Phase behavior and structure of model colloid-polymer mixtures confined between two parallel planar hard walls

Abstract

Using Gibbs ensemble Monte Carlo simulations and density functional theory we investigate the fluid-fluid demixing transition in inhomogeneous colloid-polymer mixtures confined between two parallel plates with separation distances between one and ten colloid diameters covering the complete range from quasi-twodimensional to bulklike behavior. We use the Asakura-Oosawa-Vrij model in which colloid-colloid and colloidpolymer interactions are hard-sphere like, while the pair potential between polymers vanishes. Two different types of confinement induced by a pair of parallel walls are considered—namely, either through two hard walls or through two semipermeable walls that repel colloids but allow polymers to freely penetrate. For hard
semipermeable walls we find that the capillary binodal is shifted towards higher
lower polymer fugacities and lower
higher colloid fugacities as compared to the bulk binodal; this implies capillary condensation
evaporation of the colloidal liquid phase in the slit. A macroscopic treatment is provided by a symmetric Kelvin equation for general binary mixtures based on the proximity in chemical potentials of statepoints at capillary coexistence and the reference bulk coexistence. Results for capillary binodals compare well with those obtained from the classic version of the Kelvin equation due to Evans and Marini Bettolo Marconi, J. Chem. Phys. 86, 7138
1987 and are quantitatively accurate away from the fluid-fluid critical point, even at small wall separations. However, the significant shift of the critical polymer fugacity towards higher values upon increasing confinement, as found in simulations, is not reproduced. For hard walls the density profiles of polymers and colloids inside the slit display oscillations due to packing effects for all statepoints. For semipermeable walls either similar structuring or flat profiles are found, depending on the statepoint considered.