Insights into the transport of aqueous quaternary ammonium cations: A combined experimental and computational study

This study focuses on understanding the relative effects of ammonium substituent groups (we primarily consider tetramethylammonium, benzyltrimethylammonium, and tetraethylammonium cations) and anion species (OH^-, HCO₃^-, CO₃^2-, Cl^-, and F^-) on ion transport by combining experimental and computational approaches. We characterize transport experimentally using ionic conductivity and self-diffusion coefficients measured from NMR. These experimental results are interpreted using simulation methods to describe the transport of these cations and anions considering the effects of the counterion. It is particularly noteworthy that we directly probe cation and anion diffusion with pulsed gradient stimulated echo NMR and molecular dynamics simulations, corroborating these methods and providing a direct link between atomic-resolution simulations and macroscale experiments. By pairing diffusion measurements and simulations with residence times, we were able to understand the interplay between short-time and long-time dynamics with ionic conductivity. With experiment, we determined that solutions of benzyltrimethylammonium hydroxide have the highest ionic conductivity (0.26 S/cm at 65 C), which appears to be due to differences for the ions in long-time diffusion and short-time water caging. We also examined the effect of CO₂ on ionic conductivity in ammonium hydroxide solutions. CO₂ readily reacts with OH^- to form HCO^-₃ and is found to lower the solution ionic conductivity by almost 50%.