Polymerized Ionic Liquids-based Hydrogels as Ion Exchangers
Johanna ROMISCHKE1, Rebecca MAHNEKE1, Jenny SEIFERT1, Johanna MEYER2, Udo KRAGL1
1University of Rostock, Rostock, Germany
2University of Hannover, Hannover, Germany
Polymerized ionic liquids (pIL) are polymers based on ionic liquids (IL) as monomeric units. Compared to conventional polymers, pIL´s have similar properties and, in contrast to solid electrolytes, a soft character.  They are generally synthesized by initiated polymerizations or condensation reactions. Three-dimensional pIL networks that are able to swell in water without dissolving themselves can be described as hydrogels. The absorption of water is reversible and closely related to the structure of the polymer network. Hydrogels in general can have very different properties based on their origins (natural or synthetic), their composition (homo-, co- or multipolymeric), their physical appearance (matrix, film or microspheres), their electrical charge (neutral, anionic, cationic, ampholytic or polybetaines), their method of polymerization (e.g. emulsion, suspension) and their type of crosslinking.[1,2] Due to this high variance of possible properties, hydrogels also have a very broad field of application, including medical, industrial and agricultural sectors. Based on the polyelectrolytic structure of pIL, they are ideally suited for ion exchange.
In this work, cation exchange with 3-sulfopropylmethacrylate potassium salt (MAESO3) gels and anion exchange with [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MAETMA) gels, each crosslinked with N,N'-methylenebis(acrylamide) (Mbis), and different exchange ions were investigated. Ion exchange with charged dyes, such as methylene blue which is a competitive inhibitor of the glutathione reductase of Plasmodium falciparum and used in malaria therapy, proved to be very successful.
Compared to the common exchange materials, which are also based on (meth)acrylic structures, our hydrogels proved to be excellent ion exchangers with comparable capacity, which can be controlled by the temperature. In direct comparison with the conventional exchange resin Amberlite IR120, a strongly acidic cation exchanger, our gels already showed more than six times faster uptake of the dye from the solution within the first 10 min (68% removal instead of 10%). Even when the amount of gel particles used is reduced by a factor of five, the ion exchange is more than twice as fast in comparison (26% removal instead of 10%). Also worth mentioning is the impressive recyclability of the hydrogel particles, which show consistent replacement capacities over more than five cycles. Further investigations are still in progress.
After completion of previous investigations, our hydrogels appear to be promising ion exchangers which can compete with conventionally used materials. Further investigations are still in progress, but due to the already known antibacterial properties of some of these materials, an application in medicine or water treatment is indeed conceivable.
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