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

The apparent molar volumes V-phi of aqueous alpha-alanine, beta-alanine, an d proline have been determined with platinum vibrating tube densitometers a t temperatures from 298 to 523 K and at pressures in excess of steam satura tion. Values of the standard partial molar volumes V degrees for the aqueou s amino acids increase with temperature then deviate toward negative values at temperatures above 398 K, consistent with a lowering of the critical te mperature in the solutions relative to water. This is opposite to the behav ior 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 sug gests that either the nonelectrostatic hydration effects are of similar mag nitude to the solvent polarization effects or that the equilibrium is signi ficantly shifted toward the nonzwitterionic species at 523 K. Preliminary c alculations suggest that the former is the case.