Phase behavior of a mixture of platelike colloids and nonadsorbing polymer

Suspensions of sterically stabilized colloidal gibbsite platelets have recently been found to exhibit both an isotropic-nematic (I-N) and a nematic-columnar (N-C) phase transition. In the present paper we show that depletion attraction, brought about by the addition of nonadsorbing polymer, enriches the phase behavior of these platelet suspensions even further. Pronounced broadening of the isotropic-nematic gap occurs, with purely nematic samples re-entering the biphasic state by the addition of nonadsorbing polymer. At the same time, depletion attraction enhances size fractionation between coexisting phases, which actually provides an effective means for reducing the polydispersity of the suspensions. An additional isotropic phase emerges, which leads to the appearance of several three-phase equilibria and even a four-phase equilibrium. We explain the observed topology of the phase diagram by the interplay between depletion attraction and the platelets' polydispersity.