Reactive Monte Carlo Simulations for Charge Regulation

We present a reactive Monte Carlo simulation method to study acid-base equilibrium that controls charge regulation in colloidal systems. The simulations use a semi-grand canonical ensemble in which colloidal suspension is in contact with a reservoir of salt and strong acid. The surface reactions are controlled by the equilibrium constants, which correspond to either the internal partition function of a protonated acid surface group or a group with a specifically adsorbed cation. The interior of colloidal particles is modeled as a low dielectric medium, different from the surrounding water. The effective colloidal charge is calculated for different number of surface acidic groups, pH, salt concentrations, and types of electrolyte. The results are compared with the experimental measurements of the effective colloidal charge obtained using potentiometric titration. Excellent agreement is found between simulations and experiments.