Surface tension and nucleation rate of phases of a charged colloidal suspension

The square gradient approximation is used to calculate the surface tension between two phases of differing density in a charged colloidal suspension, and the results are compared with experimental and theoretical evidence from various colloidal systems. The nucleation rate of a colloidal liquid cluster from a metastable colloidal gas is estimated using a version of classical nucleation theory. We explain in terms of nucleation phenomena the recently described "Swiss cheese effect," which involves the formation of crystals from an initial disordered state, followed by the formation of disordered regions within the crystals and at the interfaces between them. We argue that this sequence of events shows evidence both of homogeneous and of heterogeneous nucleation. The experimental prominence of homogeneous nucleation suggests that metastability is very important in colloidal systems, and therefore that the consideration of nucleation rates is essential to the study of phase behavior in such systems. We also predict that the occurrence or nonoccurrence of phase separation into a dense and a rarefied phase is governed by the ratio of the macroion charge to the macroion radius.