Induced Amphotropic and Thermotropic Ionic Liquid Crystallinity in Phosphonium Halides:  “Lubrication” by Hydroxyl Groups

Abstract

The influence of covalently attaching hydroxymethylene to the methyl groups of methyl-tri-n-alkylphosphonium halides (where the alkyl chains are decyl, tetradecyl, or octadecyl and the halide is chloride or bromide) or adding methanol as a solute to the salts on their solid, liquid-crystalline (smectic A2), and isotropic phases has been investigated using a variety of experimental techniques. These structural and compositional changes are found to induce liquid crystallinity in some cases and to enhance the temperature range and lower the onset temperature of the liquid-crystalline phases in some others. The results are interpreted in terms of the lengths of the three n-alkyl chains attached to the phosphorus cation, the nature of the halide anion, the influence of H-bonding interactions at the head group regions of the layered phases, and other solvent-solute interactions. The fact that at least 1 molar equiv of methanol must be added to effect complete (isothermal) conversion of a solid methyl-tri-n-alkylphosphonium salt to a liquid crystal demonstrates a direct and strong association between individual methanol molecules and the phosphonium salts. Possible applications of such systems are suggested.

Thermotropic Ionic Liquid Crystals

“…It has been stated that noncovalent interactions between hydroxyl groups of a solvent and charged phosphorus centers induce mesomorphism and attenuate ion pairing [36,37]. Recently, new phosphonium mesogens 14 , containing β-hydroxyethenyl groups, have been prepared (Scheme 7) [38]. Due to intermolecular OH∙∙∙P + interactions in these compounds, they displayed self-enhanced amphotropic behavior, similarly to it was earlier observed upon the addition of methanol to simple tetraalkylphosphonium mesogens 13 .…”

“…Due to intermolecular OH∙∙∙P + interactions in these compounds, they displayed self-enhanced amphotropic behavior, similarly to it was earlier observed upon the addition of methanol to simple tetraalkylphosphonium mesogens 13 . In the absence of solvents, the phosphonium salts 13 showed melting points around 100 °C and clearing points between 106.2 and 114.0 °C, while phosphonium mesogens 14 , with embedded hydroxyl groups and the methanol solvates 13∙CH 3 OH (m = 1, X – = Cl – , Br – ; Scheme 7), featured considerably lower melting points (at 43.5–85.0 °C; for 13∙CH 3 OH 49.7–54.9 °C) and clearing points (73.4–99.0 °C; for 13∙CH 3 OH 27.5–82.5 °C) [38]. In another approach, the phosphonium-“ate” zwitterionic structures 15 have been designed (Scheme 5), with an idea to weaken cation-anion contacts within an ionic layer and, at the same time, to establish new weak interactions between counteranions and “ate” groups in the phosphonium cations [39].…”

Liquid crystalline phases of 1,2-dimethyl-3-hexadecylimidazolium bromide and binary mixtures with water

“…The textures are ascribed to intrinsic spontaneous alignment. Fan-shaped focal conic textures, a common feature of the smectic A phase, were observed when the isotropic sample was cooled [16]. The micrographs taken at 230, 200, and 42°C show the existence of a fan-shaped focal conic texture appearing in the isotropic phase, a pure fan-shaped focal conic texture, and a mixture of a solid and a fan-shaped form, respectively, indicating the isotropic-mesophase and mesophasesolid phase transitions clearly.…”

Self-assembly of lyotropic liquid crystal phases in ternary systems of 1,2-dimethyl-3-hexadecylimidazolium bromide/1-decanol/water

“…9 that enthalpy changes of transition for solid-LC transitions are greater than the corresponding ones for the LC-solid transitions. This is because the solid-LC transitions involve primarily melting of IL and 1-decanol with long alkyl chains, and the LC-solid transitions concern primarily disruption of the ordering by ionic interactions and hydrophobic effect [23,24]. Both the enthalpy changes increase with increasing the content of IL.…”