Apparent molar heat capacities C p,φ of aqueous morpholine and morpholinium chloride were determined with a Picker flow microcalorimeter at temperatures from 10 to 55 °C. The apparent molar volumes V φ have been determined with platinum vibrating tube densitometers at temperatures from 10 to 300 °C and pressures in excess of steam saturation. Values of V φ 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 φ data and low-temperature values of C p,φ 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.

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Apparent molar heat capacities C p,φ and apparent molar volumes V φ for aqueous diethanolamine (HOC 2 H 4 ) 2 NH, diethanolammonium chloride (HOC 2 H 4 ) 2 NH 2 Cl, N,N′-dimethylethanolamine (HOC 2 H 4 )(CH 3 ) 2 N, and N,N′-dimethylethanolammonium chloride (HOC 2 H 4 )(CH 3 ) 2 NHCl were determined from 283.15 to 328.15 K with a Picker flow microcalorimeter and vibrating tube densimeter. The experimental results have been analyzed in terms of Young's Rule with the Guggenheim form of the extended Debye-Hückel equation and appropriate corrections for chemical relaxation effects. These calculations lead to standard partial molar heat capacities and volumes for the neutral amines, + (aq) over the experimental temperature range.

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