Equilibrium swelling behavior of thermally responsive metal affinity hydrogels, Part II: Solution effects

In a previous study the effect of compositional variations on the equilibrium swelling of co-polymer hydrogels designed and synthesized from N-isopropylacrylamide (NIPAAm) and vinyl iminodiacetic acid (VIDA) monomers was investigated [Iyer G, Tillekeratne LMV, Coleman MR, Nadarajah A. Polymer, in this issue, doi:10.1016/j.polymer.2008.06.037]. The gels have both thermally responsive swelling and metal affinity properties and the effect of solution conditions on the equilibrium swelling of copper chelated and unchelated gels is studied here. In contrast to their sharp phase transition behavior in DI water, buffer solutions unexpectedly caused swelling of both gels to be the same and lose the sharp phase transition. Imidazole solutions had the expected phase transition behavior with increasing swelling and loss of phase transition of the unchelated gels which were partially reversed by copper chelation. Other small non-binding molecules, such as phenol, had minimal effects on the swelling behavior. Chicken egg white lysozyme solutions caused both gels to have reduced but equal equilibrium swelling and lose their sharp phase transition. These solution effects are explained in terms of salting in/salting out phenomena, the polarization of amide groups in VIDA, the solution pH and protein adsorption on hydrogel surfaces.