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 (H₁) and related O/H₁ 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, H₁; bicontinuous cubic, V₁; lamellar, L_α; 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 H₁ phase also exhibits behavior that is dependent on the surfactant hydrophilicity, i.e., high values of the elastic modulus G′ and complex viscosity |η*| have been observed in the C₁₂EO₉ system, possibly due to the strong neighboring head group interaction. Hexagon structure of the H₁ phase was calculated by using the Bohlin model. The viscosity of the O/H₁ emulsions decreases with increasing oil concentration, possibly due to the low volume fraction of the continuous phase (the H₁ phase).