On the Partitioning of Cosurfactant in Mixed Micelles: Size Enhancement and Nematic Stability

We consider a role for nonionic cosurfactant (e.g., alcohol) in ionic surfactant/water solutions according to which cosurfactant interposes itself between surfactant molecules at the micellar surface, thereby diluting the charge density. This relief of electrostatic strain is most pronounced in regions of low curvature (small head-group area). Accordingly, the cosurfactant can be shown to go preferentially into the body rather than the caps (rim) of finite-size prolate (oblate) aggregates. As a consequence of this partitioning of cosurfactant between different environments in mixed anisotropic micelles, the standard chemical potential per molecule is found to decrease faster with aggregation number (s) than in the pure surfactant/water situation: in the case of rods, for example, μ_s° = μco. + (δ₁/s) + (δ₂/S²). The presence of the 1/s²term leads to a stronger increase of average size with overall amphiphile concentration and to an enhancement of nematic (vs. isotropic) phase stability. Rough estimates of these effects are made for a wide range of cosurfactant-to-surfactant ratios.