Polymer-surfactant interaction and stability of amorphous colloidal particles

The behavior of mixtures of polymer and surfactant as peptizing agents for controlling Ostwald ripening and solvent-mediated phase transformations of amorphous colloidal particles is studied by equilibrium dialysis and electron spin resonance (ESR) spectroscopy using nitroxide spin-labeled compounds. It is found that amorphous phase stability is dependent on the interplay between polymer-surfactant complexation in the bulk fluid and the conformation of adsorbed molecules at the particle-fluid interface. Optimum stability is obtained over a range of compositions in which both polymer and surfactant are strongly bound to the surface of the particles. The conformation of the adsorbed polymer is characterized by a high fraction of train segments and relatively few loops. The shorter chain surfactant functions as a filler to fill vacancies on the surface. The results suggest that the combination of polymer and surfactant may provide a defect-free monolayer that offers better protection against solvent-mediated transformations than either polymer or surfactant alone.