Correlation between colloidal stability and surfactant adsorption/association phenomena studied by light scattering

The stability of a colloidal system composed of styrene-acrylate copolymer particles and potassium stearate (KS) anionic surfactant molecules has been determined in terms of the Fuchs stability ratio, W, as a function of the surfactant concentration, by measuring the initial aggregation kinetics using the small-angle light scattering (SALS) technique. The structure of the particle surface is peculiar, being irregularly patterned, and thus represents a model system to investigate colloidal stability of nonsmooth colloidal particles. From the SALS kinetic experiments, it is found that the stability increases dramatically with KS concentration until the saturation of the available surface occurs. At concentrations higher than the saturation concentration, the W value decreases markedly with KS, as a consequence of attractive depletion forces induced by formation of micelles in the water phase. The adsorption isotherm, determined through the surface tension technique, agrees with the W vs KS behavior, with respect to the onset of saturation and the surface-per-molecule value, and it can be described by the two-step Langmuir isotherm. Static light scattering spectra of the particles at different adsorbed amounts of KS have been fitted by means of the Lorenz-Mie theory and accounting for the experimentally determined particle size distribution. The increase in the particle diameter imputable to KS adsorption is sizable. Stability data measured under high fluid shear in a turbulent capillary (in the absence of any screening salt) fit well into this scenario. However, depletion forces are shown to be noncooperative with turbulent shear in the absence of screening electrolytes.