Thermodynamic Properties of Aqueous Dimethylamine and Dimethylammonium Chloride at Temperatures from 283 K to 523 K: Apparent Molar Volumes, Heat Capacities, and Temperature Dependence of Ionization

Abstract

Data for the apparent molar volumes of aqueous dimethylamine and dimethylammonium chloride have been determined with platinum vibrating tube densimeters at temperatures 283.15 K ≤ T ≤ 523.15 K and at different pressures. Apparent molar heat capacities were measured with a Picker flow microcalorimeter over the temperature range 283.15 K ≤T ≤ 343.15 K at 1 bar. At high temperatures and steam saturation pressures, the standard partial molar volumes $V_2^ \circ$ of dimethylamine and dimethylammonium chloride deviate towards positive and negative discontinuities at the critical temperature and pressure, as is typical for many neutral and ionic species. The revised Helgeson-Kirkham-Flowers (HKF) model and fitting equations based on the appropriate derivatives of solvent density have been used to represent the temperature and pressure dependence of the standard partial molar properties. The standard partial molar heat capacities of dimethylamine ionization $\Delta _{{\text{ion}}} C_{{\text{p,2}}}^ \circ$, calculated from both models, are consistent with literature data obtained by calorimetric measurements at T ≤ 398 K to within experimental error. At temperatures below 523 K, the standard partial molar volumes of dimethylamine ionization $\Delta _{{\text{ion }}} V_2^ \circ$ agree with those of morpholine to within 12 cm³-mol^-1, suggesting that the ionization of secondary amine groups in each molecule is very similar. The extrapolated value for $\Delta _{{\text{ion }}} V_2^ \circ$ of dimethylamine above 523 K is very different from the values measured for morpholine at higher temperature. The difference is undoubtedly due to the lower critical temperature and pressure of (CH₃)₂NH(aq).