Amino acids under hydrothermal conditions:: Apparent molar heat capacities of aqueous α-alanine, β-alanine, glycine, and proline at temperatures from 298 to 500 K and pressures up to 30.0 MPa

The apparent molar heat capacities C-p(o) of aqueous alpha -alanine, beta -alanine, glycine, and proline have been determined using a differential flow calorimeter and a Picker flow microcalorimeter at temperatures of 298 K less than or equal to T less than or equal to 500 K and at pressures from steam saturation to 30 MPa. Comprehensive equations to describe the standard-state properties over this range are reported. Values of the standard partial molar heat capacities C-p(o) for the aqueous amino acids increase with temperature and then deviate toward negative values at temperatures above about 390 K, consistent with increasing the critical temperature in the solutions relative to water, i.e., negative Krichevskii parameters. This is opposite to the behavior predicted by correlations reported in the geochemical and chemical literature. The temperature dependence of C-p(o) predicted using the very recent functional group additivity model of Yezdimer et al. (Chem. Geol. 2000, 164, 259-280) is only in qualitative agreement with the experimental results. The results are consistent with a simple solvation model in which the zwitterions are represented by point dipoles.