Melting temperature of colloidal crystals of polystyrene spheres

The melting temperature (Tm) of colloidal crystals of polystyrene spheres has been measured by the reflection spectroscopy as a function of sphere concentration. Seven kinds of spheres their diameters ranging from 85 to 212 nm, and their monodispersity indices (standard deviation of spheres divided by the mean diameter) between 0.014 and 0.26, are used after further purification and deionization processes. A phase diagram, including liquid-like and crystal-like (body-centered-cubic and face-centered-cubic lattices) structures, is obtained as a function of Tm against sphere concentration. The data of Tm are analyzed successfully with the theory of Williams, Crandall, and Wojtowicz, and the heat and entropy of melting are estimated. Tm values are consistent with the theory of Robbins and co-workers which treats the repulsive Yukawa potential between colloidal spheres.