The effect of N-dodecanol on the thickness of soap films

NOTES approximation the behaviour of the blood flow along a vein in which pulsed flow has but little influence. Moreover, it must be emphasized that the experimental method is simple and can be used whenever one attempts to find flow-rate curves for a non-Newtonian fluid. REFERENCES 1. BARRAS, J. P. (1967), Thesis. 2. BENIS, A. M., Thesis, Mass. Inst. Teehnol. Boston (1964). 3. BENIS, A. M., LACOSTE, J., ANDLOCKHART,A., Bull. Phys. Path. resp. 4, 103 (1968). 4. BOMBLED,J. P., Cahiers du Groupe franfais de Rhdologie, 1, 35 (1966). 5. CASSON, N., in "Rheology of disperse systerns." C. C. Mill, Ed. Pergamon Press, London (1959). 6. CERNY, L. C. Biorheology 1,159 (1963). 7. ISENBERG, I. Bull. math. Biophys., 15, 149 (1953). 8. JoLY, L., Hemorheology, edited by Copley, Pergamon Press, New York, 1967, p. 41. 9. KRIEGER, I. M., AND MARON, S. H., dr. Appl. Physiol. 23, 147 (1952). 10. LARCAN, A., STOLTZ, J. F., STOLTZ, M., AND GENETET, M., Agressologie, 8, 471 (1967). 12. MERRILL, E. W., BENIS, A. M., GILLILAND, E. R., SHERWOOD, AND SALZMAN, E. W., or . Appl. Physiol. 20, 5 (1965). 13. MERI~ILL, E. W., ~NI) PELL~TI~R, G. A., J . Appl. Physiol. 23, 178 (1967). 14. 0LDROYD, J. G., in " R h e o l o g y - - T h e o r y and applications." Edited by Eirich, Academic Press Inc., New York 1, 663 (1962). 15. RIVLIN, R. S., Hemorheology, edited by Copley, Pergamon Press, New York, 1967, p. 157. 16. ScHvaz, J., Rheologica Acta 1, 58 (1958). 17. SCOTT-BLAIR,G. W., Nature, 183,613 (1959). 18. Soo, S. L., " F l u i d Dynamics of Multiphase Systems." Blaisdell, London (1967). 19. STOLTZ, J. F., GENETIflT, B., AND. LARCAN, A, C. R. Acad. Sei. Paris 3-7-1967, sdr. D, p. 64. 20. STOLTZ, J. F., GENETET, B., AND LARCAN, A., C. R. Soc. Biol. Nancy 161, 1342 (1967). 21. STOLTZ, J. F., AND LARCAN, A., in "Cahiers du groupe fran~ais de Rh~ologie," (In Press) (1969). 22. WILI(INSON, W. L. " N o n - N e w t o n i a n fluids." VoI. 1. Pergamon Press, London (1960). J. F. STOLTZ A. LARCAN

Section d'HOmorh@ologie, Clinique mOdicale A, C.H.U. de Nancy (France) Received October 30, 1968 Revised March 18, 1969

577

The Effect of N-Dodecanol on the Thickness of Soap Films A large proportion of recent experiments carried out to investigate the properties of soap films have been made on films stabilized with sodium n-dodeeylsulphate (SDS). The choice of this surfactant is regrettable since it has been reported (1) that hydrolysis to a more surface active compound n-dodecanol (DOH) occurs in aqueous solution. Corkill et al. (2) have stated that they were unable to form stable soap films from pure SDS at any concentration and in a recent experiment by Razouk and Mysels (3) SDS rigorously purified by foaming was found to form films which were insufficiently stable for long term studies. The indications are that many of the reported measurements on stable films formed from " p u r e " SDS solutions may have in fact been made on solutions containing traces of D O H as a stabilizing impurity. We have carried out some experiments to find out what influence D O H has on the thickness of films formed by SDS solutions with films of approximately 100-A thickness. EXPERIMENTAL METHODS SDS was prepared from Fluka " p u r i s s " D O H by the procedure of Dreger et al. (4). Purification was made by continuous diethyl ether extraction for 3 weeks followed by two crystallizations from aqueous solution. The surface tension-concentration curve of freshly prepared aqueous solutions in the absence of salt showed no minimmn (5). The sample was always stored in the solid state in a vacuum dessicator. Fluka " p u r i s s " D O H and the analytical grade sodium chloride was used in the experiments. Solutions were made up from deionized water which was distilled from alkaline potassium permanganate and then redistilled (specific conductivity 1-3 × 10-6 ~-1 cm-1). Thickness measurements were made using a reflectometer similar to that described by Lyklema et al. (6) operating at a wavelength of 5460 A. The films were formed on a glass rectangular grame (5 X 2 cm) in an all glass apparatus consisting of a pyrex glass cylindrical cell 15 cm long and 6.5 cm in diameter. The apparatus was placed in a water thermostat at 25 4- 0.01°C. The equivalent water thicknesses of the films were calculated as described by Lyklema et al. (6). Surface tension measurements were made with a Du Noiiy tensiometer at room temperature (~22°C) and corrected by the method of Harkins and Jordan (7).

