J. Soc. Cosmet. Chem., 35, 45-57 (January/February 1984) The ratio of interlamellarly fixed water to bulk water in O/W creams HANS JUNGINGER, Department of Pharmaceutical Technology, University of Leiden, Gorlaeus Laboratories, P.O. Box 9502, 2300 RA Leiden, The Netherlands, A.A.M.D. AKKERMANS, Europe Data, Bredestraat 24, 6211 HC Maastricht, The Netherlands, and WALTER HEERING, Institute of Pharmaceutical Technology, Technical University of Braunschweig, 3300 Braunschweig, Federal Republic of Germany. Received.July 28, 1983. Synopsis O/W creams with crystalline gel structures are supposed to be four-phase systems. Their dominant structural elements are the hydrophilic and lipophilic gel phases. The hydrophilic gel phases have--depending on the hydrophilic character of their polar groups--a very strong swelling capacity. Further, a dynamic equilibrium is maintained between the water interlamellarly inserted into the hydrophilic gel phase and the bulk water phase. The latter is mainly fixed mechanically by the hydrophilic gel phase. The lipophilic gel phases, which immobilize the dispersed phase, however, have no or only an extremely weak swelling capacity. By means of thermogravimetry (TG) a quantitative differentiation between water interlamellarly fixed in the hydrophilic gel phase and bulk water is possible. Through the use of dynamic TG, the curves of water release and the results obtained are discussed. In the water-containing hydrophilic ointment DAB 8 and in the non-ionic hydrophilic ointment DAC the fractions of interlamellarly fixed water are substantially higher than those of the bulk water. In the investigated stearate creams, however, the fraction of the bulk water is much higher than the fraction of interlamellarly fixed water. Consequently, the water release from these stearate creams is higher than that of the other O/W creams mentioned above. It is concluded that the ratio of interlamellarly fixed water and bulk water is an important criterion for the properties of such O/W creams and that TG can be suitably applied not only as in process control during product manufacturing but also as a useful tool in the development of O/W creams with a controlled water release. INTRODUCTION X-ray investigations by the Kratky low angle technique and by goniometry in combination with quantitative differential scanning calorimetry led to structure models of the water-containing hydrophilic ointment DAB8* (1,2). The formulation of this *German Pharmacopoeia, 8th ed. (1978), Govi-Verlag, Frankfurt am Main, FRG. 45

46 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS cream is as follows: Emulsifying wax 9 parts Liquid paraffin 10.5 parts White petrolatum 10.5 parts Water 70 parts It was found that such O/W creams may be regarded as four-phase systems (Figure 1). Figure 1. Gel structures of the water-containing hydrophilic ointment DAB 8. a: mixed crystal bilayer of cetostearyl alcohol and cetostearyl alcohol sulfate b: interlamellarly fixed water layer a + b: hydrophilic gel phase c: cetostearyl alcohol semihydrate lipophilic gel phase d: bulk water phase e: lipophilic components (dispersed phase). The dominant matrices are the hydrophilic and the lipophilic gel phases. Both phases build up bilayers of mixed crystals. The bilayers are oriented in such a way that the hydrocarbon tails are directed towards each other, as are the polar groups (Figure 1, region a). The hydrophilic gel phase consists of cetostearyl alcohol and the whole amount of the ionic sodium-n-alkylsulfates thus strong hydrophilic moieties and hydrophobic cores relieve each other (Figure 1, region a). If water is present, water molecules are inserted between the polar sulfate and alcoholic groups of the surfactant molecules (Figure 1, region b). The regions a and b together form the hydrophilic gel phase. The water molecules interlamellarly fixed in the hydrophilic gel phase are in equilibrium with the molecules of the bulk water phase (Figure 1, region d). Both water phases form the continuous phase of the system. It is assumed, however, that the