The role of the cosurfactant in the CTAB/water/n-pentanol/n-hexane system: pentanol effect on the phase equilibria and mesophase structure

We report on the effect of 1-pentanol loading on the phase behaviour of mixtures of CTAB/water/n-hexane at fixed mole ratios 1:80:47. The cosurfactant induces changes in the interfacial film curvature. By increasing the pentanol/CTAB mole ratio, the system evolves from oil-in-water to water-in-oil structures. For very large 1-pentanol loading some water is expelled from the reverse micelles resulting in a L2 plus water equilibrium (emulsification failure). In the range of compositions investigated most of the phase equilibria reveals the coexistence of liquid crystals and hexane/pentanol solutions. In these cases, we estimate the interface composition by assuming a constant 1-pentanol concentration in all the oil domains, and by tacking into account the alcohol solubility in water. In the case of single-phase sample the interfacial composition was determined by means of turbidimetric titration. The adsorption of cosurfactant at the interface of direct micelles, planar lamellae, bicontinuous microemulsions, and spherical reverse micelles follows the same adsorption isotherm (independently from the curvature of the interface). Moreover, the results obtained unambiguously show that the interface composition dictates the spontaneous curvature of interfacial film. Actually positive, null, and negative curvatures correspond to different compositions of the interfacial film. Once the influence of cosurfactant on the spontaneous curvature of the interface is understood, the appearance of the emulsification failure upon pentanol loading can be rationalised within the framework of the flexible surface model.