DIELECTRIC-CONSTANT EFFECTS ON THE ENERGETICS OF OPPOSITELY CHARGED COLLOIDS - A LATTICE FIELD-THEORY STUDY

A recently developed lattice field theory formulation of the statistic al mechanics of a classical Coulomb gas interacting with a fixed charg e distribution [R. D. Coalson and A. Duncan, J. Chem. Phys. 97, 5653 ( 1992)] is extended to deal with systems having a spatially varying die lectric constant. To illustrate the utility of the formalism, electros tatic free energies of two oppositely charged macroions suspended in i onic solution are calculated for several geometric configurations. In many cases, it is found that the free energy is minimized at a separat ion between macroions corresponding to a ''nontouching'' configuration . The value of the dielectric inside the macroions has an important ef fect on the free energy of near-touching configurations. In particular , lowering the dielectric constant inside the macroion relative to the solvent dielectric constant leads to higher free energies of near-tou ching configurations, and hence promotes a stable state where the macr oions are farther apart. Our findings also indicate that a large diffe rence between the magnitude of the charges on the positive and negativ e macroions enhances the stability of a ''nontouching'' configuration.