Journal of Colloid and Interface Science,

Vo|. 30, No. 4, August 1969

578

NOTES

loo i- ....

-I---?o

........

v:9

_

_

_

o

80

2'0

,_

21

212

~

L

44

L

45

I

46

~V,

L

,^,

74 ¢9G

73

T i me (hrs)

FIG. 1. Film thickness against time of t h e r m o s t a t i n g . RESULTS AND DISCUSSION Films formed from pure SDS solutions of conc e n t r a t i o n 0.25% were not s t a b l e long enough for thickness m e a s u r e m e n t s to be made ( ~ m i n u t e s ) even after t h e r m o s t a t t i n g for 48 hours. A d d i t i o n of sodium chloride e n h a n c e d t h e films s t a b i l i t y and all t h e m e a s u r e m e n t s were m a d e on solutions of c o n c e n t r a t i o n 0.15 M sodium chloride a n d 0.032% SDS (0.0011 M). The l a t t e r c o n c e n t r a t i o n is j u s t above t h e critical micelle c o n c e n t r a t i o n . We h a v e found t h a t macroscopic films are n o t stable below the c.m.c, of the system. Experiments in the absence of DOH. I n t h e absence of D O H films formed were n o t stable for longer t h a n a b o u t 30 minutes, and m a n y r u p t u r e d before this time. I t was found t h a t t h e b l a c k films c o n t i n u e d to t h i n w i t h time. This effect is illust r a t e d in Fig. 1 where film thicknesses are p l o t t e d as a f u n c t i o n of the time from first placing the cell in t h e t h e r m o s t a t b a t h . E a c h curve was o b t a i n e d b y forming a film and measuring its thickness over a period of a p p r o x i m a t e l y 30 m i n u t e s at 5-minute intervals. T h e glass cell which h a d a t o t a l volume of 350 ml c o n t a i n e d 100 ml of t h e freshly p r e p a r e d solution. T h e reflectometer was focussed near t h e top of the film which was 2 cm high and the refiected light was recorded as soon as the b l a c k film covered t h e film area seen b y the p h o t o multiplier. The t o t a l area of the film became b l a c k after a p p r o x i m a t e l y 25 minutes. One definite conclusion which can be d r a w n from Fig. I is t h a t t h e initial thickness of the b l a c k film is i n d e p e n d e n t of t h e time of t h e r m o s t a t t i n g w i t h i n e x p e r i m e n t a l error. These a n d similar Journal of Colloid and Interface Science, Vol. 30, No. 4, August 1969

TABLE I DOH~ S~-%

0 0.1 0.5 1.0 2.0 2.0

No. of films 12 9 9 9 5~ (Mobile) 1 (Rigid)

~(~.) 103.7 103.6 109.1 110.9 109.1 132.7

± 444±

3(£) 1.4 1.7 0.7 2.5 1.1

A3'b (dynes Cm-l)

