Cooperative Interaction of Anionic Dyes with Imidazole-Containing Polymers

Abstract

The binding behavior of methyl orange to several homopolymers and copolymers containing imidazole and imidazolium salts was investigated as a function of polymer structure and counterion. The polymers were fractionated and characterized by light scattering, viscosity, and turbidimetry. Dye binding was measured by equilibrium dialysis, spectrophotometric titration, and microcalorimetry.

The extent of counterion binding to the polycations increases in the order Ac⁻ (acetate)< Cl⁻ < Br⁻ < I⁻ < SCN⁻. The reverse order was observed, however, for the dye-binding strength of these polycations in the various salt media. These results demonstrate the competitive effect of the counterion on dye binding. The dye binding constant and the enthalpy of dye binding increase with decreasing [Polymer]/[Dye], suggesting a positive cooperative effect caused by dye stacking along the polymer chain. Incorporation of a nonbinding comonomer decreases binding strength, a result attributed to the lack of dye stacking. Our ΔG° and ΔH° data indicate that dye binding is driven both enthalpically and entropically. The constancy of ΔH° suggests that the electrostatic contribution to ΔH° is not predominant. Van der Waals attraction between the bound dye molecules and the dye and the polymer side groups, however, may play an important role in dye binding.