380 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS All three types of surface active agents (anionic, cationic, and non- ionic) form liquid crystals in aqueous systems regardless of whether they are co•nplexed with a long-chain polar compound or not (9-11). Surface active agents generally form the "s•nectic" type of liquid crystal structure which consists of regular layers of surface active agent molecules separated by water •nolecules (12). The polar heads of the surface active agents are oriented towards the aqueous phase. The "smectic" type of structure is regarded by Mulley (13) as a fully developed McBain la•nellar micelie. There see•ns to be no convincing reason why the same type of structure in a •nodified spherical form cannot exist around a dispersed oil droplet and function essentially as a solid stabilizer. Ionic surface active agents with short alkyl chains may not separate as a liquid crystal phase until the concentration reaches 20-30%, but as the chain length increases, the concentrations at which liquid crystal structures form decrease rapidly. Liquid crystals may occur at concentra- tions of 1-2% when the chain length reaches C• (13). In aqueous systexns containing a surface active agent and long-chain fatty alcohol or acid complex, liquid crystal structures form at low con- centrations, possibly because the repulsion between the ionized heads of the surface active agent is reduced by the presence of the alcohol mole- cules. This allows closer packing of the molecules (13). Ekwall et al. (14) have indicated that mesomorphic phases xnay be formed below the critical xnicelle concentration (cmc) in systeIns containing surface active agents and polar cronpounds. Lawrence and Hyde (15) have also sug- gested that the breaks occurring in the conductivity curves of cationic detergents in the presence of organic additives near the cmc may be due to the formation of a •nesoxnorphic phase rather than changes in the cmc. When the third component is nonpolar, liquid crystals form only at high concentrations because the nonpolar component does not aid in the orientation. Mulley (13) visualizes that the addition of water-insoluble compounds to aqueous systexns of surface active agents containing Hartley xnicelles gradually changes the micelle structure until precipitation of the liquid crystal occurs. This concept has been discussed by Winsor (16). Liq•tid Crystal Structures and Pearlescence The pearlescence in aqueous systems containing certain molecular complexes is often associated with liquid crystal structures. For example, the pearliness in creams formulated with an excess of stearic acid in so-

EMULSIONS AND FOAMS 381 dium or potassium stearate systems is considered to be due to the ordered structure of the acid soap complex of free stearic acid and sodium stearate (17). The 1:1 complex of sodiron pallnitate and palmitic acid has a crystal pattern and a liquid crystal structure in aqueous systems (18). The molecular complexes from the triethanolamine salts of the fatty acids and free fatty acids also show a slight pearlescence by them- selves in the aqueous phase and are much more effective stabilizers for aerosol emulsions and foams than the triethanolamine salts by them- selves (19). Very pearlescent aqueous systems are produced by certain nonionic polyoxyethylene fatty ether-fatty alcohol combinations (20). The pearlescence is believed to result from the liquid crystal structure of the complexes in the aqueous phase. The complex structures were judged to be liquid crystalline in nature, not only because of their pearlescent properties but also because the pearlescence disappeared over a specific transition temperature range (Table I) which apparently is due to the gradual "melting" of the liquid crystalline structures responsible for the pearlescence. This change is reversible and can be considered analogous to the two melting point transitions exhibited by typical liquid crystals. TABLE I Temperature Dependence of Pearlescent Structures Polyoxyethylene (10) Cetyl Ether Systems Fatty Alcohol Transition Temperature Range (øF) Lauryl 77-82 Myristyl 93-101 Cetyl 95-115 Stearyl 100-125 Becher and Del Vecchio (21) determined film drainage transition tem- peratures for several polyoxyethylene lauryl ethers in the presence ot• lauryl and cetyl alcohols by means of surface viscosity measurements. The curves illustrating the change in surface viscosity with temperature were regarded as being in the nature of melting point depression curves possibly attributable to complex formation. Conceivably the two-dimen~ sional melting points could also be interpreted on the basis of the liquid crystalline nature of the structures.