Effect of polymer nonideality in a colloid-polymer mixture

We present a theory for the effect of polymer nonideality on the phase behavior of a hard-sphere colloid plus nonadsorbing polymer mixture. Starting from the behavior at the theta temperature, which has been treated previously within a mean-field framework. [H. N. W. Lekkerkerker et al., Europhys. Lett. 20, 559 (1992)], we use perturbation theory to take into the account the polymer-polymer interaction up to the second virial level. Theoretical predictions of the phase behavior are compared with data obtained in one particular model system: sterically stabilized polymethylmethacrylate particles and random-coil polystyrene dispersed in cis-decahydronaphthalene. Within the same theoretical framework the potential of mean force is calculated between a pair of plates immersed in a nonideal polymer solution. The result, showing a potential barrier in front of an attractive depletion well, is in agreement with a recent calculation by Walz and Sharma