An electric double layer is studied by means of Monte Carlo simulations and mean-field theory. The counterions of the uniformly charged surfaces are modeled as flexible polyelectrolytes. For this particular model system it turns out that the traditional double layer repulsion becomes attractive for a wide range of systems. The main reason for this attraction is an entropically driven bridging mechanism, and its magnitude is significant compared to ordinary double layer or van der Waals forces. The polyelectrolyte Poisson–Boltzmann theory developed here behaves in a qualitatively correct manner, also predicting an attractive interaction extending over several nanometers. These results may have some relevance to technical and biological systems, where sometimes puzzling force behavior is seen in the presence of polyelectrolytes.

A density functional theory for polymer fluids is derived. The form of the theory is identical to a continuum self-consistent-field theory but we show that with the correct ‘‘mean field,’’ the theory is exact. We apply the theory to a polymer fluid made up of tangential hard spheres and borrow from a successful density functional theory for simple hard spheres to obtain a nonlocal functional expression for this mean field. The latter depends only upon the monomer density. This approximation is used to treat the problem of a mixture of such polymers and simple hard spheres, contained in slitlike pores. Comparison with simulations show that the theory, though simple in structure, and containing a single adjustable parameter, is surprisingly accurate.

A theory for nonuniform polymer melts is presented, which combines density functional theory with Monte Carlo methods. The theory treats the ideal gas functional exactly via a single chain simulation and uses the weighted density approximation for the excess free energy functional. The bulk fluid properties required in the theory are obtained from a generalized Flory equation of state. The predictions of the theory are compared to Monte Carlo simulations for the density profiles of semiflexible polymer melts confined between flat plates. Good agreement between theory and simulation is found for 3mers and 20mers and for several densities and molecular stiffnesses.

We investigate theoretical models of room temperature ionic liquids, and find that the experimentally observed camel-shape of the differential capacitance is strongly related to dispersion interactions in these systems. At low surface charge densities, the loss of dispersion interactions in the vicinity of the electrodes generates depleted densities, with a concomitant drop of the differential capacitance. This behavior is not observed in models where dispersion interactions have been removed.