SOAP FILMS AXD COLLOIDAL BEHAT'IOUR BY A . S. C. LAWRENCE
Aqueous soap solutions, which have the peculiar property of forming stable films, bubbles and foams, are also the best known example of a rather unusual type of colloidal solution. In addition, many other colloids, such as the proteins, form stable films. For these reasons confusion exists between these two sets of properties although they actually have no closer connection than a common origin in the peculiarities of the soap molecule. Much of this confusion has arisen through failure to differentiate between liquid soap films and films which are not truly liquid. The undoubted colloidal structure of these latter has led to the idea of a colloidal structure for soap films. As opposed to these, structures have been proposed which certainly err on the side of simplicity.' Bubbles can be blown from mercury and selenium but they are small, unstable, and in no way comparable with soap films. The real problem of the soap film is that it exists at all. To consider, for simplicity, a plane film; why does the liquid composing it not fall freely under gravity? Or, alternately, why does the film not contract to a spherical drop under the influence of surface tension? Kow, as a matter of fact, gravity does exert some influence on the rate of thinning of a vertical film; but the fact that a horizontal film thins shows that this process is largely independent of gravity. To explain the existence of the soap film, it has been suggested that it has a sandwich structure; that its stability depends upon the presence of a surface layer at each face between which is included a variable amount of liquid.* Thinning will be regulated by the laws of fluid viscosity; that is, according to Poiseuille's law, the rate will be proportional to the fourth power of the distance between the two surface layers. It is clear that in very thin films this viscous resistance will be very large. This structure also explains why there are no effects due to gravity in a horizontal film. It has been suggested that these surface layers possess colloidal p r o p e r t i e ~ ;it~ has also been suggested that the interior liquid has a colloidal structure.' I shall show that the characteristic properties of the liquid soap film require neither of these proposals.
Adsorption in Soap Films This sandwich structure was suggested for the soap film inany years ago but it is only revntly that accurate information has accumulated concerning the surface layers required. Of course this surface layer does not only exist a t the surface of the soap film but also at the surface of any soap solution. But, whereas in a soap solution there is one surface layer covering a very large depth of liquid, in the soap film there are two surface layers enclosing a very small amount of interior liquid.
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The first experimental evidence of the existence of this surface layer on soap solutions was provided by some observations made by Lord Rayleigh many years ago.2 I t had long been known that soap solutions were charactensed by very small surface tension-about one third that of water-when measured by the usual methods. He compared these values of the “static” surface tension with those obtained by a “dynamic” method in which the surface whose tension was being measured was continuously renewed during the measurement. He found that the dynamic values were considerable larger than the static ones, the difference being most marked in the more dilute solutions; thus indicating that the lowering of the surface tension of water by soap is due to the formation of a specific surface layer which takes a definite though small time to form. Confirmation of this view is provided by the recent work of du Nouy on the static surface tension of soap solutions a t enormous dilutions. He found that a millionth part of soap does not affect the surface tension of water a t all initially but that on standing quite undisturbed for two hours a drop of 2 0 dynes ensue^.^ The application of the Gibbs adsorption equation to soap solutions is a matter of extreme complexity. The surface tension concentration curves of the soaps fall very steeply until a minimum of about one third of the surface tension of water is reached a t a concentration of about 0.5 per cent of soap. The curves then bend round sharply and the surface tension remains constant or else increases very slightly with increasing concentration of soap. The Gibbs equation would therefore suggest that positive adsorption only takes place in solutions containing less than 0 . 5 per cent soap and that above this Concentration adsorption becomes slightly negative; this in spite of the fact that the actual value of the surface tension remains small-a fact which presupposes the continued existence of the adsorbed surface layer. It appears, however, that the Gibbs equation is not applicable in the usual simplified form ;
‘I he assumptions made in its derivation (that only two components are present); that the osmotic pressure is a linear function of the concentration of the solute; neglect of electrical effects), are not fulfilled in soap solutions owing to their complexity. Furthermore, no allowance is made for possible variation of the extent of the adsorption layer; thus introducing an unwarranted (L posteriori assumption concerning the layer itself. This complexity is due to part of the solute being molecularly dispersed and part colloidally; both parts are also in hydrolytic equilibrium. An aqueous soap solution therefore contains:
’””li’: 7
Fatty Anions Sodium Cations
Neutral Colloid
Colloid Anions
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A. S. C. LAWRENCE
