Amino acids under hydrothermal conditions:: Apparent molar volumes of aqueous α-alanine, β-alanine, and proline at temperatures from 298 to 523 K and pressures up to 20.0 MPa

The apparent molar volumes V-phi of aqueous alpha-alanine, beta-alanine, and proline have been determined with platinum vibrating tube densitometers at temperatures from 298 to 523 K and at pressures in excess of steam saturation. Values of the standard partial molar volumes V degrees for the aqueous amino acids increase with temperature then deviate toward negative values at temperatures above 398 K, consistent with a lowering of the critical temperature in the solutions relative to water. This is opposite to the behavior predicted by the correlations developed by Shock and Helgeson (Geochim. Cosmochim. Acta. 1990 54, 915-945) and Amend and Helgeson (J. Chem. Sec., Faraday Trans. 1997 93, 1927-1941). The contribution to V degrees from the solvent polarization by the large dipole moment of the zwitterions deviates toward negative infinity as T-c is approached, in a manner similar to the V degrees values for each of the aqueous amino acids. While this agreement is qualitatively consistent, it is not quantitatively consistent, which suggests that either the nonelectrostatic hydration effects are of similar magnitude to the solvent polarization effects or that the equilibrium is significantly shifted toward the nonzwitterionic species at 523 K. Preliminary calculations suggest that the former is the case.