FIXED AND BULK EMULSION WATER 55 Palmitic acid 12 parts Triethanolamine 1.2 parts Glycerol, 85% 13.5 parts Water 10-61.3 parts The results of thermogravimetry are shown in Figure 9. It becomes quite clear, above all, that at a high water content only one third is fixed interlamellarly but that two thirds of the water represents the bulk water phase, which is directly available for skin hydration. These facts give a good explanation of the results of Tronnier (13), who stated that the rate of hydration of the skin by stearate creams is much higher in comparison with that of other O/W creams. At a total water amount higher than 55% these systems become unstable and the transition takes place from a cream with a coherent three-dimensional hydrophilic gel network to a liquid emulsion without these structural elements. Finally, thermogravimetry may also be useful in order to characterize creams of the W/O type. Figure 10 represents the TG- and DTG-curves of such W/O cream 3.0 2,5' 2.0' 1.5 1.0 [mg/s] •G 0.0 0 dt AG 0.04 I I I I I I • I 20 30 40 50 60 70 80 90 loci Figure 10. TG-curve (AG-curve) and DTG-curve (AG/dt - curve) of a W/O cream used in cosmetics (NIVEA-Creme ©, Beiersdorf, Hamburg, FRG). Total water content: 68% (w/w).

56 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (NIVEA-Creme ©, Beiersdorf, Hamburg, FRG), 68 percent of which consists of the dispersed water phase. First of all, the DTG-curve makes it clear that until a temperature of 50øC has been reached and the cream has melted, the rate of evaporation is quite small. Thereafter, the rate of evaporation increases linearly. At 70øC a maximum is reached and the rate of evaporation decreases almost linearly as well. Both types, the TG- and DTG-curve, are representative and typical of W/O creams. CONCLUSIONS Through the use of thermogravimetry the measurement of the ratio of interlamellarly fixed water and bulk water in O/W creams is possible. The procedure is quite reproducible (error _ 2%) and owing to the short time needed for one measurement (approx. 40 min) it is a suitable in process control during the manufacturing of the creams. Aging of the creams (displacing of the interlamellarly fixed water fraction in favor of the bulk water) is also detectable with this technique (14). Furthermore, the development of cosmetic products with controlled water release becomes quite feasible. ACKNOWLEDGEMENTS The accurate performance of the thermogravimetric measurements by G. Schmidt- Habelmann and G.J. Gooris and the grant allowed by the Deutsche Forschungsge- meinschaft are gratefully acknowledged. REFERENCES (1) C. Fiihrer, H.Junginger, and S. Friberg. Strukturuntersuchungen von Salben, RiSntgenstrukturuntersu- chungen an der Hydrophilen Salbe DAB 7, J. Soc. Cosmet. Chem. 29, 703-716 (1978). (2) H. Junginger, C. Fiihrer, J. Ziegenmeyer, and S. Friberg, Strukturuntersuchungen an der Wasserhal- tigen Hydrophilen Salbe DAB 7,J. Soc. Cosmet. Chem. 30, 9-23 (1979). (3) H.Junginger, Strukturuntersuchungen an Stearatcremes. Pharm. Ind. (in press). (4) H. Junginger, C. Fiihrer, and J. Ziegenmeyer, Strukturunte•suchungen an O/W-Cremes. Congressus Pharmaceuticus Hungaricus VII, Budapest (1979). (5) H. Junginger, The structures of amphiphilic creams, FIP abstracts, 43rd International Congress of Pharmaceutical Sciences, Montreux 1983, p 87. (6) T. de Vringer, thesis Leiden (in preparation). (7) T. de Vringer, J. Joosten, and H. Junginger, Characterization of the gel structure in a nonionic ointment by small angle X-ray diffraction, Coil. Polym. Sci. (accepted for publication). (8) H.Junginger, W. Heering, C. Filhrer, and I. Geffers, Elektronenmikroskopische Untersuchungen ilber den kolloidchemischen Aufbau von Salben und Cremes, Coil. Polym. Sci. 259, 561-567 (1981). (9) H. Junginger, w. Heering, C. Fiihrer, and I. Geffers, Transmissions-elektronenmikroskopisch Untersuchungen kolloider Salben- und Cremestrukturen, Beitr. Elektronenmikroskop. Direktabb. Oberfl. 14, 465 (1981). (10) H. Junginger and W. Heering, Darstellung kolloidaler Strukturen von Salben, Cremes, Emulsionen und Mikroemulsionen mittels Gefrierbruch-Atztechnik und TEM, Acta Pharm. Technol. 29, 85-96 (1983). (11) F. Franks, M. H. Asquith, C. C. Hammond, H. B. Skaer, and P. Echlin, Polymeric cryoprotectants in the preservation of biological ultrastructure, J. Microsc. Oxford 110, 223-238 (1977).