Phase behavior and concentration fluctuations in suspensions of hard spheres and nearly ideal polymers

Abstract

The phase behavior and concentration fluctuations in suspensions of hard sphere colloids and nonadsorbing polymers under nearly ideal solvent conditions is studied experimentally. A remarkably different qualitative behavior compared to the athermal solvent case is observed for the dependence on polymer/particle size asymmetry of both the gelation and fluid–fluid phase separation boundaries. Near the theta state the effect of increasing the range of depletion attractions leads to a weak monotonic destabilization of the homogeneous phase at high particle volume fractions, with a reversal of the trend at lower volume fractions. In stark contrast to athermal solvent behavior, this nonmonotonic behavior results in multiple “curve crossings” of gel and phase separation boundaries as the polymer/particle size ratio is varied. Quantitative comparisons with no adjustable parameter PRISM integral equation theory for the fluid–fluid spinodals and osmotic compressibilities show good qualitative or semiquantitative agreement with all the experimental trends. The differences between good and ideal solvent conditions are largely attributed to changes in the polymer–polymer pair correlation functions due to the enhanced ability of coils to interpenetrate and cluster in theta solvents. Even for ideal solvent conditions the simplifying polymer model and statistical mechanical assumptions adopted by prior classic free volume and related approaches appear to miss fundamental aspects of the experimental behavior, especially for large size asymmetry ratios and/or moderate-to-high colloid volume fractions. The primary error can be identified with the approximation of a polymer chain by a phantom sphere with no conformational degrees of freedom.