Potential of mean force between charged colloids: Effect of dielectric discontinuities

The potential of mean force between two spherical and like-charged macroions in a salt-free aqueous solution has been determined using an extended primitive model and canonical Monte Carlo simulations. The systems considered covered the range from a purely repulsive to a purely attractive potential of mean force as the electrostatic coupling was increased. The macroions were modeled as spherical dielectric cavities, and the polarization surface charge densities occurring at the dielectric discontinuities were expanded in spherical harmonics. The surface polarization gave rise to (i) an attenuation of the counterion accumulation at the macroion surfaces at all cases considered, (ii) an enhanced repulsive potential of mean force in the weak to intermediate electrostatic coupling regime, and (iii) a less attractive at short separation and an enhanced attractive potential of mean force at longer macroion-macroion separation in the strong electrostatic coupling regime.