Depletion and structural forces in confined polyelectrolyte solutions

Monte Carlo simulations and density functional calculations have been performed for charged macromolecules confined to planar slits. The force between the confining walls has been evaluated as a function of separation, while keeping the chemical potential of the macromolecules constant. Highly charged spherical particles and flexible polyelectrolyte chains in confinement give rise to depletion and structural oscillatory forces as a function of surface separation. The sign and magnitude of the surface charge of the confining walls have no dramatic effect on the qualitative behavior of the confined liquid. With neutral or oppositely charged surfaces, an accumulation of charged macroions is seen in the slit driven by the repulsive interaction between the macroions, while equally charged surfaces give rise to a pure depletion. The net charge, the range of interaction, and the particle density affect the details of the force curve. For spherical macroions, the period of the oscillations scales approximately as the bulk aggregate concentration, C-bulk(-1/3). Confined polyelectrolyte chains share some of these properties, but they partly display a different behavior. One clear difference is that the polyelectrolyte net charge, that is, the degree of polymerization, has no effect on the osmotic pressure. This is an indication that polyelectrolyte chains pack not as spheres but rather as cylindrical objects. Another difference is that the effective repulsive interaction between polyelectrolyte chains can be more long ranged and oscillatory forces can appear more readily than for a corresponding solution of equally charged spherical macroions.