Thermodynamic properties of aqueous morpholine and morpholinium chloride at temperatures from 10 to 300 degrees C: Apparent molar volumes, heat capacities, and temperature dependence of ionization

Apparent molar heat capacities C-p,C-phi of aqueous morpholine and morpholinium chloride were determined with a Picker flow microcalorimeter at temperatures from 10 to 55 degrees C. The apparent molar volumes V-phi have been determined with platinum vibrating tube densitometers at temperatures from 10 to 300 degrees C and pressures in excess of steam saturation. Values of V-phi for morpholine approach large positive values at elevated temperatures, consistent with a lowering of the critical temperature in the solutions relative to water. The effect in aqueous morpholinium chloride is reversed, confirming the profound effect of ionic charge on the high-temperature thermodynamic properties of aqueous solutes, even for large organic molecules. Standard partial molar heat capacity functions were estimated from the high-temperature V-phi data and low-temperature values of C-p,C-phi using an empirical model based on the appropriate solvent density derivatives and the revised Helgeson-Kirkham-Flowers model. The results from both models are consistent with literature values for the heat capacity of ionization determined from high-temperature potentiometric measurements to within the combined experimental uncertainties. The results show that the effects of solvent expansion by the neutral species is significant at elevated temperatures. The effective Born radius of ions containing organic groups could be significantly larger than the radius calculated from the formula for simple cations because of this effect.