Nanoscale Carbon Greatly Enhances Mobility of a Highly Viscous Ionic Liquid

TL;DR

This study demonstrates that carbon nanotubes can dramatically increase the mobility of highly viscous ionic liquids, potentially improving energy storage technologies through nanoscale confinement effects.

Contribution

It reveals that ionic liquids exhibit up to a fivefold increase in diffusion within carbon nanotubes, a novel finding supported by computer simulations.

Findings

Ionic diffusion in CNTs can increase fivefold.

Confined ILs retain ionic phase with enhanced mobility.

Simulation explains the energetic basis of increased diffusion.

Abstract

Ability to encapsulate molecules is one of the outstanding features of nanotubes. The encapsulation alters physical and chemical properties of both nanotubes and guest species. The latter normally form a separate phase, exhibiting drastically different behavior compared to bulk. Ionic liquids (ILs) and apolar carbon nanotubes (CNTs) are disparate objects; nevertheless, their interaction leads to spontaneous CNT filling with ILs. Moreover, ionic diffusion of highly viscous ILs can increase 5-fold inside CNTs, approaching that of molecular liquids, even though the confined IL phase still contains exclusively ions. We exemplify these unusual effects by computer simulation on a highly hydrophilic, electrostatically structured, and immobile 1-ethyl-3-methylimidazolium chloride, [C2C1IM][Cl]. Self-diffusion constants and energetic properties provide microscopic interpretation of the observed phenomena. Governed by internal energy and entropy rather than external work, the kinetics of CNT filling is characterized in detail. The significant growth of the IL mobility induced by nanoscale carbon promises important advances in electricity storage devices.