Download - Mixed monolayers. I. Adsorbed films at air-water surfaces

MIXED MONOLAYERS. I. ADSORBED FILMS AT AIR-WATER SURFACES

Eric Hutchinson 1

From the Department of Chemistry, Stanford University, Calif. Received June 8, 1948

INTRODUCTION

A considerable amount of information is available about mixed monolayers of insoluble films placed on water surfaces (8), but very little is available for corresponding films adsorbed from solution.

Much attention has been focused recently (10,12,15) on the so-called type I I I surface tension curves which are frequently obtained with solutions of synthetic detergents and in which minima are often observed in the surface tension-concentration curves, although frequently there is no minimum but the surface tension after rapid lowering in very dilute solution remains nearly constant. In the case of at least one such detergent, sodium dodecyl sulfate, Miles and Shedlovsky (14) have clearly demonstrated that the minimum is absent provided the detergent is highly purified, and that the minimum is restored by the addition of very small amounts (ca. 0.1%) of "impurity" to the solution. These findings have been confirmed by Brady (4) using a different method of purifying the sample, and by Robinson 2 who added further amounts of lauryl alcohol to intensify such minima.

Further, with highly purified samples of sodium dodecyl sulfate time effects on surface tension are almost entirely absent, as found by Brady (4) and by the author. So much discussion has taken place about the surface aging of detergents of this kind (2,3,16), and so many theories have been proposed to account for the long time effects on surface tension, that it i s of prime importance to study systems in which the materials are really pure, and then deliberately to add impurities and observe the effects.

While experiments of ~ Miles and Shedlovsky (14) and Brady (4) demonstrate such effects qualitatively, it was thought desirable to make a quantitative study of the effects of a known impurity while applying the Gibbs adsorption theorem to such a system. The materials used were sodium dodecyl sulfate, with n-octyl alcohol as the impurity, Sodium dodecyl sulfate was chosen (a) because of the data available on the pure

Bristol-Myers Company Postdoctorate Fellow and Research Associate in Chemistry. 2 Unpublished results, Stanford University.

413

414 ERIC HUTCHINSON

compound from the work of Miles and Shedlovsky, and (b) because a fairly large sample of the highly purified material, kindly supplied by E.I. du Pont de Nemours & Co., was available. Although the most likely source of impurity is dodecyl alcohol, octyl alcohol was used because of the greater ease in the preparation of the solutions, for which the low solubility of dodecyl alcohol would have proved troublesome.

EXPERIMENTAL

Measurements of the surface tension of the solutions of octyl alcohol and sodium dodecyl sulfate in distilled water and conductivity water were made by means of the ring method (9), and the Wilhelmy plate method (5,6,7) in its differential form. Usually 50 ml. of the solution was placed in a 10 cm. glass dish provided with a cover to prever~t accidental contamination from the air. No attempt was made to thermostat the solutions, but the laboratory temperature during the experiments remained at 23°C. ± 1 °. The Wilhelmy plate method showed that only very small time effects in surface tension occurred. After a preliminary drop of ca. 0.2 ~ 0.4 dynes/cm, during the first 2-3 rains., no further change in surface tension was observed, either in the solutions of octyl alcohol and sodium dodecyl sulfate :alone, or in solutions containing both components during at least 3 hrs. The slide used in this method was a square micro- scope cover slip ca. 2.4 X 2 X (J.02 cm, and was hung from the arm of a torsion balance. Harkins (7) describes two alternative methods of using the Wilhelmy plate method in its differential form, Firstly, a constant upward pull may be exerted on the slide, and the surface pressure ( i .e . ,

the decrease in surface tension) s tudied as a function of the depth of immersion of the slide. Secondly the slide may be maintained at a constant depth of immersion and the surface pressure studied as a function of the upward pull On the slide required to maintain this constant position. The second of these methods was adopted.

The ring used in the ring method was a Cenco platinum-iridium ring R

4.0 cm. in circumference and had a - factor (9) equal to 39.5. r

Agreement between the two methods was generally within 0,2 dyne/ cm.; and in the worst cases was within 0.6 dyne/cm. This agreement is significant in itself, demonstrating that, in cases like the present, where aging is almost entirely absent, the ring method may be used with solutions as well as with pure liquids. Both methods are dependent, for simple use, on the maintenance of zero contact angle at the solid:liquid interface. With solutions there exists the possibility that this may be difficult to ensure but, since the solid was glass in the one case and platinum-iridium in the other, and agreement was obtained, the assumption may reasonably be made that zero contact angle was obtained in both cases.

