Reversible adsorption of calcium ions by imprinted temperature sensitive gels

With the aim of developing polymeric gels sensitive to external stimuli and able to reversibly adsorb and release divalent ions, copolymer gels of N-i sopropylacrylamide (NIPA) and methacrylic (MAA) monomers were prepared. We chose calcium as a target divalent ion. Two MAAs form a complex with a calc ium ion, and the NIPA component allows the polymers to swell and shrink rev ersibly in response to temperature. The adsorbing site develops an affinity to target ions when the adsorbing molecules come into proximity, but when they are separated, the affinity diminishes. To enhance the affinity to cal cium, an imprinting technique was applied using Ca2+ and Pb2+ ions as templ ates in methylsulfoxide and dioxane media, respectively. The adsorption cap acity of the imprinted gels was compared with that of the nonimprinted gels , and the effects of the templates, the solvents, and the amount of methacr ylic monomers used in the synthesis and the medium temperature over the Ca2 + adsorption capacity of the gels from aqueous solutions were evaluated. Th e analysis of the adsorption revealed that (a) the adsorption can be descri bed by the Langmuir isotherms; (b) there is an approximately linear relatio nship between saturation and methacrylic monomer concentration; (c) the aff inity depends on the degree of gel swelling or shrinkage that can be switch ed on and off by temperature; (d) in the shrunken state, the affinity depen ds approximately linearly on the MAA concentration in the imprinted gels, w hereas in the nonimprinted gels it is proportional to the square of MAA con centration; (e) the imprinted gels adsorb more than the nonimprinted gels w hen MAA concentration is less than that of permanent cross linkers. The suc cess of imprinting of CaMAA(2) and PbMAA(2) complex is evidence for memory of such complex onto the weakly cross-linked gel. (C) 2001 American Institu te of Physics.