Hybrid Liquid Crystals from the Self-Assembly of Surfactant-Encapsulated Polyoxometalate Complexes

Ionic-surfactants-based thermotropic liquid crystals

Ionic surfactants can be combined with various functional groups through electrostatic interaction, resulting in a series of thermotropic liquid crystals (TLCs).

Thermal stability of mesoscopic compounds of cetyltrimethylammonium and Keggin metatungstates

A hybrid surfactant/polyoxometalate compound was synthesized by combining isopolytungstate anions with the cationic surfactant cetyltrimethylammonium bromide (CTA-Br) to produce a hierarchical compound that we identify as (CTA)₇[H₂W₁₂O₄₀]Cl·2H₂O. At room temperature the compound consisted of hexagonally ordered sheets of Keggin ions, with an intervening gallery containing alkyl-chains of the organic cations. The synthesis was highly dependent on solution pH, reaction time and the order in which the reactants were added. We examined the effect of temperature on the stability of (CTA)₇[H₂W₁₂O₄₀]Cl·2H₂O using thermal gravimetric analysis, differential scanning calorimetry, FT-IR spectroscopy and in situ synchrotron X-ray diffraction, and found a step-wise conversion to monoclinic WO_xvia a series of intermediates. Heating under nitrogen atmospheres accelerated transition events by ∼100 °C when compared to heating in air. During heating, the interplanar gallery at first expanded in a series of steps starting at 90 °C as the CTA⁺ amphiphiles changed orientation, before collapsing rapidly at 240 °C, a temperature coinciding with the removal of about 40% of the organic material. Between 240 and 320 °C, the material consisted of fragments of the Keggin ion cores, arranged in 2D hexagonally-packed sheets. At ∼330 °C, the Keggin ions were completely destroyed and replaced by bulk W₁₇O₄₇ which, upon further heating, transformed to bulk WO₂ or WO₃ depending on the environment.

Ionic Liquid Crystals: Versatile Materials

This Review covers the recent developments (2005-2015) in the design, synthesis, characterization, and application of thermotropic ionic liquid crystals. It was designed to give a comprehensive overview of the "state-of-the-art" in the field. The discussion is focused on low molar mass and dendrimeric thermotropic ionic mesogens, as well as selected metal-containing compounds (metallomesogens), but some references to polymeric and/or lyotropic ionic liquid crystals and particularly to ionic liquids will also be provided. Although zwitterionic and mesoionic mesogens are also treated to some extent, emphasis will be directed toward liquid-crystalline materials consisting of organic cations and organic/inorganic anions that are not covalently bound but interact via electrostatic and other noncovalent interactions.

Synthesis and characterization of metal-substituted tetraalkylphosphonium polyoxometalate ionic liquids

Trihexyl(tetradecyl)phosphonium and Keggin polyoxotungstates [PW₁₁O₃₉M(H₂O)]ⁿ⁻, M = Fe, Co, Mn, or related lacunary [PW₁₁O₃₉]⁷⁻ and [P₂W₂₀O₇₀(H₂O)₂]¹⁰⁻ anions yield compounds that are liquids at room temperature.