyl sulfate should be less than in correspond- ing experiments at air-water surfaces.

The properties of the mixed adsorbed films are intermediate between those of the two pure components.

REFERENCES

1. AD~, N. K., AND SHUTE, H. L., Trans. Faraday Soc. 34, 758 (1938). 2. BURDON, R. S., ibid. 35, 727 (1939). 3. Gouv, G. Ann. phys. 6, 5 (1916). 4. GUGGENHEIM, E. A., AND ADAM, N. K., Proc. Roy. ,.%c. London 139A, 218 (1933). 5. HARKINS, W. D., AND BROWN, F, E., J. Am. Chem. Soc. 41, 499 (1919). 6. HUTCHINSON, E., J. Phys. Colloid Chem. 52, 897 (1948). 7. HUTCHINSON, E., J. Colloid Sc/. 3, 219 (1948). 8. HUTCHINSON, E., ibid. 3, 235 (1948). 9. KOENIG, F. O., to be published.

10. McBAIN, J. W., AND JOHNSTON, S. A.} Proe. Roy. ~OC. London 181A, 119 (1942). 11. MIL~s, G. D., AND SHEDLOWSKY, L., J. Phys. Colloid Chem. 48, 67 (1944); 49, 71

(1945). 12. POWNEY, J., AND ADDISON, C. C., Trans. Faraday Soc. 33, 1246 (1937).