Abstract
We present a Molecular Dynamics study of mixtures of charged Gay-Berne (GB) ellipsoids and spherical Lennard-Jones (LJ) particles as models of ionic liquids and ionic liquid crystals. The GB system is highly anisotropic (GB(4.4,20.0,1,1)) and we observe a rich mesomorphism, with ionic nematic and smectic phases in addition to the isotropic mixed phase and crystalline phases with honeycomb structure. The systems have been investigated by analyzing the orientational and translational order parameters, as well as radial distribution functions. We have directly compared 1:1 mixtures, where the GB and LJ particles have a charge equal in magnitude and opposite in sign, and 1:2 mixtures where the number of LJ particles is twice as large compared to the GB and their charge half in magnitude. The results highlight the role of the long-range isotropic electrostatic interaction compared to the short-range van der Waals anisotropic contribution, and the effect of the stoichiometry on the stability of ionic mesophases.
Highlights
Ionic liquid crystals (ILC) are interesting materials composed of ions, as are their analogues ionic liquids (IL), and exhibiting thermotropic mesomorphism as non-ionic liquid crystals (LC)
We note that a general effect of increasing the charge, for a given packing fraction, is to increase the stability of the isotropic mixed phase, and to destabilize the ionic liquid mesophases. This result has been experimentally confirmed recently by Laschat and co-workers [54]: the authors investigated a series of amino acid/crown ether ionic LCs and found that increasing the charge is detrimental for the stability of ionic LC phases. In this manuscript we have presented a direct comparison of 1:1 and 1:2 mixtures of charged GB and LJ particles as models of ionic liquids and ionic liquid crystals
Despite the simplicity of the model, we observe a rich polymorphism with isotropic, nematic, smectic and crystal phases of ionic type. Both increasing the charge above a certain limit, as well as increasing the ratio of LJ over GB particles, leads to a destabilization of the ionic mesophases
Summary
Ionic liquid crystals (ILC) are interesting materials composed of ions, as are their analogues ionic liquids (IL), and exhibiting thermotropic mesomorphism as non-ionic liquid crystals (LC).A comprehensive review on this subject has been published [1] where the synthesis and technological applications of ILCs have been thoroughly discussed. Very few examples of an ionic nematic phase have been reported in the literature [14,15,16] The reason of this behavior can be traced back to the structure of the cations: this is usually formed by a charged moiety and a relatively long alkyl chain and these two parts tend to micro-segregate, promoting the formation of layered, that is smectic, phases instead of isotropic phases. Such micro-segregation is well-known to affect the structure of the isotropic phase of ILs [17]: Systems with relatively short alkyl chains do not form ILC phases, a non-homogeneous structure of the isotropic phase of the IL has been observed first from MD simulations [18,19,20] and later confirmed experimentally [21]
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