Solubilization and phase equilibria of water-in-oil microemulsions: II. Effects of alcohols, oils, and salinity on single-chain surfactant systems

The solubilization and phase equilibria of water-in-oil (w/o) microemulsions have been studied to elucidate how the molecular structure of various components of microemulsions and salinity influence two interfacial parameters of the system, namely the spontaneous curvature and elasticity of interfacial films. Most solubilization and phase equilibria data presented are explained by the change of these two parameters. It is indicated that oil can influence the property of an interfacial film by a solvation effect. Oil molecules with small molecular volume or high aromaticity produce a strong solvation effect and consequently lead to a greater penetration of oil molecules into the surfactant chain layer, thus increasing the rigidity and curvature of the interface. The addition of alcohol exhibits strong effects on the elasticity and spontaneous curvature of an interface. Decreasing alcohol chain length can increase the fluidity of the interfacial film and hence increase the attractive interdroplet interaction. On the other hand, increasing alcohol chain length often increases the rigidity and curvature of an interface. For alcohols which increase the interfacial fluidity, increasing alcohol partitioning at the interface will increase the fluidity and natural radius of the interface. The addition of electrolytes results in tighter molecular packing at the interface and a decrease in the natural radius and interfacial fluidity. There often exists an optimal salinity for a given w/o microemulsion system at which a maximum brine solubilization occurs. The maximum solubilization is interpreted as a result of counteracting effects of attractive interdroplet interaction of fluid interfaces and the bending stress of rigid interfaces. As a result of this study, phenomenological guidelines for the formulation of microemulsions have been proposed. It is inferred that the addition of a small amount of alcohol at optimal salinity together with the condition of chain length compatibility can result in the largest possible brine solubilization in a given w/o microemulsion.