SPECTROSCOPIC MEASUREMENT OF LOCAL COMPOSITIONS IN BINARY-LIQUID SOLVENTS AND COMPARISON TO THE NRTL EQUATION

In this paper we compare the local compositions of solvents around a s olute predicted from the nonrandom two-liquid (NRTL) equation with exp erimental spectroscopic measurements of the local compositions to test the ability of the thermodynamic equation to model the true local env ironment. Shifts in the wavelength of maximum absorption of the solvat ochromic probe phenol blue are used to estimate the local compositions . Mixtures investigated include acetone/cyclohexane, triethylamine/cyc lohexane, ethyl butyrate/cyclohexane, cyclohexanone/cyclohexane, tolue ne/cyclohexane, acetophenone/cyclohexane, acetophenone/toluene, and et hyl butyrate/cyclohexanone. Comparison with the NRTL equation requires the interaction energies between the phenol blue and each of the solv ents, and these parameters are determined from the solubility of pheno l blue in the pure solvents. In most cases the measured local composit ions compare reasonably well with those predicted from the NRTL equati on. Spectroscopic measurements such as these are very useful in testin g and refining thermodynamic models.