Ionic fluids withr−6pair interactions have power-law electrostatic screening

Abstract

The decay behaviour of radial distribution functions for large distances r is investigated for classical Coulomb fluids where the ions interact with an r−6 potential (e.g. a dispersion interaction) in addition to the Coulombic and the short-range repulsive potentials (e.g. a hard core). The pair distributions and the density–density (NN), charge–density (QN) and charge–charge (QQ) correlation functions are investigated analytically and by Monte Carlo simulations. It is found that the NN correlation function ultimately decays like r−6 for large r, just as it does for fluids of electroneutral particles interacting with an r−6 potential. The prefactor is proportional to the squared compressibility in both cases. The QN correlations decay in general like r−8 and the QQ correlations like r−10 in the ionic fluid. The average charge density around an ion decays generally like r−8 and the average electrostatic potential like r−6. This behaviour is in stark contrast to the decay behaviour for classical Coulomb fluids in the absence of the r−6 potential, where all these functions decay exponentially for large r. The power-law decays are, however, the same as for quantum Coulomb fluids. This indicates that the inclusion of the dispersion interaction as an effective r−6 interaction potential in classical systems yields the same decay behaviour for the pair correlations as in quantum ionic systems. An exceptional case is the completely symmetric binary electrolyte for which only the NN correlation has a power-law decay but not the QQ correlations. These features are shown by an analysis of the bridge function.