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 solute predicted from the nonrandom two-liquid (NRTL) equation with experimental spectroscopic measurements of the local compositions to test the ability of the thermodynamic equation to model the true local environment. Shifts in the wavelength of maximum absorption of the solvatochromic probe phenol blue are used to estimate the local compositions. Mixtures investigated include acetone/cyclohexane, triethylamine/cyclohexane, ethyl butyrate/cyclohexane, cyclohexanone/cyclohexane, toluene/cyclohexane, acetophenone/cyclohexane, acetophenone/toluene, and ethyl butyrate/cyclohexanone. Comparison with the NRTL equation requires the interaction energies between the phenol blue and each of the solvents, and these parameters are determined from the solubility of phenol blue in the pure solvents. In most cases the measured local compositions compare reasonably well with those predicted from the NRTL equation. Spectroscopic measurements such as these are very useful in testing and refining thermodynamic models.