Heating-induced freezing and melting transitions in charged colloids.

We examine influence of temperature on the phase behavior of dilute aqueous dispersions of charged colloidal silica and polystyrene particles. They undergo either freezing or melting transitions with increasing temperature. Freezing occurs in the case of low-charge, low-salt colloids, and melting is observed in the case of high-charge, high-salt colloids. All of these phase transitions are thermoreversible. These intriguing behaviors can be qualitatively explained in terms of the decrease in the permittivity of water at elevated temperatures.