CONSISTENCY DEVELOPMENT OF A MODEL CREAM 343 cream transforms into the unstable state of an emulsion droplet suspension. Further, after this transition to the suspension state, the lipophilic gel phase (also known as coagel) still surrounds the dispersed internal phase. The agglomerates and oil droplets in the photomicrographs of Figure 3 indicate that the wax and oil components of the internal phase form distinct entities when phase-separated. According to both Eccleston (1) and Junginger (8), pearlescence in creams is attributable to crystallization in the continuous phase in the form of light-refracting platelets. Junginger (8) further states that these platelets are structurally similar to lipophilic gel phase (i.e., coagel). In the case of the model cream discussed here, the phase-separated wax components are re- sponsible for the pearlescence in the 16-month-old sample. IDENTIFICATION OF PHASE-SEPARATED COMPONENTS In order to further identify the phase-separated components, thermal optical videomi- croscopy was performed on the 16-month-old cream using slides specifically selected to have minimal or extensive agglomeration. Figure 4 shows the photomonitor recordings of the melting transitions of 16-month-old cream samples from slides with and without extensive agglomeration. The slide with relatively few agglomerates forms a smooth transition to an initial arrest at 45øC followed by a final arrest at 48.5øC. The videotape of the melting transition indicated that these arrests correspond to the melting point of the cream matrix and trace amounts of agglomerate, respectively. The initial arrest in the slide with extensive agglomeration occurs at the same temperature as the slide with trace agglomerates and, according to the videotape, also corresponds to the melting point of the cream matrix. A broad transition in the photomonitor recording occurs over the range from 47øC to 58.5øC. The videotape indicates that this corresponds to the agglomerate melting transition. Examination of the formula indicates that the agglom- erates could be cetyl alcohol, cetyl palmitate, and/or stearyl alcohol, which have liter- ature melting points at 49øC, 54øC, and 56-60øC, respectively (10). The values at the 90 80 70 60 50 40 30 25 Temperature øC Figure 4. Photomonitor recordings of the model cream: (a) model cream with few agglomerates (b) model cream with agglomerates.

344 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS thermal arrests of the photomonitor recordings are sufficiently consistent with these values to verify the suspected identity of the agglomerates. Thermal and infrared trace substance analysis of an isolated agglomerate indicated cetyl palmitate to be the major component and stearyl and cetyl alcohol to be the minor components (11). The absence of isopropyl myristate in the trace substance analysis verifies the photomicroscopy observation that the oil and wax components phase separate as distinct entities. CONCLUSIONS The model cream was a white lotion at the time of manufacture that slowly converted to a semisolid consistency and subsequently became softer again as aging and the phase separation process progressed. The consistency development and destabilization were both associated with phase changes in the surfactant/fatty amphiphile systems. In the former case, surfactant interpenetrates the fatty amphiphiles in a semihydrate crystalline arrangement to form a fully hydrated, lameliar liquid crystalline gel network. In the latter case, the lameliar liquid crystal breaks down, expelling the fatty amphiphiles, which revert back to the crystalline semihydrate arrangement. The consistency changes reflect the integrity of the gel network since it is a highly viscoelastic system. The extent of destabilization is apparent in the degree of pearlescence, since the phase-separated fatty amphiphiles and waxes of the internal phase form light-refracting platelets. The consistency development and destabilization process can be monitored using rhe- ology and microscopy. Structural development is apparent in the rheograms by an increase in hysteresis and a shift toward higher yield values. However, once a semisolid consistency is established, the overall shape of the rheogram remains basically un- changed except for the shift to higher shear stress values. Destabilization appears in the rheograms through formation of spurs and secondary inflections, shifts to lower maxi- mum shear stress values, and changes in the size and shape of the hysteresis loop. General trends in the destabilization process show a low shear rate inflection, which gradually becomes more pronounced and shifts to higher shear stress values, and a recoil following the maximum shear stress, which shifts to lower shear rates. These changes reflect the destruction of a shear-resistant network and formation of a less shear-resistant network and can be correlated with breakdown of the lameliar liquid crystalline gel network, droplet coalescence, and agglomerate formation. The phase changes apparent in photomicrographs support the rheological findings. As the cream consistency develops, polarized light photomicrographs show a decreased presence of grainy textures (coagel or lipophilic gel) and increased formation and thick- ness of birefringent, lameliar structures (lameliar liquid crystalline gel phase or hydro- philic gel). Ordinary light shows gradual replacement of the grainy textures with emulsion droplets and improved definition of the droplets. These changes have been attributed to increased surfactant penetration of the fatty amphiphiles. Destabilization has been associated with formation of diffuse, weakly birefringent structures when viewed with polarized light. These areas also appear diffuse in ordinary light. The concentration of the diffuse, weakly birefringent structures has been found to increase as phase separation progresses. The pearlescence apparent in phase-separated creams has been attributed to these agglomerates that, according to theory (1,8), are structurally similar to coagel (i.e., lipophilic gel). The small oil droplets associated with the ag-