The lack of selectivity in the transport of transition metal ions exhibited by pyridyl- and 2,2′-bipyridyl-based hydrogel membranes

Abstract

A range of hydrogel copolymer membranes of poly(2-hydroxyethyl methacrylate) with 4-vinylpyridine or 4-vinyl-4′-methyl-2,2′-bipyridine have been made and their permeabilities to a series of transition metal salts determined. Ion transport is characterized by an initial induction period prior to the onset of steady-state permeation. This period varies depending on the ion under study and can be related to the relative stabilities of the complexes formed within the membrane. The lag time is attributed to the saturation of coordination sites within the membrane, which also affects the rate of permeation of the metal ion relative to that through unmodified membranes. The potential of these systems for selective transition metal ion transport is assessed by competitive studies using binary salt solutions. The membranes do not exhibit any selectivity; in fact, their behaviour can even be described as being antiselective. In pairs, different cations display similar lag times and permeabilities, characteristic of the more strongly coordinating ion. Membrane ligand density has little influence on the final permeability of the salt, which precludes a site to site mechanism as the main mode of transport. However, complexation to the membrane coordination sites is implicated as the cause of modifications to the permeation properties of the membranes.