Structural and Dynamical Properties of Tetraalkylammonium Bromide Aqueous Solutions: A Molecular Dynamics Simulation Study Using a Polarizable Force Field.

Understanding the behavior of aqueous solutions containing tetraalkylammonium (TAA) cations is of great significance in a number of applications, including polymer membranes for fuel cells. In this work, a polarizable force field has been used to perform atomistic molecular dynamics (MD) simulations of aqueous solutions containing tetramethylammonium (TMA) or tetrabutylammonium (TBA) cations and Br counterions. Extensive MD simulations of TMA-Br/water and TBA-Br/water systems were conducted as a function of solution composition (ion pair:water molar ratios of 1:10, 1:20, 1:30, 1:63, and 1:500) at atmospheric pressure and 298 K. Our simulations demonstrate excellent agreement with available experimental data for solution densities and diffusion coefficients of different species as a function of solution composition, providing us confidence in analyzed structural and dynamic correlations. Various ion-ion and ion-water spatial distributions and the extent of cation aggregation are discussed in light of changes in the structure of cations hydration shells. The delicate balance between cation ionic core interactions with water and the hydrophobic interactions of alkyl tails leads to nontrivial self-assembly of TAA cations and the formation of an interpenetrating cationic network at higher concentrations. The ions and water dynamics are strongly coupled with the observed structural correlations and are analyzed in terms of various residence time, diffusion coefficients, and ionic conductivity.