OIL-IN-WATER CREAM STABILITY 225 tained after melting this sample on the XRD hot stage and allowing the material to congeal. This new pattern is far simpler than originally observed and consists of at least two features. The width and location of the XRD peaks suggest that they do not represent individual d-spacings, but instead reflect the ordering of the material as a whole. Quite different behavior was observed for U.K.-sourced glyceryl monostearate. As evi- dent in Figure 6, the material consisted of the same four-peak system, although the relative intensities of these were not equivalent to the U.S.-sourced material. Melting and congealing the U.K.-sourced material caused the total collapse of all peaks into a single gaussian peak, centered at 22 ø 2-theta. This observation is interpretated to imply that the U.K.-sourced glyceryl monostearate congeals into a single-phased material. The implications of the XRD studies are that a chemical difference between the U.S.- and U.K.-sourced glyceryl monostearate yields structural differences after these are melted and congealed. A material capable of solidifying into a single phase would contract significantly upon cooling, while a material that would solidify into a multi- phasic mixture would not be able to contract upon solidification. This behavior has indeed been observed for bulk glyceryl monostearate: if the slide holding the U.K.- sourced material is inverted after the melt congeals, the material will actually fall out of the XRD slide holder of its own accord. The U.S.-sourced material is extremely diffi- cult to remove from the slide holder and requires mechanical scraping for its removal. It 10.0 15.0 •0.0 •5.• 30.8 Degrees 2-Theta Figure 6. X-ray powder diffraction patterns obtained for U.K.-sourced glyceryl monostearate as received (solid trace) and after being melted and congealed (dashed trace).
226 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Degrees 2-Theta Figure 7. X-ray powder diffraction patterns obtained for U.K.-sourced glyceryl monostearate to which sufficient stearic acid had been added so as to raise the acid value to approximately 5. Data are shown for the unheated material (solid trace) and for the sample after it had been melted and congealed (dashed trace). would appear that the chemical composition of the U.K.-sourced glyceryl monostearate is such that all non-idealities cancel and the overall material behaves as if it were a chemically pure material. To test this hypothesis, U.K.-sourced glyceryl monostearate spiked with stearic acid was subjected to the variable temperature XRD study. As evident in Figure 7, the complicated initial XRD pattern simplified after the material was melted and con- gealed, but the resulting XRD pattern now contained several scattering peaks. In addi- tion, the congealed material remained firmly attached to the XRD slide holder after solidification and was removable only with difficulty. These observations indicate that addition of the stearic acid altered the composition of the glyceryl monostearate suffi- ciently that it could not congeal into a single phase capable of contraction. The effect on cream properties resulting after chemical modification of the glyceryl monostearate composition was then investigated. U.K.-sourced glyceryl monostearate (acid value 1.1) was modified by the addition of stearic acid until the final acid value was adjusted to approximately 5. Upon manufacture of a 20-kg cream batch, no initial bleed was observed. Relative to a cream made with U.K.-sourced glyceryl monostea- rate, the consistency of the modified formulation (with stearic acid) was firmer initially, and the gel structure developed more rapidly (see Figures 8 and 9).
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