Solvation forces in a model mixture of ions and dipoles

The forces between two charged thick plates immersed in a model fluid are investigated using the density-functional method. The model fluid comprises oppositely charged spherical ions together with neutral spherical particles with fixed dipoles µ at their centres. The thermodynamic potential is obtained as a quadratic functional of the local density, local electric charge and polarisation of the fluid with kernels which depend on the direct correlation functions of the bulk liquid. The latter are approximated by the functions obtained by means of the mean spherical approximation. The equilibrium densities which minimise the thermodynamic potential are computed and from these the forces between the plates are derived. When the dielectric constant of the bulk fluid is large, the liquid structure has a significant effect on these forces and differences from the results obtained using Debye-Hückel theory are apparent. It is suggested that these results have implications for the theory of colloidal stability.