Interaction of anionic surfactant with polymeric nanoparticles of similar charge.

The formation of micelle-like nanosize aggregates above a critical aggregation concentration (CAC) by a water-soluble, amphiphilic, and statistical copolymer poly(SAMPS/DA) of sodium N-acrylamidomethylpropanesulfonate (SAMPS) and N-dodecylacrylamide (DA) was studied. The structural changes that result from the interactions between the polymeric nanoparticles and sodium dodecylsulfate (SDS), an anionic surfactant, were studied with the aid of surface tension, viscosity, steady-state fluorescence, dynamic light scattering, and transmission electron microscopic techniques. In dilute solution with polymer concentration C(p)<CAC, the copolymer does not interact with SDS at concentrations lower than its CMC value. The polymer only binds to SDS micelles to produce polymer-decorated micelles. In polyelectrolyte solutions with C(p)>CAC, strong interactions between the polyelectrolyte and SDS were observed even at a very low level of surfactant addition. The interaction is purely hydrophobic in nature. The surfactant monomers bind to the polymer micelles to form smaller spherical aggregates (polymer-SDS complex). When surfactant was added above its saturation concentration, the association complexes are disrupted and only surfactant micelles decorated by polymer chain(s) were observed. The microenvironment of the polymer-SDS complexes was observed to be much less polar than that of neat polymer aggregates and SDS micelles. Also, the internal rigidity of the polymer-SDS complexes was found to be higher than that of the pure polymer or SDS micelles. It was observed that the neat polymer aggregates and polymer-decorated SDS micelles are more stable than the polymer-SDS complexes.