EMULSIONS AND FOAMS 387 surements. He assumed that if the density of the emulsion differed from that of the benezene and aqueous phases, the difference was due to the in- terfacial fihn which formed a third phase. He concluded from his data that the interfacial film in the emulsion was polymolecular rather than monomolecular. Cockbain (33) reported a study of the aggregation of dispersed ben- zene and paraffin hydrocarbon droplets in soap stabilized emulsions using the rate of creaming as the criterion of aggregation. The aggregation and disaggn:egation phenomenon became very complex as the soap concentra- tion increased beyond the cmc. The most reasonable explanation Cock- bain found for this behavior was that polymolecular adsorbed films formed when the soap concentration exceeded the micelle concentration. Additional evidence for polymolecular interfacial films was obtained by Dixon et al. (34, 35) who showed by a radiotracer method that the ad- sorption of the anionic agent, di-n-octyl sodium sulfosuccinate, and the cationic stearamido-propyldimethyl-2-hydroxyethyl ammonium sulfate was greater than that which could be accounted for by a monolayer. The adsorption of the latter was about ten times that calculated for a mono- layer. Other workers, such as Delay (36), have also postulated polymolec- ular interfacial fihns in order to interpret and explain interfacial phe- nomena. Structure of Water in Interfacial Regions Thus far very little has been said about the contribution of water to the structure of the interfacial films. There seems to be little doubt, how- ever, that the structure of water in the immediate vicinity of interfaces is different from that in the bulk phase. Derjaguin and Landau (37) believe that thick layers of water are immobilized at interfaces while Davies and Rideal (38) report that layers of water may be oriented at liquid surfaces to form what they term "soft ice" with a viscosity about like that of butter and a density greater than one. Drost-Hansen (39) states that the thick- ness of surface layers at aqueous interfaces is probably larger than is gen- erally realized and may be greater than 20 molecular layers. Henniker (40) reports that in many liquids other than liquid crystals, the orienta- tion may extend 1000 A or more below the surface, the precise depth being determined by the specific material with which the liquid is in con- tact. Ions have a considerable effect upon the orientation of water mole- cules. McBain (41) suggests that the dipoles of a surface active agent in

388 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS the interface can orient dipoles of water around the polar head groups of the surfactants. Low (42) has also discussed a model for ion-water inter- actions. In this structure an ion is surrounded by three regions. In the region immediately adjacent to the ion, the water is strongly oriented and immobilized by the electric field of the ion. The second region contains water in which the structure is broken down and more random than normal water, while the third region consists of normal water polarized by the ionic field which is relatively weak at this distance. Thus, considering the orientation of water in the vicinity of inter- [aces and ions, it is not surprising that hydration layers separate the alter- nating layers of surface active agents in the liquid crystal structures. Boffey et al. (12) report that the thickness of the ,rarer layer in the smectic liquid crystal may reach 110 A and they consider that it is held by solution forces to the ionic heads of the surface active agents and the hydroxyl groups of the polar additives. Similar regions are shoxvn by systems with nonionic surface active agents. DISCUSSION The most effective emulsion and foam stabilizers for aerosol systems formulated with the fiuorocarbon propellants are surface active materials that form an oriented, polymolecular structure at the interface with es- sentially solid properties. The surface active materials must have a low solubility in both the aqueous and propellant phases and also the proper wettability characteristics so that they remain in the interfacial regions instead of dispersing or dissolving in either of the t,vo phases. It is possi- ble that the poor solvent properties of the fluorocarbon propellants are a factor in the necessity for a stabilizer to function as a finely divided solid. Many molecular complexes derived from combinations of surface active agents and long-chain alcohols or acids are excellent stabilizers for aerosol systems. Since it has been established that molecular complexes form liquid crystals in aqueous systems at the concentrations used for stabilizing aerosol systems, it seems reasonable to assume that the com- plexes also form liquid crystal structures at the propellant-water inter- [ace. The liquid crystal molecular complexes, with their oriented poly- molecular structure, would be expected to stabilize aerosol emulsions and foams by somewhat the same mechanism as finely divided inorganic solids. Many water-soluble or dispersible surface active agents are relatively poor stabilizers for aerosol systems. The addition of a long-chain polar