Smectic phases, liquid crystal ionic liquids

Prominent exceptions are studies on the liquid crystal phase formation and self-assembly of two-dimensional disc- or sheet-like nanomaterials such as the organization of nanodiscs or nanoplatelets into nematic, smectic, or columnar morphologies [263-270] (see Fig. 2 for an example of the self-assembly of nanoclay in aqueous suspensions) or the synthesis of CuCl nanoplatelets from ionic liquid crystal precursors as described by Taubert and co-workers [271-273]. 

Evans et al. also showed that the 1 1 mixture of BAN and (3, y-distearoyl-phos-photidylcholine (DSPC) gives a smectic A texture in the temperature range of 57.3 to 100°C [21]. This is the first notice of lyotropic lamellar liquid crystals formed in the ionic medium. Additionally, Seddon et al. [28] and Neve et al. [29] have described the long-chained A-alkylpyridinium or l-methyl-3-alkylimidazolium ions to display smectic liquid-crystalline phases above their melting points, when Cl or tetrachloro-metal anions like CoCl " and CuCl " are used as the counter ions. Lin et al. have also noted the liquid crystal behavior of 1-alkylimidazolium salts and the effect on the stereoselectivity of Diels-Alder reactions [30]. However, liquid crystals are classified as ionic liquid crystals (ILCs), and they are distinguished from liquid crystals that are dispersed in ionic liquids. Although the formation of micelles and liquid crystal phases in ionic liquids have been thus reported, there has been no mention of the self-assembly of developed nano-assemblies that are stably dispersed in ionic liquids. In the next section the formation of bilayer membranes and vesicles in ionic liquids is discussed. 

Metal-based ionic liquid crystals containing a tetrahedral tetrahalometalate ion (MX X = Cl, Br, I M = Co, Ni, Cu, Pd, etc.) 7 [41, 54] and 8 [55-57] were studied (Figure 25.1). They provide various coordination geometries. For example, Al-aUcylpyridinium salts 8 often denoted as [C Py]2[CuCl4] (n= 12-18) show a variety of liquid crystalline phases from smectic to columnar or even cubic phases... 

Bipyridinium cation 9 [58], phosphonium [59-62], guanidinium [63], and vina-midinium [64] cations were used to form thermotropic ionic liquid crystals. For example, the mixture of diheptyl and dioctyl viologens 9 (20 80 by wt%) with TFSI anion forms a smectic A phase between 22° and 132°C. It was reported that the viologens show fluorescence in both non-polar and polar organic solvents, which can be useful for the development of biological and chemical sensors [65]. 

Two-dimensional ion conduction was also achieved for the smectic liquid crystals consisting of the mixture of ionic liquids and hydroxyl-functionalized rod molecules [49-51] and smectic LC imidazolium salts having a long alkyl chain [52-55]. One-dimensional ion conduction was performed for aligned columnar LC phases forming ion-channels of fan-shaped imidazolium salts having a tris(alkoxy)phenyl moiety [56]. 

Fig. 3.1 -4 Changes in melting points for l-aikyi-3-methyiimidazoiium tetrafluoroborate [25] ionic liquids as a function of chain iength, showing true melting points ( ) and glass transitions ( ) with thegenerai trends for decreasing melting points with n = 1 to n = 8, followed by an increase in melting point and formation of a smectic liquid crystal phase (o) with increasing n clearly visible.

An excellent example of counterion influence is the quite different thermal behavior of double-chain l-methyl-3,5-bis(n-hexadecyloxycarbonyl)pyridinium ion in crystals with iodide or chloride as counterion [4]. The iodide salt revealed three phase transitions solid crystalline-solid crystalline at —326 K, solid crystalline-liquid crystalline at —358 K, and liquid crystalline-isotropic liquid at —378 K. The X-ray diffraction pattern of the liquid crystalline phase could be best rationalized in terms of a smectic-H phase. The chloride anion could be unfavorable for liquid crystalline behavior because of its smaller ionic radius relative to the iodide anion. Less shielding of the positive charges of the pyridinium rings by the chloride counterion leads to increased electrostatic repulsion between headgroups. 

The conductivity of the smectic phase is important rather than rely on impurities in the liquid crystal or degradation products of the dynamic scattering, it is desirable specifically to add ionic dopants. In the early work, the homeotropic aligning dopant [hexadecyltrimethyl ammonium bromide (HMAB)] conveniently also acted as the... 

Furthermore, we succeeded in the tmiform alignment between the ITO electrodes and gold comb electrodes in this two-component system liquid crystal and found that the ion conductivity parallel to the smectic layer structure is about 3,000 times higher than in the perpendicular direction. We could also easily adjust the ionic conductivity and the phase transition temperature by changing the composition mixture [172]. It is a great advantage to use such mixtures that form assembled structures by non-covalent intermolecular interactions. 

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