MIXED MONOLAYERS. I 415

No appreciable differences could be found for the surface tensions of solutions prepared in distilled water from those of solutions prepared in conductivity water.

RESULTS

Surface tensions for a number of solutions of octyl alcohol and sodium dodecyl sulfate in Water are given in'Tables I -VI? In Fig. 1 are given curves for the surface tension as a function of the mole fraction N2 of the octyl alcohol in water and in solutions of sodium dodecyl sulfate of various concentrations. Fig. 2 presents corresponding data for the variation of surface tension as a function of the mean mole fraction N3:~ of the sodium dodecyl sulfate in water and in various solutions of octyl alcohol.

TABLE I Surface Tension of Mixed Solutions of Octyl Alcohol and Sodium Dodecyl Sulfate

Conc. of alcohol

molality M~

2.52 X 10 -3 2.016 1.512 1.008 0.504 0.252

Cone. of NaLS G

molality M,

F2

5.57 5.17 5.22 5.22 5.30 3.00

Surface excess

g./mole X 10 ~°

r~

5.57 5.17 5.22 5.22 5.30 3.00

Area

2 9 . 6 32.0 31.6 31.6 31.2 52.0

Film pressure

36.9 33.8 29.3 24.6 14.8 6.6

a Sodium dodecyl sulfate.

T A B L E I I

Surface Tension of Mixed Solutions of Octyl Alcohol and Sodium Dodecyl Sulfate

Surface excess

Conc. of alcohol molality

Ms

2.52 X 10 -3 2.016 1.512 1.008 0.504 0.252 0

Cone. o~ NaLS a

molality Ms

0.5 X 10 -3 0.5 X 10 -3 0.5 X 10 -3 0.5 X 10 -3 0.5 X 10 -3 0.5 X 10 -3 0.5 X 10 -3

F2

5.28 5.08 4.57 4.16 3.23 2.21 0

g. /mole X 101°

F~

0.53 .49 .64 .64 .93

.90

F

5.81 5.57 5.21 4.80 4.16

.90

Area

28.4 29.6 31.7 34.4 39.7

183.0

Film pressure

38.7 35.9 32.9 28.0 21.0 13.9 4.9

Sodium dodecyl sulfate.

3In Tables I - V I , r2 = surface excess of octyl alcohol~ F3 = surface excess of so- d ium dodecyl sulfate.

