Non-isoplethic measurement on the solid-liquid-vapor equilibrium of binary mixtures at cryogenic temperatures

We measured the solid-liquid-vapor (SLV) equilibrium of binary mixtures during experiments that alternated between cooling the mixture and injecting the more-volatile component into the sample chamber; thus, the composition of the mixture changed (non-isoplethic) throughout the experiment. Four binary mixtures were used in the experiments to represent mixtures with miscible solid phases (N₂/CO) and barely miscible solid solutions (N₂/C₂H₆), as well as mixtures with intermediate solid miscibility (N₂/CH₄ and CO/CH₄). We measured new SLV pressure data for the binary mixtures, except for N₂/CH₄, which are also available in the literature for verification in this work. While these mixtures are of great interest in planetary science and cryogenics, the resulting pressure data are also needed for modeling purposes. We found the results for N₂/CH₄ to be consistent with the literature. The resulting new SLV curve for CO/CH₄ shows similarities to N₂/CH₄. Both have two density inversion points (bracketing the temperature range where the solid floats). This result is important for places such as Pluto, Triton, and Titan, where these mixtures exist in vapor, liquid, and solid phases. Based on our experiments, the presence of a eutectic is unlikely for the N₂/CH₄ and CO/CH₄ systems. An azeotrope with or without a peritectic is likely, but further investigations are needed to confirm. The N₂/CO system does not have a density inversion point, as the ice always sinks in its liquid. For N₂/C₂H₆, new SLV pressure data were measured near each triple point of the pure components.