Density Dependence of Homopolymer Adsorption and Colloidal Interaction Forces in a Supercritical Solvent: Monte Carlo Simulation

The complex relationship among polymer solution density, adsorption, and colloidal interactions in a supercritical solvent is investigated by means of Monte Carlo simulation. The expanded grand canonical method is applied to simulate homopolymer adsorption from solution on two impenetrable flat surfaces. Adsorption isotherms indicate that chain adsorption increases as bulk density is lowered toward the bulk solution phase boundary because chains adsorb to escape an increasingly poor bulk solvent. The force between two surfaces coated with adsorbed chains changes dramatically as a function of bulk solution density. Bridging attraction is observed above the bulk upper critical solution density (UCSD). As adsorption increases, this bridging attraction decreases as the bulk solution density is decreased due to chain crowding in the pore. At the UCSD the force becomes repulsive due to entropically unfavorable chain overlaps, and bridging attraction is no longer present. At densities below the UCSD, the force becomes attractive again, due to LCST-type (entropically driven) phase separation driven by the increase in entropy of solvent expelled from the pore. Lattice−fluid self-consistent field theory agrees with our simulation results for the density dependence of adsorption isotherms and force profiles.