Vapor pressure isotope effects in aqueous systems. VIII. The system dimethyl sulfoxide/H2O/D2O

Vapor pressures for the system I (dimethyl sulfoxide/H₂O=DMSO/H₂O) and isotopic differential pressures I-II (II=DMSO/D₂O) have been measured between 25 and 70°C at DMSO concentrations of 0.05, 0.15, 0.30, 0.45, 0.60, 0.70, 0.80, 0.87, and 0.92 mole fraction. A high-precision differential method was used. The total pressures over the solutions, I, have been fitted to a relation derived from the Duhem-Margules equation, P ^T =P ₁ ^o X₁γ₁+P ₂ ^o X₂γ₂, with γ₁=exp∑_kα_kX ₂ ^k ] and $$\gamma _2 = exp\sum \alpha _k X_2^k - \sum (\alpha _k /(k - 1))(kX_2^{k - 1} - 1)].$$ . The α_k are parameters andk is a number ≥2. The α_k were taken as temperature dependent. Four parameters sufficed to fit the data within experimental error. Excess partial molal properties derived from the fits are in quantitative agreement with earlier literature 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 relation above. The excess free energies and enthalpiesG _I ^E andH _I ^E are large and negative. The isotope effects ΔG _I,II ^E =G _I ^E ?G _II ^E and ΔH _I,II ^E are negative. 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. A small amount of enthalpy data for the solution of DMSO in HOH and DOD is reported.