Cholesterol-based low-molecular mass gelators towards smart ionogels

Abstract

Ionic liquids are solvents of future. One of the promising methods to boost their uses is to solidify them in a physical way but with little interruption of their properties. It is therefore of interest to create gels of ionic liquids by using low-molecular mass compounds as gelators (LMMGs). Herein, we report a number of ionic liquid gels (ionogels) of which specially designed and synthesized cholesteryl derivatives were employed as gelators. The ionogels as obtained are thermo-reversible. In particular, the one with 1-butyl-3-methylimidazolium tetrafluoroborate (IL2) as solvent and a cholesteryl derivative containing a D-phenylalanine residue (1D) as a gelator is very stable both in neutral and acidic mediums as demonstrated by a yield stress of 76 Pa for a self-standing cylinder of the ionogel. Furthermore, the ionogel can be easily converted into a hydrogel via simple replacement of the solvent with water in situ. More interestingly, the conversion is reversible, a phenomenon never reported before. At the same time, the critical gelation concentration (CGC) of 1D for IL2 is only 0.06%, w/w, which is almost the lowest value reported for ionogels till now, and falls into the category of “super-gelator”. Magnetization of the ionogel has been realized by introduction of micro-/nano-Fe₃O₄ particles. As expected, the magnetic gel as obtained is responding to external magnetic field. Specifically, it changes into fluid with the presence of a magnetic field exceeding certain strength, and retains to gel upon removing the magnetic field and with a treatment of sonication and heating–cooling cycle. SEM and TEM observations revealed the continuous fibrous network structures of the molecules of the gelator in the ionogels. To the best of our knowledge, this is the first report on ionogels possessing stimulus-responsive properties, good mechanical strength, and super-gelation talent.