Association of nonionic polymers with micelles, bilayers, and microemulsions

Abstract

A unified thermodynamic theory is developed here to explore the nature of association of nonionic polymer molecules with surfactant aggregates such as globular micelles, rod-like micelles, bilayers, oil in water microemulsions, and water in oil microemulsions. The association complex is visualized as consisting of the polymer molecule wrapped around the aggregate at its interface with water. The association with polymer provides enhanced shielding of the hydrophobic aggregate surface from water, increases the steric repulsions at the aggregate surface and gives rise to hydrophobic interactions between the polymer and the aggregate surface. Simple free energy models are formulated to account for these polymer-induced effects. Illustrative calculations have been carried out to examine the behavior of monoalkyl and dialkyl surfactants with nonionic, anionic, and zwitterionic polar head groups. The results show that polymer–aggregate association is almost always preferred in the case of anionic or zwitterionic surfactants. As a result of the association with the polymer, globular micelles transform into smaller globular micelles; large, polydisperse rod-like micelles change into small, monodispersed globular micelles; spherical vesicles and planar bilayers transform into smaller discoids. In all the cases, the polymer-bound aggregates are formed at a critical micelle concentration (CMC) that is significantly lower than the CMC of the polymer-free systems. In marked contrast, aggregates of nonionic surfactants may or may not associate with the polymer molecule. If association occurs, the polymer-induced changes in the aggregate morphologies are similar to those for the anionic surfactants. However, the CMC for the formation of bound aggregates is usually very close to that for the formation of the free aggregates. In many situations, association is not favored and polymer-free aggregates coexist with free polymer molecules in solution. It is found that subtle variations in the hydrophobic characteristics of the polymer molecule can tilt the equilibrium between polymer-bound and polymer-free aggregates. In the case of microemulsions, calculations show that the volume ratio of the dispersed phase to the surfactant is significantly altered by the presence of the polymer. Also, the condition for phase inversion from oil in water to water in oil type of microemulsion is modified by the presence of the polymer. The theoretical results suggest ways by which one may control the morphologies of surfactant aggregates using polymer additives.