Entropy driven phase transitions in colloid-polymer suspensions: Tests of depletion theories

The phase behavior of model athermal silica (radius R=50 nm)-polystyrene-toluene suspensions has been determined over nearly two orders of magnitude in polymer or colloid size asymmetry. Fluid-gel, fluid-crystal, and fluid-fluid transitions are observed as R-g, the polymer radius of gyration, increases. Based on the polymer concentration relative to the dilute-semidilute crossover density, c(p)/c(p)*, as the relevant measure of depletion attraction, we find that suspension miscibility monotonically improves as R-g increases for all colloid volume fractions. This trend is in contradiction to all classic depletion theories of which we are aware. However, the predictions of fluid-fluid spinodal phase separation by the microscopic polymer reference interaction site model integral equation theory of athermal polymer-colloid suspensions are in agreement with the experimental observations. Polymer-polymer interactions, chain fractal structure, and structural reorganizations are implicated as critical physical factors. A fluid-gel transition is observed in the one-phase region for R-g=0.026R. The recently proposed dynamic mode-coupling theory is found to provide a nearly quantitative prediction of the gel line. With increasing R-g/R, gelation is predicted to require larger values of c(p)/c(p)* such that the nonergodicity transition shifts into the metastable region of the phase diagram in agreement with experiment. Comparison of the gelation behavior predicted based on the assumption that it is coincident with the static percolation line is also examined, with mixed results. (C) 2002 American Institute of Physics.