The role of counterions in ionic liquid crystals

TL;DR

This study uses simulations to explore how counterions influence the structure of ionic liquid crystals, confirming the existence of a wide smectic-A phase with potential applications in ionic conductivity.

Contribution

It extends previous models by explicitly including counterions, validating the wide smectic-A phase formation through grand canonical Monte Carlo simulations.

Findings

Wide smectic-A phase forms with charged mesogens at low temperatures.

Counterions accumulate between smectic layers in the wide smectic-A phase.

Phase transition behavior aligns with earlier effective one-species models.

Abstract

Previous theoretical studies of calamitic (i.e., rod-like) ionic liquid crystals (ILCs) based on an effective one-species model led to indications of a novel smectic-A phase with a layer spacing being much larger than the length of the mesogenic (i.e., liquid-crystal forming) ions. In order to rule out the possibility that this wide smectic-A phase is merely an artifact caused by the one-species approximation, we investigate an extension which accounts explicitly for cations and anions in ILCs. Our present findings, obtained by grand canonical Monte Carlo simulations, show that the phase transitions between the isotropic and the smectic-A phases of the cation-anion system are in qualitative agreement with the effective one-species model used in the preceding studies. In particular, for ILCs with mesogenes (i.e., liquid-crystal forming species) carrying charged sites at their tips, the wide smectic-A phase forms, at low temperatures and within an intermediate density range, in between the isotropic and a hexagonal crystal phase. We find that in the ordinary smectic-A phase the spatial distribution of the counterions of the mesogens is approximately uniform, whereas in the wide smectic-A phase the small counterions accumulate in between the smectic layers. Due to this phenomenology the wide smectic-A phase could be interesting for applications which hinge on the presence of conductivity channels for mobile ions.