Thermodynamics of aqueous methyldiethanolamine and methyldiethanolammonium chloride over a wide range of temperature and pressure : apparent molar volumes, heat capacities, compressibilities, and excess molar heat capacities

摘要

Alkanolamines are an industrially important class of compounds that are used in a wide variety of applications because of their thermal stability, volatility, and basicity. Since they are ideal for probing solvation behavior in high temperature water, they are of considerable interest in the development of theoretical models for ionic solvation. Standard partial molar volumes V°, heat capacities C°p, and compressibilities K°are important in this context because they define the temperature and pressure dependence of important parameters such as the ionization constant. This comprehensive study presents thermodynamic data leading to standard partial molar properties and excess properties of the tertiary alkanolamine, methylchethanolamine (MDEA) and its chloride salt over a wide temperature range at various pressures. -- Apparent molar volumes of aqueous methyldiethanolamine and its salt were determined with platinum vibrating tube densitometers over a range of temperatures from 283 to 573 K and at pressures from 0.1 MPa to 20 MPa. Apparent molar heat capacities were obtained using a Sodev Picker flow microcalorimeter at a pressure of 0.1 MPa and within the temperature range of 283 to 328 K. The experimental results for the neutral amine were well represented using linear expressions with corrections for partial ionization and then extrapolated to infinite dilution to obtain values for V° and C°. The experimental results for the salt were determined as a function of ionic strength and analyzed by means of the Guggenheim form of the extended Debye-Hiickel equation to obtain values for V° and C°p. Apparent molar isothermal compressibilities kt° at a pressure of 0.1 MPa were obtained at temperatures from 283 to 313 K with speed of sound measurements. Compressibilities were important in this study since they account for the small pressure dependence observed in the measured apparent molar volumes. -- The standard partial molar volumes V° for the neutral amine and its ionized form show increasingly positive and negative values at high temperatures and pressures, as predicted by corresponding states and group additivity arguments. The standard partial molar volume data were used to successfully extrapolate low temperature C°p data to elevated temperatures. The density model and the revised Helgeson-Kirkham-Flowers (HKF) model have been used to represent the temperature and pressure dependence of the standard partial molar properties to yield a full thermodynamic description of the systems. Further, using appropriate expressions, the density model was employed to obtain the concentration dependence of the apparent molar properties. -- Excess heat capacities CpEX from 278 to 373 K were determined with a CSC 4100 differential scanning calorimeter over the entire mole fraction range for the water- methyldiethanolamine system and the results were analyzed using Redlich-Kister type equations. Calculations were made in accordance with appropriate water vapor corrections based on Raoult's law and the Clausius-Clapeyron equation. The excess heat capacities are positive in magnitude over the entire mole fraction range which is typical of completely miscible water + polar organic systems and indicates a large positive deviation from ideality. In general, the CpEX become progressively more positive as the temperature increases with the maxima shifting to larger mole fractions in the water-rich region. Direct data reduction yields reduced excess heat capacities which appear to be consistent with solute-induced structural changes in the intermolecular structure of the water/MDEA system.