Phase equilibria for binary n-alkanenitrile-n-alkane mixtures IV. Liquid-liquid phase equilibria for propanenitrile with C5C8 and C10n-alkanes

The orthobaric liquid-liquid phase diagrams for the five binary mixtures of propanenitrile with the n-alkanes C5C8 and C10 have been determined by visual observation of the separation temperatures of mixtures of known mass composition. The results have been fitted to a conventional modern expression incorporating where needed Wegner extended-scaling terms. Various aspects of coexistence curve symmetry have been explored, particularly symmetry of the critical composition, i.e., whether it falls close to 0.5, and symmetry of the diameter, i.e., whether it falls parallel to the temperature axis. Neither mole fraction nor volume fraction fully meets either requirement. A new variable τ based on a forced fit to τc = 0.5 reveals both kinds of symmetry and the data for all five mixtures when expressed in terms of a reduced temperature TTc as a function of τ fall on a universal curve. The implications of this for the coexistence curves for bihomologous mixtures are briefly discussed.