Amino acids under hydrothermal conditions: Apparent molar volumes, apparent molar heat capacities, and acid /base dissociation constants for aqueous alpha-alanine, beta-alanine, glycine, and proline at temperatures from 25 to 250°C and pressures up to

Amino acids are members of a unique group of compounds that exist in solution as zwitterions. Yet the thermodynamic properties of aqueous amino acids have not been measured at temperatures above 343 K. The amino acids studied in this work have been chosen based on their hydrothermal stability and their solubility in water. A series of batch experiments confirmed that aqueous alpha-alanine, glycine, and proline were stable on the time scale required for our measurements at the temperatures, pressures, and molalities required for this work.;The apparent molar volumes V&phis; of aqueous alpha-alanine, beta-alanine, and proline have been determined using platinum vibrating tube densitometers at temperatures from 298 K to 523 K and at pressures from steam saturation to 30 MPa. Values of the standard partial molar volumes V° for the aqueous amino acids increase with temperature, then deviate toward negative values at temperatures above 398 K, consistent with an increase in the critical temperature in the solutions relative to water. The apparent molar heat capacities Cp,&phis; of aqueous alpha-alanine, beta-alanine, glycine, and proline have been determined using a differential flow calorimeter and a Picker flow microcalorimeter at temperatures from 298 K to 498 K and at pressures from steam saturation to 30 MPa. Values of the standard partial molar heat capacities C&j0;p for the aqueous amino acids increase with temperature, then deviate toward negative values at temperatures above 373 K to 423 K, also consistent with an increase in the critical temperature in the solutions relative to water. The values of both V° and C&j0;p increase with increasing pressure. Comprehensive equations to describe the standard-state properties over the experimental temperature range are reported.;In this work, the first experimentally determined apparent molar volumes V&phis; for aqueous alpha-alanine, beta-alanine, and proline were obtained at T ≥ 343 K. The first experimentally determined apparent molar heat capacities Cp ,&phis; for aqueous amino acids at T ≥ 328 K were obtained in this work. The first experimentally determined acid/base dissociation constants for aqueous alpha-alanine obtained at T ≥ 423 K were also obtained in this work. (Abstract shortened by UMI.).