PENETRATION AND COMPLEX-FORMATION IN MONOLAYERS* By J. H. SCHULM^• and J. A. FR•E•r Department of Colloid Science, Cambridge University (England) IF ^ MONOLAYER OF an in- soluble substance is spread at an air-water interface, and a soluble substance containing a polar or ion- ized group attached to a non-polar structure (e.g., organic acids, soaps) is introduced into the underlying solution, in low concentrations (1 - 10 X 10 -6 g./cc.), there may be considerable changes in the surface pressure and potential of the film. If this is so, the soluble substance is said to penetrate the monolayer (1, 2). Be- cause of the chemical nature of the substances concerned, it is clear that there may be interaction between the polar head groups, or between the non-polar portions (van der Waals forces), or between both portions, and the nature of the phenomenon is determined by the relative importance of the two kinds of forces (3). In the first place, there may be strong polar interaction associated with weak van der Waals forces, as when benzoic acid is injected be- Dr. Schulman addressing the Society of Cosmetic Chemists of Great Britain, November 9, 1949. * Basis of an informal talk by Dr. Schul- man, presented at the Society of Cosmetic 'Chemists of Great Britain, Meeting, No- vember 9, 1949, London, England. neath a film of a long-chain amine (4). In this case a process of solu- tion of the soluble substance in the monolayer takes place, which may be reversed by compressing the film, 381

382 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS when ejection of the penetrating molecules occurs, followed by their adsorption under the monolayer, which is thus stabilized, by polar forces. No stoichiometric associa- tion is observed between the two components, and fairly large con- centrations of the penetrating sub- stance are required. If there is no interaction between the two components of the system, the behavior depends on their rela- tive surface activities. If the mono- layer substance is the more active, nothing happens above the collapse pressure of the soluble compound, but if not, it is displaced from the surface, with the formation of a monolayer of the soluble substance. This occurs when soaps are injected beneath films of triglycerides, and consequently accounts for the de- tergent action of soaps (5). When both van der Waals and polar forces are strong, definite stoichiometric complexes are formed, often analogous to complexes formed in bulk solution, e.g., between cho- lesterol and digitonin (1). The ex- tent of penetration of the film is shown by a sharp rise in the surface pressure of the film at constant area. On compression of the film, the pres- sure rises still more, and it is found that compression can be carried to a pressure higher than the collapse pressures of the components. For example, a complex of cetyl alcohol and sodium cetyl sulfate will stand a pressure of 60 dynes/cm. com- pared with the collapse pressures of cetyl alcohol (40 dynes/cm.) and sodium cetyl sulfate (15 dynes/ cm.). Similarly, the collapse pres- sure of the cholesterol-digitonin complex is 60 dynes/cm. compared with those of its components (40 and 20 dynes/cm., respectively). When the film of the complex collapses, it may do so as a unit (e.g., elaidyl alcohol-sodium cetyl sulfate) or by o o • I I I I I I I •o •o 30 40 •,0 •0 Min. Figure 1.•Sodium cetyl sulfate 3.3 X 10 -* injected at pH 7.2 under various films, at 10 dynes surface pressure. ejection of one component (e.g., oleyl alcohol-sodium cetyl sulhte, the latter being ejected). The sta- bility of the complexes is increased by the adsorption of a layer of sol- uble component beneath the mixed film. The formation of complexes shows remarkable specificity. Saponin, for example, forms a complex with cholesterol but not with cholesteryl acetate, because of the considerably reduced hydrogen-bond activity :.• .