Balance of hydrophobic and electrostatic interaction of polymers and surfactants: Case of anionic surfactant and hydrophobically modified polymer

We investigated the structure of polymer-surfactant aggregates and their pH-dependent structural evolution using hydrophobically modified poly(vinyl-pyrrolidone) (h-PVP) and sodium dodecyl sulfate (SDS). The structure of the complexes in the weak (pH ≃ 9) and strong (pH ≃ 2) interaction regimes was studied using small-angle X-ray scattering, with the data analysed on an absolute intensity scale, using molecular parameters as constraints. At pH 9, where self-assembly was driven by hydrophobic interactions, we have found that, at low surfactant concentrations, elongated aggregates were formed. At excess surfactant concentrations, the aggregates became more compact with a smaller aggregation number, resembling free micelles with the hydrophobic domains of the polymer incorporated into the surfactant core. In all cases, aggregates formed a continuous network, with polymer serving as a weak cross-linker between aggregates. Finally, we have compared the structure of these weakly interacting aggregates with the precipitates formed at low pH, where the electrostatic attraction dominates.