Concentration fluctuations in a model colloid-polymer suspension: Experimental tests of depletion theories

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 integral equation approach reveal systematic deviations. The free volume approach strongly underestimates depletion attraction effects for small polymers and massively overestimates them for large polymers. The direction of the errors of liquid state theory are similar, but much better agreement is found and only modest quantitative errors are present over the entire range of polymer-colloid size asymmetry studied. The importance of physical correlation effects such as fractal coil structure and colloid-induced polymer clustering is deduced. The conclusions based on these homogeneous phase compressibility measurements are consistent with our prior experimental and theoretical studies of phase behavior and polymer insertion chemical potentials.