94.5 94.4 99.9 101.7 99.9

0 0.7 4.7 7.6 9.2

119.3

9.2

a After 21 hours in t h e r m o s t a t . b D e p t h of m i n i m u m in v against c o n c e n t r a t i o n curve after p r e p a r i n g t h e solution and after 50 hours. m e a s u r e m e n t s we h a v e made also indicate t h a t t h e change in film thickness w i t h time decreases w i t h the time of t h e r m o s t a t t i n g . T h e average initial thickness (96.9 4- 0.5 -~) is slightly higher t h a n the result quoted in T a b l e I for zero D O H concentration which was o b t a i n e d w i t h a different b a t c h of NaC1 solution. Since there is a s h a r p decrease in thickness between the silver and b l a c k films at a salt concent r a t i o n of 0.15 M it seems reasonable to conclude t h a t the initially formed b l a c k film has the t r u e equilibrium thickness and a n y s u b s e q u e n t t h i n ning is due to evaporation. E v a p o r a t i o n becomes less noticeable as t h e s y s t e m approaches t h e r m a l equilibrium b u t t h e process is v e r y slow. All the m e a s u r e m e n t s recorded below refer to

NOTES the initial (zero time) thicknesses since we believe t h e y most n e a r l y correspond to true equilibrium. Ezperiments in the presence of DOH. T a b l e I records t h e thickness m e a s u r e m e n t s for films at several c o n c e n t r a t i o n s of DOH. The a m o u n t of DOI-I is expressed as a percentage b y weight based on t h e weight of SDS present. A d d i t i o n of as little as 0.1% D O H greatly e n h a n c e d the s t a b i l i t y against r u p t u r e and all the films c o n t a i n i n g D O H were stable for longer t h a n t h e period of observation. E a c h solution was freshly p r e p a r e d from a single stock solution of NaC1 and m e a s u r e m e n t s of the thickness were commenced after t h e cell h a d been in the t h e r m o s t a t for a p p r o x i m a t e l y 20 hours and c o n t i n u e d up to 48 hours. T h e thicknesses were m e a s u r e d over 30-minute periods and showed the t h i n n i n g b e h a v i o r as described above. T h e average zero time thickness is recorded t o g e t h e r w i t h the s t a n d a r d deviation. T h e surface tension-concent r a t i o n curves were also m e a s u r e d on the freshly p r e p a r e d solutions and after a period of 50 hours. No difference in the two curves was found. In this connection it m a y be n o t e d t h a t I t a r r o l d found t h a t hydrolysis became significant in SDS solutions c o n t a i n i n g no salt after aging for 24 hours (1). T h e results show t h a t the e q u i v a l e n t w a t e r thickness increases significantly as the a m o u n t of D O I t increases up to a c o n c e n t r a t i o n of 1% ( D O t { / S D S ) . I t is necessary to correct the equivalent w a t e r thickness for the " s a n d w i c h " s t r u c t u r e of the film. F o r a mobile film t h e true film thickness a can be calculated from the e q u a t i o n (8). (no~ -

n ~ ~)

no~ -- 1 where x refers to the m o n o l a y e r thickness of ref r a c t i v e index nm, and no is the refractive index of the aqueous core. N u m e r o u s values (2, 9) h a v e been r e p o r t e d for the area per molecule of SDS at t h e liquid-Mr interface and from a review of the l i t e r a t u r e a v a l u e of a p p r o x i m a t e l y 40 A 2 per molecule seems to be a reasonable average. Assuming a m o n o l a y e r density of 1 g. cm -a gives a v a l u e of x = 11 A. If n~ = 1.33 a n d n m = 1.45, a = aw - 9.2. T h e values of a so calculated are listed in T a b l e I. This correction is most p r o b a b l y an undere s t i m a t e since it would seem likely t h a t as t h e c o n c e n t r a t i o n of D O H increases the m o n o l a y e r thickness increases. T h e m a x i m u m m o n o l a y e r thickness corresponding to fully o u t s t r e t c h e d hyd r o c a r b o n chains has been e s t i m a t e d (6) at 16 for r i g ~ films which would give a correction of --13.4 A. Hence, t h e m a x i m u m change in the optical correction is 4.2 A and since the fihns were mobile this m u s t be an overestimate. Thus, while the increase in m o n o l a y e r thickness would reduce the observed increase in fihn thickness with D O H

