Thermodynamics Research Center / ThermoML | Fluid Phase Equilibria

ABSTRACT

Calculated ideal-gas heat capacities and experimental data for vapor pressure, condensed phase heatcapacities, and calorimetric vaporization enthalpies of alkane-alpha, omega-diamines, NH2-(CH2)n-NH2 (2 less than n less than 8), were treated simultaneously to obtain a consistent thermodynamic description. Vapor pressures for the members with 3 less than n less than 8 were measured using the static method in the temperature range from 233 to 314 K. The thermodynamic properties in the ideal gaseous state for the members with n = 2 and 3 were obtained using the fundamental vibrational frequencies, molecular structure data, and potential energy profiles of internal rotations calculated using the density functional theory (DFT) at the B3LYP/6-311+G(d,p) level of theory and four statistical thermodynamic models of different complexity. Based on the results of the simultaneous correlation, the sublimation and vaporization thermodynamic properties for 2 less than n less than 8 and the triple-point coordinates for n = 3, 4, 5 and 7 were calculated. The thermodynamic properties for liquid-vapor equilibrium display a linear dependence on number of methylene groups n, while the sublimation thermodynamic properties show an evident odd-even effect with higher values for even members of a homologous series. The alternating pattern is interpreted on the basis of the literature crystallographic data and corresponding intermolecular interactions. Based on the observed trends, the correlations allowing the prediction of the thermodynamic properties for the crystal-vapor and liquid-vapor equilibria and crystalline and ideal-gas heat capacities within the homologous series are suggested.