378 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Schulman and Cockbain (3) subsequently studied the effect of com- plex formation upon the stability of oil-in-water emulsions prepared with tnineral oil. Emulsions with sodium cetyl sulfate alone as the emulsi- fying agent were unstable but combinations of sodium cetyl sulfate and cetyl alcohol or cholesterol gave excellent emulsions. The increase in emulsion stability achieved with the alcohols was attributed to complex formation between the ionized surfactant and alcohol at the droplet in- terface. Later investigators reported an increase in foam stability when lauryl alcohol was added to sodium lauryl sulfate solutions. Epstein et al. (4) considered this to be due to the formation of a crystalline molecular complex from two molecules of sodium lauryl sulfate and one molecule of lauryl alcohol. A number of other molecular complexes have been isolated and analyzed in recent years. Kung and Goddard have reported various alkyl sulfate-fatty alcohol complexes with tool ratios of 1:1 or 9:1 as well as fatty acid-potassium salt complexes with a tool ratio of 1:1 (5-7). The work of Goddard, Kung, and others established the compositions of complexes that were prepared and isolated in various ways, but the actual structure of a complexed interfacial film around an emulsified oil droplet or at the interfacial regions in foams is considerably less certain. I t could have a definite composition with the surfactant and fatty alcohol (or acid) present in the same stoichiometric ratios as those found in the complexes that were isolated and analyzed, or it could have an indefinite and varying composition. According to Alexander (8), Lawrence has suggested a liquid crystal structure at the interface for the ionized surfac- tant and fatty alcohol complex. Alexander also pointed out that in this event, the interface, instead of consisting of a mixed monolayer, probably would have a layered structure closer to that of solid stabilizers such as zinc and aluminum stearate. Little basic information about aerosol emulsions and foams is avail- able but some of their general properties suggested that in the interfacial regions in aerosol systems, molecular complexes could have the liquid crystal structure proposed by Lawrence. Aerosol emulsion and foam systems therefore were examined to determine to what extent this hy- pothesis applied. THEORY The literature indicates that many molecular complexes from sur- face active agents and long-chain alcohols possess liquid crystal structures in aqueous systems, even at very low concentrations. It seems reasonable

EMULSIONS AND FOAMS 379 to assume, therefore, that molecular complexes also have a liquid crystal structure around emulsified propellant droplets or in aerosol foams. The most effective surface active agents for stabilizing aerosol emul- sions and fomns are practically insoluble in both the aqueous and pro- pellant phases. From this it is concluded that the surface active agents are present in the interfacial regions essentially in solid form and stabilize emulsions and foams in the same way as finely divided inorganic sta- bilizers. Since molecular complexes are effective stabilizers for aerosol systems, this suggests that the molecular complexes are also acting as solid stabilizers. The Lawrence-Alexander hypothesis that molecular com- plexes have a liquid crystal structure in the interfacial regions and act as finely divided solids appears to be compatible with the experimental data for aerosol emulsions and foams. One reason that certain molecular complexes may be effective as stabilizers for aerosol systems is that the addition of the fatty alcohol or acid converts the normally water-soluble or dispersible surface active agent into a liquid crystal structure considerably less hydrophilic than the surface active agent itself which is able to function at the interface as a solid stabilizer. Evidence for the polymolecular structure in the interfacial regions is based primarily upon the known polymolecular orientation o.f the mole- cules in liquid crystals and the experimental data in the literature which indicate that many common emulsion systems have polymolecular inter- facial films. EXPERIMENTAL DATA Liquid C•ystal Structures of Surface Active Agents There is considerable evidence in the literature that molecular com- plexes from surface active agents and long-chain alcohols or acids form liquid crystal structures in aqueous systems. Most of the data were ob- tained from aqueous systems without a dispersed oil phase. In order for the results to apply to the Lawrence-Alexander hypothesis, it is neces- sary to assume that the same forces that cause molecular complexes to form liquid crystal structures in a bulk aqueous phase also cause the for- mation of liquid crystal structures in the interfacial regions around dis- persed oil droplets. The structures in the bulk phase and at the oil-water interface probably would be different, possibly laminar in the aqueous phase and spherical at the interface, but still liquid crystalline in both cases.