416 E R I C H U T C H I ~ T S O N

T A B L E I I I


Cone. of alcohol

molality M2

2.53 X 10 -3 2.016 1.512 1.008 0.504 0

Conc. of NaLS~

molality Ma

1.0 X lO-a 1.0 X 10 -3 1.0 X 10 -3 1.0 X 10 -3 1.0 X 10 -3 1.0 X I0 -s

r2

4.37 4.36 4.37 4.25 3.02 O

Surface excess

g.tmole X 10 ta

r t

0.78 .86

1.00 0.95 1.25 1.80

5.15 5.22 5.37 5.20 4.27 1.80

Area

32.0 31.6 30.7 31.6 38.7 91.7

Film pressure

40.3 39.0 36.1 32.0 24.8

9.5


T A B L E IV


Surface excess

Cone. of alcohol mo~:ty

2.56 X 10 -3 2.016 1.512 1.008 0.504 0

Conc. of NaLS~

molality Ma

1.50 X 10 -3 1.50 X 10 -3 1.50 X 10 -3 1.50 X 10 -s 1.50 X 10 -3 1.50 X 10 -3

r t

4.47 3.95 4.15 3.77 2.85 0

g./mole X 10 l°

rz

1.03 1.02 1.11 1.18 1.69 2:70

5.50 4.97 5.26 4.95 4.54 2.70

Area

30.0 33.1 31.4 33.2 36.3 61.4

Film pressure

42.5 40.8 37.7 34.2 28.3 14.2

a Sodium dodecyl sulfate. T A B L E V


Surface excess

Cone: of alcohol

molality M~

2.56 X 10 -s 2.016 1.512 1.008 0.504 O

Conc. of NaLS~

molality Mz

2.0 × 10 -3 2.0 X 10 -3 2.0 X 10 -3 2.0 X 10 -3 2.0 X 10 -3 2.0 X 10 -3

rz

4.04 3.66 3.48 3.28 2.62

g./mole X 10 ~

r, I 1.08 1.07 1.18 1.38 1.90 3.66

5.12 4.73 4.66 4.66 4.52 3.66

Area

32.2 34.9 35.9 35.9 36.5 45.9

Film pressure

43.8 41.5 39.6 36.1 30.5 17.9



TABLE VI


Conc. of alcohol molality

M j

2.52 X 10 -3 2.016 L512 1.008 0.504

Conc. of NaLSo

molality M*

2.50 X 10 -s 2.50 X 10 -3 2.50 X 10 -a 2.50 X 10 -a 2.50 X 10 -3 2 .50X 10 -3

r l

3.82 3.05 2.76 3.05 2.68

S u r f a c e e x c e s s

g . / m o l e X 10 ~°

r,

1.26 1.25 1.33 1.66 2.23 4.50

5.08 4.25 5.09 4.68 4.91 4.50

Area

32.5 (38.8) 32.4 35.3 33.6 36.7

Film pre~ure

45.2 42.5 40.7 37.9 32.5 22.1

40

W z 121

t,

c~20 3

J

'TIC

F-vN 2 0 C T Y L ALCOHOL IN WATER

V I I I I I'O 2.0 3"0 4-0

MOLE FRACTION OF ALCOHOL N~ 10 6

Fro. 1. Surface pressure as a function of octyl alcohol concentration. Curve 1, oetyl alcohol in 2.5 X 10 -3 M sodium dodecyl sulfate; Curve 2, octyl alcohol in 2.0 X 10 -a M sodium dodecyl sulfate; Curve 3, octyl alcohol in 1.5 × 10 -a M sodium dodecyl sulfate; Curve 4, octyl alcohol in 1.0 × 10 -a M sodium dodecyl sulfate; Curve 5, octyl aloehol in 0.5 × 10 -a M sodium dodecyl sulfate; Curve 6, octyl alcohol in water.

418 E R I C H U T C H I N S O N

I

4 O

bJ Z :>-

b_ w2C

ee

~, vc t.L

I 0 20 30 4 0 MOLE FRACTION OF DETERGENT N~IO ~

3

Fro. 2. Surface pressure as a function of sodium dodecyl sulfate concentration. Curve 1, sodium dodecyl sulfate in 2.52 N 10-3M oetyl alcohol; Curve 2, sodium dodecyl sulfate in 2.01 N 10 -a M octyl alcohol; Curve 3, sodium dodecyl sulfate in 1.51 X 10 .3 M octyl alcohol; Curve 4, sodium dodecyl sulfate in 1.008 ~E 10 -3 M octyl alcohol; Curve 5, sodium dodecyl sulfate in 0.504 × 10 -3 M octyl alcohol; Curve 6, sodium dodecyl sulfate in water.

In Fig. 3 are g iven force-area curves for the adsorbed films of octy l alcohol, sodium dodecyl sulfate, and a mixed film containing both com-

ponents . The areas per molecule in the film were obtained as described in fol lowing sections.

D I s c u s s z o ~

The generalized form of the Gibbs adsorption theorem m a y be written:

k

ds' = - ~ d~r~ , (1) 1

MIXED MONOLAYERS, I 419

4C

w Z >-

bJ

~D W n-

U ,-710

I 2 0

AREA

I I I . ] I I i 4-0 60 . 80 .~_

PER MOLECULE L,,~NGST RO M SJ

Fro. 3. (1) Force-area curve for film adsorbed from alcohol solutions; (2) Force-area curve for film adsorbed from solutions containing 1 : 1 proportions of alcohol and sodium dodecyl sulfate; (3) Force-area curve for film adsorbed from sodium dodecyl sulfate solutions.

where

- sur face t ens ion = ~ ( T , x2, x3 . . . x~),

#~ -- chemica l p o t e n t i a l of t h e i t h c o m p o n e n t ,

£~ = sur face excess of t h e i t h c o m p o n e n t ,

x~, . . . x~ -~ a n y i n d e p e n d e n t i n t e n s i v e v a r i a b l e s of t he s y s t e m ,

k -- n u m b e r of c o m p o n e n t s in t he sys t em.

Also :

- ~ r, . (2) Oxi 1

T h e d i f f icu l ty in a p p l y i n g the G i b b s t h e o r e m lies in t he ca l cu l a t i on of t h e

420 ERIC HUTCHINSON

various derivatives 0-~z~." Koenig (11) 4 has calculated these derivatives for

a numbe~ of systems• According to the scheme set up by Koenig the present case may be

considered as a two-phase system (a -b B) containing 3 components all of which are present in at least one phase, ~, the solution, and in which is a dilute vapor.

Taking as the independent intensive variables, T, N~, N~-,

"r -= "r(T, N~, N~.), where

N~ = mole fraction of water in phase a,

N~ -- mole fraction of octyl alcohol in phase a,

N ~ = mean mole fraction of sodium dodecyl sulfate in phase a.

