FLUIDEQ · A new equation of state for solutes in high-temperature fluids

A new equation of state for solutes in high-temperature fluids

Aqueous fluids are key agents of heat and mass transfer in the Earth, playing pivotal roles in the hydrothermal ore deposit formation, continental and submarine geothermal systems, volcanoes, and the evolution of Earth and other planetary interiors. Therefore the physical and chemical properties of aqueous fluids, over a wide range of pressure-temperature-composition (PTX) conditions, are key to understanding geologic processes. At present, the thermodynamic properties of aqueous solutions can be modeled reliably only within a restricted range of PTX conditions relevant to geologic systems. Importantly, conditions near the critical point of water (H2O), which are key conditions in a variety of geologic environments (and at which heat and mass transport properties are maximized), cannot presently be modeled. The goal of this project is to develop a new thermodynamic model for solutes throughout the PTX range of natural hydrothermal systems, extending from liquid-like to vapor-like fluids and including near-critical conditions. To do so, this project will link macroscopic thermodynamic properties with insights from atomistic modeling, in order to form a rigorous basis for interpreting the properties of aqueous solutions in terms of molecular-scale properties. Molecular dynamics simulations provide insight by linking observable thermodynamic properties to the underlying solute-solvent interactions. The two main approaches employed in this research are: Firstly, atomistic simulations of non-electrolyte and electrolyte solutes in aqueous fluids to seek relationships between thermodynamic properties and solute-solvent interactions; and secondly, testing various correlation protocols using a large data set combining published thermodynamic quantities for solutes with molecular-dynamics results over wide ranges of physical conditions. In this project, extensive simulations were conducted on pure H2O as well as solutions of electrolytes (sodium chloride) and non-electrolytes (argon). The thermodynamic properties of these solute were quantified, as well as the solvation structure of solutes and solvent. Particular focus was given to conditions near the critical point of water, as well as 'supercritical' conditions, at fluid densities ranging from liquid-like fluids at high pressures, to vapor-like fluids at low pressures. The results show that solvation free energies are systematically correlated to the solvent density, asymptotically approaching zero at the ideal gas limit. Moreover, various structural parameters (for example, coordination numbers and partial molar volumes) are correlated to both solvent density and solvation free energy. These correlations provide a basis for a new predictive model relating molecular-scale insights and macroscopic thermodynamic properties. The results of molecular simulations have been combined with published experimental data on solute properties to investigate strategies for predicting thermodynamic properties of solutes. Based on molecular-scale insights from atomistic simulations, clear correlations have emerged that provide a foundation for modeling hydrothermal fluid physical and chemical properties. Predictions based on this approach will have wide applications in understanding fluid-rock interactions and thus geologic systems in general, as well as potential applications in industrial processes involving hydrothermal fluids.

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