The phase diagram of charged colloidal suspensions

Abstract

The thermodynamics of a system of identical electrically charged spheres in colloidal suspension in a polar fluid (typically, H₂O), are calculated within a model of point particles interacting via a screened Coulomb (Yukawa) potential of suitably renormalized strength. The particles are taken to be sufficiently massive that a classical theory is appropriate. The free energies of the crystalline phases are calculated within a mean field theory which is an extension of the self‐consistent harmonic approximation. The melting curve is estimated from the semiempirical Lindemann rule, with mean squared displacements again calculated self‐consistently. As functions of the relevant parameters: electrical charge of each sphere, density of spheres and added salt concentration, stable predicted phases include fcc and bcc crystals and a disordered fluid. Under suitable circumstances a reentrant bcc→fcc→bcc transition is predicted as salt is added, and bcc is predicted always likely to be the stable phase just below the melting transition.