579

c o n c e n t r a t i o n , it could n o t account for it completely. Experiments with 2% DOH. T h e b e h a v i o r of films w i t h 2% D O H p r e s e n t was not reproducible. I n i t i a l l y after the freshly p r e p a r e d solution h a d been t h e r m o s t a t t e d for 4 hours, a p r e c i p i t a t e formed. The p r e c i p i t a t e was most p r o b a b l y a D O H - S D S adduct, thus removing some D O H from the solution and the thickness m e a s u r e m e n t s made after 21 (99.9 A), 45 (97.5 A) and 53 (97.4 A) hours correspond to a solution c o n t a i n i n g less t h a n 0.5% D O t t and indicate t h a t t h e p r e c i p i t a t i o n was complete after a b o u t 50 hours. On redissolving t h e p r e c i p i t a t e b y h e a t i n g and replacing the cell in t h e t h e r m o s t a t no p r e c i p i t a t i o n was observed for 20 hours and t h e film formed was rigid and came to an equilibrium thickness of 119.3 A after a d r a i n i n g period of 140 hours and remained c o n s t a n t for a t least a n o t h e r 30 hours. D u r i n g this time precipitation occurred and when t h e rigid film was r u p t u r e d a mobile film could be formed w i t h a thickness of 104.1 ~_. These results are explicable if the solution was below the film drainage t r a n s i t i o n t e m p e r a ture (10) at 25°C and when some D O N is removed b y p r e c i p i t a t i o n the fihn drainage t r a n s i t i o n temp e r a t u r e is lowered and the film becomes mobile. A l t h o u g h t h e e x t e n t of p r e c i p i t a t i o n a f t e r a given time is n o t reproducible, the results are significant in showing v e r y clearly t h a t a rigid film has a greater thickness t h a n a mobile film at the same ionic s t r e n g t h (the slight change in ionic s t r e n g t h due to p r e c i p i t a t i o n of some SDS w i t h D O H will n o t significantly effect the overall ionic s t r e n g t h ) . This conclusion has been theoretically explained b y L y k l e m a a n d Mysels (11) b u t t h e i r experimental m e a s u r e m e n t s did n o t confirm t h e theoretical conclusions. REFERENCES

1. I-]ARROLD,S. P., dr. Colloid Sci. 15, 280 (1960). 2. COl%KILL, J. M., GOOD]VfAN, J. F., HAISMAN, D. R., AND gARROLD, S. R., Trans. Faraday Soe. 57, 821 (1961). 3. RAZOUK, R. I., AND MYSELS, 2 . J., J. Am. Oil Chemists' Soe. 45, 381 (1968). 4. DnEGER, E. E., KEIM, G. I., MILES, G. A., SHEnLOVSI~Y, L., ANn ROSS, J., Ind. Eng. Chem. 36, 610 (1944). 5. HAnnOLD, S. P., J. Phys. Chem. 63, 317 (1959). 6. LYKLEMA, J., SCHOLVEN, P. C., AND MYSELS, K. J., J. Am. Chem. Soe. 69, 116 (1965). 7. HARKIN8, W. D., AND JORDAN, H. F., J . Am. Chem. Soc. 52, 1751 (1930). 8. FRANKEL, S. P., AND MYSELS, 2 . J., J. A p p l i e d Phys. 37, 3725 (1966). 9. NILSSON, G., ]. Phys. Chem. 61, 1135 (1957). WILSON ET AL., J. Colloid Sci. 12,345 (1957). WIEL, I., J. Phys. Chem. 70, 133 (1966). Journal of Colloid and Interface Science, Vo]. 30, No. 4, August 1969

580

NOTES

VAN VOOST VADER, F., Trans. Faraday Soc. 56, 1067 (1960). MURAMATSV, M. ET AL., Bull. Chem. Soc. (Japan) 41, 1279 (1968). 10. EPSTEIN, M. B., WILSON, A., GERSttMA.N,J., AND ROSS, J., J. Phys. Chem. 60, 1051 (1956). 11. LYKLEMA,J., AND MYSELS, •. J., J. Am. Chem. Soe. 87, 2539 (1965).

M. N. JONES ]:). A_. REED Department of Chemistry University of Manchester Manchester, M13 9PL England Received March 19, 1969; revised April 24, 1969

Journal of Colloid and Interface Science, Vol. 30, No. 4, August 1969