Physical adsorption of hydroxide ion at mineral–water interface

By using first-principles simulations, we recently identified an interfacial charging phenomenon on oxide minerals via physical hydroxide ion adsorption that is distinctly different from the traditional chemical bonding of protons to surface hydroxyl groups. In this study, to deepen understanding of this adsorption phenomenon, the basal surfaces of kaolinite (the gibbsite plane) and brucite were selected as model surfaces of aluminosilicates and divalent metal hydroxide, respectively. The pKa values of surface groups and orientational dynamics of interfacial water were determined through first-principles molecular dynamics simulations and the adsorption free energies of OH– were obtained via reactive molecular dynamics technique. The new results confirm that the microscopic mechanism underlying physical OH– adsorption originates from the slow reorientation dynamics of interfacial water through which OH– ions are effectively trapped. Based on the so far available results, it appears that trivalent metal (hydr)oxides are the minerals on which hydroxide ion can physically adsorb most favorably, implying that minerals with trivalent metal (hydr)oxide structure may also exhibit this special adsorptive capability. ... mehrWe discuss the more general implications of this interfacial charging mechanism.