Dynamic behavior of surfactant films

Dynamic Behavior of Surfactant Films R. B I E N K O W S K I AND ~'~. S K O L N I C K

Department of Physics and Health Sciences Center, State University of New York, Stony Brook, New York 11790* Received January 19, 1970; accepted June 25, 1971 The hysteresis in @-A) exhibited by surfactant films, which are mechanically expanded and compressed, has been studied as a function of cycling frequency. The shape and area of the hysteresis loops varied with frequency. The surfactants used were lung surfactant, vegetable lecithin, laurie acid, and sodium dodecyl sulfate. I. INTRODUCTION The properties of lung surfactant (LS) have been extensively studied during the past ten years (1). Figure 1 is a schematic diagram of apparatus used to measure the surface tension of a film of LS on an aqueous substrate; the area covered b y the film can be varied b y moving the teflon blades. The surface tension of the LS film depends on the area between the blades and also on whether the area is decreasing or increasing, i.e., the relation between surface tension and area is double valued or hysteretic. The shape and size of the hysteresis loop varies with the cycling frequency. Typical loops (surface tension vs. area) for LS are shown in Fig. 2. We have investigated how the hysteresis depends on frequency for some common surfactants: LS, vegetable lecithin, sodium dodecyl sulfate, and laurie acid. In this paper we present some preliminary results.

equilibrium; indeed, it is not even close to equilibrium (2). The actual state of the system is influenced by the succession of previous states. Furthermore, the system exhibits hysteretic response only over a certain range of frequencies of the driving force. In the limit of high frequencies the mechanisms responsible for the hysteresis will not be able to respond; in the limit of low frequencies the " m e m o r y " of the system will fade. Thus a graph of the integral

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* The authors' present address is: The University of Texas Medical School at Houston, Texas Medical Center, Houston, Texas 77025.

where ~ is the surface pressure; 7 is the surface tension; 70 is the surface tension of substrate; and A is the area of surface, plotted as a function of frequency (whieh will be called a "dispersion curve") will show a peak. T h a t is, the " a r e a " bounded by the hysteresis loop will vanish at very low and very high frequencies. This "area," which has the dimensions of work, is a measure of the irreversibility of the process; it is equal to the work lost in taking the system through a complete eyele. Nothing can be said, however, about the detailed eharaeter-

Copyright © t972 by Academic Press, Inc.

Jo'arnal of Colloid and Interface Science, Vol. 39, No. 2, May 1972

II. COMMENTS ON THE NATURE OF HYSTERESIS If a system's response to a periodic driving force is hysteretie then the system is not at

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istics of the dispersion curve, e.g., whether it is sharply peaked, or whether it is skewed, without a detailed knowledge of the physical mechanisms responsible for the hysteresis. III. MATERIALS AND METHODS The apparatus used is similar to t h a t described b y Mendenhall and Mendenhall (3); it is manufactured b y Cahn (4) (see Fig. 1). A Cahn R G Electrobalance measured the downward pull (proportional to surface tension) on a partially immersed glass cover slip. The cover slip was etched with hydrofluoric acid to increase wettability. The cont a c t angle is assumed to be zero (3). For this reason the cos 0 term, which takes into account the effect of the contact angle, has not been included in Eq. [1]. The area between the teflon barriers is changed b y an approxim a t e l y sinusoidal drive. Cycling frequencies ranged from 0.08 to 27 cycle/rain. I n the work reported here the area varied from a m a x i m u m of 60 cm 2 to a m i n i m u m of 20 am 2. The surfactants and substrates used are described in Table I. The LS was obtained b y washing the excised lung of a healthy adult male rabbit with 0.9 % saline solution. (See Ref. 1, p. 60.) No effort was made to purify this extract. Similarly, the vegetable lecithin and sodium dodecyl sulfate are relatively " i m p u r e . " Substrates were prepared using deionized water and commercial buffers. I t will be noted in the table t h a t the concentration of LS is not expressed in m g / m l but rather in terms of the area t h a t oro~rnal of Colloid and ~nterface Science, Vol. 39, No. 2, M a y 1972

a drop will displace on an aqueous surface t h a t has been lightly dusted with tale (5). We have observed t h a t the degree of hysteresis depends on the p H and ionic strength of the substrate. For example, the hysteresis exhibited b y sodium dodecyl sulfate is greater on acidic substrates; Scarpelli (6) has reported t h a t the hysteresis of LS is greatly reduced when no counter-ions are present in the substrate. We are presently investigating the p H dependence of hysteresis for the f a t t y acids. Drops of surfactant were gently placed on the substrate using a pipet while the wiper blades were held at m a x i m u m area. Cycling was begun when the change in surface tension with time was "slight" (~<0.5 d y n e / cm/min). D a t a were recorded when consecutive hysteresis loops were superimposable. 1 T h e temporal evolution of the hysteresis loops is in itself an interesting phe1 In the case of lauric acid this criterion was not rigourously applied. Lauric acid is slightly soluble ; a (v-A) curve for a newly spread film will exhibit a discontinuity (indicating a phase transition in the surface layer) whereas an adsorbed film will not. [See the discussion by Ter Minassian-Saraga (7).] After a sufficiently long aging time, however, the spread and adsorbed films should be indistinguishable. Continuous cycling should decrease the aging time; at a cycling frequency of 12 cycle/ rain we have found that the time for evolution to an adsorbed film is of the order of 1 hr. The data reported here are for a spread film of lauric acid; consecutive hysteresis loops were approximately superimposable.

