CHEMICAL-EQUILIBRIUM IN SIMPLE FLUIDS - SOLVENT DENSITY DEPENDENCE OF THE DIMERIZATION EQUILIBRIUM OF 2-METHYL-2-NITROSOPROPANE IN ARGON AND XENON

We have studied the dimerization equilibrium of 2-methyl-2-nitrosopropane (MNP) at 60 and 35-degrees-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-degrees-C decreases in the low-density region, whereas it increases in the high-density region. The inversion of the density dependence occurs around rho(r) congruent-to 0.8 in argon and approximately 1.4 in xenon, where rho(r) means the density reduced by the critical density of the solvent fluid. The equilibrium constant in xenon at 35-degrees-C increases with increasing solvent density in the low-density region (rho(r) < approximately 0.5), while it decreases in the medium-density region (0.5 < rho(r) < 1.5). The equilibrium constant in argon at 35-degrees-C has a similar density dependence to that at 60-degrees-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.