Standard Partial Molar Volumes of Some Aqueous Alkanolamines and Alkoxyamines at Temperatures up to 325 °C:  Functional Group Additivity in Polar Organic Solutes under Hydrothermal Conditions

Apparent molar volumes of dilute aqueous solutions of monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), N,N −dimethylethanolamine (DMEA), ethylethanolamine (EAE), 2-diethylethanolamine (2-DEEA), and 3-methoxypropylamine (3-MPA) and their salts were measured at temperatures from 150 to 325 °C and pressures as high as 15 MPa. The results were corrected for the ionization and used to obtain the standard partial molar volumes,. A three-parameter equation of state was used to describe the temperature and pressure dependence of the standard partial molar volumes. The fitting parameters were successfully divided into functional group contributions at all temperatures to obtain the standard partial molar volume contributions. Including literature results for alcohols, carboxylic acids, and hydroxycarboxylic acids yielded the standard partial molar volume contributions of the functional groups >CH−, >CH 2, −CH 3, −OH, −COOH, −O−, ⪫N, >NH, −NH 2, −COO - Na +, −NH 3 + Cl -, >NH 2 + Cl -, and ⪫NH + Cl - over the range (150 °C ≤ t ≤ 325 °C). These allow predictions of the standard partial molar volume of aqueous organic solutes composed of these groups at temperatures up to ∼310 °C and pressures of 10−20 MPa to within a precision of ±5 cm 3 ·mol -1 . The model could not be extended to higher temperatures because of uncertainties caused by thermal decomposition. At temperatures above ∼250 °C, the order of the group contributions tochanges from that observed at 25 °C, to become increasingly consistent with the polarity of each functional group. The effect of the dipole moment of each molecule on the contribution tofrom long-range solvent polarization was calculated from the multipole expansion of the Born equation using dipole moments estimated from restricted Hartree−Fock calculations with Gaussian 03 (Gaussian, Inc., Wallingford, CT) and the Onsager reaction-field approximation for solvent effects. Below 325 °C, the dipole contribution was found to be less than 2 cm 3 ·mol -1 for all the solute molecules studied. At higher temperatures and pressures near steam saturation, the effect is much larger and may explain anomalies in functional group additivity observed in small, very polar solutes.

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