THE SCREENED COULOMB (YUKAWA) CHARGED HARD-SPHERE BINARY-FLUID

The screened Coulomb (Yukawa) restricted primitive model (YRPM) of a b inary fluid is treated within the one-pole generalized mean-spherical approximation (GMSA). Fluid phase behaviour and structure are compared with the corresponding results for the unscreened Coulombic RPM. Scre ening increases the critical density and temperature above the Values for the RPM and decreases the width of the liquid-gas coexistence curv es. The asymptotics of pairwise correlation functions and the genesis of disorder lines in the phase diagram are discussed. These lines are where the ultimate decay of total density or concentration (charge) co rrelations crosses over from monotonic to oscillatory. The location of the disorder lines is strongly dependent on the range of the Yukawa p otentials. We show that very short-range interionic potentials (Yukawa models) can give rise to correlation functions which are difficult to distinguish (at all interionic separations) from those obtained for t he long-range Coulombic case for structurally equivalent thermodynamic states, reinforcing the idea that pronounced charge ordering is not u nique to Coulombic interactions but can occur whenever the interaction potential between different species (AB) is strongly attractive and t he potentials between identical species (AA and BB) are repulsive. Alt hough the GMSA. generally improves upon the MSA, we show that it leads to artificial peaks (not found in simulation) for correlations g(ii)( r) between identical ionic species at separations r close to ionic con tract when the density is high and the temperature is low. This failin g, which occurs for both the RPM and YRPM, reflects the sensitivity of g(ii)(r) near contact to approximations.