The screened Coulomb (Yukawa) charged hard sphere binary fluid

Abstract

The screened Coulomb (Yukawa) restricted primitive model (YRPM) of a binary 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. Screening increases the critical density and temperature above the values for the RPM and decreases the width of the liquid–gas coexistence curves. 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) correlations crosses over from monotonic to oscillatory. The location of the disorder lines is strongly dependent on the range of the Yukawa potentials. We show that very shortrange interionic potentials (Yukawa models) can give rise to correlation functions which are difficult to distinguish (at all interionic separations) from those obtained for the long-range Coulombic case for structurally equivalent thermodynamic states, reinforcing the idea that pronounced charge ordering is not unique to Coulombic interactions but can occur whenever the interaction potential between different species (AB) is strongly attractive and the potentials between identical species (AA and BB) are repulsive. Although 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 contract when the density is high and the temperature is low. This failing, which occurs for both the RPM and YRPM, reflects the sensitivity of g_ii (r) near contact to approximations.