CuCl Complexation in the Vapor Phase: Insights from Ab Initio Molecular Dynamics Simulations

Yuan Mei,A E Williams-Jones,Joël Brugger,Weihua Liu,A A Migdiov

doi:10.1155/2018/4279124

Abstract

We investigated the hydration of the CuCl0 complex in HCl-bearing water vapor at 350°C and a vapor-like fluid density between 0.02 and 0.09 g/cm3 using ab initio molecular dynamics (MD) simulations. The simulations reveal that one water molecule is strongly bonded to Cu(I) (first coordination shell), forming a linear [H2O-Cu-Cl]0 moiety. The second hydration shell is highly dynamic in nature, and individual configurations have short life-spans in such low-density vapors, resulting in large fluctuations in instantaneous hydration numbers over a timescale of picoseconds. The average hydration number in the second shell (m) increased from ~0.5 to ~3.5 and the calculated number of hydrogen bonds per water molecule increased from 0.09 to 0.25 when fluid density (which is correlated to water activity) increased from 0.02 to 0.09 g/cm3 (fH2O 1.72 to 2.05). These changes of hydration number are qualitatively consistent with previous solubility studies under similar conditions, although the absolute hydration numbers from MD were much lower than the values inferred by correlating experimental Cu fugacity with water fugacity. This could be due to the uncertainties in the MD simulations and uncertainty in the estimation of the fugacity coefficients for these highly nonideal “vapors” in the experiments. Our study provides the first theoretical confirmation that beyond-first-shell hydrated metal complexes play an important role in metal transport in low-density hydrothermal fluids, even if it is highly disordered and dynamic in nature.

Highlights

Understanding the speciation of metals in hydrothermal fluids is important as it controls metal mobility and mineral solubility in geological and man-made hydrothermal systems, such as those forming ore deposits and geothermal systems and those used in hydrometallurgy [1]
The bond distance and angles are consistent with results of the in situ X-ray absorption spectroscopy (XAS) studies of Cu(I)-Cl and Cu(I)-HS complexes [6, 12, 32]
Hydration numbers for the [CuCl(H2O)]0 moiety were derived from molecular dynamics (MD) simulations by calculating the integral of the Cu-O paired distribution function and by counting the H-bonded water molecules using a geometrical definition of hydrogen bonds (H-bonds) (Table 2)

Summary

Introduction

Understanding the speciation of metals in hydrothermal fluids is important as it controls metal mobility and mineral solubility in geological and man-made hydrothermal systems, such as those forming ore deposits and geothermal systems and those used in hydrometallurgy [1]. Copper is transported as chloride species in many hydrothermal fluids [5]. In high-density, reducedsulfur-poor brines, the dominant copper species is CuCl2− [6,7,8,9,10], and the activity of the chloride ion is the main control on the solubility of Cu minerals at constant temperature (T) and pressure (P). In low-density vapors, there is increasing experimental evidence showing that, aside from the fugacity of HCl0, the solubility of Cu minerals is controlled primarily by fluid density, which correlates with the fugacity of water. Mineral solubility increases exponentially with increasing fluid density in HCl-bearing water vapor [3, 11, 12]

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