MODELING SI-O BOND HYDROLYSIS WITH REAXFF POTENTIAL MODELS

The most physically accurate models of the mineral-water interface are firmly rooted in the axioms of quantum mechanics, but have limited applicability at extended time- and length-scales. Classical molecular dynamics methods are more practical in these respects, but rely on simplified physics and empirical interaction potentials that require validation. In this study, molecular dynamics simulations of Si-O bond hydrolysis are performed with several reactive empirical potentials of the ReaxFF variety. For each model, the well-tempered metadynamics method is used to drive bond hydrolysis and to obtain the corresponding energy landscape underlying the predicted reaction mechanism. Two competing processes, water exchange and bond hydrolysis, are found to occur in each case, with the potential models differing primarily with respect to the relative favorability of one process over the other. For each potential, bond hydrolysis proceeds through the formation of a penta-coordinated intermediate; however, for those models that favor water exchange over hydrolysis, the intermediate is not a transition state, but rather, a fairly long-lived stable state. Where hydrolysis is favored over water exchange, the intermediate is a transition state, and the reaction mechanism is qualitatively consistent with the results of previous quantum mechanical investigations.