The effect of H 2O and D 2O on colloidal properties of surfactant solutions and microemulsions

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 H 2O or D 2O. The H 2O system was significantly more hydrophilic than the D 2O 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 D 2O system was decreased by about 0.3% NaCl. Electrical resistance and dielectric relaxation measurements showed that the surface charge density of H 2O-in-oil microemulsions was three times higher than that of the corresponding D 2O-in-oil microemulsions. The higher surface charge density of the H 2O 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 D 2O is decreased by about 0.3% NaCl. The higher surface charge density of the H 2O system can be explained by the lower (more negative) free energy of hydration of surfactant ions and counterions in H 2O than in D 2O, which leads to a higher degree of dissociation of the surfactant polar groups in the H 2O system. An expression is given to estimate the free energy of transfer (from H 2O to D 2O) of interfacial surfactant anions from surface charge density measurements.