Polydispersity in Colloidal Phase Transitions

I have studied the effects of polydispersity on the phase behaviour of suspensions of PMMA colloidal spheres on their own and in the presence of non-adsorbed polymer. I systematically explored the volume fraction–polydispersity (φ − σ) phase behaviour of hard spheres (with radii R =167, 244, 300 and 303nm) through direct observations and crystallography measurements. I observed normal crystallisation for σ < ∼ 7.5%, and no crystals at σ > ∼ 18%. Samples at σ ≈ 9.5% showed crystal-fluid coexistence between 0.52 < ∼ φ < ∼ 0.56 but no fully crystalline behaviour above this region. This may be explained by slow particle diffusion in the dense metastable fluid and a glass transition, possibly involving only the larger particles. The addition of random coil polymer (radius of gyration rg) to a suspension of single-sized spherical colloidal particles induces an attractive depletion potential which, for size ratios ξ = rg/R < ∼ 0.2, has the effect of expanding the crystal-fluid coexistence region. Surprisingly, when such a polymer solution (with ξ = 0.1), with a range of concentrations cp, is added to a polydisperse colloidal suspension (σ ≈ 10%), crystal formation is actually suppressed. This can be explained by the fact that the polymer compresses the nascent crystal phase to volume fractions greater than the maximum φ permitted for polydisperse spheres. By modifying existing free energy equations to include the effects of colloidal polydispersity we also succeed in reproducing the observed phase diagram. Larger added polymer (ξ > ∼ 0.3) introduces a region of stable gas-liquid coexistence. In systems where crystallisation is suppressed due to polydispersity, this will theoretically be the only transition. By preparing many samples over a range of φ and cp this prediction was observed experimentally for ξ ≈ 0.5. Fractionation studies on coexisting phases enabled verification of a recent universal law of fractionation in slightly polydisperse systems.