A. T. Florence and K. J. Mysels
234 (3) G. V. Buxton, F. S. Dainton, T. E. Lantz, and F. Sargent, Trans. Faraday Soc., 66, 2962 (1970). (4) B. G. Ershov, Proc. Czech. Annu. Meet. Radiat. Chem., 70th, 7970, 2, 261 (1971). ( 5 ) P. Hamlet and L. Kevan,J. Amer. Chem. Soc., 93, 1102 (1971). (6) H. Hase and L. Kevan, J. Chem. Phys., 54,906 (1971). (7) H. B. Steen and J. Moan, J. Phys. Chem., 76, 3366 (1972). (8) B. G. Ershov and A . K. Pikaev, Advan. Che!. Ser.. 81, 1 (1968).
(9) T. Shida and W. H. Hamill, J. Amer. Chem. Soc., 88, 3689 (1966). (10) F. S. Dainton, G. A. Salmon, and P. Wardman, Proc. Roy. Soc., Ser. A, 31 3, 1 (1969). (11) E. M. Fielden and E. J . Hart, Radiat. Res., 32, 564 (1967); 33, 426 (1970). (12) T. Shida, S. Iwata, and T. Watanabe, J. Phys. Chem.. 76, 3683 (1972). (13) T. Shida, J. Phys. Chem., 74, 3055 (1970).
Bursting of Soap Films. VI. The Effect of Surfactant Purity Alexander T. Florence and Karol J. Mysets*’ R. J. Reynolds Tobacco Company, Winston-Salem, North Carolina 27702 (Received August 6, 7973)
Publication costs assisted by the R. J. Reynolds Tobacco Company
Velocities of the rim and of the frontal shock of the aureole were measured photographically on bursting films from solutions of sodium dodecyl sulfate undergoing purification by exhaustive foaming. The principal features of the bursting process of mobile films remained unaffected by purification and quantitative changes were observed only upon addition of a massive amount of dodecyl alcohol.
During the last few years a number of ~ a p e r s dealing ~-~ with the detailed study of the bursting of soap films have indicated the potential of such studies for the elucidation of fast desorption kinetic^,^ of the equation of state for highly compressed monolayer^,^ and hence in the study of intermolecular forces. It is therefore of some interest to know the degree to which observations correspond in fact to the postulated theoretical mode12b or are due to experimental artifacts. Thus, for very thin films formed from sodium dodecyl sulfate solution, some of the deviations of velocities of the rim of the growing hole from those expected on the basis of the model have been ascribed5 to aerodynamic drag by the surrounding atmosphere rather than to the close proximity of the two surfaces. In this note we consider another possibility; namely, that surfactant impurities play a significant role in bursting phenomena, particularly in the formation of the aureole,2a the portion of thickening and accelerating film which precedes the growing hole in the bursting film. The study of the properties of the aureole provides the information referred to above. The aureole is generally sharply delimited by two shock wavesx2b one is formed by its advancing front, the other by the rim of the hole. Their velocities can be measured accurately from a series of submicrosecond flash photographs triggered through a delay circuit by the hole-initiating spark.2a Since the triggering process involves a small but somewhat uncertain time length, a single photograph can give a ratio of velocities but several photographs involving different delays, and otherwise the same conditions, are required for absolute velocity measurements. The solutions studied were purified by foaming and foam removal. For this purpose, a submerged air inlet and a foam suction tube were incorporated into the film bursting apparatus2a used previously. The air was passed over active charcoal and humidified by dilute, aqueous CaC12 The Journal of Physical Chemistry, Vol. 78. No. 3, 7974
before entering the surfactant solution. Although foaming does have limitations which are discussed elsewhere,6 it still seems the best available method for removing surface active impurities. The concentration of other impurities has been kept low in all experiments by standard precautions. In initial experiments, two solutions of pure sodium dodecyl sulfate (NaDS), used in previous work,2a were foamed for over 10 and over 26 hr, respectively, with occasional interruptions during which photographs of the bursting process were taken. No effect of this purification could be detected. The results and experimental details shown in Figure 1 indicate that the previously reported bursting studies were not affected significantly by readily removable impurities. To learn about the effect of a massive impurity, an 8.35 X loT3 M NaDS solution was prepared to which was added 0.47% (of NaDS) of the most likely surface active impurity, namely, dodecyl alcohol (DOH). It is known that under these circumstances the DOH can lower the critical micelle concentration (cmc) and can then be solubilized in any NaDS micelles present.? It can also form a highly insoluble alcoholate of NaDSE (probably (NaDS)zDOH), and it adsorbs very strongly at the air-solution surface to give monolayers which can have a very high surface viscositye and a 1:l ratio of NaDS to DOH.’’ Such solutions may generate very slowly draining “rigid” soap films.11 The equilibrium situations is complicated and some observations suggest that, under certain conditions, the rigid films form when the solution is supersaturated with respect to the alcoholate and not when the latter has fully precipitated or when the solution is undersaturated in this respect. The solution originally contained some alcoholate crystals and gave a rigid film, whose bursting has a very different character,2a but after 5 hr of foaming the film was
