Acceleration of diffusion in ethylammonium nitrate ionic liquid confined between parallel glass plates

TL;DR

This study investigates how confinement between glass plates accelerates ionic diffusion in ethylammonium nitrate, revealing increased diffusion coefficients and relaxation rates, and suggesting a preserved sponge-like structure with enlarged channels.

Contribution

It demonstrates that confinement enhances ionic diffusion and relaxation in EAN without inducing ordering, proposing a model of an expanded sponge-like structure in confined conditions.

Findings

Diffusion of ions increases by 1.7 times in bulk.

Effective diffusion coefficient of EA cation increases by 1.86 times under confinement.

NMR relaxation of NH3 protons is 22 times faster in confinement.

Abstract

The bulk self diffusion of the ethylammonium cation and the nitrate anion can both be described by respective single diffusion coefficients. Diffusion of the anion is 1.7 times higher than that of the cation. This indicates no tight association of the ions in the ionic liquid. For the ethylammonium cation confined between glass plates the effective diffusion coefficient along, as well as normal to the confining glass plates is higher by a factor of 1.86 as compared to that in the bulk. The same time, NMR T2 relaxation of protons of NH3 group of the EA cation is faster by a factor of 22 than that in bulk. 2H NMR spectra of selectively labeled CH2 and CH3 groups of EA do not demonstrate any ordering of the EA between the glass plates. We suggested that these data favor a model where a bulk isotropic sponge like structure of EAN is saved in the confinement, but sizes of connecting channels increases. Those leads to faster translational diffusion and faster exchange processes of protons of NH3 group, in comparison with the bulk.