Binding of methyl orange and its homologs by polyion complex in ethylene glycol

Abstract

AbstractThe extent of binding of methyl orange, ethyl orange, propyl orange, and butyl orange by a polyion complex, which consists of equimolar sodium polymethacrylate and a piperidinium cationic polymer, was measured in ethylene glycol at 25, 30, and 35°C. The effects of aqueous and nonaqueous media on the binding of the dye by the polymer matrix were compared. In ethylene glycol only noncooperative binding (Langmuir type) was observed, whereas two modes of binding, noncooperative and cooperative, were involved in the aqueous environment. The first binding constants and the thermodynamic parameters associated with the binding in ethylene glycol were evaluated. The polymer matrix exhibited a higher affinity toward the dye in ethylene glycol than in water at low concentrations of free dye and bound these anionic cosolutes with different hydrophobicities but the same affinity as in ethylene glycol. These interactions are highly exothermic and characterized by relatively large negative entropy changes. Electrostatic forces are the principal contributors to the dye–polymer complex formation. The binding behavior observed in the nonaqueous solvent is reasonably accounted for in terms of disappearance of hydrophobic interactions between the dye and the polymer and an increase in electrostatic attraction between the anionic small molecule and the positively charged sites on the polyion complex.