80 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS structure formed by the cetyl palmitate, while the other rheogram characteristics are due to the primary liquid crystalline gel network structure formed by the Lexemul AR © emulsifier system. STRUCTURE DETERMINATION Knowledge of the cream composition indicates that its structure is formed by a 2/1 ratio of Lexemul AR © to cetyl palmitate. In accordance with the gel network theory, the primary network structure of the cream is formed by the Lexemul AR ©, which is a commercial brand of self-emulsifying glyceryl monostearate having a cationic surfactant, stearamidoethyl diethylamine, as the emulsifier. As will become apparent from the thermal optical analysis data, the cetyl palmitate in the formulation precipitates with the Lexemul AR © during the cooling process of the cream manufacture and contributes to the network. A series of photomicrographs of the cream were obtained at 25 ø, 30 ø, 35 ø, and 40øC and showed no visual change in structure in response to the temperature change. Figure 2A shows the structure of the cream at 25øC. The numerous irregular-shaped particles were identified by their melting point to be cetyl palmitate. At 47øC the cetyl palmitate particles began to liquefy, and a slight volume expansion occurred. This portion of the melting transition was completed at 50øC and yielded oil droplets embedded in a matrix of Lexemul AR © as pictured in Figure 2B. The structure remained visually unchanged again until at 55øC the Lexemul AR © particles began to liquefy and coalesce. Rapid flow occurred at 59.5øC, indicating completion of the melting transition. Figure 3 shows the photomonitor recordings obtained during the heating program of the cream and the raw materials constituting its network structure. The photomonitor transition temperatures associated with these recordings are given in Table II. Exami- nation of the cream recording (Figure 3D) indicates that its melting transition shows an A B Figure 2. Photomicrographs of the cream at 500X magnification. A. 25øC sample prior to heating showing solid wax particles. B. 50øC sample showing molten cetyl palmitate droplets embedded in the Lexemul AR © matrix.

MODEL CREAM RHEOLOGY 81 A i i I 60 50 40 30 25 TEMPERATURE øC Figure 3. Photomonitor recordings of the cream and its raw materials. A. Lexemul AR©/cetyl palmitate 2/1 comeIt. B. Cetyl palmitate. C. Lexemul AR ©. D. Model cream. Table II Photomonitor Transition Temperatures Material Temperature (øC) Lexemul AR©/cetyl palmitate 2/1 28, 48.5, 53, 58 Cetyl palmitate 31, 51 Lexemul AR © 50, 60.5 Cream 36, 50, 56, 60