Ion-ion correlations in quenched disordered media

Monte Carlo simulation based on Widom's test particle method is used to study the behavior of dilute ionic solutes in a quenched disordered medium with ionized obstacles. The structure of the medium is assumed to correspond to that of an ionic fluid equilibrated at certain prequenching temperature T-q and permittivity epsilon(q) different from the corresponding values at the conditions of observation. The correlations among solute ions display qualitative differences from those observed in annealed systems. The ions of equal sign are attracted to domains of the disordered material characterized by charge opposite to that of the ions. The attraction of ions of the same sign to the same domains results in an apparent attractive contribution to the disorder-averaged interionic potential. At distances sufficiently exceeding the screening length pertaining to the ionized obstacles at the prequenching conditions, the disorder-induced term prevails over the direct Coulombic repulsion and a net attraction is observed. A similar mechanism leads to a long-ranged repulsion between oppositely charged ions. These findings are in agreement with earlier calculations of the disorder-averaged ion-ion potentials based on the asymptotic Debye-Huckel description of the disordered medium. The simulations are also used to estimate the effects of the medium on thermodynamic properties of the embedded ionic solute. The energies and activity coefficients are found to decrease with the product of the prequenching temperature and permittivity, a phenomenon explained in terms of the increase of the potential fluctuations in the disordered medium with growing epsilon(q)T(q) In spite of its global electroneutrality, the quenched medium displays a strong selectivity with respect to the valency of the solute, the tendency towards absorption increasing with the charge of the ions. The selectivity rapidly increases with increasing value of the characteristic product of the prequenching temperature and the permittivity epsilon(q)T(q). (C) 1996 American Institute of Physics.