Realistic calculation of the low- and high-density liquid phase separationin a charged colloidal dispersion - art. no. 041511

A realistic statistical-mechanics model is applied to describe the repulsiv e interaction between charged colloids. The latter, in combination with the long-range van der Waals attraction simulated under excess salt environmen t, gives rise to a total intercolloidal particle potential showing a clear second potential minimum. Differing from the usual Derjaguin-Landau-Verwey- Overbeek (DLVO) model, the present model is valid at any finite concentrati on of colloids and is thus an appropriate model for investigating the low- and high-density liquid phase transition. Employing this two-body colloid-c olloid potential and in conjunction with the Weeks-Chandler-Andersen [J. D. Weeks. D. Chandler, and H. C. Andersen, J. Chem. Phys. 54, 5237 (1971)] th ermodynamic perturbation theory, we derive analytical expressions for the p ressure, chemical potential, and related thermodynamic functions. These the rmodynamic quantities were used to calculate the phase diagrams of charged colloidal dispersions in terms of the critical parameters: temperature, vol ume fraction, and electrolyte concentration parameter k(D). Compared with t he DLVO model, we find the areas enclosed within the spinodal decomposition and also the liquid-liquid coexistence curves broader in the present model for an excess salt condition kappa =k(D)sigma (0)less than or similar to 2 00, sigma (0) being the macroion diameter, in addition to exhibiting a shif t in the critical point kappa (c) to lower values; for kappa >300, the disp arities between the two models reduce. The same thermodynamic perturbation theory has been employed to study also the weak reversible coagulation whos e physical origin is attributed to the presence of the second potential min imum. We examine various colloidal parameters that affect the structure of the latter and deduce from our analysis the conditions of colloidal stabili ty. In comparison with the measured flocculation data for a binary mixture of polystyrene lattices and water, we find that our calculated results are generally reasonable, thus lending great credence to the presently used mod el.