Thermodynamic properties of aqueous mixtures of hydrophilic compounds 2. Aminoethanol and its methyl derivatives

The vapour pressures of water + 2-aminoethanol, + N-methyl-2-aminoethanol, and + N,N-dimethyl-2-aminoethanol at 298.15 and 308.15 K were measured over the whole composition range by a static method. The excess enthalpies and densities of the same mixtures at 298.15 K were also measured with an isothermal displacement calorimeter and a pyknometer. The thermodynamic excess functions: G^E, H^E, TS^E, and V^E were calculated. Except for aminoethanol-rich region in (water + N,N-dimethyl-2-aminoethanol), where G^E is slightly positive, the signs and relative magnitudes of molar excess functions were 0 > G^E>TS^E>H^E, V^E < 0. These are in accordance with general characteristics of (water + a highly hydrophilic compound). However, hydrophobicity is clearly seen for dilute aqueous solutions where the partial molar volume of aminoethanol exhibits a minimum and the partial molar enthalpy decreases rapidly to a limiting value at infinite dilution. This tendency increases with the introduction of methyl groups into 2-aminoethanol.