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 ---.

340 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Ordinary Light Polarized Light I Day 15 Days 34 Days Figure 2. Consistency development of the model cream.