Thermodynamics of hydrogen-bonding mixtures 4: GE, HE, SE and CPE and possible double azeotropy of water+N-methylethylenediamine

An ebulliometer was used to measure total vapor-pressure (PTx) data on nine mixtures of water+N-methylethylenediamine (and the pure components) between 52 and 116°C. Bubble-point data were measured at six pressures from 13.33 to 101.325 kPa. These PTx data indicate the following azeotropic behavior: at pressures below 32.7 kPa there is a single maximum-boiling azeotrope, above 46.7 kPa there is no azeotrope, and between 32.7 and 46.7 kPa there is the possibility of a double azeotrope. This type of double azeotrope is rare because the pure component vapor pressures are considerably different (Psat1/Psat1≈1.6); it may be that the apparent extrema in the Tx data are due to artifacts related to the purity of the N-methylethylenediamine (≈95 mass%). ich–Kister GE model was fitted to isotherms at 60, 85, 90 and 100°C via Barker’s method with an average standard error of 0.52% in pressure. The system exhibits large negative deviations from ideality (derived γ∞=0.05–0.67) which decrease with increasing temperature. Equimolar GE/T values thus derived increase with increasing temperature which predicts a negative HE. Equimolar CPE data, measured by differential scanning calorimetry (DSC), are positive and also increase with increasing temperature. These mixture thermodynamic data show that the system water+N-methylethylenediamine belongs to the class of mixtures where GE<0, HE<0, and TSE<0. Therefore, the data indicate that (1–2) hydrogen-bonding of water with N-methylethylenediamine is greater than either the (1–1) or (2–2) hydrogen-bonding in the pure components.