Birefringent emulsions stabilized with steareth-2 and steareth-21

In this research, birefringent emulsions stabilized with Steareth-2 and Steareth-21 were studied. Some authors attributed to them a liquid-crystalline structure, although the microphotographs presented by them as an evidence of such structure actually suggest the presence of solid crystals instead of liquid crystals. The purposes of this paper are to determine the true cause of the birefringence of this type of emulsions and to study how the composition of the oily phase influences the stability and the birefringence. Six emulsions stabilized with a mixture of Steareth-2 and Steareth-21 were prepared, each one with a different emollient. Four of them were birefringent while two were isotropic. The observed birefringence would be due to the presence of solid crystals and not of liquid-crystals. These solid crystals could be originated by the interaction between both emulsifiers, either between themselves, with stearic acid or cetyl alcohol presents in the formulations, or even because of the insolubility of stearic acid or cetyl alcohol in emollient oil. Moreover, the isotropic emulsions showed greater viscosity than the birefringent ones. This result is accorded to the fact that the birefringence is not related to the presence of a liquid-crystalline interphase, because this type of interphase leads to an increase of viscosity. Colegio de Farmacéuticos de la Provincia de Buenos Aires