Vapor pressure of partially deuterated methanes (CH3D, CH2D2, and CHD3)

Abstract

The difference between the vapor pressure of completely protiated or deuterated methane (CH4 or CD4) and partially deuterated methanes (CH3D, CH2D2, or CHD3) has been measured over the 96–121 K temperature range. The vapor pressure data obtained were fitted to equations of the type T ln(p/p)=A/T+B, where the prime always refers to the lighter molecule. Within the studied temperature range, the vapor pressure isotope effect was found to be “inverse” (p>p) for all the systems, except in the low-temperature range of the (CH4/CH3D) system. Our data agrees with other results found in the literature, both experimental and theoretical. Differences in the enthalpy of vaporization were calculated from the experimental results. In the case of the (CH4/CHD3) system, our vapor pressure isotope effect (VPIE) results were also compared with liquid–vapor isotope fractionation factor (LVIFF) data from other authors.

Isotope effects on VLE properties of fluids and corresponding states: Critical point shifts on isotopic substitution

“…These parameters each correlate nicely with tabulated [27] Pitzer acentric factors, ω (Table 2), and yield good agreement between calculated and observed vapor pressures [2,[41][42][43][44][45][46][49][50][51][52][53][54]. Calculated liquid densities lie below experiment for (0 < τ = 1 − T/T C < ∼0.3).…”

“…For CH 4 /CD 4 , critical properties, and liquid density and vapor pressure IE's have been measured by Grigor and Steele [2]. Vapor pressures have been reported from Lisbon for other methane isotopomers and for mixtures of these isotopomers [51][52][53][54]. Neither critical property nor molar density IE's are known for methane isotopomer pairs other than CH 4 /CD 4 .…”

Search for methane isotope fractionation due to Rayleigh distillation on Titan

“…If condensation occurs under Rayleigh conditions, where the condensate is immediately removed from the vapor after formation, and we assume that process is isothermal at temperature T , Calado et al (1997) liquid, Armstrong et al (1953) solid, Armstrong et al (1953) solid, Fouchet & Lellouch (2000) Figure 6: The fractionation factor, α, for vapor in equilibrium with solid and liquid methane from the two-parameter fits to the laboratory data of Armstrong et al (1953) and Calado et al (1997), along with a curve for the expression used by Fouchet and Lellouch (2000).…”

“…The fractionation factor is the ratio of vapor pressures for the two isotopologues. Laboratory measurements of the vapor pressures of methane isotopologues in equilibrium with solid and liquid methane at different temperatures are reported with various two-parameters fits (Armstrong et al, 1953;Calado et al, 1997), and are shown in Figure 6. Also included is the expression used by Fouchet and Lellouch (2000), where they assumed a maximum α = 1.004 at 71 K, which they used to calculate a maximum -6‰ fractionation on Titan for solid methane condensate.…”

“…Here we use the more recent laboratory measurements from Calado et al (1997) for the liquid methane fractionation factor for CH 3 D/CH 4 ,…”