THERMODYNAMICS OF BINARY-LIQUID MIXTURES OF PARTIALLY DEUTERATED METHANES WITH CH4 OR CD4

The vapor pressure isotope effect of samples of isotopically substitut ed methane and their mixtures was measured as a function of temperatur e and mixture composition: The differential vapor pressure between CHi D4-i (with i = 0 or 4) and CHjD4-j (with j = 1, 2, or 3), the differen tial vapor pressure between mixtures of (CHiD4-i+CHjD4-j) and CH4 (if i=4) or CHjD4-j (if i=0) and the absolute vapor pressure of CH4 (if i= 4) or CHjD4-j (if i=0), were measured simultaneously between 96 and 12 1 K for mixtures of nominal composition 0.25, 0.50, and 0.75 mole frac tion in the reference methane species. The p(x,T) data were used to ca lculate the excess molar Gibbs energy function, G(E)(x,T) and the exce ss molar enthalpy H-E(x), assuming that this last function is independ ent of temperature in the experimental range. The deviations from idea l behavior are very small, G(E) being only some tenths of J/mol for eq uimolar mixtures. The experimental GE values compare well with estimat ed results based on a modified version of the statistical theory of is otope effects in condensed phases. Comparisons with reported values of the liquid-vapor isotope fractionation factor for the CD3H-CH4 system are also made. (C) 1997 American Institute of Physics.