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 :.• .

PENETRATION AND COMPLEX-FORMATION IN MONOLAYERS 383 the ester group compared to the hydroxyl. Similarly, oleyl alcohol forms a comple3c with sodium oleate but not with oleic acid. Specificity is thus related to the polar group, and also to steric hindrance and the presence of double bonds. The effects of the polar group is shown by the variation in the equilibrium surface pressure of substances pene- trating a cholesterol monolayer, the 30 o i •. 3 4 5 6 3.3 x 10-7 g./c.c. Figure 2.--This shows that F.-/ = RT log C•/c2. z/= area of cetyl sulfate molecule. order being: R--NHa+ R--SO4- R--SOa- R--CO2- R-- NMea + bile salt artion. The effect of the presence of a double bond is shown strikingly by the effect of sodium cetyl sulfate on monolayers of the unsaturated alcohols, as compared with the sat- urated C•8 alcohol. As will be seen, acetyl alcohol film solidifies on penetration at an area of 78 A. 2 per molecule, whereas the elaidyl alcohol film (trans) is liquid down to 60 A. 2 per molecule, and the oleyl alcohol film (cis) never solidifies. It is clear that the trans-isomer conforms more closely to the shape of the saturated alcohol than does the cis-isomer, and consequently the van der Waals attraction for the long chain salt is greater. Iso- morphism between the molecules plays an important part in the pene- tration phenomenon. METHODS 0•' INVESTIOATIOl• Several methods are available for the study of the penetration phe- nomenon. It may be studied either at constant area of the film, when changes are revealed by increasing surface pressure, or by keeping the pressure constant, when the film ex- pands as penetration proceeds (21). Finally, the film may be expanded to a large area, injection carried out, and the mixed film compressed, the force-area curve being plotted. From the existence of kinks in the force-area curves of various pene- trating systems, Schulman and Sten- .hagen concluded that 1:1 and 1:3 complexes existed, and that 1:2 complexes were unstable. Work by Matalon and Schulman (6) has since shown that owing to the rigidity of some of the films studied (e.g., cetyl alcohol-sodium cetyl sulfate) hysteresis effects are involved which make some of the conclusions doubt- ful. However, work done by these authors using an expansion tech-