Ab-initio assessment of conventional standard-state thermodynamic properties of geochemically relevant gaseous and aqueous species

Abstract

After some introductory remarks on the essential basic concepts concerning isolated (gas state) atoms and molecules (adiabatic vs. vertical ionization potential and electron affinity; dissociation energy), we will show how their (absolute) internal energy and enthalpy (i.e. “thermal corrections” to the energy of the molecule) may be converted into the conventional counterparts of common use in thermochemistry through an appropriate thermochemical cycle. The first-principles acceptation of entropy, in terms of translational, rotational, vibrational, and electronic contributions to the bulk partition function, will then be briefly restated, emphasizing that, for practical calculations, the V-dependency of the translational terms can be converted into a P-dependency with the perfect gas law. The entropy of “aqueous” species will then be discussed after some preliminary remarks on the significance of the “solvation entropy” in terms of a Polarized Continuum Model (PCM) conceptual framework, and some comments on the energy of the “hydronium ion” H 3O +. Some comments on the nature of the electrostatic portion of the solvation entropy, in terms of the Born solvation theory, and the non-electrostatic portion (solvent collapse+cavity formation+dispersion+repulsion+liberation entropy terms), will then be made, showing how this last term may be computed practically by exploiting its analogy with the Helgeson–Kirkham–Flowers electrostatic approach. After some elementary application of the various concepts to the gaseous state, we will present a potential application of major use in geochemistry, i.e. the possibility of computing ab-initio the interaction parameters (i.e. individual activity coefficients) of complex ions in solution through PCM procedures. We will then discuss the state of the art attained in determining ab-initio the p K a scale of organic and inorganic acids, perhaps the most demanding task, due to the high level of internal precision required. Finally, we will give some examples of the application of the assessment of the Gibbs free energy of formation from the elements ( G f °), the enthalpy of formation from the elements ( H f °), and entropy ( S°) at standard state (hypothetical 1-molal solution referred to infinite dilution at 298.15 K and 10 5 Pa) on geochemically relevant aqueous species of heavy metals.