Formation and structure of slightly anionically charged nanoemulsions obtained by the phase inversion concentration (PIC) method

Abstract

While nanoemulsions (10–200 nm) are not thermodynamically stable systems they can exhibit quite long term stability. In this paper oil/surfactant mixtures, containing diethylhexyl carbonate/phenoxyethanol/parabens as oil and polyglyceryl-4 laurate/dilauryl citrate as surfactant, form nanoemulsions simply by dilution with water, i.e. by means of the phase inversion concentration (PIC) method. In order to study this highly interesting phenomenon an investigation at constant oil-to-surfactant (O/S) ratio was done by means of viscosity, conductivity, and UV/Vis-transmittance measurements. This phase study as a function of the dilution by water shows that at an intermediate water content a two-phase system of bicontinuous structure is formed, which exhibits a very pronounced viscosity and conductivity maximum shortly before the homogeneous nanoemulsion phase is reached. In the same region SANS shows a high degree of ordering of this bicontinuous structure. SANS and cryo-TEM investigations of the nanoemulsion regime show an increasing average size with dilution and, more interestingly, the presence of two populations with different average particle sizes around 10–15 nm and 25–40 nm. The relative proportion of each population depends on the amount of added water, leading to an average growth of the particle size with increasing dilution.