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DYNAMIC BEHAVIOR OF SURFACTANT FILMS

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TABLE I Surfactant

Lung Surfactant Vegetable lecithin (commercial grade) Sodium dodecyl sulfate (commercial grade) Laurie acid (99-b% pure)

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Concentration

Amount Used

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8 drops 5 mg

pH 7/0.9% NaC1 pH 7

1 mg

pH 4

0.1 mg

0.01 N HC1

chloroform-methanol 1 mg/ml 2:1 (vol:vol)

nomenon which has not yet been investigated. The amount of surfactant used in each experiment is indicated in Table I. In the ease of LS and vegetable lecithin, it is evident t h a t much more surfaetant was present than was needed to form a simple monolayer. The areas of the hysteresis loops were determined with a planimeter and plotted against the logarithm of the cycling frequency (Fig. 3). IV. DISCUSSION The data presented in Fig. 3 are in qualitative agreement with the discussion in II;

Substrate

the hysteresis is most evident only over a range of frequencies. The dispersion curves exhibit a high-frequency peak a n d / o r a broad plateau at low frequencies. Figure 4 shows a generalized dispersion curve which incorporates these features. I t is likely that two different mechanisms are responsible for the hysteresis. (1) Desorption-diffusion: During compression of the film, soluble molecules may detach from the surface and diffuse into the substrate. Upon reexpansion these molecules m a y diffuse back to and enter the surface. The Journal of Colloid and Interface Science, Vol. 39, No. 2, May 1972

328

BIENKOWSKI AND SKOLNICK

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FREQUENCY (cpm) FIG. 3. Area of hysteresis loop vs. cycling frequency for various surfactants. Journal of Colloid and Interface Science, Vo]. 39, No. 2, May 1972

DYNAMIC BEHAVIOR OF SURFACTANT FILMS

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(2)

cycle of desorption and readsorption will lag behind the mechanical cycling of the film. Insoluble molecules will not be dispersed in the substrate. Willis (8) has shown that molecules in a mechanically cycled film of stearie acid do not penetrate as much as 1 mm below the surface. The desorption process has been treated in detail by Ter Minassian-Saraga (7). Surface structures: The other m~jor phenomenon contributing to hysteresis is the formation and relaxation of surface structures such as micelles and sheet-like laminae. (Phase changes in the film and collapse of the film would both fall in this category.) Upon compression, surface tension is decreased as individual molecules are crowded together and the surface concentration is increased. This is followed by structure formation and effective reduction in molecular adsorption. Surface tension would tend to increase. During the expansion phase of the cycle, these structm'es would relax.

Both mechanisms reduce the number of molecules adsorbed at the surface. This reduction lags the initial crowding of molecules if the cycling time of the driving force is less than the relaxation time of the mechanism. Because the dispersion curve for a very soluble species like sodium dodecyl sulfate shows a sharp peak at high frequencies while the dispersion curve for a relatively insoluble species like lauric acid exhibits only the low frequency plateau (see Fig. 3), we think that the peak is associated with desorption, whereas the plateau is associated with the formation and decay of surface structures. The characteristic times for these processes, as given by the inverse frequencies, would be desorption: surface structures:

10-L101 min 10°-103 min

The upper limit of 103 rain can only be regarded as approximate. Rabinovitch et al. (9) reported that a collapsed film of stearic acid will respread itself in about 7 hr. The general dispersion curve shown in Fig. 4 may be considered in terms of components based upon the two mechanisms discussed before. Decomposition of this Journal o/Colloid and Interface Science, VoL 39, No. 2, M a y 1972

330

BIENKOWSKI AND SKOLNICK

curve is shown using a relatively broad " s t r u c t u r e " component, which dominates at low frequencies, and a more sharply peaked "desorption-readsorption" component at the higher frequencies. I t m u s t be emphasized t h a t the preceding discussion is of a speculative nature. A clear distinction m u s t be made between the experimental data of Fig. 3 and the generalized dispersion curve shown in Fig. 4. ACKNOWLEDGMENTS This work was supported by grants from Long Island Jewish Medical Center and the State University of New York Research Foundation.

Journal of Colloid and Interface Science, Vol. 39, No. 2, May 1972

REFERENCES 1. SOAaPELLI,E., "The Surfactant System of the Lung", Lea and Febiger, Philadelphia, 1968. 2. BRIDG~AN,P. W., Rev. Mod. Phys. 22, 56 (1950). 3. MENDENHALL, R. M., AND MENDENttALL, JR., A. L., Rev. Sci. Instrum. 34, 1350 (1963).

4. Cahn Division, Ventron Instruments Corp., 7500 Jefferson St., Paramount, CA 90723. 5. LA~GMVla, I., J. Chem. Ed. 8,850 (1931). 6. SCARPELLI, E., GARBAY~ K., AND KOCHEN, J., Science 148, 1608 (1965). 7. TEa MI~ASSlAN-SAnAGA, L., J. Chim. Phys. 52, 181 (1955); J. Colloid Sci. 11,398 (1956). 8. WILLIS, R. F., d. Colloid Interface Sci. 35,

1 (1971). 9. RABINOVITCH, W., ROBERTSON, R. F., ANn MASON, F. G., Can. J. Chem. 38, 1881 (1960).