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The interaction of N-methylpyrrolidone with azobenzene in aqueous solutions containing 1 mol/l NaCl, LiCl, NaClO4 and NaSCN has been investigated by means of the solubility measurement of azobenzene. The thermodynamic parameters for the process which consists of the transfer of 1 mole azobenzene from the salt solution to the salt solution containing 6 mol/l N-methylpyrrolidone have been calculated. In the presence of the inorganic salts, the values of ΔF°u-trans were negative. However, the contributions of enthalpy and entropy changes to the free energy change of the process varied with the salt species. It was shown that in the presence of a water structure-breaking salt such as NaClO4 or NaSCN, the value of ΔH°trans was negative and ΔS°u-trans was positive, whereas in the presence of a water structure-making salt such as NaCl or LiCl, the value of Δ°trans was nearly zero and ΔS°u-trans was positive. These facts were interpreted in terms of hydrophobic interactions between the non-polar parts of azobenzene and N-methylpyrrolidone.

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 ΔH° and ΔS° values is not pronounced. Also temperature dependences of the ΔH° and ΔS° 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.

The extent of binding of methyl orange, ethyl orange, propyl orange, and butyl orange by poly(vinylpyrrolidone) has been measured in aqueous solutions of inorganic electrolytes such as NaCl, LiCl, NaSCN, and NaClO4 by an equilibrium dialysis method. The effect of the salts on the first binding constants and the thermodynamic functions which are accompanied by the dye—polymer association process was investigated relative to the corresponding values in the absence of such salts. It was found that in aqueous solutions of NaCl and LiCl the enthalpy change accompanying the binding is small and the largest contribution to the free energy of binding is from the positive entropy gain. For NaSCN and NaClO4, the values of ΔF° and ΔH° were both large and negative and the value of ΔS° was small and negative. Thus, the favorable free energy for the complex formation was due entirely to the negative enthalpy term. These characteristics of the thermodynamic quantities are discussed in terms of changes in structural properties of water in the vicinity of the binding entities and conformational changes of the polymer to which the dye is bound due to the added foreign electrolytes.