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

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.