Characterization of Water-in-Oil Microemulsions Formed in Silicone Oils

The phase stability and structure of water-in-oil microemulsions stabilized by the surfactant Aerosol OT have been examined in the low molecular weight silicone oils hexamethyldisiloxane (HMDS) and diphenyltetramethyldisiloxane (DPTMDS). The solubilization capacity ωmax (where ω = [H2O]/[AOT]) determined as a function of temperature defines a limited single phase microemulsion region with relatively low water solubilization (ωmax < 40). Addition of NaCl shifts this single phase region to a higher temperature. SANS and dynamic light scattering measurements show the presence of strong attractive interdroplet interactions in HMDS, which are relatively absent in DPTMDS. Addition of n-octanol as a cosurfactant dramatically increases the solubilization of water in HMDS, giving an optimal solubilization capacity at a specific cosurfactant:surfactant molar ratio, x. Small-angle neutron scattering (SANS) measurements made close to the optimized condition show elimination of attractive interactions. A Porod analysis of the SANS data demonstrates an increase in the area of the surfactant/cosurfactant layer and commensurate reduction in droplet size with increasing x.