Two-Component Hydrogels Formed by Cyclodextrins and Dicationic Imidazolium Salts

In the framework of our recent interest in studying the gelling behaviour of imidazolium salts, we took into account the possibility of obtaining two-component hydrogels by combining cyclodextrins and dicationic imidazolium salts. In particular, we used diimidazolium salts differing in the alkyl chain length and in the size, shape, and coordination ability of the anion, namely the 3,3′-di-n-decyl- and 3,3′-di-n-dodecyl-1,1′-(1,4- phenylenedimethylene)diimidazolium dibromide, and 3,3′-di-n-dodecyl-1, 1′-(1,4-phenylenedimethylene)diimidazolium ditetrafluoroborate and dihexafluorophosphate. As far as cyclodextrins are concerned, to gain information on the effects of different sized cyclic oligosaccharides, we used both α- and β-cyclodextrins. The interaction between the organic salts and the oligosaccharides was investigated by using polarimetry, competitive fluorescence measurements, and 1H NMR spectroscopic analysis. In the case of gel formation, properties of the obtained soft materials were studied by determining their thermodynamic stability, analyzing their optical properties by means of UV/Vis spectroscopy, and studying their morphology by using scanning electron microscopy. The gelation process was studied by resonance light scattering measurements and this approach has allowed us to gather information on the size of the aggregates characterizing the gel phases. Finally, the soft materials were also investigated with respect to their ability to self-repair after the action of external stimuli, such as magnetic stirring or ultrasound irradiation. Taken together, the data collected demonstrate that the properties of these two-component gels can be controlled by changing the nature of the cyclodextrin, the salts, or, more simply, by changing the host/guest ratio. Two-component hydrogels having different properties have been obtained from cyclodextrins and dicationic imidazolium salts. Pseudorotaxane formation seems to be the key factor favouring the self-assembly process. As a main advantage, the approach allows the features of the soft materials to be changed simply by introducing minor structural changes to the salt or varying the host-guest ratio.