J. Soc. Cosmetic Chemists, 17• 801-830 (196(i) Molecular Complex Formation in Aerosol Emulsions and Foams PAUL A. SANDERS, Ph.D.* Presented May 10, l•66, New York City Synopsis--Previous studies in nonaerosol systems have shown that certain detergents, such as sodium lauryl sulfate, form molecular complexes with long chain alcohols or acids at air/ water or oil/water interfaces. In the present investigation it has been shown that molecular complexes are also formed in aerosol emulsion systems. It was found that, in many cases, the addition of a long-chain alcohol to an aerosol emulsion system prepared with the tri- ethanolamine salt of a fatty acid or sodium lauryl sulfate as the surfactant produced a marked increase in emulsion and foam stability and a decrease in foam drainage. In some cases foam viscosity was increased. These effects of the long-chain alcohols occur in nonaerosol and aerosol systems and are indicative of molecular complex formation. INTRODUCTION It has been known for a long time that combinations of certain surfactants produce unexpected effects in emulsion systems. The reason for this remained obscure for many years, and in many cases the causes are still unknown. However, the effect of combinations of surfactants and long-chain polar compounds, such as the fatty alcohols, has been shown to be due to the formation of molecular complexes between the surfactants and the alcohols. The study of these molecular complexes has clarified much of the phenomena observed with these combinations. The investigation of molecular complexes was initiated in 1937 by Schulman and Rideal (1) with the discovery that sodium cetyl sulfate and cholesterol formed stable complexes at air/water interfaces. These complexes formed because of the attraction between the polar groups of * "Freon" Products Laboratory, E. I. du Pont de Nemours and Co., Wilmington, Del~ 801

8O2 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS the two compounds and the association between the hydrophobic por- tions of the molecules which resulted from the Van der Waal's forces of attraction. The strength of the complexes was found to be dependent upon the stereochemical configurations of the two interacting molecules (2). Sodium cetyl sulfate and cholesterol, cetyl alcohol, or elaidyl alcohol formed strong complexes. However, sodium cetyl sulfate and oleyl alcohol formed weak complexes because the cis-configuration of oleyl alcohol prevented a close association between the two molecules. The effects of the complexes at oil/water interfaces in emulsion sys- tems were found by Schulman and Cockbain (3) to be similar to those observed previously at air/water interfaces. Sodium cetyl sulfate alone or with oleyl alcohol gave poor emulsions of mineral oil in water, while combinations of sodium cetyl sulfate with cetyl alcohol or cholesterol gave good emulsions. Studies of molecular complexes have also been carried out in con- nection with an investigation of the drainage rates of liquids through foams and the drainage properties of films from aqueous solutions. Miles et al. (4, 5) observed that the drainage rates of liquids through sodium lauryl sulfate foams were markedly decreased by the presence of lauryl alcohol. It was also noted that aqueous solutions containing combinations of sodium alkyl sulfates and cetyl alcohol gave slow drain- ing films, while those with sodium alkyl sulfates alone or with oleyl alcohol gave fast draining films. The slow draining property of the foams and films was attributed to a high surface viscosity which resulted from complex formation between the alkyl sulfates and alcohols. When the temperature of the slow draining films was increased, the film ultimately became fast draining at a specific temperature, called the film drainage transition temperature. The transition from slow to fast draining of the foams and films oc- curred at a temperature at which the molecules in the complex had sufficient thermal energy to overcome the bonding energies (6). Similar experiments were carried out on nonionic ethoxylated lauryl alcohols with lauryl or cetyl alcohol by Beeher and Del Vecchio (7). These com- binations also were found to give slow draining fihns. The compositions of a number of molecular complexes have been determined. Epstein et rd. (8) reported crystalline intermolecular products in which the sodium alkyl-sulfates and long-chain alcohols were combined in a tool ratio of 2:1. Kung and Goddard (9, 10) have also investigated the composition of complexes. Lithium, potassium, am- monium, and magnesium lauryl sulfates formed complexes in which the