Jacobson’s intermolecular free lengths in liquid mixtures – A thermodynamic approach

[Display omitted] •New thermodynamic formulae for liquid mixtures were reported.•Intermolecular free length in the thermodynamic ideal mixture was calculated.•Thermodynamic deviation of the free length corresponds to the excess volume. Jacobson’s intermolecular free length is a quantity related to molar volume and the internal surface of a liquid. A difference between intermolecular free length in a mixture and that calculated from the free lengths in its pure components results from molecular interactions. Commonly, the mole-fraction-weighted average of the intermolecular free lengths is a reference function in such calculations. The reference state lacking physical meaning leads to questionable conclusions. A new, correct formula for the intermolecular free length in thermodynamic ideal mixture was derived and discussed in this paper. The free lengths of the components in a thermodynamic ideal mixture are additive in a new “internal surface fraction” concentration scale. Examples illustrated significant differences between the deviations calculated from the two formulae for model systems and real binary mixtures. Since the values of the intermolecular free length deviations and the excess volumes have the same sign, the conclusions about intermolecular interactions based on these functions will be the same.