Molecular dynamics simulation of barite and celestite ion-pairs

Abstract

The presence of ion-pairs in electrolyte solutions affects the activity of dissolved species as well as the solubility of minerals. The extent of ion-pairing in a system is predicted by an association constant, K[subscript A], which for sparingly soluble salts are frequently determined experimentally in binary or ternary systems. This introduces complex activity coefficient calculations that often require unavailable parameters. Barite (BaSO₄) and celestite (SrSO₄) are sparingly soluble minerals with interest in the oil and mining industry, yet the values of K[subscript A] for the ion-pairs BaSO₄(aq.) and SrSO₄(aq.) are still uncertain. Molecular dynamics simulations are used to obtain the K[subscript A] values for these two salts through potential of mean force (PMF) calculations. The molecular mechanisms involved in the association reactions are also explored, in particular the role of the association intermediates in the overall reaction as described by the Eigen mechanism. Additionally, the kinetics of water exchange around the free and paired ions is examined and the residence time of a water coordinated to the free and paired cation is calculated.