Effects of Surfactant Hydrophilicity on the Oil Solubilization and Rheological Behavior of a Nonionic Hexagonal Phase

The effects of surfactant hydrophilicity on the phase behavior and rheology of a hexagonal phase (H-1) and related O/H-1 emulsions in a water/C12EO (n) /isohexadecane system (n = 7, 9) have been investigated. In this phase behavior study, a variety of surfactant aggregates (micellar, W-m; hexagonal, H-1; bicontinuous cubic, V-1; lamellar, L-alpha; inverse micellar, O-m) have been observed in water-C12EO (n) binary systems. The oil solubility of different surfactant aggregates was strongly affected by the surfactant hydrophilicity, i.e., surfactant aggregates in the C12EO7 system could solubilize a larger amount of oil than those in the C12EO9 system. It can be inferred that the surfactant with long hydrophilic head groups could be closely packed in the interface, leading to a reduction in the oil solubility in the micellar core due to the strong head group interaction. Interestingly, the rheology of the H-1 phase also exhibits behavior that is dependent on the surfactant hydrophilicity, i.e., high values of the elastic modulus G' and complex viscosity |eta*| have been observed in the C12EO9 system, possibly due to the strong neighboring head group interaction. Hexagon structure of the H-1 phase was calculated by using the Bohlin model. The viscosity of the O/H-1 emulsions decreases with increasing oil concentration, possibly due to the low volume fraction of the continuous phase (the H-1 phase).