Thermodynamics of liquid mixtures of krypton and methane

Abstract

The vapour pressure of liquid mixtures of krypton and methane has been measured as a function of composition at 103.94, 115.77 and 144.79 K. The excess Gibbs energy of mixing G^E has been evaluated from the vapour pressure measurements, and the heat of mixing H^E and the excess entropy of mixing S^E have been estimated from the temperature dependence of G^E. The volume change on mixing, V^E, has been measured at 115.77 K, and is very small and negative. Values of the excess functions have been calculated from several recent theories. Even though the system only departs slightly from ideal behaviour, none of the theories applied predicts excess functions in good agreement with experiment if the geometric mean (Berthelot) rule is adopted for the intermolecular energy of a pair of unlike molecules. The differences between the theoretical and experimental values of G^E can be annulled by departures from the geometric mean rule of only a few tenths of 1 %. Boron trifluoride, which was used as a cryostat material to give the constant temperature of 144.79 K, appears to crystallize in two forms with triple-points only 0.09 K apart.