The excess thermodynamic properties of liquid (CH4+CD4)

Abstract

The vapor pressure differences between a mixture of (CH₄+CD₄) and CH₄ and between CD₄ and CH₄ were measured simultaneously with the CH₄ vapor pressure. This was done at 29 temperatures between 97 and 122 K, and for six different mixtures, of compositions 0.150, 0.250, 0.375, 0.500, 0.625, and 0.750 mole fraction in CD₄. These mixtures exhibit very small positive deviations from Raoult’s law. Calculated excess Gibbs energies for equimolar mixtures were 0.60 J mol⁻¹ at 100 K and 0.42 J mol⁻¹ at 120 K. These values of G^E are 2 to 3 orders of magnitude smaller than those typically found in binary mixtures of simple nonisotopic species. The molar excess enthalpy, calculated from the temperature dependence of G^E, is H^E(x=0.5)=(1.5±0.2) J mol⁻¹. The experimental results were used to test three theoretical models: the vdW‐1 fluid theory, 1cLJ perturbation theory, and the theory of isotope effects in mixtures. While the first two proved to be inadequate, the isotope effect theory agrees well with experiment.