Thermodynamics of mixtures containing alkoxyethanols: Part XVII — ERAS characterization of alkoxyethanol + alkane systems

Abstract

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, CPE, of solutions formed by a self-associated compound and an inert solvent has been derived. The CPE and the molar excess enthalpies (HE) and excess volumes (VE), as well as the molar enthalpies of vaporization of the pure alkoxyethanols, are represented accurately by ERAS. The calculated curves for HE and VE are skewed towards high mole fractions of the alkane. The experimental curves are more symmetrical. The opposite behaviour is observed for CPE 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 HE (except at high temperatures and pressures) and for CPE. 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 HE and VE 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. Key words: alkoxyethanol, intermolecular, intramolecular, H-bond, dipole–dipole interactions.