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 GE 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 GE, is HE(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.

Thermodynamics of binary liquid mixtures of partially deuterated methanes with CH4 or CD4

“…The literature sources or, when lacking, the methodology followed to estimate them was previously described in detail. 1 Values of G E are also shown in Table I along with the calculated values of liquid and gas phase composition, x a and y a , respectively, as well as the activity coefficient, ␥ a , of the reference species. Values for the excess pressure p E , defined as…”

“…1͑a͒-1͑f͒, two cross sections at constant temperature ͑100 and 120 K͒ of the fitted surfaces are shown, the results obtained being compared with those found for the (CH 4 ϩCD 4 ) system. 1 The points in Figs. 1͑a͒ and 1͑b͒ rep-resent values of G E taken from each experimental run ͑mix-ture͒, interpolated for the selected temperatures.…”

“…1,10,12 and where the isotopic composition of each sample is known. The vapor pressure data were measured using a doubly differential manometric technique described elsewhere.…”

Vapour pressure of liquid deuterium sulphide

“…Results for the vapour pressure isotope effect (VPIE) of isotopically substituted methane, ammonia and hydrogen chloride (all as pure components) have already been reported (1)(2)(3) and the data on methane isotopic mixtures have also appeared recently. (4) The protiated and deuterated hydrogen sulphide molecules were chosen as the next system. The VPIE of H 2 S and D 2 S (H, D substitution) has been determined together with the deviations from ideal behaviour of their mixtures.…”

Are isotopic mixtures ideal?

“…The vapor pressure differences between the CH 4 /CD 4 mixtures and CH 4 have been determined as a function of concentration and temperature [15]. The G E value obtained from the measured excess vapor pressure is, for example, 0.6 J/mol for the equimolar mixture at 100 K. The theoretical analysis has shown that the models generally used for simple mixtures, the "vdW-1 fluid theory" [24] and "1cLJ perturbation theory" [25], are inadequate in explaining the experimental results, while the isotope effect theory agrees well with experiment ( Table 2).…”