Bursting of Soap Films
I
235
B 270
.----
0
I
-
B Newton black
''\o
L
u, misec
Hours of Foaming
I I
I
10 20 Figure 1. The lack of effect of purification by foaming on the bursting velocities of films from solutions of standard purity: circles, solution A , 0.51% NaDS; squares, solution B, 0.19% NaDS, 0.3 M NaCI; open symbols, frontal shock; filled symbols, rim. Thicknesses in nm as indicated: Newton Black films have a thickness of -4.5 nm. mobile, and after 10.3 hr the alcoholate crystals were no longer visible, showing the effectiveness of the purification process which was continued for a further 10.2 hr. Throughout this purification the character of bursting of mobile films remained qualitatively the same, showing that its general features do not depend on accidental impurities. There were, however, quantitative changes as shown in Figure 2. The rim velocities increased as alcohol was removed whereas those of the frontal shock decreased a t the same time, leading to a considerable narrowing of the aureole. This seems to be the expected behavior. Both theory2b and e x p e r i m e n t ~ ~ ashow , ~ , ~ that for not-too-thin .~ 6 mobile films the rim velocity is close to ( 2 ~ / p 6 ) O where is the film thickness, p the density of the solution, and u its surface tension. It is well known that removal of DOH leads to an increase in surface tension so that it should cause an increase in rim velocity, as observed. The change in rim velocity of 17% should correspond to a change in surface tension of 37%, which is very close to the change measured by Miles and Shedlovsky12 under comparable conditions (from about 29 to 39 dyn/cm or about 34%). The velocity of the frontal shock depends on the initial modulus of elasticity of the monolayer. It is given quantitatively by which is a rearrangement of eq 3-12 of ref 4. Here a = 60/6 = A / & is the fractional shrinkage of the film as its area changes from the original A0 to A , and A denotes the changes occurring across the shock. At constant p and 60,
Hours o f ,Foaming
I
Figure 2. T h e effect of purification by foaming on the bursting velocities of films from a solution of 0.24% NaDS originally containing 0.00516% of DOH and forming rigid films. The ratio plotted is that of aureole width-to-hole radius. Film thickness 480 nm. it is clearly A a / A a , the modulus of elasticity, that determines us. Though no independent measurement of this modulus is available, it can be expected to be much high: er in the presence of DOH since the monolayer, though mobile, is close to the rigid one in which the area per molecule islo only about 20 A2; i.e., extremely crowded. Thus, removal of DOH should reduce the surface concentration and hence the modulus of elasticity, and the frontal shock velocity should decrease which is again the observed behavior. Thus surface impurities affect only the quantitative aspects of the bursting process, and these only when present in high concentration, and they are not responsible for the existence of the main features of the bursting process which has been discussed in earlier papers.
References and Notes (1) Present address, 8327 La Jolla Scenic Drive, La Jolla, Caiif. 92037. (2) (a) W. R. McEntee and K. J. Mysels, J. Phys. Chem., 73, 3018 (1969); (b) S. P. Frankel and K . J. Mysels, ibid., 73, 3028 (1969). (3) A. T. Florence and G. Frens, J. Phys. Chem., 76, 3024 (1972). (4) G. Frens, K. J. Mysels, and 8. R. Vijayendran, Spec. Discuss. FaradaySoc., 1, 12 (1970). (5) K. J. Mysels and B. R. Vijayendran, J. Phys. Chem., 77, 1692 (1973). (6) K. J. Myseis and A. T. Florence, J. Coiloid Interface Sci.. 43, 577 (1973). (7) L. H. Princen and K. J. Mysels, J. Phys. Chem., 63, 1781 (1959); P. Mukerjee and K. J. Mysels, Nat. Stand. Ref. 3ata Ser., N a t Bur. Stand.. No. 36. 15 . - 11971) -. (8) M. B. Epstein, A. Wilson, C. W. Jacob, L. E. Conroy, and J. Ross, J. Phys. Chem., 58, 860 (1954); A . Wilson, J. SOC. Cosmetic Chem., 6. 392 (1955). (9) A. G. Brown, W.G.'Thuman, and J. W. McBain, J. Colloid Sci., 6, 491 (1953); J. Ross, J. Phys. Chem., 62, 531 (1958). (IO) A. Wilson, M. B. Epstein, and J. Ross, J. Colloid Sci., 12, 345 (1957). (11) G. D. Miles, J. Ross, and L. Shedlovsky, J. Amer. Oii Chem. Soc., 27, 268 (1950); K. J. Mysels, K. Shinoda, and S. P. Frankel, "Soap Films, Studies of Their Thinning and a Bibliography," Pergamon ' Press, New York, N. Y . , 1959. (12) G. D. Miles and L. Shedlovsky, J. Phys. Chem., 48, 57 (1944). - - a
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