Determination of the molecular size from measurements of vapour pressure of binary liquid mixtures. Theory, experiments and quantum chemical calculations

In the present work we focus on some fundamental organic solvents. The total and partial vapour pressure of 30 binary systems made of hexane, cyclohexane, and benzene with 10 organic molecules, namely chloroform (Chl), chlorobenzene (Cl-Ben), tetrahydrofuran (THF), dichloromethane (DCM), acetone (Ac), ethanol (EtOH), N, N-dimethylformamide (DMF), methanol (MeOH), acetonitrile (MeCN) and dimethylsulphoxide (DMSO), were measured using the method developed by Sameshima. A simple electrostatic model was derived to express the change of the Helmholtz free energy as a function of dielectric properties of the binary mixtures, as well as the dielectric constant, dipole moment, dipole polarizability, and radius of a spherical cavity hosting the aforementioned 10 organic molecules. Through measurements of the partial vapour pressure of these 10 organic liquids, the size of the cavity was extracted, which approximates their molecular size. The molecular size obtained from the experiments exhibits a good agreement with the results as obtained from the cavity size calculated using the Polarizable Continuum Model (PCM). The partial vapour pressure of some compounds such as Ac, DMF, DMSO, and MeCN presented severe deviation from the Raoult law. The intermolecular interaction among the molecules of each compound was believed to take responsibility for their exceptional behaviours in vapour pressure.