Standard Partial Molar Heat Capacities and Volumes of Aqueous Dimethylethanolamine and 3-Methoxypropylamine from 283 to 393 K, and Thermodynamic Functions for Their Ionization Equilibria to 598 K and 20 MPa
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Abstract
Densities for dilute aqueous solutions of N,N-dimethylethanolamine (DMEA), 3-methoxypropylamine (3-MPA), and their protonated salts were measured relative to water at 283.15 K ≤ T ≤ 363.15 K and 0.1 MPa using vibrating tube densimeters. Volumetric heat capacities of the same solutions were measured at 283.15 K ≤ T ≤ 393.15 K and 0.4 MPa using twin fixed-cell, power compensation, differential temperature scanning nano calorimeters. From these measurements apparent molar volumes, Vϕ, and heat capacities, Cp,ϕ, were calculated and corrected for speciation to obtain standard partial molar properties Vo and Cpo, for the species DMEA(aq), DMEAH+Cl-(aq), 3-MPA(aq) and 3-MPA+Cl-(aq). The experimental values for Vo and Cpo, measured in this work were combined with high temperature values reported by Bulemela and Tremaine (J. Phys. Chem. B 2008, 112, 5626-5645) over the range 423.15 to 598.15 K to derive parameters for the semi-empirical “density” model that expresses the standard partial molar thermodynamic properties of aqueous species as a function of temperature and solvent density. The fitted parameters were used with critically evaluated literature values for the standard partial molar Gibbs energy and enthalpy of reaction at 298.15 K to obtain values for the ionization constant of N,N-dimethylethanolamine and 3-methoxypropylamine from 283.15 to 598.15 K.