Charge ordering and the structure of ionic liquids: Screened Coulomb versus Coulomb interionic potentials

The authors have calculated the radial distribution functions g _{alpha beta}(r) and the partial structure factors S _{alpha beta}(k) for two different charged hard-sphere models of a molten alkali halide. In the first model the interionic potentials are taken to be Coulombic outside the hard core; this is the conventional restricted primitive model. In the second the potentials are assumed to be screened Coulomb outside the core. Their calculations, which are based on the mean spherical approximation and on the hypernetted-chain approximation, demonstrate that very short-ranged screened potentials can produce g _{alpha beta}(r) and S _{alpha beta}(k) which are very close to those obtained for the conventional restricted primitive model of an alkali halide. In particular, the distribution functions for the screened Coulomb model exhibit 'charge ordering'; g ₊₊(r) oscillates out of phase with g _+-(r) and the first peak in S ₊₊(k) lines up with the first minimum in S _+-(k). 'Charge ordering' is not a unique feature of Coulombic interactions as is sometimes supposed, but will occur whenever the potential between unlike species is strongly attractive.