Abstract
Abstract The effects of hydration on solubilization and colloidal phenomena have been studied by comparing the differences between two oil-brine-surfactant-alcohol systems containing H2O or D2O. The H2O system was significantly more hydrophilic than the D2O system in relation to coacervation of aqueous surfactant solutions, formation of middle phase microemulsions, surfactant partitioning in oil and brine, and solubilization capacity of brine in oil-external microemulsions. The two oil-brine-surfactant-alcohol systems appeared to be macroscopically similar if the salinity of the D2O system was decreased by about 0.3% NaCl. Electrical resistance and dielectric relaxation measurements showed that the surface charge density of H2O-in-oil microemulsions was three times higher than that of the corresponding D2O-in-oil microemulsions. The higher surface charge density of the H2O system caused its higher interfacial alcohol concentration, which was confirmed by alcohol titration measurements. It is interesting to note that the interfacial alcohol concentration of the two systems can have similar values if the salinity of D2O is decreased by about 0.3% NaCl. The higher surface charge density of the H2O system can be explained by the lower (more negative) free energy of hydration of surfactant ions and counterions in H2O than in D2O, which leads to a higher degree of dissociation of the surfactant polar groups in the H2O system. An expression is given to estimate the free energy of transfer (from H2O to D2O) of interfacial surfactant anions from surface charge density measurements.
Published Version
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