ION-ION CORRELATIONS IN QUENCHED DISORDERED MEDIA

Monte Carlo simulation based on Widom's test particle method is used t o 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 preque nching temperature T-q and permittivity epsilon(q) different from the corresponding values at the conditions of observation. The correlation s among solute ions display qualitative differences from those observe d in annealed systems. The ions of equal sign are attracted to domains of the disordered material characterized by charge opposite to that o f the ions. The attraction of ions of the same sign to the same domain s results in an apparent attractive contribution to the disorder-avera ged interionic potential. At distances sufficiently exceeding the scre ening length pertaining to the ionized obstacles at the prequenching c onditions, the disorder-induced term prevails over the direct Coulombi c repulsion and a net attraction is observed. A similar mechanism lead s to a long-ranged repulsion between oppositely charged ions. These fi ndings are in agreement with earlier calculations of the disorder-aver aged ion-ion potentials based on the asymptotic Debye-Huckel descripti on 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 dec rease with the product of the prequenching temperature and permittivit y, a phenomenon explained in terms of the increase of the potential fl uctuations in the disordered medium with growing epsilon(q)T(q) In spi te of its global electroneutrality, the quenched medium displays a str ong selectivity with respect to the valency of the solute, the tendenc y towards absorption increasing with the charge of the ions. The selec tivity rapidly increases with increasing value of the characteristic p roduct of the prequenching temperature and the permittivity epsilon(q) T(q). (C) 1996 American Institute of Physics.