Development of Water Reactive Potentials for Molecular Dynamics Simulations of Sodium Phosphate Glasses

The diffuse charge reactive potential (DCRP) developed in the recent past has been used successfully in reactive Molecular Dynamics (MD) simulations of bulk and surface glass structures, interfacial reactions, ion dissolution and transport in silicate glasses under aqueous conditions. In the present work, DCRP is parametrized to investigate sodium phosphate glasses interacting with water. The parametrization process was done with the aim of reproducing the correct density and structural features of sodium phosphate glasses and phosphoric acids. One of the important structural features in phosphate glasses is the PO₄ unit that contains at least one double bonded terminal oxygen, which is indicated by a distinct double peak in the P–O pair distribution function, in most of ultraphosphate compositions. The relative proportion of the bridging oxygen (BO) and nonbridging oxygen (NBO) varies with the alkali content and has implications in reactivity of the glass with water. The parameters optimized in this work reproduce the density and structural features of ultraphosphate glasses and phosphoric acid with reasonable accuracy. The reactivity of the glass was tested by calculation of the reaction barrier of the hydrolysis reaction of pyrophosphoric acid and exposing phosphate glass surface to water and allowing equilibration and reaction for 4 ns. Formation of P–O–H linkages and sodium dissolution in water indicate that DCRP shows the appropriate reactions for phosphate glasses with water.