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 that 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 mesogens (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 hexagonal crystal phases. 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.
Highlights
Ionic liquid crystals (ILCs) are versatile materials that exhibit, on the one hand, properties of ionic systems, such as the capability of charge transport, and on the other hand, they are able to form mesophases, which is the distinctive feature of liquid crystals
After having analyzed the differences of the smectic structures associated with the charge distributions D = 0 and D/R⊕ = 1.8, we focus on the phase behavior, i.e., identifying those regions in the (T∗, η) plane, which corresponds to the aforementioned distinct structures
Our analysis aims at investigating how the phase behavior and the structural properties of an effective onespecies model, which has been employed in previous theoretical studies of ionic liquid crystals (ILCs),3,5,6 are affected by taking the counterions explicitly into account
Summary
Ionic liquid crystals (ILCs) are versatile materials that exhibit, on the one hand, properties of ionic systems, such as the capability of charge transport, and on the other hand, they are able to form mesophases, which is the distinctive feature of liquid crystals.. Several numerical studies aimed at gaining insight into the link between the underlying molecular features of ILCs and their resulting properties, such as the phase behavior or the formation of nanostructures. Molecular dynamics (MD) computer simulations can be performed with a rather detailed description of the underlying molecules.. Molecular dynamics (MD) computer simulations can be performed with a rather detailed description of the underlying molecules.7 Such simulations can provide a rich and detailed picture of the structures of the formed mesophases. The complexity of the underlying models makes it still difficult to pinpoint basic characteristics of these systems, which give rise to the observed properties (or at least being essential for their appearance)
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