J. Soc. Cosmet. Cbem., 21, 377-391 (May 27, 1970) Stabilization of Aerosol Emulsions and Foams PAUL A. SANDERS, Ph.D.* Presented September 8-9, 1969, Seminar., St. Louis, Mo. Synopsis--It is proposed that the INTERFACIAL REGION around PROPELLANT droplets in AEROSOL EMULSIONS stabilized with MOLECULAR COMPLEXES is polymo~ lecular and consists of alternating layers of oriented water molecules and bimolecular layers of the molecular complex. The molecular complex has a LIQUID CRYSTAL STRUC- TURE and, since it is almost insoluble in either the propellant or aqueous phase, concen- trates at the interface and stabilizes the emulsion by forming essentially a solid interfacial fihn in somewhat the same way as finely divided iuorgauic solids. This hypothesis is based upon the evidence that molecular complexes have liquid crystal structures in aqueous sys- tems and that almost all aerosol emulsions and FOAMS are stabilized by substances which are practically insoluble in both the aqueous and propellant phases. INTRODUCTION The formation of molecular complexes between ionized surfac- rants and long-chain alcohols or acids has been postulated for many years (1) but it was the work of Schulman and his coworkers that provided the real .stimulus in this field. Schulman and Rideal (2) reported in 1937 that when a monomolecular film of cetyl alcohol or cholesterol was spread over an aqueous solution of sodium cetyl sulfate, the cetyl sulfate molecules penetrated the surface film of alcohol and formed a complex with the alcohol. The existence of the complex was suggested by the fact that the mixed film of cetyl sulfate and alcohol was more resistant to disruption by pressure than the film from either component alone. * Freon Products Laboratory, E. I. du Pont de Nemours & Co., Inc., Wilmington, Del. 19898. 377

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