Reentrant phase transition in charged colloidal suspensions.

We report the observation of a novel phase transition in dilute aqueous suspensions of polystyrene particles as a function of ionic impurity concentration C. The suspension phase separates into dense and rare phases only for a restricted range of C which depends on particle concentration n. The dense phase has liquidlike or crystalline order depending on n and C. Free energies of the homogeneous and the phase-separated states are calculated with a n effective interparticle potential. The calculated phase diagram is i n qualitative agreement with the present experimental results.