The ratio of free and fixed interlamellar water as quality criteria for OW creams
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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, however, which immobilize the dispersed phase, 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. Using 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.