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 Tb3+ ion as a fluorescent probe. The fluorescence intensity and lifetime of the Tb3+ ion depend directly on the number of water molecules bound to their inner coordination sphere. The more efficiently ligands coordinate to Tb3+ ion, the more water molecules are expelled and, consequently, the greater the fluorescence intensity and lifetime observed. Lifetime measurements in H2O and D2O showed that the number of water molecules coordinated to the Tb3+ ion are 6.5, 6.1, 4.9, and 3.6 for monomer, dimer, trimer model compounds, and polyacrylate, respectively. The viscosities of Tb3+-polyacrylate complexes were measured in the presence of a large excess of sodium bromide. Viscosities (eta(sp)/c) of the polyacrylate solutions follow Huggins' equation, and on addition of Tb3+ 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 Tb3+-polymer complexes were calculated using the Flory-Fox equation and were found to be reduced from 125 to 48 nm on addition of Tb3+. These results indicate that of the nine water molecules coordinated to Tb3+ ion in aqueous solution, five to six are replaced with carboxylate groups attached to the polymer chain wrapped around the Tb3+.