Limiting activity coefficients and gas-liquid partition coefficients of alkylbenzenes in hydro-organic solvents

The limiting activity coefficients and gas-liquid partition coefficients of a series of n-alkylbenzenes (benzene to n-butylbenzene), cumene and tert.-butylbenzene have been measured in mixtures of water with methanol, acetonitrile, isopropanol and tetrahydrofuran over a wide range in composition. The reproducibility of the measured data for 3-4 replicate runs was generally better than 5%. Activity coefficients have been studied as a function of volume fraction, π* (Kamlet-Taft solvent polarity/polarizability scale), Et (solvent polarity scale based on Reichardt's betaine) and surface tension. None of these solvent properties gives a universal curve for the different solvent systems. The overall free energy of the solution process was computed using the activity coefficient and the free energy of cavity formation was computed via the Sinanoglu-Reiss-Moura Ramos solvophobic model. The individual free energy terms were examined in terms of their dependence on solute size and solvent composition. The results were generally consistent with chemical expectations. The UNIFAC model of activity coefficient was examined. The predicted values of the limiting activity coefficients for the non-polar solutes in these aqueous solvents were disappointing. We suspect that the UNIFAC parameters needed to predict these systems are based on poor data.