Amino acids under hydrothermal conditions : apparent molar volumes, apparent molar heat capacities, and acid/base dissociation constants for aqueous α-alanine, β-alanine, glycine, and proline at temperatures from 25 to 250°C and pressures up to 30.0 MPa

Clarke, Rodney George Francis (2000) Amino acids under hydrothermal conditions : apparent molar volumes, apparent molar heat capacities, and acid/base dissociation constants for aqueous α-alanine, β-alanine, glycine, and proline at temperatures from 25 to 250°C and pressures up to 30.0 MPa. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

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 α-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 Vɸ of aqueous α-alanine, β-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,ɸ of aqueous α-alanine, β-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°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°p increase with increasing pressure. Comprehensive equations to describe the standard-state properties over the experimental temperature range are reported. -- The deviation toward negative values by V° and C°p is opposite to the behavior predicted by the correlations developed by Shock and Helgeson (Geochim. Cosmochim. Acta. 54, 915-945, 1990) and Amend and Helgeson (J. Chem. Soc, Faraday Trans. 93, 1927-1941,1997). The temperature dependence of V° and C°p predicted using the very recent functional-group additivity model of Yezdimer et al. (Chem. Geol. 164, 259-280, 2000) is only in qualitative agreement with the experimental results. The contribution to V° and C°p from the solvent polarization by the large dipole moment of the zwitterions deviates toward negative infinity as Tc is approached, in a manner similar to the experimental values of V° and C°p for each of the aqueous amino acids. While this agreement is qualitatively consistent, it is not quantitatively consistent, which suggests that the non-electrostatic hydration effects are of similar magnitude to the solvent polarization effects. -- The acid/base dissociation constants for aqueous α-alanine have been determined from 423 K to 523 K using a UV-visible spectrophotometer and the colorimetric indicators developed by Xiang and Johnston (J. Sol. Chem. 26, 13-30, 1997) and Ryan et al. (J. Phys. Chem. 101, 1827-1835, 1997). The dissociation constants that were estimated with the functional-group additivity model of Yezdimer et al. (Chem. Geol. 164, 259-280, 2000) and those obtained from the isocoulombic extrapolation of room temperature data were found to be an upper limit for the measured values. The contribution of non-zwitterionic forms of the aqueous amino acids to the experimentally determined values of V° and C°p were negligible at all but the highest temperatures. -- In this work, the first experimentally determined apparent molar volumes Vɸ for aqueous α-alanine, β-alanine, and proline were obtained at T ≥ 343 K. The first experimentally determined apparent molar heat capacities Cp,ɸ for aqueous amino acids at T ≥ 328 K were obtained in this work. The first experimentally determined acid/base dissociation constants for aqueous α-alanine obtained at T ≥ 423 K were also obtained in this work.

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