Ethylene glycol and fatty acid have a profound impact on the behavior of water-in-oil microemulsions formed by the surfactant Aerosol-OT

The addition of ethylene glycol and lauric acid had a significant effect on the phase behavior of water/Aerosol-OT/n-alkane water-in-oil microemulsion systems. Despite the fact that ethylene glycol solubilizes in the micellar water pool, its addition increased the solubilization of water. The addition of lauric acid further increased the solubilization of water and ethylene glycol, apparently by screening attractive interactions. The water-ethylene glycol solubilization curve featured two phase boundaries. The approach of the upper phase boundary was accompanied by an increase in the magnitude of attractive interactions (determined from dynamic light scattering experiments) and percolation, but no such increase occurred at the approach of the lower phase boundary. The effects of several parameters, such as lauric acid concentration, alkane chain length (isooctane versus tetradecane), addition of 1-butanol, and aqueous pseudophase ionic strength, are consistent with the conceptual model developed by Shah (Langmuir 1987, 3, 1086), which indicates that the upper phase boundary occurs due to the presence of strong attractive interactions and micellar clustering, while the lower phase boundary represents the approach of the spontaneous radius of curvature. Fourier transform infrared spectroscopy analysis revealed that microemulsions from our system contained a larger percentage of ''free'', unbound solubilizate in the water pool compared to that of microemulsions in the absence of ethylene glycol and a fatty acid.