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    The dissociation mechanism and thermodynamic properties of HCl(aq) in hydrothermal fluids (to 700 °C, 60 kbar) by ab initio molecular dynamics simulations

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    Access Status
    Open access
    Authors
    Mei, Y.
    Liu, W.
    Brugger, J.
    Sherman, D.
    Gale, Julian
    Date
    2018
    Type
    Journal Article
    
    Metadata
    Show full item record
    Citation
    Mei, Y. and Liu, W. and Brugger, J. and Sherman, D. and Gale, J. 2018. The dissociation mechanism and thermodynamic properties of HCl(aq) in hydrothermal fluids (to 700 °C, 60 kbar) by ab initio molecular dynamics simulations. Geochimica Et Cosmochimica Acta. 226: pp. 84-106.
    Source Title
    Geochimica Et Cosmochimica Acta
    DOI
    10.1016/j.gca.2018.01.017
    ISSN
    0016-7037
    School
    School of Molecular and Life Sciences (MLS)
    Funding and Sponsorship
    http://purl.org/au-research/grants/arc/DP160100677
    URI
    http://hdl.handle.net/20.500.11937/66792
    Collection
    • Curtin Research Publications
    Abstract

    HCl is one of the most significant volatiles in the Earth’s crust. It is well established that chloride activity and acidity (pH) play important roles in controlling the solubility of metals in aqueous hydrothermal fluids. Thus, quantifying the dissociation of HCl in aqueous solutions over a wide range of temperature and pressure is crucial for the understanding and numerical modeling of element mobility in hydrothermal fluids. Here we have conducted ab initio molecular dynamics (MD) simulations to investigate the mechanism of HCl(aq) dissociation and to calculate the thermodynamic properties for the dissociation reaction at 25–700 °C, 1 bar to 60 kbar, i.e. including high temperature and pressure conditions that are geologically important, but difficult to investigate via experiments.

    Our results predict that HCl(aq) tends to associate with increasing temperature, and dissociate with increasing pressure. In particular, HCl(aq) is highly dissociated at extremely high pressures, even at high temperatures (e.g., 60 kbar, 600–700 °C). At 25 °C, the calculated logKd values (6.79 ± 0.81) are close to the value (7.0) recommended by IUPAC (International Union of Pure and Applied Chemistry) and some previous experimental and theoretical studies (Simonson et al.., 1990; Sulpizi and Sprik, 2008, 2010). The MD simulations indicate full dissociation of HCl at low temperature; in contrast, some experiments were interpreted assuming significant association at high HCl concentrations (≥1 m HCltot) even at room T (logKd ∼0.7; e.g., Ruaya and Seward, 1987; Sretenskaya, 1992; review in Tagirov et al., 1997). This discrepancy is most likely the result of difficulties in the experimental determination of minor (if any) concentration of associated HCl(aq) under ambient conditions, and thus reflects differences in the activity models used for the interpretation of the experiments. With increasing temperature, the discrepancy between our MD results and previous experimental studies, and between different studies, becomes smaller as the degree of HCl association increases. The MD simulations and available experimental studies show consistent results at hydrothermal conditions (300–700 °C, up to 5 kbar). The new thermodynamic properties based on the MD results provide an independent check of the dissociation constants for HCl(aq), and the first dataset on HCl dissociation in high P-T fluids (up to 60 kbar, 700 °C) beyond available experimental conditions. Our results will enable prediction of the role of HCl in controlling element mobility in deep earth hydrothermal systems, including fluids associated with ultra-high pressure metasomatism in subduction zones.

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