Nonionic oil-in-water microemulsions: the effect of oil type on phase behaviour

The formation of oil-in-water (o/w) microemulsions stabilized by the nonion ic surfactants, polyoxyethylene-10-dodecyl ether, polyoxyethylene-10-oleyl ether, N,N-dimethyldodecylamine-N-oxide and N,N-dimethyloleylamine-N-oxide and containing a variety of pharmaceutically acceptable oils, namely ethyl butyrate, ethyl caprylate, ethyl oleate and the triglycerides, soybean oil, Miglyol 812 and tributylin, has: been examined at 298 K. The effect on mic roemulsion formation of replacing water with phosphate buffered saline (PBS ) and complete PBS has been established. In addition, the effect of changin g temperature (from 298 to 310 K) on the phase behaviour of microemulsions formulated using PBS as continuous phase has been determined. Although some small differences in phase behaviour were noted when altering the continuo us phase, the greatest difference in phase behaviour was observed when chan ging the experimental temperature, particularly for microemulsions stabiliz ed by polyouyethylene-10-oleyl ether. Regardless of the temperature and aqu eous phase used, however the larger molecular volume oils (soybean oil, Mig lyol 812 and ethyl oleate) were solubilized to a lower extent than the smal ler molecular volume oils (namely, ethyl butyrate and ethyl caprylate). The only exception to this rule was when polyoxyethylene- 10-oleyl ether was u sed as surfactant, particularly at 298 K, where it was the larger molecular volume oils that were solubilized to the greatest extent. Cloud point/phas e inversion temperature experiments suggested that the higher molecular vol ume oils were incorporated into the microemulsions prepared using the polyo xyethylene-based surfactants in a different way than the smaller molecular volume oils and suggest that the smaller molecular volume oils are acting i n much the same way as a cosurfactant in that they interchelate with their hydophilic group interspersed in the surfactant head group region. As N,N-d imethyldodecylamine-N-oxide does not exhibit a cloud point it was not possi ble to determine the mode of oil incorporation in microemulsions prepared w ith this surfactant. (C) 2000 Elsevier Science B.V. All rights reserved.