found no less than five. This appears to be the maximum but there still remains a large gap between the thickest black and the thinnest “coloured” film-the silvery white as the following table shows:First order black Second ” ” Third ” ’”
6P11.L I2
,’
28
”
Fourth order black Fifth ” ” Silvery-whit e
24pp 30 ” IO0
,’
I t is important to observe that the films and bubbles of great stability* prepared by Sir James Dewar were all blacks of the second order.9 The first order blacks are much less stable while the higher orders are quite ephemeral and never large in area.lo According to the suggested constitution of the soap film the black will consist of the two surface layers; most if not all the interior liquid having drained away. This will be the second order of black. When the thickness is compared with the length of the soap molecule, it is clear that there does still remain some of the interior liquid; without its presence the film could not be more than 8 to I O p p thick. The higher orders of black appear during the thinning of films made from rather dilute solutions and require undisturbed development. Their origin is, no doubt, the chains which McBain has suggested occurring beneath the surface-oSiented layer. This view puts the maximum effective depth of the adsorbed layer a t five molecules (long). It is unusual, if indeed it ever occurs, for the first order of black to be formed from the second without other manifestations of a disturbed development.** My view of the process is that it involves the rupture of one of the surface layers; the film then either breaks or else coalescence of the interior liquid takes place according to surface tension requirements on the remaining surface layer. An orientation of the adsorbed molecules beneath the remaining surface layer will be required to form the first order of black which only consists of two monomolecular oriented surface layers containing a small amount of interior liquid. Even if the film survives this very considerable transformation, it will still be much less stable than the second order of black owing to the lack of any reserve of adsorbed fatty material beneath the two surface layers. Strange as this inversion appears, there is considerable experimental evidence that my explanation is correct. The most striking point is that the rate of coalescence of the blacks to form the first order increases as their thickness decreases, the growth of the first order of black in the second being especially rapid. Now this is quite contrary to what would happen if the drainage were still proceeding between two surface layers; for in that case halving the thickness would reduce the rate to one sixteenth; whereas in actual facts the rate is increased several times.
Stratified Soap Films The stratified soap films discovered by Perrin’ and the bimolecular “leaflets’’ from which he supposed these films to be built up are not a simple * The solutions used contained a considerable amount of glycerin--Io times the concentration of soap: it is rare for such solutions to give 1st order black films. * * Full particulars of abnormal developments will be found in the author’s “Soap Films.”
SOAP FILMS AND COLLOIDAL BEHAYIOUR
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extension of Johonnott’s five blacks but form a class by themselves. I t camot be too strongly urged that these films are abnormal and results obtained from their examination are not necessarily applicable to soap films in general. I have shown elsewhere that the formation of these stratifications is essentially the separation of a solid component although the stratifications behave at first as though liquid.* It differs from adsorption in that it is not restricted to the surface but goes on throughout the film. It is rather of the nature of a crystallisation. It is noteworthy that the formation of these stratified films requires the use of very concentrated soap solutions. Perrin assumed that the adsorbed layer in soap solutions contains no alkali because it has an acid reaction and that, in consequence, his elementary “leaflets” were bimolecular layers of oleic acid. More recently McBain has shown that there is in the adsorption layer three-fifths of the alkali required for neutrality. The process of stratification suggests the formation of an insoluble component and its subsequent separation in lamellar form. I t s insolubility requires the suppression of the carboxyl groups to form a sheet similar to those formed by the solid fatty acids. It appears probable that the stratifications are actually acid soap. A small quantity of caustic soda solution added to a solution giving good stratified films, inhibits this behaviour. Acid potassium oleate has been prepared by crystallisation from alcoholic solution and has the requisite lamellar form.” There is not sufficient evidence to decide whether there is any water present in these stratifications or not. A completely analogous state of affairs has been observed in surface films by Lyons and Rideal.’* Palmitic acid, which forms monomolecular films on acid water surfaces, is found to form an insoluble bimolecular film on alkali. The solution of the monomolecular film on addition of alkali results in the formation of molecules of soap which become submerged in the water and then link up with the remaining fatty acid molecules on the surface thus forming a sheet of acid soap of twice the thickness of the original monomolecular film. There is no reason for the formation of a molecule of acid soap of the formula; Ha;K-di; but it is easy to see how a crystal could be formed in which the unit cell contained two molecules of fatty acid and two of soap instead of four of fatty acid. It seems therefore that the formation of these stratifications is a purely crystalloidal and not a colloidal phenomenon.
