Equation of state of normal alkanes and thermodynamics of their mixtures

Abstract

The Prigogine principle of corresponding states for chain-molecule liquids is tested using published density, thermal expansion coefficient and isothermal compressibility data for the n-alkane series from methane to polymethylene. The principle is well obeyed allowing reduction parameters P^*, V^* and T^* to be determined. The equation-of-state data for the pure liquids and the corresponding states theory applied to their mixtures permit the accurate prediction of excess volumes and their temperature and pressure dependence. Theoretical equations of state based upon the cell partition function (including the new Flory theory) show systematic errors. Analyses of experimental data using these theories produce, as artefacts, a variation of the reduction parameters with T, and an important decrease of P^* with decreasing chain index. The present analysis show P^* to be constant to ±1 % from polymethylene to butane indicating that the terminal methyl groups and the interior methylene groups are similar in intermolecular contact energies. Without end-effects, the cell model theories predict and incorrect negative sign for H^E of n-alkane mixtures at ordinary temperatures.