Interaction of poly(vinylpyrrolidone) with methyl orange and its homologs in aqueous solution over the temperature range 60–90°c

Abstract

Abstract The binding of methyl orange, ethyl orange, propyl orange, and butyl orange by poly(vinylpyrrolidone) has been examined by a technique of equilibrium dialysis over a high temperature range (60–90°C). The first binding constants and the thermodynamic parameters in the course of the binding were evaluated. The results obtained at these temperatures were compared to those at lower ones (5–35°C) described previously in order to estimate the contribution of hydrophobic bonds to the binding. It was found that at the 60–90°C range complex formation between the dye and the macromolecule is associated with an exothermic enthalpy change and a positive entropy change. The enthalpy and entropy changes of the binding are of the order of −4.5 kcal/mole and 6 eu, respectively, for each dye measured. Thus the binding is mainly enthalpy‐controlled. Furthermore the effect of the alkyl chain length of the dye on both the Δ H ° and Δ S ° values is not pronounced. Also temperature dependences of the Δ H ° and Δ S ° terms were not observed. All these observations in the higher temperature range can be explained as a result of the disruption of water structure in the binding environment and hence a decrease in hydrophobic bond formation between the dye and the polymer.