Local Hydration Structures and Dynamic Properties of Tetra-Alkyl Ammonium Aqueous Solution Systems: An Insight from Molecular Dynamics Simulations

Understanding the behavior of aqueous solutions containing tetra-alkyl ammonium cations is of great importance in a number of applications, including polymer membranes for fuel cells. We have developed and validated new polarizable force field for atomistic molecular dynamics (MD) simulations of aqueous solutions containing tetra-methyl ammonium (TMA) and tetra-butyl ammonium (TBA) cations with OH^- or Br^- anions. The force field was parameterized against density functional theory calculations (M052X/aug-cc-pvDz) of binding energies and optimal geometries of different molecular clusters.  Extensive MD simulations of TMA/Br/water, TBA/Br/water and TMA/OH/water systems were conducted as function of solution composition (ion pairs-water molar ratios of 1:10, 1:20, 1:30, 1:63, 1:500). The comparison of ions local coordination shell and analysis of species self-diffusion coefficients showed a very good agreement with existing experimental data for Br anion-containing systems, especially at low ion concentrations, where previous simulations failed to reproduce consistent trends with experiments. The differences in self-diffusion coefficients of species between the experimental data and our simulations results are within 10%-15%.