Modeling self-assembly of surfactants at solid/liquid interfaces. I. Hydrophobic surfaces

A theory of surfactant self-assembly on isotropic hydrophobic surfaces is presented by extending the well established treatment of self-assembly in solution. The free-energy model for the formation of surface aggregates includes an additional term (beyond those used for bulk aggregates) to account for the replacement of the solid surface-water contact by the solid surface-aggregate core contact. This free-energy contribution is characterized by a single parameter, the displacement tension γ. Illustrative calculations of the critical aggregation concentration (CAC), aggregate shape and size are presented for anionic, zwitterionic, and nonionic surfactants. For all types of surfactants, the CAC is much lower than the bulk CMC. Regardless of the value of γ, surface aggregates of ionic surfactant are always smaller than the bulk-phase analogs. Conversely, surface aggregates of zwitterionic and nonionic surfactants can be either smaller or larger than those in solution, depending on the interplay between headgroup repulsion and aggregate core-solid surface attraction. A rich variety of aggregate morphologies including hemispheres, hemicylinders, finite disks, and continuous monolayers are predicted depending upon the surfactant and the solid surface. More interestingly, increasing the chemical potential of the surfactant (by increasing the total surfactant concentration) can induce the morphological transformation of surface aggregates to less energetically favorable structures. Thus the same surfactant on a given solid surface can self-assemble into various shapes depending upon the total surfactant concentration.