Metal complexation and ion hydration in low density hydrothermal fluids: ab initio molecular dynamics simulation of Cu(I) and Au(I) in chloride solutions (25-1000 degrees C, 1-5000 bar)

Yuan Mei, Weihua Liu, David M Sherman, Joel Brugger

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Abstract

Low-density supercritical fluids are suspected of being able to transport metals, but it is unclear what the speciation/complexation would be in such conditions. In this work, we used ab initio molecular dynamics simulations to investigate the complexation, ion association and hydration of Cu+ and Au+ in NaCl brines as a function of solution density, from ambient to supercritical conditions (to 1000 °C, 5000 bar). Cu(I) and Au(I) form distorted linear complexes with two chloride ligands (i.e., CuCl2− and AuCl2−) in subcritical chloride brines. We have discovered that these charged complexes remain in high density supercritical fluids even at high temperature; however, with decreasing density, these complexes become progressively neutralized by ion association with Na+ to form low-charge (NanCuCl2)n−1 and (NanAuCl2)n−1 complexes. In these species, the Na+ ion is very weakly bonded in the outer coordination sphere, resulting in highly disordered structures and fast (few picoseconds) exchange among coordinated and solvent Na+ ions. Thermodynamic models to predict the solubility of metals in low-density magmatic or metamorphic fluids must account for these species. In addition, we found that the number of water molecules (i.e., the hydration number) surrounding the Cu+, Au+, Na+ and Cl− ions decreases linearly when fluid density decreases; this supports empirical thermodynamic models that correlate the stability constants of complexation reactions with solvent density. The traditional Born-model description explains the ion association as resulting from the decreased dielectric constant of the solvent. However at a molecular level, the increased ion association results from the increase in translational entropy associated with ion dehydration.
Original languageEnglish
Pages (from-to)196-212
Number of pages17
JournalGeochimica et Cosmochimica Acta
Volume131
DOIs
Publication statusPublished - 15 Apr 2014
Externally publishedYes

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