OIL-IN-WATER CREAM STABILITY 223 (+) (-) Temperature (oC) Figure 4. Differential scanning calorimetry thermograms obtained for U.K.-sourced glyceryl monostea- rate as received (solid trace) and after being melted and congealed (dashed trace). yielded a melting point around 55øC. This behavior of glyceryl monostearate is consis- tent with previous findings (5,6). The DSC profile of U.K.-sourced glyceryl monostearate (see Figure 4) was found to exhibit a slightly different trend. The DSC melting point of material as received aver- aged around 65øC, and after being allowed to melt and solidify the DSC melting point decreased to 59øC. This material also appears to undergo a structural change upon melting and congealing, but the different temperature values noted between U.S.- and U.K.-sourced glyceryl monostearate suggests that the structural changes are not equiva- lent. The acid values of the two sourced glyceryl monostearates were found to differ signifi- cantly (see Table III). The acid value of the U.S.-sourced material was found to be 5.1 eq. KOH/gm, while the acid value of the U.K.-sourced material was measured as 1.1 eq. KOH/gm. These differences in acid value are significant and definitely indicate differences in chemical composition between the two lots. In fact, when U.K.-sourced glyceryl monostearate was spiked with approximately 2% w/w stearic acid (raising the acid value of this material to about 5), its DSC melting point was found to decrease down to 60øC. The ester value, which is the difference between the saponification value less the acid value, was 166.9 for the U.K. material and 158.9 for the U.S. material, which indicated that the U.K. glyceryl monostearate was slightly more lipophilic. Fur-
224 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table III Comparative Properties of Glyceryl Monostearate Sourced From U.S. and U.K. U.S. U.K. Appearance Flakes Powder Total % monoglyceride 41.0 36.3 Iodine value 1.6 0.6 Acid value 5.1 1.1 Saponification value 164.0 168.0 Ester value 158.9 166.9 Melting point By DSC 60øC 65øC ther, the total monoglyceride and diglyceride contents were lower and the triglyceride content was higher for the U.K. source, which indicated fewer free hydroxyl groups and therefore a more lipophilic character. Since the DSC experiments implicated the existence of a structural change upon melting and congealing, variable temperature XRD experiments were carried out to identify the nature of this structural alteration. Although the material is of relatively low crystallinity, sufficient structure exists to yield some x-ray scattering at 2-theta angles between 18 ø and 25 ø. As shown in Figure 5, the XRD pattern of U.S.-sourced glyceryl monostearate consists of at least four features. A new XRD pattern was ob- 18.8 15.6 Degrees 2-Theta Figure 5. X-ray powder diffraction patterns obtained for U.S.-sourced glyceryl monostearate as received (solid trace) and after being melted and congealed (dashed trace).
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