Insights into the Transport of Aqueous Quaternary Ammonium Cations: A Combined Experimental and Computational Study

Abstract

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%.

Tetraalkylammonium Cations Conduction through a Single Nanofluidic Diode: Experimental and Theoretical Studies

“…Note also that the cation hydrophobicity increases with the alkyl chain, thus decreasing the ionic conductivity. This experimental trend has also been found experimentally and theoretically in free aqueous solutions [42] Fig. 4(A).…”

Unexpected hydroxide ion structure and properties at low hydration

“…However, as we will argue below, vehicular diffusion is likely to dominate at low hydration. 16,17 .…”

Atomistic simulations of ammonium-based protic ionic liquids: steric effects on structure, low frequency vibrational modes and electrical conductivity

“…[1][2][3][4][5][6][7][8] Improvement in thermal and electrochemical properties of fuel cells through the use of cost-effective ammonium based protic and aprotic ILs as electrolytes has spurred interest in them. 7,[9][10][11][12][13][14][15] Nakamoto and Watanabe synthesized a series of protic ILs (PILs) from Brønsted acid-base combination of aliphatic amines with oxoacids. They obtained remarkable electrolytic properties under anhydrous conditions for diethylmethylamine trifluoromethanesulfonic acid ([dema][TfO]) PIL.…”