Colloidal Soap Films The clearest proof that freely-thinning soap films are not colloidal is provided by the abnormalities of films which do develop a colloidal structure. First however, it is necessary to distinguish between a colloidal structure for the surface layers and for the film as a whole.** As regards the former, whatever its properties, it is rather a strain on colloid nomenclature to suggest that the surface layer two molecules thick, just described, has a gelstructure. There * “Soap Films,” p. 58 (rgzg). ’* There does not seem to be any reason for expecting surface activity in polypeptides The simplest amino-acids are only slightly adsorbed in aqueous solution. Freundlich: “Colloid and Capillary Chemistry,” 641 (1926).
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A. S . C. LAWRESCE
is evidence that colloidally dissolved substances do not lower the surface tension of their solvents to any great extent; it has also been suggested that the fatty anion cannot lower it either." The work of duNouy already mentioned completely refutes this idea; a t the enormous dilutions employed by him, hydrolysis is certainly complete and the surface activity must be due to the fatty anion. I n any case experience of black films supports the view that they can remain truly liquid and mobile. I t has also been suggested that the soap film has a colloidal structure as a whole.l* It is known quite definitely that there must be interior liquid even in the thinnest films. Reinhold and Rucker observed that when the thickness of a soap film becomes less than about j o o p p , its electrical conductivity begins to rise in an abnormal manner but that disturbance of the film reduces it. They also found that addition of nitre or other electrolyte prevents this abnormal rise. Since electrolytes are negatively adsorbed, their presence in the black film definitely proves the inclusion of interior liquid. There can be no doubt that, when first formed, the soap film contains colloid particles, since all soap solutions contain them. But the ease with which these particles are oriented by a flow of liquid14 owing to their characteristic thread-like shape makes it highly probable that they are oriented in the plane of a film just by the action of forming it. Drainage will keep them oriented in this manner and draw them out from between the surface layers in the ordinary course of drainage. When the black films obtained from aqueous soap solutions in exhausted tubes break, a peripheral ring of gel frequently remains. I t sometimes happens, also, that a bubble, normal at first, develops a colloidal structure and becomes gelatinous. This occurs in bubbles blown from sodium oleate containing saturated impurities such as stearic and palmitic acids, Certain colloidal solutions have a viscosity which is variable depending on the rate of shear; if this is small enough the viscosity is enormous. Pure sodium oleate is not elastic; pure sodium stearate is slightly so but a mixture of the two is elastic out of all proportion to it constituents. Bubbles bloxn from such solutions thin partially and then no further; they become gelatinous and have a characteristic vertically striped appearance which supports my view of the orientation of the colloid particles. I t is obvious that owing to the enormous viscous resistance t o flow of the interior liquid in a soap film, and the consequently small shearing rate, the conditions are exceptionally favourable to the appearance of this abnormal elasticity. The reason for such bubbles thinning partially and then becoming gelatinous is usually the result of blowing up a bubble with a newly prepared solution. Ageing goes on for a day or so and after this time a bubble will not usually increase in viscosity in situ. It is interesting to observe that there is no improvement in the film-forming capacity of the solution as a result of this ageing; another piece of evidence that this property does not depend on the colloidally dissolved components of the solution. One final piece of evidence against any form of gel structure for soap films may be included. Ammonium oleate solutions lose their base rather easily and the viscosity of the solution increases but no oleic acid separates. If,
-.0.1P FILMS .ANI> COLLOIDAL BXHAVIO1:H
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