338 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS of the cream. Thermal optical videomicroscopy and trace substance analysis are used to characterize the cream and to identify the composition of the structures observed in the photomicrographs. EXPERIMENTAL FORMULA The cream is based on a commercially available blend of cetearyl alcohol and ceteareth- 20 in a 75/25% (w/w) ratio. The ceteareth-20 is the surfactant that penetrates the fatty amphiphile, cetearyl alcohol, to provide body to the cream in accordance with the gel network theory (1-6). Cetyl palmitate is a wax used to assist in adding body to the cream via formation of a secondary network structure (7). Isopropyl myristate is the liquid internal phase. The identity of the destabilizing ingredient is proprietary, but it can be described as an anionic surfactant. The cream was manufactured using a variable speed mixer with a marine propeller. Emulsification was via the water-to-oil phase inversion technique. A water ring was used to force cool the cream to room temperature. RHEOLOGY AND MICROSCOPY A Ferranti-Shirley cone and plate viscometer was used to determine the rheology of the cream. Samples were tested at 25øC with the instrument in low gear using the 7-cm truncated cone, a 60-sec sweep time, and the 1x scale expansion on the recorder. A shear rate range of 0-164 sec- • was achieved with these instrument settings. Photo- micrographs were obtained with a Zeiss Universal microscope at 500x magnification using both ordinary and polarized light on the same sample. THERMAL OPTICAL VIDEOMICROSCOPY Thermal optical videomicroscopy was performed using polarized light and a 250x magnification. The melting behavior of the cream and its raw materials was studied using a Mettier FP 82 hot stage equipped with photomonitor at a heating rate of 5ø/minute over the range of 25øC to slightly above the melting point. Samples were also studied by videotaping the melting transition. The melting point was defined as the temperature at which a thermal arrest appears in the photomonitor recording and/or the temperature at which the cream or crystals liquefied and flow began. In order to obtain a thin film suitable for microscopy, raw materials were premelted on the slide prior to monitoring their behavior. Cream samples were monitored directly from the initial slide preparation since a thin film was easily produced and because preliminary heating destroyed the sample. RESULTS AND DISCUSSION CONSISTENCY DEVELOPMENT The model cream was a white lotion at the time of manufacture, but it slowly developed

CONSISTENCY DEVELOPMENT OF A MODEL CREAM 339 a semisolid consistency. The rheograms in Figure 1 reflect the structural development of the cream by an increase in hysteresis and a shift toward higher shear stress values. The narrow hysteresis loop and low yield value at day 1 are very characteristic of a lotion consistency, while the broad hysteresis loop and high yield values at days 15 and 34 are characteristic of creams. Note that the overall shapes of the rheograms at days 15 and 34 remain the same while the yield value increases in response to the structural devel- opment. Photornicrographs of the cream corresponding to the rheogram time intervals are pre- sented in Figure 2. An overall grainy texture appears in both ordinary and polarized light at day 1 and corresponds to the "coagel" or "lipophilic gel" phase described by Eccleston (1) and Junginger (8), respectively. This phase consists of partially hydrated, crystalline fatty amphiphiles with no surfactant penetration. Photos at days 15 and 34 show a gradual replacement of the grainy texture with emulsion droplets. As the consistency develops, ordinary light shows better definition of the emulsion droplets and polarized light shows formation of birefringent lameliar structures. Patel et al. (9) referred to these birefringent structures as "onion ring" lameliar phase and correlated them to the liquid crystalline gel phase of the gel network theory. Successive increases in lameliar thickness correspond to increased surfactant penetration of the fatty am- phiphile with consequential gel phase formation and subsequent consistency develop- ment as demonstrated by the rheograms. 160 120 80 40 I 1 day / 16 mo 15 days 99 days• - I ,• 5-• •.•\ // ," ', //// I I I I 266 532 798 1064 days I 1333 Shear Stress (dynes/cm 2) Figure 1. Rheograms of the aging model cream. Consistency development . Destabilization ---.