EFFECT OF COUNTERION CONDENSATION ON THE FLUORESCENCE QUENCHING OF PHENANTHRENE-LABELED SODIUM-SALT OF POLY(ACRYLIC ACID-CO-ACRYLAMIDE) IN SALT-FREE SOLUTION

A small mole fraction of phenanthrene (Phen) moieties were covalently incorporated into copolymers of sodium acrylate and acrylamide varying Manning's dimensionless charge density parameter xi (0.67 < xi < 2.8). In aqueous solution with no added salt (except for quenchers), fluorescence of the Phen moieties was efficiently quenched by Tl+ ions at very low quencher concentrations. The quenching efficiency markedly increased with increasing xi, which was interpreted as due to counterion condensation. A kinetic model of mixed static and dynamic quenching in conjunction with Manning's condensation theory was proposed. In this model, fluorescence was postulated to be statically quenched by condensed Tl+ ions existing in the active sphere according to the Perrin formulation and also dynamically quenched by atmospheric Tl+ ions according to the Stern-Volmer formulation. The quenching data were satisfactorily fit to the kinetic model. The fraction of the static quenching decreased sharply with decreasing xi. It was estimated that 34% of the condensed Tl+ ions existed in the active sphere for the polymer with xi=2.8, while only 1.2% were estimated to be in the active sphere for xi=1.5.