Towards the phase diagram of a polydisperse mixture of charged hard spheres

¹ Institute for Condensed Matter Physics - Svientsitskoho 1, 79011 Lviv, Ukraine
² Center for Computational Materials Science and Institut für Theoretische Physik TU Wien - Wiedner Hauptstraße 8-10, A-1040 Wien, Austria
³ Department of Chemical Engineering, Vanderbilt University Nashville, TN 37235-1604, USA
⁴ Chemical Sciences Division, Oak Ridge National Laboratory Oak Ridge, TN 27831-6110, USA

Received: 17 May 2005
Accepted: 29 July 2005

Abstract

In an effort to approach a quantitative determination of the phase diagram of polydisperse mixtures of (possibly size-asymmetric) charged hard spheres (CHS), we propose a concept that explicitly takes into account the association of the ions into dimers, i.e., an effect that is known to be of high relevance close to the liquid-vapor phase boundary in the restricted primitive model (RPM). We use the polymer mean spherical approximation (PMSA) to calculate the properties of this polydisperse mixture of dimers. The concept turns out to be a truncatable free energy model, thus the phase diagram can be calculated by solving a highly non-linear set of equations. We present the full phase diagram (in terms of cloud and shadow curves and binodals) and discuss fractionation effects for a model system.