The effect of H/D Substitution and pressure on liquid liquid equilibrium for cyclohexane and methanol

The coexistence curve of cyclohexane + methanol system and its isotopic variants has been measured near the upper critical solution temperature at atmospheric pressure as well as at pressures up to 13 Mpa. For this purpose a computer controlled high pressure apparatus, using laser light scattering as phase transition indicator has been built. The systems studied are C₆H₁₂ + CH₃OH, C₆D₁₂ + CH₃OH, C₆H₁₂ + CH₃OD and C₆H₁₂ + CD₃OD. The parameters of the coexistence surface, i.e., critical temperatures, amplitudes, exponents and critical line slopes are reported. The effect of fractional isotopic dilution of each component of the mixture on the critical solution temperature has also been studied.

The effect of pressure and isotopic composition on the coexistence curve parameters have been discussed in relation to the predictions of thermodynamic and scaling theories of the critical solution phenomenon. The statistical mechanical theory of condensed phase isotope effects is applied to rationalize the observed isotope effect on the critical solution temperature. The observed non-linearity in the plot of critical solution temperature with respect to fractional isotopic composition is shown to be a consequence of non-ideality in isotopic mixtures.