Re-entrant phase transitions and dynamics of a nanoconfined ionic liquid

TL;DR

This study uses molecular dynamics simulations to explore how nanoscale confinement affects the phase behavior and dynamics of ionic liquids, revealing reentrant phase transitions and decoupled ion dynamics relevant for energy applications.

Contribution

It provides a detailed theoretical and numerical analysis of the complex phase and dynamic behavior of confined ionic liquids, highlighting reentrant transitions and ion decoupling.

Findings

Identification of reentrant phase transitions under confinement

Observation of decoupled counterion dynamics

Impact of confinement on ionic organization and transport

Abstract

Ionic liquids constrained at interfaces or restricted in subnanometric pores are increasingly employed in modern technologies, including energy applications. Understanding the details of their behavior in these conditions is therefore critical. By using molecular dynamics simulation, we clarify theoretically and numerically the effect of confinement at the nanoscale on the static and dynamic properties of an ionic liquid. In particular, we focus on the interplay among the size of the ions, the slit pore width, and the length scale associated to the long-range organization of polar and apolar domains present in the bulk material. By modulating both the temperature and the extent of the confinement, we demonstrate the existence of a complex reentrant phase behavior, including isotropic liquid and liquid-crystal-like phases with different symmetries. We show how these changes impact the relative organization of the ions, with substantial modifications of the Coulombic ordering, and their dynamical state. In this respect, we reveal a remarkable decoupling of the dynamics of the counterions, pointing to very different roles played by these in charge transport under confinement. We finally discuss our findings in connection with very recent experimental and theoretical work.