Thermodynamic Properties of Dilute Aqueous Electrolytes and Non-Electrolytes at Elevated Temperatures, from Molecular Dynamics Simulations

English Abstract:

Thermodynamic properties of aqueous solutes are key parameters in geochemical modeling of fluid-driven and fluid-mediated processes. Properties at infinite dilution comprise the most commonly used solute standard state convention, and are thus required for essentially all fluid-rock reaction modeling. In this study we use classical molecular dynamics simulations to estimate thermodynamic quantities for ions, ion pairs and non-electrolytes at elevated temperatures and low to high pressures (i.e., from vapor-like to liquid-like fluid densities). Simulations are done using the SPC/E water model, and solutes including Na⁺, Cl^–, NaCl⁰and Ar are included with concentrations set to 0.01 molal. Hydration free energies for atoms, ions or ion pairs are calculated by lambda dynamics.

Hydration free energies of aqueous solutes at near-critical and super-critical temperatures show a systematic dependence on fluid density under isothermal conditions, with more pronounced density dependence at low pressures and only weak density dependence at high pressures. Hydration free energies also show relatively weak isochoric temperature dependence at super-critical temperatures. The density dependence seems to reflect short-range and long-range density fluctuations resulting from solvation. Free energies of hydration for non-electrolytes show trends opposite to those for electrolytes. Differences in hydration free energies for neutral ion pairs compared to free ions are used to estimate dissociation constants at these conditions.

One advantage of the methods used here is that experimentally determined solvation free energies of electrolytes are difficult to deconvolute into cation and anion contributions due to the charge balance restriction, whereas the present data are implicitly resolved into contributions from individual ions. Moreover, thermodynamic data (hydration free energies) can be linked to other calculated properties, such as the solvation structure around ions and non-electrolytes. Correlations between free energies and structural data thus can be used to interpret the factors controlling thermodynamic properties.