Small-Angle X-ray Scattering Studies of Liquid Crystalline 1-Alkyl-3-methylimidazolium Salts

Abstract

A series of long-chain 1-alkyl-3-methylimidazolium salts ([C n -mim]X, n ) 12-18) containing the anions, chloride, bromide, trifluoromethanesulfonate (OTf), and bis(trifluoromethanesulfonyl)imide (TFI), have been synthesized and characterized. The salts have amphiphilic characteristics, and the thermotropic phase behavior of these salts and the analogous tetrafluoroborate salt has been investigated by variable temperature small-angle X-ray scattering, polarizing optical microscopy, and differential scanning calorimetry. The salts form lamellar, sheetlike arrays in the crystalline phase and an enantiomeric smectic liquid crystalline phase at higher temperatures, except for the salts containing the bis(trifluoromethanesulfonyl)imide anion which melt directly to form isotropic liquids. The nature of the anion influences the size of the interlayer spacing in both the crystal and in the mesophase. The interlayer spacing in the mesophase is largest for the anions with the greatest ability to form a three-dimensional hydrogen-bonding lattice, following the order [TFI] -< [OTf] -< [BF 4 ] -< Br -< Cl -.

Raman Spectroscopy and Ab-Initio Model Calculations on Ionic Liquids

“…18, broad Raman features are observed for longer-chain [C n mim][BF 4 ] liquids (n ¼ 7-10) in the wavenumber region of 800-950 cm À1 , where hydrogen rocking and bending vibrations of the methylene groups are located. These broad features are indicative of aliphatic interactions between the alkyl chains [108], similar to the interactions found for certain [C n mim][Cl] and [C n mim][PF 6 ] salts and meso-phase liquid crystals when n>12, see [52,114,138]. This interaction through the alkyl chains is likely to operate also in the [C 4 mim][X] RTILs.…”

“…Early studies probed the nature of interactions in socalled first-generation chloroaluminate ionic liquids [40], and now also information becomes available for second generation, air-stable systems, such as the imidazolium mixtures. Many RTIL studies have been directed towards possible applications [8, 11, 41]: a number of reports have been published covering theoretical aspects [42][43][44][45][46][47], X-ray crystallography of the frozen melts [48][49][50][51][52], NMR relaxation, conductivity, viscosity, diffusion [4,28,[53][54][55][56][57][58][59], and gas solubilities in ionic liquids [22,60]. Hydrogen-bonding is known to occur between the cations and anions in most ionic liquids, as demonstrated by several investigators on e.g., systems containing 1-alkyl-3-methylimidazolium.…”

“…Although other techniques such as infrared spectroscopy, X-ray, and neutron diffraction studies have been used to study these ions in the liquid or solid state or at surfaces [6,52,[109][110][111][112][113][114][115][116][117][118][119][120], a real gain in our understanding came with the combination of crystal structure solution, Raman spectroscopy and ab-initio DFT calculations [108,121]. We concentrate the story on the instructive example of the l-butyl-3-methylimidazolium cation, [C 4 mim] þ (see Fig.…”

Supramolecular Structures in the Presence of Ionic Liquids

“…This concept is supported by the results of IR (Dieter et al, 1988;Ozawa et al, 2003;Talaty et al, 2004), Raman (Katayanagi et al, 2004;Talaty et al, 2004), neutron diffraction analysis (Deetlefs et al, 2006;Hardacre et al, 2003), X-ray reflectivity experiment (Carmichael et al, 2001), small-angle X-ray diffraction analysis (Bradley et al, 2002) and large-angle X-ray scattering experiment (Kanzaki et al, 2009), indicating that 1,3-dialkylimidazolium ILs have analogous structural patterns in the solid and liquid phases. It is the long-range Coulomb interactions in ILs that can lead to longer spatial correlations than those in comparable classic van der Waals organic liquids (Cang et al, 2003).…”

“…As for imidazolium ILs with long alkyl chains (C n ≥ C 12 ), liquid crystal (LC) phase often appears in the course of increasing temperature (Bowlas et al, 1996;Bradley et al, 2002;De Roche et al, 2003;Getsis & Mudring, 2008;Holbrey & Seddon, 1999;Lee et al, 2000;Lee et al, 1997;Li et al, 2005b;Zhou & Antonietti, 2004). If an anion has a long alkyl chain, the long alkyl chains on the imidazolium ring are not necessary for the formation of LC .…”