Interactions in mixtures of colloidal silica spheres and polystyrene molecules in cyclohexane: II. Light scattering

Abstract

Light scattering measurements have been conducted on homogeneous systems containing lyophilic, spherical silica particles and polystyrene molecules in cyclohexane at the theta temperature. From the decrease in the apparent molar mass of the silica particles in solutions with increasing concentrations of polystyrene, the negative adsorption of polystyrene on silica was obtained. The negative adsorption, which is caused by the volume restriction effect, increases with increasing molar mass of the polymer. The negative adsorption can be converted formally to an equivalent depletion layer surrounding the particles, in which essentially no polymer is present. The thickness of this layer proves to be comparable with the radius of gyration of the polymer molecules. The apparent optical radius of gyration of the silica particles decreases with increasing polymer concentration and polymer molar mass. This observation is also explained by a negative adsorption of polymer molecules. Calculations show that the decrease in the optical radius of gyration is in agreement with the magnitude of the negative adsorption determined from the apparent molar mass of the silica. The second virial coefficient of the silica particles, determined from the concentration dependence of the light scattering, can become negative, reflecting an effective attraction between the particles. This attraction has been predicted theoretically and can result in phase separation at higher concentrations of polymer.