Chemical equilibrium in fluids from the gaseous to liquid states: Solvent density dependence of the dimerization equilibrium of 2-methyl-2-nitrosopropane in carbon dioxide, chlorotrifluoromethane, and trifluoromethane

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Abstract
We have studied the dimerization equilibrium of 2‐methyl‐2‐nitrosopropane in carbon dioxide (CO2), chlorotrifluoromethane (CClF3), and trifluoromethane (CHF3) at 60 °C from the gaseous to liquid states. The dimerization equilibrium constants are not monotone functions of the solvent density in all solvent fluids. The density dependence of the equilibrium constant shows two inversions: The equilibrium constants in CO2 and CHF3 increase with increasing solvent density up to ρr≂0.3, where ρr is the reduced density by the critical density of solvent, whereas the equilibrium constant in CClF3 shows little dependence on the solvent density in this density region. From ρr≂0.3 to about 1.4, the equilibrium constants decrease with increasing solvent density. In the higher‐density region, the equilibrium constants increase again. We measured the equilibrium constants in CClF3 and CHF3 at 35 °C, and obtain the density dependence of the equilibrium constant which is almost the same as that at 60 °C. The effects of the temperature and the solvent species on the equilibrium constant are larger in the low‐density region than in the high‐density region. The differences between the low‐ and high‐density regions suggest that the energetic factor is dominant in the low‐density region, and that the packing effect is dominant in the high‐density region. The experimental results are qualitatively reproduced by a simple reaction model where two spherical molecules dimerize in a fluid composed of spherical molecules, although the reaction model overestimates the equilibrium constant.

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