The experimental phase diagram of charged colloidal suspensions

We present a study of the order-disorder and BCC-FCC transitions of a model colloidal suspension of highly charged polystyrene spheres as a function of the particle volume fraction and the ionic strength. We analyze Kossel lines, produced by backscattered light diffraction, to identify the crystal structures and determine the interparticle spacings. Our results consist of phase diagrams showing that the BCC structure is stable at ionic strengths lower than 2.7 × 10−6 M KCl and volume fractions less than 0.008. We compare our experimentally determined phase diagrams with recent analytical models and with the molecular dynamics (MD) simulations of Kremer, Robbins, and Grest. The effective hard sphere prediction of the order-disorder boundary overestimates the phase transition at low ionic strengths and underestimates it at high ionic strengths for both the constant-charge and constant-potential approximations. Analytical models predicting the BCC-FCC transition boundary from either the internal energy or the free energy difference between the two states are in clear disagreement with both the MD results and our data. Using a renormalized charge, we find very good agreement between our experimental results and the MD predictions for both the solid-liquid and solid-solid transitions.