Phase behavior of a model of colloidal particles with a fluctuating internal state

Colloidal particles are not simple rigid particles; in general an isolated particle is a system with many degrees of freedom in its own right, e.g., the counterions around a charged colloidal particle. The behavior of model colloidal particles, with a simple phenomenological model to account for these degrees of freedom, is studied. It is found that the interaction between the particles is not pairwise additive. It is even possible that the interaction between a triplet of particles is attractive while the pair interaction is repulsive. When this is so the liquid phase is either stable only in a small region of the phase diagram or absent altogether.