The interactions of methyl orange with methyl, hexyl, and benzyl quaternized poly(JVvinylimidazoles) were studied to assess the structural factors and the nature of the driving forces for dye binding. The stoichiometry of the polymer-dye complex is dictated by charge. The apparent binding constant increases with polymer charge density as well as polymer saturation r (where r = [Db]/[Pt], and [Db] and [Pt] are the molar concentrations of bound dye and total polymer quaternary sites, respectively), but decreases in the presence of competing counterions, indicating that Coulombic forces operate. The van der Waals forces between the dye and the aromatic or long-chain hydrocarbon group at the polymer quaternary site also enhance binding strength. Cooperative dye binding was observed in the region 0 < r < 0.7. This behavior is discussed in terms of the dimerization tendency of the dye molecules. In the high-saturation region (0.7 < r < 1.0), however, steric hindrance prevents further aggregation of the dye, and cooperativity was lost.
Molecular modeling was used to study the intramolecular conformational behavior of neutral and fully quaternized poly(jV-vinylimidazole) and the anionic form of methyl orange. Subsequently, intermolecular modeling of polymer-dye interactions was performed on the basis of the intramolecular findings. The purpose of the intermolecular studies was to identify a molecular geometry of the polymer-dye complex whose thermodynamic properties are consistent with experimental observations. This was achieved by assuming an isotactic chain segment of the polymer in a 3/1 helical conformation and an effective dielectric constant (in the Coulomb atom-pair potential) of 30. The polymer-dye binding complex is characterized by partial insertion (intercalation) of a phenyl ring of the dye between spatially adjacent polymer side-chain rings with the sulfonate of the dye located as close as possible to the charged imidazole nitrogen of the polymer.
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