Concentration Fluctuations in a Model Colloid−Polymer Suspension: Experimental Tests of Depletion Theories

Abstract

A light scattering turbidity method has been employed to measure the dimensionless colloidal osmotic compressibility of carefully characterized model athermal colloid (silica)−polymer (polystyrene) suspensions. Mixture thermodynamics is controlled by purely repulsive, hard sphere interactions between the particles while the polymer is in a good solvent. Polymer size is varied by nearly 2 orders of magnitude from far below (1.3 nm) to larger than (70 nm) the particle radius (50 nm). Polymer concentrations are systematically increased up to the solubility limit, and a wide range of colloidal volume fractions up to 0.35 are studied. The measured amplitude of long wavelength colloidal concentration fluctuations provides a sensitive probe of polymer-induced entropic depletion attractions and serves as a demanding test of theoretical descriptions. Quantitative, no adjustable parameter comparisons of experiment with both the classic phantom sphere free volume theory and the recently proposed polymer liquid state...