INTERACTION OF N-METHYLPYRROLIDONE WITH AZOBENZENE: THE EFFECT OF INORGANIC SALTS

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.