CRITICALITY AND PHASE-TRANSITIONS IN IONIC FLUIDS

Recent experimental investigations of criticality and phase separation in ionic fluids have revealed behavior of great theoretical interest. In seeking to understand the experiments, some of which appear to exh ibit argonlike criticality and some of which exhibit ''classical'' (me an-field) criticality, a convenient starting point is the restricted p rimitive model (RPM) of symmetrically charged hard spheres, all of equ al diameter sigma, each sphere bearing a positive or negative charge o f magnitude q. There is overall charge neutrality, so that the expecte d number densities of the anions and cations are equal, rho(+) = rho(- ). Studies of RPM charge-charge and density-density correlation functi ons indicate that the fluctuation-suppressing mechanism that yields me an-field critical behavior in nonionic systems with long-range interpa rticle potentials is not operative in the RPM. On the basis of plausib le assumptions, Ising-like behavior is instead expected. The above wor k is summarized. New work of Zhang and the author is outlined, showing that when one loses the RPM symmetry (through, e.g., different valenc e, diameter, or dipole moment of anions and cations) a strong coupling between charge-charge and density-density correlation ensues. The way in which this can be expected to give rise to mean-field or mean-fiel d-like behavior is noted. Other new observations concern the mean-fiel d analogy found by Hoye and the author between the parameter 2/(d-2) ( d is the dimensionality) in that model and the monomer number in high polymers, with respect to the coexistence-curve shape dependence on th ose parameters.