Preferential Binding of Thallium Cations onto Poly(sodium acrylate-co-acrylamide) in the Presence of Alkali Metal Salts in Aqueous Solution As Studied by Fluorescence Quenching of Polymer-Bound Phenanthrene Labels

Abstract

Binding of Tl+ ions on phenanthrene (Phen)-labeled copolymers of sodium acrylate and acrylamide with different charge densities (ξ = 2.5 and 0.94) was studied by fluorescence quenching in aqueous solution with added alkali metal salts. The polyanion with ξ = 2.5 showed very efficient quenching of Phen fluorescence with an upward curvature in a Stern−Volmer plot. The fluorescence decays, in the presence of Tl+, consist of two components; fast and slow decays are attributed to quenching by bound and free Tl+ ions, respectively. Both the steady-state and time-dependent fluorescences indicate that the fluorescence quenching by Tl+ (8.0 × 10-5 M) is not affected at all by the addition of Li+, Na+, and K+ ions (5.0 × 10-4 M), indicating that polymer-bound Tl+ ions are not replaced by the added alkali metal cations. In contrast, the polymer with ξ = 0.94 shows much less effective quenching with a straight line in a Stern−Volmer plot. There is no fast decay component due to the quenching by bound Tl+ in the Phen fluorescence. Unlike the case of the polymer with ξ = 2.5, fluorescence quenching is caused by atmospheric Tl+. The fluorescence quenching by Tl+ was significantly reduced by the addition of the alkali metal cations, indicating that the added alkali metal cations can displace the atmospheric Tl+. These data show that the binding of Tl+ onto the polyanion with a high charge density is preferential over the binding of Li+, Na+, and K+ ions.