Ab initio molecular dynamics simulation and free energy exploration of copper(I) complexation by chloride and bisulfide in hydrothermal fluids

Chloride and bisulfide are the primary ligands believed to control the transport of copper in hydrothermal fluids. Ab initio molecular dynamics (MD) simulations based on density functional theory were conducted to predict the stoichiometries and geometries of Cu(I) complexes in mixed chloride and hydrosulfide (HS− and H2S(aq)) fluids at ambient temperature and at 327 °C and 500 bar, and to assess the relative importance of the chloride and hydrosulfide ligands for Cu transport. The simulations accurately reproduce the identity and geometries of Cu(I) chloride and bisulfide species derived from experimental solubility, UV–Vis, and in situ XAS results. The simulations indicate the following ligand preference: HS− > Cl− > H2S for Cu(I) complexes, but predict a high stability of the mixed-ligand complex, CuCl(HS)−, a species similar to NaClCuHS species in vapour phase suggested by Zajacz et al. (2011). The thermodynamic properties (formation constants, log Ks) of Cu(I) chloride and bisulfide complexes were investigated by distance-constrained MD simulations using thermodynamic integration. The predicted log Ks of the following reactions are in good agreement (within 1 log unit) with the experimental values ( Brugger et al., 2007 and Liu et al., 2001):Cu++Cl-=CuCl(aq);logK327°C,calc=+3.87±0.14;logK325°C,exp=+4.12;CuCl(aq)+Cl-=CuCl2-;logK327°C,calc=+2.84±0.09;logK325°C,exp=+1.98;CuCl2-+Cl-=CuCl32-;logK327°C,calc=-1.23±0.21;logK325°C,exp=-2.17.The fair agreements between the predicted log Ks with those derived from experimental data demonstrate the potential of predicting thermodynamic properties for transition metal complexes under hydrothermal conditions by MD simulations. The formation constant for the mixed-ligand complex CuCl(HS)− is calculated for the first time:Cu++Cl-+HS-=CuCl(HS)-;logK327°C,calc=+11.47.Determination of the formation constants for Cu(I) complexes enabled the construction of activity–activity diagrams entirely based on the MD simulation data. The results suggest that the mixed-ligand complex plays an important role in Cu transport in hydrothermal fluids.