The effect of H/D substitution and pressure on the liquid–liquid equilibrium between cyclohexane and methanol

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Abstract
Coexistence curves for the systems C6H12+CH3OH(i), C6D12+CH3OH(ii), C6H12+CH3OD(iii), and C6H12+CD3OD(iv) have been studied as a function of pressure [0.1<(P/MPa)<13] and reduced temperature t=(1−T/Tc) (0<t<1.3×10−2). A multiple sample technique was employed. The amplitudes and critical exponents and their pressure and isotope dependences are reported. The effect of isotopic dilution of each component on the critical solution temperature Tc has also been studied. The critical exponents show neither isotope nor pressure dependence. Critical temperatures show singnificant isotope dependence [Tc(i)−Tc(ii)]= i−ii=3.91 K, i−iii=−2.50 K, i−iv=0.23 K, and a significant pressure dependence dTc/dP=0.317 K/MPa, which over the range of conditions is independent of pressure and isotopic substitution. The amplitude factors, which carry larger experimental errors, show both isotope and pressure dependences. Isotopic dilution studies were carried out only at ambient pressure. Their interpretation leads to the conclusion that C6H12/C6D12 binary solutions are ideal within the experimental error of these measurements, but CH3OH/CH3OD mixtures show significant nonideality. The pressure and isotope dependence of the coexistence parameters for solutions i–iv are discussed in terms of thermodynamic and scaling theories of critical solution phenomena, and in terms of the statistical theory of isotope effects in condensed phases. The classical part of the analysis employed the Guggenheim theory of symmetrical mixtures, which quantitatively succeeded in rationalizing the isotope and pressure dependences of the effects.