I t may then be shown (11) that, generally, for such a system: 0° 0=

l r ' = - ' j = 2 , 3 , . . . k . ( 3 )

For ideal solutions of non-electrolytes we may write:

(0.1 (01nNr

_ ~ , * o

N7 .~ R T

- - - Y~l' (4) and

#~ = ~ -b R T In N~;

• O~g R T l ON~ - N~ j = h.

= 0 I j ~ h Hence

R T R T O.r - N~ r~ + N---f~ r~ = ON~ (5)

The equation for ~ thus contains only two unknowns F~ and r~. In

4 The author wishes to acknowledge his gratitude to Dr. F. O. Koenig for great assistance in the interpretation of the Gibbs theorem, and for the calculations of the derivatives. In this paper the theoretical portions are due to Dr. Koenig. A generalized scheme for the application of the Gibbs theorem is to be published shortly by this author•

M I X E D M O N O L A Y E R S . I 421

the case tha t one of the components, e.g., the lth is an ideal electrolyte, we have

~7 = ~7 ° -~ vRT In N?-~, and

and

0~? vRT

ON?± N? '

0u~ vR T ] ONT~- NT+ for h = l,

0 for h ~ l.

~RT . vRT F? = 0~, Thus ~ FI -~- /V3---~ - - ONT-----=e for l = ideal electrolyte. For the

system water, octyl alcohol, sodium dodecyl sulfa te /water vapor,

Component 1 = water,

2 = octyl alcohol,

3 = sodium dodecyl sulfate.

• R T ,~ R T O,y • - - N ~ F I - b N--~2 I '~ = - ON--~2 (6)

and 2 R T 2 R T 0"1

- N-Y r ? + ~ r ~ = - - - - ~ , (7 ) N3± ONa±

assuming the solutions to be ideal. These two equations contain three unknowns F1, r~, r3 and to solve for these we require a third equation, viz., a normal convention to define the position of the Gibbs surface•

In this particular case, where the solutions are dilute, a suitable con= vent ion is F1 = 0: the so-called Gibbs' convention.

Then N~ 0~/ N~ OF

F~ = R T ON~ R T ON~ (8)

where

and F = the film pressure = ~0 _

N ~ : OF r3 = 2 R T ONe± (9)

Values for F2 and 1~3 obtained by measuring the slopes of the F - N curves are given in Tables I -VI . Values of the total surface excess 1 ~ = F2 W Fa

1 and the corresponding areas per molecule in the films. A = ~-~ are given

in Tables I - VI also.

422 ERIC HUTCHINSON

Any study of adsorbed films, and particularly those involving detergents such as sodium dodecyl sulfate, is critically dependent on the purity of the materials used. The n-octyl alcohol used here was a pure sample and was well shaken up with "Florisil" prior to use. The sodium dodecyl sulfate showed practically no signs of aging and this may be taken as indicative of a high degree of purity. The surface tension curve of aqueous solutions of this material, however, was some 1-3 dynes/cm. lower than those of Brady (4) and of Miles and Shedlovsky (14), depend- ing on the Concentration. Hence, it is possible that very small traces (<<0.05%) of impurity still remained in the material. This does not in- validate some of the more important conclusions which may be drawn from the results.

By virtue of Eq. (8) the 1~ values quoted for the octyl alcohol are independent of any impurities in the sodium dodecyl sulfate (except in the event that the adventitious impurity is itself octyl alcohol). The values quoted for 1~, will, in general, be greater than the true values for

OF F~, in the absence of impurity, since the measured slope ~ ought

OF strictly to be written 0 (N3. -k hrlm,u~ty) " Hence, the F3 values quoted

represent Upper limits for this quantity. Even so, it is immediately apparent tha t the number of molecules of oetyl alcohol in the film is at least equal to that of the sodium dodecyl sulfate, and is generally 2 or 3 times greater: Even when the bulk concentration of alcohol is' only one-fifth that of the sodium dodecyl sulfate, there are rather more molecules of alcohol than of detergent in the film. Hence, the alcohol is relatively more surface active than the detergent containing 4 more carbon atoms, and it is, therefore, highly probable that, owing to the Traube rule effect, dodecyl alcohol would be very much more surface active than sodium dodecyl sulfate. Thus, even when present at only small concentration as impurity, the alcohol would form a significant portion of the surface film. Beyond the critical concentration of the sodium dodecyl sulfate, when micelles are present, the alcohol will be largely, if not entirely, removed from free solution by solubilization, and the film will now consist almost entirely of sodium dodecyl sulfate. Thus, in terms of the mixed film it becomes apparent why type I I I curves are obtained with impure materials. It is still a problem why type II I curves without a minimum exhibit a constant low surface tension over a wide range of concentration.

