The phase diagram of charged colloidal suspensions

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 fee 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.