Structure and Diffusivity of Oxygen in Concentrated Alkali-Metal Hydroxide Solutions: A Molecular Dynamics Simulation Study

Molecular dynamics simulations of oxygen molecules in NaOH and KOH solutions at different temperatures (25-120 degrees C) and concentrations (1:100-1:5, molar ratios) were performed in this study. The interactions of oxygen molecules with the surrounding solvent and solute were clarified by considering the solvent-solvent, oxygen-solvent, and oxygen-solute radial distribution functions. The self-diffusion coefficients of the oxygen molecules and the solute were both determined by analyzing the mean-squared displacement (MSD) curves, using Einstein's relationship. It was concluded that at all concentrations, the diffusion coefficient of oxygen in NaOH solution is smaller than that in the corresponding KOH solution. The diffusion coefficients for hydroxide, Na+, and K+ decrease with increasing solute concentration, following similar trends to those of oxygen. The oxygen diffusion coefficient obtained in this study is in good agreement with the reported experimental value, suggesting that MSD is an attractive approach to study the oxygen diffusion behavior in strong alkaline solutions at elevated temperatures, which are experimentally extremely challenging.