Ion association and hydrogen bonding in potassium dihydrogen phosphate solutions: Insights from molecular dynamics simulations

Potassium dihydrogen phosphate (KDP) is a critical material in non-linear optics, with significant applications in electro-optical and laser technologies. Despite its importance, the solution properties of KDP remain poorly understood, and to the best of our knowledge, no prior molecular dynamics (MD) simulation studies have directly probed the structure and behavior of KDP in aqueous solutions. This study presents results from MD simulations of KDP in both solution and solid states and compares four dihydrogen phosphate (DP) force-field models in five different water models. Our results reveal that the solution structure is strongly dominated by the association of the DP anions through direct hydrogen bonding, where the degree of association exhibits a marked concentration dependence in accord with the experiment. Of the four DP models evaluated, two are much better able to reproduce experimental data, including from neutron scattering and ab initio MD simulation results, demonstrating their effectiveness in capturing the hydrogen bonding patterns that appear to govern local solution structure. We find that the extent of hydrogen bonding between DP anions is also sensitive to the choice of water models, with stronger hydration reducing DP-DP association. We have also examined the two models for both tetragonal and monoclinic crystal structures of KDP and found that these models are able to reproduce the experimental parameters relatively well. The findings of this study enhance our understanding of KDP solutions and lay the groundwork for future investigations into its solution and solid-state properties and behavior.