Chemical equilibrium in simple fluids: Solvent density dependence of the dimerization equilibrium of 2-methyl-2-nitrosopropane in argon and xenon

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Abstract
We have studied the dimerization equilibrium of 2‐methyl‐2‐nitrosopropane (MNP) at 60 and 35 °C in argon and xenon up to densities about twice the critical density of the solvent. With an increase in the solvent density, the dimerization equilibrium constant at 60 °C decreases in the low‐density region, whereas it increases in the high‐density region. The inversion of the density dependence occurs around ρr≂0.8 in argon and ∼1.4 in xenon, where ρr means the density reduced by the critical density of the solvent fluid. The equilibrium constant in xenon at 35 °C increases with increasing solvent density in the low‐density region (ρr<∼0.5), while it decreases in the medium‐density region (0.5<ρr<1.5). The equilibrium constant in argon at 35 °C has a similar density dependence to that at 60 °C. The internal energy change for the dimerization shows a large density dependence in the low‐density region of xenon, while it changes little with density in argon. The theoretical calculation by the Percus–Yevick (PY) approximation for a simple reaction model gives a poor result for the density dependence of the equilibrium constant, although the temperature dependence of the equilibrium constant is reproduced qualitatively. The calculation for a more realistic model including the molecular anisotropy of MNP suggests that the density dependence at the low‐density limit is sensitive to the interaction model.