Inferences of Acidity and Basicity from the Asymmetric Master Equation Method

The asymmetric master equation method (MEM2) [Ind. Eng. Chem. Res., 2022, 61, 15724] is implemented using the Elliott-Suresh-Donohue (ESD) equation of state as the basis for describing interactions by dispersion forces. The energies of self- and cross-association are inferred from the vapor-liquid equilibrium (VLE) and excess enthalpy (H^E) experimental data. Through the MEM2 theory, these association energies can be interpreted as electron donor (basicity) and acceptor (acidity) energies inherent to individual molecules or functionalities within those molecules. In this way, acidity and basicity are characterized for 20 functionalities, plus water, carbon dioxide, ammonia, and a few other common solvents. VLE data are usually insufficient to assign unambiguous values of acidity and basicity. Through simultaneous correlation of VLE and H^E data, the ESD-MEM2 model sets the stage for significant progress in accurately modeling combined properties with a self-consistent model. In most cases, VLE are modeled with less than 2% deviation in bubble point pressure, and H^E is modeled with deviations less than 0.1 kJ/mol. Comparison of the MEM2 acidity and basicity to parameters based on the linear solvation free energy relationship shows trends that are not quantitative. It is demonstrated that a functionality like a phenyl ring may exhibit a range of basicity depending on the acidic probe molecule. From this perspective, most values tabulated here represent initial estimates of properties that should be averaged over many probe compounds. Nevertheless, the insensitivity of VLE to the exact value of acidity or basicity means that these average values can still be useful in predictions of phase behavior, while the customized values can be used to characterize specific binary mixtures when sufficient experimental data are available.