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

82 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS initial deviation from baseline at 36øC, which arrests at 50øC, begins a gradual ascent again at 56øC, then forms a final arrest at 60øC. Each of these arrests can be correlated with the visual observations. Upon examination of the photomonitor recordings of the raw materials, it is apparent that the arrests in the cream recording also correlate with the melting point of the individual raw materials. Cetyl palmirate (Figure 3B) begins melting at 3 IøC and completes the transition at 5 IøC, while Lexemu! AR © (Figure 3C) melts over the range 50-60.5øC. The comelt of these ingredients (Figure 3A) gives a recording showing arrests near the melting point of the individual ingredients, thus indicating that the components remain as distinct entities rather than coprecipitating as a solid solution. The first transition of the comelt begins at 28øC, is completed at 48.5øC, and is then followed by the second transition, which begins at 53øC and is completed at 58øC. These transitions correspond to the melting of the cetyl palmirate and Lexemu! AR © components, respectively. Based on the melting behavior of the raw materials and the comelt, it can be concluded that the wax particles in the first melting transition of the cream are cety! palmirate and that the higher melting structures are Lexemu! AR ©. Thus, the network structure of the cream consists of individual cety! palmirate particles dispersed throughout the matrix formed by the Lexemul AR © ternary system. As is apparent from the rheograms and x i values, the cetyl pa!mitate forms a secondary network structure within the cream, which is very sensitive to thermal changes and gradually disappears as the temperature is increased over the range 25-40øC. The thermal destruction of this network structure can be explained upon closer examination of the melting behavior of cetyl pa!mitate. As the melting transition in Figure 4 indicates, the cety! palmirate particles gradually decrease in their overall dimension as the temperature increases. Of particular note is that in response to the dimension reduction, the number of crosslinks between particles is substantially reduced at 40øC in comparison to 25 øC and that the crosslink destruction accelerates as the melting point is approached. The photomonitor recording indicates that the melting transition of cetyl palmirate begins at 3 IøC. Thermal optical analysis has shown that as part of the network structure of the cream, the cety! pa!mitate maintains its individual chemical integrity and follows the same melting transition. However, the particles are smaller than those pictured in Figure 4, and network crosslinks occur by contact with the Lexemu! AR © matrix particles as well as by self-association. The photomonitor recording of the cream indicates initiation of the cety! palmitate melting transition at 36øC rather than at 31øC, which would be ex- pected. This is probably an artifact of the limits of the microscope magnification, the lower birefringence of the smaller wax particles, and the limits of the photomonitor sensitivity. In other words, the actual initiation of the cety! pa!mitate melting transition in the cream is probably 3 iøC, but the limits of resolution of the instrumentation do not allow detection until 36øC because of the smaller particle sizes being observed. Thus, the thermal destruction of the cetyl palmirate secondary structure in the cream follows the same sequence as cetyl palmirate alone and is due to crosslink destruction in response to particle dimension reduction. CONCLUSIONS According to the gel network theory, the oil phase is mechanically entrapped by the