The solubility of solids in near-critical fluids. Vi. CHI3 in CO2 revisited

The possibility of a complete thermodynamic description of the behaviour of dilute solutions on the basis of the Krichevskii function J = (∂p/∂x)V,T∞, which performs well in the near-critical region, is hampered by its weak temperature dependence. A previous determination of the solubility of CHI3 in near-critical CO2 showed little change in J over a temperature interval of 30 K. Using a high-pressure spectrophotometric technique, we have re-investigated (CHI3 + CO2), covering the temperature range 273 K to 373 K with seven isotherms. The maximum pressure was 25 MPa and, depending on temperature, the density of CO2 studied was between 0.5 mol · dm-3 and 25 mol · dm-3. As the maximum solubility of CHI3 was 1.7 · 10-2 mol · dm-3 (at the highest temperature and pressure), the solutions were sufficiently dilute to be considered in the Henrian range. The solubility data expressed as ln E, where E is the enhancement factor, were successfully fitted (with the exception of the 305.4 K isotherm) to polynomials of the reduced solvent density and of the square root of the reciprocal reduced temperature. The solubility of CHI3 at low fluid densities was used to obtain information about the intermolecular parameters of the solute using the second virial coefficient limiting expression for ln E. The experimental data allowed reliable estimation of the temperature dependence of J in the whole temperature and pressure ranges studied. The 305.4 K isotherm, which is only 0.4 per cent above the critical temperature of CO2, shows a different dependence on the fluid density, and consequently had to be analysed separately. A careful analysis of the experimental errors suggests that the difference is not an artifact, but is due to the near-critical behaviour of the solvent coupled with the solvent-solute intermolecular interactions