  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.
