Effect of colloid surface charge on the phase diagram of colloïdal suspensions for different thermodynamic conditions

Abstract

Abstract The paper is an extensive calculation to understand, the effect of the surface charge of colloids on the phase diagram of colloidal suspensions, under different conditions of colloid packing fraction, screening parameter of a salt reservoir, and zeta potential, using the Poisson-Boltzmann theory. Two models, Constant Surface Potential (CSP) and Charge Regulation (CR) are used to calculate the surface charge of colloids. Our findings suggest that both models give a closed description of the colloid's charge behavior, except in dense systems where the CR model predicted a slightly higher surface charge compared to the CSP model. Additionally, for fixed screening parameter (salt concentration), the colloid surface charge exhibits a density-dependent reduction when increasing the packing fraction. This density-dependence appears earlier for low screening parameter. A phase diagram is constructed using a mapping of colloidal suspension to a corresponding point Yukawa potential. It seems that, depending on the zeta potential values, electrostatic repulsion causes crystallization structures in a limited range of intermediate values of the screening salt reservoir. Increasing zeta potential lead to moving the crystal structures to a low packing fraction, which expands the stable FCC solid region. The study observes re-entrant fluid states related to the reduction in colloid surface charge when the salt reservoir screening parameter is decreased and colloidal density is increased.