The interaction between macroparticles in molecular fluids

Potentials of mean force for pairs of charged and neutral macroparticles immersed in simple dipolar hard sphere and waterlike solvents are obtained using the reference hypernetted-chain theory. In general, the results depend rather strongly upon both the solvent and the size of the macroparticles. For smaller neutral macroparticles, the potential of mean force in both solvents is essentially a short-ranged hydrophobic interaction with very little structure. For larger neutral particles in water, short-ranged structural effects likely due to solvent ordering near the flatter surfaces are evident. For charged macroparticles, the potential of mean force is found to be essentially a superposition of three distinct contributions: a short-ranged structural contribution which clearly depends upon the granular nature of the solvent; a repulsive ‘‘cavity’’ term which decays asymptotically as r−4; and of course there is the long-ranged Coulombic interaction. At intermediate separations, this repulsion is the principal correction to Coulomb’s law and it is shown to be much more important in molecular solvents than predicted by comparable dielectric continuum calculations. Furthermore, it is shown that this contribution can be represented with reasonable accuracy by simple analytical approximations.