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-fluiddemixing transition in inhomogeneous colloid-polymer mixtures confined between two parallel plates withseparation distances between one and ten colloid diameters covering the complete range from quasi-twodimensionalto bulklike behavior. We use the Asakura-Oosawa-Vrij model in which colloid-colloid and colloidpolymerinteractions are hard-sphere like, while the pair potential between polymers vanishes. Two differenttypes of confinement induced by a pair of parallel walls are considered—namely, either through two hard wallsor through two semipermeable walls that repel colloids but allow polymers to freely penetrate. For hardsemipermeable walls we find that the capillary binodal is shifted towards higher lower polymer fugacitiesand lower higher colloid fugacities as compared to the bulk binodal; this implies capillary condensationevaporation of the colloidal liquid phase in the slit. A macroscopic treatment is provided by a symmetricKelvin equation for general binary mixtures based on the proximity in chemical potentials of statepoints atcapillary coexistence and the reference bulk coexistence. Results for capillary binodals compare well withthose 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 atsmall wall separations. However, the significant shift of the critical polymer fugacity towards higher valuesupon increasing confinement, as found in simulations, is not reproduced. For hard walls the density profiles ofpolymers and colloids inside the slit display oscillations due to packing effects for all statepoints. For semipermeablewalls either similar structuring or flat profiles are found, depending on the statepoint considered.