Surface charging behavior of nanoparticles by considering site distribution and density, dielectric constant and pH changes--a Monte Carlo approach.

Monte Carlo simulations are used to describe the charging behavior of metal oxide nanoparticles thus enabling a novel and original approach to predict nanoparticle reactivity and the possible interactions with biological and environmental molecules. The charging behavior of spherical nanoparticles is investigated by adjusting the pH of the media and the influence of surface site distribution, density and dielectric constant as well as the acid/base properties of the surface sites and ΔpKa(0) values (difference between two successive deprotonation constants) is systematically studied using a grand canonical Monte Carlo method. A primitive Coulomb model is applied to describe the interaction energies between the explicit discrete sites. Homogeneous/heterogeneous surfaces and patches with homogeneous and heterogeneous distributions are considered in order to reproduce possible site distributions of metal oxide nanoparticles. Two models are used. In the 1-pKa(0) model (one deprotonation step) the results indicate that the deprotonation process is controlled by inter-site distances which are defined by site distributions and densities. It is shown that the homogeneous surface is the most efficient site distribution to obtain high ionization degrees. In the 2-pKa(0) model (two deprotonation steps), the ΔpKa(0) value is found to control the surface charge properties with regard to pH changes. By considering the variation of the total nanoparticle surface charge as a function of pH our results help in the distinction between the zero charge and the isoelectric point and interpretation of experimental NP titration curves.