Phase separation in mixtures of two sizes of silica particles dispersed in DMF on the addition of polystyrene

Phase separation, induced by attractive depletion interactions, in mixtures of two sizes of charged silica particles (30 and 200 nm diameter), dispersed in dimethylformamide (DMF), upon the addition of a non-adsorbing, monodispersed polymer (polystyrene) has been studied, for two polymer molecular weights (number-average 115,000 and 543,000). The repulsive electrostatic interaction between the silica particles was reduced through addition of 40 mM lithium chloride. This paper is a follow-up to a recent short communication by the same authors (J. Zhou, J. S. van Duijneveldt and B. Vincent, Phys. Chem. Chem. Phys. 13, 110-113 (2010)), for the same silica particles, plus the lower molecular weight polymer, in DMF. In that paper it was reported that, for certain concentration ranges of the ternary system, three co-existing phases are formed at equilibrium, and in others two. In this work it is shown that, despite the well-known importance of the polymer/colloid size ratio on the phase behaviour of their mixtures, using the higher molecular weight polymer leads to broadly similar behaviour, although the phase boundaries are shifted slightly. The main new finding is that of distinct regions in the phase separation process for systems resulting in two co-existing equilibrium phases (fluid + solid). On increase of polymer concentration, initially only the large particles aggregate. At higher polymer concentration, mixed aggregates are formed consisting of both large and small particles. While this finding can be rationalized in terms of relative strengths of depletion interactions, such scenarios have not been predicted theoretically to the authors' knowledge.