Amphiphile self-assembly based on ionic liquids

Abstract

Ionic liquids (ILs), as a class of compounds composed of ions with melting points at or near room temperature, have unusual physicochemical properties including low volatility, high temperature stability, high ionic conductivity and easy recyclability. Thus ILs have been applied widely in the range of organic synthesis, catalysis, chromatography, analytical chemistry, biochemistry and so on. Perhaps the most unique capability of ILs is to support the self-assembly of amphiphiles. Due to the extraordinary properties of ILs, the self-assembled aggregates based on ILs have attracted a lot of attentions during the last decades. ILs are now not only considered as important alternative solvents, but also as materials with unique and tuneable properties which can be easily adjusted by suitable selection of cations and anions for a specific function. Based on the unique designability of ILs, it will be expected to achieve the controllability of the nanostructures and properties of IL-based self-assembled aggregates by tailoring the structures of ILs. Here, the research progress of amphiphile self-assembly based on ILs was reviewed. Firstly, the room-temperature ILs can be considered as solvents to participate in the self-assembly process and classified as protic ionic liquids (PILs) and aprotic ionic liquids (AILs), respectively. Up to now, a number of AILs based on imidazolium cations and many alkylammonium PILs, such as ethylammonium nitrate (EAN), have been used as self-assembly media to support amphiphile to form various aggregates including micelles, vesicles, microemulsions and liquid crystals. Different from AILs, the PILs can build up hydrogen-bonding networks which are similar to water molecules due to their protic nature and general properties. The different solvent properties of PILs and AILs have significant influence on the aggregation behavior of amphiphilic molecules. Secondly, the long chain analogues of common ionic liquids, named as surfactive ionic liquids (SAILs), could possess inherent surface active properties and self-assemble to form different aggregates with specific structures, shapes and properties in aqueous solutions like traditional amphiphilic molecules. Various long-chain SILs with different cationic headgroups based on imidazolium, pyrrolidinium, piperidinium etc. have been synthesized and investigated. Except for cationic SAILs, the aggregation behaviors of a series of anionic SAILs based on organic surfactant anions and imidazolium cations have also been studied to expand the application of SAILs in the filed of colloid and interface chemistry. SILs, as a novel kind of surfactants, can self-assembly into different kinds of aggregates driven by intermolecular interactions including hydrophobic interaction, electrostatic interaction, hydrogen bonding and so on. Thus, the type of cations, the alkyl chain length, and the nature of the counterions have key effect on the aggregation behavior of SAILs. Thirdly, the physicochemical properties of the given surfactant solutions can also be modified by the addition of external ionic liquids. Due to the tailoring properties of ILs, “task-specific” ILs (TSILs) have emerged by introducing functional groups as a part of substituent to impart a particular capability to ILs and further modulate the aggregation behavior of surfactant solutions. This is also a practical way for us to determine the role of one specific intermolecular interaction on the self-assembly process. The present work summarized the amphiphile self-assembly based on ILs, including room-temperature ILs as solvents, long-chain ILs as amphiphiles and ILs as external additives. Our aim is to establish the dependence of aggregation behavior of amphiphiles on the unique structures of ILs. This review is helpful to achieve the controllability and functionalization of traditional self-assembled aggregates by the incorporation of functional ILs.