THERMODYNAMIC PROPERTIES OF AQUEOUS MORPHOLINE AND MORPHOLINIUM CHLORIDE AT TEMPERATURES FROM 10-DEGREES-C 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 mor pholinium 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 a t temperatures from 10 to 300 degrees C and pressures in excess of ste am saturation. Values of V-phi for morpholine approach large positive values at elevated temperatures, consistent with a lowering of the cri tical temperature in the solutions relative to water. The effect in aq ueous morpholinium chloride is reversed, confirming the profound effec t of ionic charge on the high-temperature thermodynamic properties of aqueous solutes, even for large organic molecules. Standard partial mo lar heat capacity functions were estimated from the high-temperature V -phi data and low-temperature values of C-p,C-phi using an empirical m odel based on the appropriate solvent density derivatives and the revi sed Helgeson-Kirkham-Flowers model. The results from both models are c onsistent with literature values for the heat capacity of ionization d etermined from high-temperature potentiometric measurements to within the combined experimental uncertainties. The results show that the eff ects of solvent expansion by the neutral species is significant at ele vated temperatures. The effective Born radius of ions containing organ ic groups could be significantly larger than the radius calculated fro m the formula for simple cations because of this effect.