Ion Binding Properties of Polycarboxylates Using Terbium(III) as a Fluorescent Probe: Viscosities and Coordinated Water Molecules in Polycarboxylate−Terbium(III) Complexes in Aqueous Solutions

The binding properties of trivalent ions to polyacrylate and its low molecular weight analogs (monomer, dimer, and trimer) were investigated in dilute aqueous solution (10 -2 −10 -3 M) using Tb 3+ ion as a fluorescent probe. The fluorescence intensity and lifetime of the Tb 3+ ion depend directly on the number of water molecules bound to their inner coordination sphere. The more efficiently ligands coordinate to Tb 3+ ion, the more water molecules are expelled and, consequently, the greater the fluorescence intensity and lifetime observed. Lifetime measurements in H 2 O and D 2 O showed that the number of water molecules coordinated to the Tb 3+ ion are 6.5, 6.1, 4.9, and 3.6 for monomer, dimer, trimer model compounds, and polyacrylate, respectively. The viscosities of Tb 3+ −polyacrylate complexes were measured in the presence of a large excess of sodium bromide. Viscosities (η sp / c ) of the polyacrylate solutions follow Huggins' equation, and on addition of Tb 3+ ion, the intrinsic viscosities decrease abruptly from 700 to 40 mL/g. The mean end-to-end distances, 〈 r 2 〉 1/2, for polyacrylate in Tb 3+ −polymer complexes were calculated using the Flory−Fox equation and were found to be reduced from 125 to 48 nm on addition of Tb 3+ . These results indicate that of the nine water molecules coordinated to Tb 3+ ion in aqueous solution, five to six are replaced with carboxylate groups attached to the polymer chain wrapped around the Tb 3+ .