Surface Charge Induced Reentrant Crystalline−Liquid Transition in Colloidal Systems: The Role of the Microion Disposition

Abstract

The liquid → crystalline → liquid transition as a function of surface charge density (σ) for colloidal dispersions reported by Yamanaka, Yoshida, Koga, Ise, and Hashimoto [Phys. Rev. Lett. 1998, 80, 5806] is examined in detail by Brownian dynamics simulations of the microion distribution. In the absence of added electrolyte, two different distribution functions are determined: Gpc(r) centered at the center of the computation cell and gpc(r) centered at a selected macroion. The “thermal radii” rGtherm and rgtherm are determined for the respective distribution functions at the value exp(1), i.e., when the total electrostatic energy of the system, 〈Esys〉, is equal to the thermal energy kBT. The methods of juxtaposition of potential fields [Langmuir 1997, 13, 5849] and Brownian dynamics simulations are used to describe the disposition of the counterions, or an “orbital” picture of colloidal structures. It is suggested from this orbital model that the crystalline regime obtains when the thermal radii extend far from the parent macroions as this leads to maximum “sharing” of the counterions, and that the reentry transition occurs because of a drastic decrease in rgtherm with high surface charge densities.