Predicting miscibility of binary liquids from small cluster QCE calculations.

The quantum cluster equilibrium method is applied to model binary systems of molecular solvents. We minimize the computational effort as well as the experimental input and present the results obtained for the completely miscible acetonitrile/acetone, benzene/acetone, and water/acetone systems, as well as for the hardly miscible water/benzene system. Only clusters of sizes up to n = 3 are applied and these are optimized employing the low-cost functional PBEh-3c. The thermodynamic functions of the pure liquids are in reasonable agreement with experiments. For both non-water containing binary systems, the Gibbs energy of mixing can be reproduced with an accuracy of ≈0.25 kJ/mol. Water containing systems are not sufficiently described by small clusters. The empirical mean-field parameter amf and exclusion volume scaling parameter bxv which depend on the experimental input are approximated by linear interpolation between their neat liquids' reference values. This makes the approach independent from the experimental data of the binary system. Despite the roughness of the approximation as well as the small size of the cluster sets, the approach is able to correctly predict the mixing behavior of all acetone systems. The benzene/water system is correctly predicted to be non-miscible at most mole fractions. A small range at high benzene concentrations (x> 0.8) is falsely predicted to be miscible.