Inasfar as the absolute magnitudes of the r values are concerned it must be borne in mind that these were calculated on the assumption that the solutions are ideal. From the work of McBain and Johnston (13) it is clear that, in dilute solutions, sodium dodeeyl sulfate is almost ideal,


yielding solutions with an osmotic coefficient g ~ 0.98-1.00. No data are available to show how good an approximation holds in the case of octyl alcohol. However, the values of F~ quoted for octyl alcohol represent a lower limit, for, if the solutions are non-ideal, the true values of F~ must necessarily be greater than these since with non-ideal solutions the slopes of the F ~ N~ curves will be greater than for ideal solutions.

The force-area curve for n-octyl alcohol alone shows the film to be a highly compressed gaseous film with large deviations from ideality above ca. 10 dynes/cm. : this is in agreement with the generally observed higher degree of condensation of alcohols as compared with corresponding acids in insoluble films (1).

The force-area curve for sodium dodecyl sulfate shows that this compound forms a "gaseous" type film with a roughly constant value for the product (Force × Area) ~ 800-900. A large number of force-area curves may be drawn for films adsorbed from solutions of the two solute components. Curve 2 in Fig. 3 shows that for the film adsorbed from approxi- mately equimolar solutions, and it is seen that the film has properties intermediate between the condensed film of the alcohol and the gaseous film of the sodium dodecyl sulfate. Inspection shows that all of the mixed films have intermediate properties, approximating more or less closely to the behavior of the major component of the film. This is essentially the same as was found for a number of mixed insoluble films by Harkins and Florence (8). There is no obvious variation in the properties of the system, e.g., rapid changes of slope in the F ~ N curves, to suggest surface compound formation such as was suggested by Schulman (17) for insoluble films of cetyl alcohol penetrated by sodium cetyl sulfate; nor is there any evidence, except possibly in extreme dilution, that one component sensibly increases the adsorption of the other by any kind of "boosting" action.

The probable accuracy of the values of A, the area per molecule in the film is of course much lower than in the case of insoluble films. Not only are the measurements of the surface tension themselves relatively inaccurate (± 0.2-0.3 dynes/cm.) but the determination of the F~ involve drawing tangents to curves which in some places have a small radius of curvature, and errors are easily introduced at this stage. Thus, since the areas are subject to a probable error of ca. 1-2 A s, it is felt that quantitative deductions from the F - A curves might not be justifiable.

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 surface tensiens of solutions containing octyl alcohol and sodium dodecyl sulfate. By the application

424 ERIC HUTCHINSON

of the Gibbs adsorption theorem for 3-component systems, values have been determined for the surface concentrat ion of alcohol and detergent. Even in solutions in which the alcohol concentration is low relative to tha t of the sodium dodecyl sulfate, the alcohol remains the main constitu- ent in the mixed film. By analogy, reasons are adduced to explain " type I I r ' surface tension curves for detergents of this kind, T h e properties of the mixed films are intermediate between those of the pure components, and there is no evidence for interaction in the mixed film. There is apparent ly mere competi t ion for the available surface in the monolayer.

REFERENCES

1. ADAM, The Physics and Chemistry of.Surfaces. Oxford, 1944. 2. ADAM AND SHVTS, Trans. Faraday Soc. 34, 1 (1938). 3. AL~XAN])ER, ibid. 37, 15 (1940). 4. BRADY, Navy Report#4, N6ori--154--T.O., II. 5. DERVICHI~N, g. phys. radium 6, 221,429 (1935). 6. HARKINS, in J. ALEXANDER'S Colloid Chemistry. Reinhold, 1944. 7. HARK:NS AND ANDERSOn, J, Am. Chem. Soc. 59, 2189 (1937). 8. HA~K:NS AND FLORENCE, J. Chem. Phys. 6, 847 (1938). 9. H~KINS AND JORDAN, J. Am. Chem. Soc. 52, 1751 (1930).

10. ttAvsv.~, Recent Advances in Colloid Science. Interscience, 1942. 11. KOENXG, Private communication. 12. McBAIN, Repts. Progress of Physics 5, 30 (1939). 13. McBAIN AND JOHNSTON, Proc. Roy. Soc. London 181A, 119 (1942). 14. MILES AND SHEDLOVSKY, J. Phys. Chem. 48, 57 (1944). 15. PowNEY AND ADDISON, Trans. Faraday Soc. 33, 1243 (1937). 16. Ross, J. Phys. Chem. 49, 377 (1945). 17. SCHULMAN AND STENHAGEN, Proc. Roy. 8oc. London 126B, 356 (1938).

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