Influence of the microemulsion phase structure on the phase inversion temperature emulsification of polar oils

Abstract Oil-in-water-emulsions (o/w) stabilized with non-ionic emulsifiers can show a temperature-induced phase inversion from an o/w emulsion to a water-in-oil (w/o) emulsion. This phase inversion can be utilized for the production of finely dispersed emulsions with long-term stability. A stable emulsion with a polar oil component was produced with a low content of a non-ionic emulsifier and the co-emulsifier glycerol monostearate. To find optimum emulsification routes the phase behavior of these emulsions was investigated as a function of the water content at different emulsifier/co-emulsifier ratios. At low water content the emulsifier system shows a one-phase microemulsion in the phase inversion temperature (PIT) region. This microemulsion phase was used to prepare o/w emulsions with high water content in a two-step process, by emulsifying the polar oil in this temperature region and by changing the oil/water ratio in an additional dilution step. Finely dispersed droplets are achieved only if the emulsifier/co-emulsifier ratio is adjusted so that during PIT emulsification a bicontinuous microemulsion phase or a lamellar liquid crystalline phase is passed through. In the case of a w/o microemulsion, coarse droplets are obtained. Furthermore, the influence of the emulsifier content on the phase behavior and the oil droplet size is examined, in order to find a criterion for an optimum emulsifier content. According to these results, long-term stable o/w emulsions with low emulsifier content can be obtained by a phase inversion emulsification, if the emulsifier mixture is optimized such that during emulsification a bicontinuous microemulsion or a lamellar liquid crystalline phase is passed through.