Wall-fluid and liquid-gas interfaces of model colloid-polymer mixtures by simulation and theory

Abstract

We perform a study of the interfacial properties of a model suspension of hard sphere colloids with diameter sigma(c) and nonadsorbing ideal polymer coils with diameter sigma(p) . For the mixture in contact with a planar hard wall, we obtain from simulations the wall-fluid interfacial free energy, gamma(wf) , for size ratios q =sigma(p)/sigma(c) =0.6 and 1, using thermodynamic integration, and study the (excess) adsorption of colloids, Gamma(c) , and of polymers, Gamma(p) , at the hard wall. The interfacial tension of the free liquid-gas interface, gamma(lg) , is obtained following three different routes in simulations: (i) from studying the system size dependence of the interfacial width according to the predictions of capillary wave theory, (ii) from the probability distribution of the colloid density at coexistence in the grand canonical ensemble, and (iii) for state points where the colloidal liquid wets the wall completely, from Young's equation relating gamma(lg) to the difference of wall-liquid and wall-gas interfacial tensions, gamma(wl)-gamma(wg) . In addition, we calculate gamma(wf) ,Gamma(c) , and Gamma(p) using density functional theory and a scaled particle theory based on free volume theory. Good agreement is found between the simulation results and those from density functional theory, while the results from scaled particle theory quantitatively deviate but reproduce some essential features. Simulation results for gamma(lg) obtained from the three different routes are all in good agreement. Density functional theory predicts gamma(lg) with good accuracy for high polymer reservoir packing fractions, but yields deviations from the simulation results close to the critical point.