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Oil/water (O/W) nano-emulsions have been formed in the system water/C16E6/mineral oil by the phase inversion temperature (PIT) method. The relation between the phase equilibria observed at the hydrophilic−lipophilic balance (HLB) temperature or the PIT (i.e., the nature, number, and relative volume fractions of the involved phases), the droplet sizes, and polydispersities of the resulting emulsions have been determined. Milky white emulsions were obtained when, at the HLB temperature, a three-phase equilibrium formed by water (W), shear-birefringent microemulsion (D), and oil (O) was observed. However, bluish transparent O/W nano-emulsions with droplet sizes as low as 40 nm were formed in a narrow range of oil-to-surfactant ratios in which a D or W + D phases were the initial equilibrium phases. In the W + D equilibria, droplet sizes were independent from the water content, indicating that nanodroplet formation is mainly controlled by the structure of the D phase. These results suggest that the main requirement for bluish transparent O/W nano-emulsion formation is the complete solubilization of the oil component in a bicontinuous microemulsion, independent of whether the initial phase equilibrium is single or multiphase.

Formation of oil-in-water nano-emulsions has been studied in the water/C12E4/isohexadecane system by the phase inversion temperature emulsification method. Emulsification started at the corresponding hydrophilic-lipophilic balance temperature, and then the samples were quickly cooled to 25 degrees C. The influence of phase behavior on nano-emulsion droplet size and stability has been studied. Droplet size was determined by dynamic light scattering, and nano-emulsion stability was assessed, measuring the variation of droplet size as a function of time. The results obtained showed that the smallest droplet sizes were produced in samples where the emulsification started in a bicontinuous microemulsion (D) phase region or in a two-phase region consisting of a microemulsion (D) and a liquid crystalline phase (L(alpha)). Although the breakdown process of nano-emulsions could be attributed to the oil transference from the smaller to the bigger droplets, the increase in instability found with the increase in surfactant concentration may be related to the higher surfactant excess, favoring the oil micellar transport between the emulsion droplets.

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