Attraction or repulsion between charged colloids? A connection with Debye-Huckel theory

We discuss the phase behaviour of suspensions of charged colloidal particle s, specifically at low salt concentrations. The total free energy F of a th ree-component system of charged colloids and positive and negative salt ion s can be expressed as F = F-DLVO + F-0. where F-DLVO is the free energy of a one-component System of colloidal particles interacting via the DLVO pair potential, and F-0 consists of density-dependent contributions from the sa lt ions. We argue that F-0 drives a gas-liquid spinodal at low salt concent rations, in accordance with several experimental observations. The underlyi ng mechanism is equivalent to that of the well-established gas-liquid trans ition in simple electrolytes. The present theory connects directly the clas sic linearized Poisson-Boltzmann theories for simple electrolytes (Debye-Hu ckel) and colloidal suspensions (DLVO).