386 JOURNAL OF THE SOCIETY the original area. This can be shown to be independent of the sur- face pressure or concentration of the penetrating molecules in the under- lying solution (2). Similarly, a cholesterol monolayer expands to double its area on penetration by saponin, showing that the area per molecule of saponin is also •,0 A. •- Surface solution effects shown at low surface pressures by excess of pene- trating agent (above the amount required to form the 1:1 complex) can be taken into account by extra- polation of the expansion-time curve to the starting time (7, 22). ADSOP. PTIOS AND TANNINO If in the underlying solution the soluble interacting molecule has two or more appropriately spaced polar groups, the penetration of the non-polar portion of the mole- cule is prevented, and adsorption in the form of a double layer takes place.This results in the film-form- ing molecules becoming spaced on the lattice of the polar groups of the adsorbed molecules, producing a solid film of the insoluble film- forming molecules at very large areas, usually at least twice the nor- mal area of solidification of the monolayer alone [e.g., tanning of an amine film (4:, 23)]. Should the area of the film-form- ing molecule be greater than the area taken up by two of the spaced polar groups in the adsorbed mole- cule, no expansion of the insoluble film takes place, but a marked in- crease in rigidity is observed, due to OF COSMETIC CHEMISTS intermolecular interlacing by the ad- sorbed molecules [e.g., tanning of protein• (1)]. Adsorption results in big changes of surface potential of the insoluble monolayer, either a rise or a fall according to the nature of the ad- sorbed dipole. REVERSlSlLITY OF ADSOP.?TION Proteins may adsorb on to lipold monolayers, either at the air-water or at the oil-water interface, in the latter case as protein-stabilized emulsions. Since this adsorption is pH-conditioned, it can be easily reversed. The structural changes of the protein molecule before ad- sorption, as an adsorbed monolayer and after desorption are very inter- esting, especially in relation to the biological activity of protein mole- cules in the three structural forms (7-9). THE OIL-WATER INTERFACE Analogous molecular interactions to those at the air-water interface can be shown by an emulsion tech- nique to exist at the oil-water inter- face. The stability and ease of formation of emulsions are related to complex-formation, surface viscosity and rigidity and surface charge. As has been shown, complex-forma- tion at the interface between an oil- soluble agent in the oil phase and a water-soluble agent in the aqueous phase can radically alter all these factors (10). ".'Since the resultant interfacial tension depends on the

::i':Surface tensions of both components, .::i' very low interfacial tensions are obtained when complex-formation : is observed between the oil-soluble :• and water-soluble stabilizing agents (11). At the oil-water interface, there are interesting phenomena which ill:suggest similar associations result- ing in mixed-film formation (12). In the first place, it is known that the ..:• interfacial tension between an aque- ous soap solution and a hydrocarbon is independent of the oil used. : This indicates the presence of a monolayer of the soap alone at the interface., On the other hand, the interfacial tension between an oil solution of oleic acid and water is strongly dependent on the oil used. The lowering of the interfacial tension between the oil and water is least with benzene, intermediate with cyclohexane and decalin and greatest with hexane and long- chain paraffins. This effect is shown very definitely, particularly in the difference between the aro- matic and saturated hydrocarbons. In the case of the latter, minima occur in the surface-tension-con- centration curves, the explana- tion of which is doubtful. If they are due tO the presence of two components in the interfacial film, the second component can only be an oxidation product of oleic acid or the hydrocarbon itself. Now, it is known that benzene is the best solvent for long-chain alcohols, to which undissociated fatty acids would no doubt approxi- mate in their intermolecular inter- PENETRATION AND COMPLEX-FORMATION IN MONOLAYERS 387 actions, and it is noticeable that it produces the least effect in these experiments, perhaps because of a low surface-bulk partition ratio of the surface-active oleic acid. In the case of emulsification, the inversion of phase continuity from oil- to water-continuous, occurs in decreasing order of readiness in the sequence benzene, cyclohexane, hex- ane, higher paraffins. This may be due to the fact that this is the de- scending order of interaction energy between solvent and solute, so that penetration of the polar heads by water becomes more pronounced than penetration of'the hydrocarbon chains by the solvent, which would favor oil-continuity from steric con- siderations.. If we now take a three-component system consisting of oil, water, and a soap such as potassium oleate and add to it a substance which from monolayer experiments would be expected to form a complex with the soap (and penetrate the soap mono- layer), the mixture liquefies and, on adding sufficient of the fourth component, clears giving a trans- parent, fluid dispersion, which does not show streaming birefringence. A suitable complex-forming agent is an alcohol such as hexyl alcohol or (best) 2•ara-methyl cyclohexanol. If benzene is used as the oil, the dispersions are oil-continuous if nujol (long-chain paraffins), they are water-continuous, as judged by their electrical conductivity. It is possible to make such systems containing equal volumes of oil and water, and indeed their sta-