Vapour pressure isotope effects in aqueous systems. Part 7.—The system [1H5]pyridine +[2H5]pyridine + H2O

Abstract

Vapour pressures for the system I (pyridine + H₂O) and isotopic differential pressures; I—II, (II =[²H₅]pyridine + H₂O) have been measured between 25 and 65°C at pyridine concentrations of 0.05, 0.10, 0.20, 0.35, 0.50, 0.65, 0.80, 0.90 and 1.0 mole fraction. The isotope effect on pure pyridine is inverse and small. It is described by the least square fit ln R= ln (P_h/P_d)= 22 518.5/T²—62.944 8/T—4.762 2 × 10⁸/T⁴. The total pressures over the solutions, I, have been fitted to a relation derived from the Duhem—Margules equation, P_Tot=P^°₁X₁γ^*₁+P^°₂X₂γ^*₂ with γ^*₁= [graphic omitted] and γ^*₂= exp[[graphic omitted](α_k/(k–1))(kX^k–1₂–1)]. The α_k are parameters and k is a number 2. The α_k were taken as temperature dependent. Seven parameters sufficed to fit the data within experimental error. Excess partial molal properties derived from the fits are in quantitative agreement with earlier results derived from the directly measured partial pressures but the present data extend over a wider temperature range. The isotopic differential pressures, I—II, were similarly fitted to the above relation. The excess free energies (positive) and enthalpies (negative), G^E_I and H^E_I, are large. The isotope effects, ΔG^E_I,II=G^E_I–G^E_II, and ΔH^E_I,II are positive. They are discussed in detail in terms of the theory of isotope effects in condensed phases and demonstrated to be consistent with that theory and with the available spectroscopic data. The measurements of Rabinovich for the system III, (pyridine + DOD) are discussed in the same context. The isotope effects ΔG_I,III and ΔH_I,III are negative. This observation is demonstrated to be spectroscopically and theoretically consistent with the present measurements. The effects I, I–II and I–III, for the pyridine water system, and by analogy for substituted pyridine water systems, are discussed in terms of the closeness to phase separation and the trends to be expected for isotope effects on the consolute properties.