COMPLEX FORMATION IN AEROSOLS 803 sulfate/alcohol ratio was 1:1, while calcium and tetramethyl ammonium salts did not form complexes. Complexes between stearie acid and sodium stearate with mol ratios of acid to salt of 1:1, 3:2, and 2:1, respectively, were reported by Ryer (11), while other acid/soap com- plexes with ratios of 1:2 were found by John and MeBain (12). The investigation of molecular complexes in aerosol emulsions and foams appeared to be a promising field of study from both a fundamental and practical aspect. Aerosol systems offered a unique opportunity to study complex formation in both an emulsion system and a foam derived from the same emulsion system. Aerosol emulsions consist of disper- sions of liquefied propellants in an aqueous phase. When these products are discharged, the liquefied propellants vaporize and produce a foam. From a practical point of view, the use of emulsion systems for formu- lating aerosol products is now well established, and the number of aerosol products formulated as emulsion systems is increasing each year. Aero- sol foams are particularly useful for cosmetic and pharmaceutical appli- cations. These applications vary widely, and so do the properties desired for the aerosol products. Some basic information on aerosol emulsions and foams is already available (13-22), but the present knowledge is relatively small compared to what remains to be learned about these systems. If molecular complexes formed in aerosol emul- sions and foams, this could provide an additional method for varying the properties of the systems. This in turn could lead to new and more effective applications for aerosol products. }•XPERIMENTAL Complex formation was judged by the effect of the alcohols on the properties of the aerosol emulsions and foams. determined were as follows: 1. Emulsion viscosity ,5. 2. Emulsion stability 6. 3. Foam stiffness 7. 4. Foam drainage The properties that were Foam stability Foam density Type of discharge All of the properties, except foam stability, are summarized in tables. As a result of the difficulty of attempting to assign a numerical value to foam stability, the foam stability properties of the systems are discussed in the body of the report and are not present in the tables. The reasons for selecting the preceding properties and the methods used to determine them were as follows:

8O4 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Emulsion Viscosity An increase in the viscosity of the bulk phase when an alcohol is added to an aqueous surfactant solution is generally considered an indi- cation of complex formation between the surfactant and alcohol (4). In the present study, samples were prepared in glass bottles for visual observation. The approximate viscosity of the emulsions was esti- mated by inverting the bottles slowly and noting the flow characteristics of the emulsions. The viscosity was rated "low" if it was close to that of water and "high" if the emulsion was very thick. Products with viscosities in between these two extremes were rated medium. Emulsion Stability Emulsion stability was judged visually by the time required for phase separation to occur after the samples had been hand shaken. The samples were emulsified by a modification of the Briggs intermittent method of emulsification (23). The samples were shaken 20 times by hand, allowed to stand overnight, and reshaken 20 times immediately before stability determination. Phase separation, or creaming, is a common method for judging emulsion stability, even though it does not necessarily indicate coalescence of the droplets and breaking of the emulsions (24). Becher (25) has pointed out that, although creaming does not represent actual breaking of the emulsion, creaming is favored by large droplet size, and this may be an indication of a process which will lead to demulsification. Schulman considered phase separation to be sufficiently valid for comparison purposes when all the samples were prepared in the same way (3). As mentioned previously, Schulman found that complex formation could cause a considerable increase in emulsion stability. Foam St{l•ness Foam stiffness was determined with a Cherry-Burrell Curd Tension Meter as described in Reference 16. Foam stiffness values are reported in grams and indicate the relative resistance of a foam to deformation resulting from the downward penetration of a curd knife. Although it is difficult to place an exact physical interpretation upon foam stiffness, it is assumed to be an aspect of foam viscosity. Complex formation has been reported to increase the surface viscosity of foams, and in the present case an increase in foam stiffness resulting from the addition of a unionized polar compound was interpreted as an indication of complex formation