Lyotropic liquid crystals fabricated in ionic liquids

Abstract

The lyotropic liquid crystals (LLCs) are one important kind of aggregates formed by surfactants in solvent media and have wide applications in nanomaterial systhesis, chemical reaction environment, drug delivery, lubricating material, oil recovery and so on. Meanwhile, the ionic liquids (ILs) have currently widely used as novel nonaqueous media in the areas of organic synthesis and catalysis, biochemical engineering, materials science, electrochemistry, carbohydrate chemistry, and separation techniques for their unique characteristics of low melting temperature, negligible vapor pressure, wide electrochemical window, nonflammability, good catalytic properties, high thermal stability, and ionic conductivity. For these reasons, more and more attention has been paid to ILs as the self-assembly media. Among them, the LLCs formed by surfactants in ILs have received increasingly studies. Exploration on the LLC behaviors of surfactants in ILs may provide not only a better understanding of intermolecular actions in self-assembly, but also certain new properties to LLCs, like better thermostability compared to those constructed in aqueous media. For these reasons, this account here reviews the LLC phase behaviors in ILs of various surfactants, including quarternary ammonium cationic surfactant, alkyl polyoxyethylene nonionic surfactant and Pluronic block copolymer. For the cationic surfactants, several single chain molecules like the hexadecyltrimethylammonium bromide, 1-hexadecyl-3-methylimidazolium chloride, and N-hexadecyl-N-methylpyrrolidinium bromide, their phase behaviors in various ILs were summaried. Various LLC phases including the normal hexagonal, lamellar, and reverse bicontinuous cubic LLC phases could be detected in ethylammonium nitrate (EAN). The hexagonal LLC phase with better thermostability was formed in EAN compared to that in water. For the molecule with two alkyl chains on one head, the dimethyldidodecylammonium bromide, the sponge and lamellar phases were observed in EAN and the sponge phase could be extended to larger region than that in aqueous system. Unlike the single chain surfactants, the reverse hexagonal phases were observed in the quarternary ammonium Gemini surfactants/EAN systems, which were not observed in corresponding water media. With increasing the chain length of one or two ethylene groups in cation of EAN, that is, from EAN to PAN or BAN, the normal LLC phases were formed back. As for the polyoxyethylene nonionic surfactants (C n E m ) in EAN, their phase behaviors were similar to those in water. Due to the weaker solvophobic interactions in EAN than in water, the longer alkyl chains were necessary for the formation of LLC. While in aprotic ILs, no LLC phase was observed. However, for the phytosterol ethoxylate surfactants (BPS-n), the LLC phases were observed both in EAN and also in aprotic ILs like [Bmim]BF4 and [Bmim]BF6. Similar aggregation behaviors were also observed for Pluronic block copolymer in EAN and [Bmim]BF4. The difference in phase behaviors of these surfactants in various ILs was analyzed by Gordon parameter, which was a measure of the cohesive energy density of the solvent. Generally speaking, ILs with higher Gordon parameters are correspondent to greater LLC diversity and better thermostability. The protic ionic liquids, which are capable of accepting protons and form hydrogen bonding network, would have a stronger ability of supporting surfactant self-assembly than the aprotic ionic liquids. This is the reason why richer phases are usually formed in EAN than in [Bmim]BF4 and [Bmim]BF6. Based on the review, the perspective for future research in this area has been also provided, such as synthesis of novel kinds of ILs and surfactants and investigation on new self-assembly systems.