Thermodynamics of mixtures containing alkoxyethanols:: Part XVII -: ERAS characterization of alkoxyethanol plus alkane systems

Alkoxyethanol + alkane systems have been examined in the framework of the ERAS model. An exact expression for the molar excess heat capacity at constant pressure, C-P(E), of solutions formed by a self-associated compound and an inert solvent has been derived. The C-P(E) and the molar excess enthalpies (H-E) and excess volumes (V-E), as well as the molar enthalpies of vaporization of the pure alkoxyethanols, are represented accurately by ERAS. The calculated curves for H-E and V-E are skewed towards high mole fractions of the alkane. The experimental curves are more symmetrical. The opposite behaviour is observed for C-P(E) in solutions with 2-ethoxyethanol, 2-propoxyethanol, or 2-butoxyethanol. The differences between the experimental and theoretical values arise because ERAS does not properly take into account the enhanced dipole-dipole interactions due to the formation of intramolecular H-bonds in alkoxyethanols. As in previous applications, ERAS cannot simultaneously represent molar excess Gibbs energies and liquid-liquid equilibria. DISQUAC, a purely physical theory, improves ERAS predictions for H-E (except at high temperatures and pressures) and for C-P(E). Liquid-liquid equilibria are also described more consistently. The self-association of alkoxyethanols via intramolecular H-bonds and the strong dipole-dipole interactions lead to values of the self-association enthalpy and of the adjustable parameter of the physical contribution to H-E and V-E that are higher than those of the homomorphic 1-alkanols. In contrast, the equilibrium constants are lower. There is good agreement between the partial molar excess enthalpies at 298.15 K and infinite dilution of 2-alkoxyethanol in 2-alkoxyethanol(1) + n-heptane(2) mixtures and the values of the self-association enthalpies.