Ionosilicas recently emerged as a new family of functional organosilicas. These materials, defined as silica based materials containing covalently anchored ionic groups, are synthesized via sol-gel reactions starting from ionic precursors. Ionosilicas combine high porosity, regular architecture on the mesoscopic scale with an unmatched chemical versatility, induced by the high number of incorporated ionic sites and the high variability of possible anion-cation combinations. In analogy to ionic liquids, which are considered as ‘designer solvents’, ionosilicas can be regarded as ‘designer materials’.
The synthesis of ionosilicas takes place under very mild reaction conditions via sol-gel reactions. A variety of ionosilica mesophases was obtained starting exclusively from different trialkoxysilylated ionic liquid precursors. Furthermore, ionosilica phases displaying different morphologies (nanoparticles, monoliths, fibers), textures and architectures were obtained via a variation of the syntheses parameters. In particular, we observed that the formation of mesostructured ionosilicas can only be achieved from suitable surfactant-precursor ion pairs. The interfacial properties of ionosilicas can be tuned both via the chemical constitution of the silylated precursor and the reaction conditions, thus allowing accessing a whole ‘library’ of materials.
We will illustrate the versatility of ionosilicas in the area of anion exchange. Ionosilicas efficiently adsorb high quantities of both organic and mineral pollutants. A first study concerning the adsorption of chromate ions revealed a very high adsorption capacity combined with rapid ion exchange kinetics. The polyvalence of ionosilicas was then shown in the area of the adsorption of drugs, i.e. p-aminosalicylate (PAS). Isothermal titration calorimetry (ITC) experiments indicated different displacement enthalpies depending on the substitution of the cationic group of the ionosilica. Specifically, PAS shows a particularly high affinity toward ionosilica material containing aromatic groups. We attribute this result to a - stacking contribution of the sorbent material with PAS.
This work shows that tailor made ionosilica based adsorbents can be designed in view of the development of selective anion exchange materials.