The dissociation mechanism and thermodynamic properties of HCl(aq) in hydrothermal fluids (to 700 °C, 60 kbar) by ab initio molecular dynamics simulations
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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DOI：
10.1016/j.gca.2018.01.017
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发表时间：
2018-04
影响因子：
5
通讯作者：
Y. Mei;Weihua Liu;J. Brugger;D. Sherman;J. Gale
中科院分区：
文献类型：
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作者：
Y. Mei;Weihua Liu;J. Brugger;D. Sherman;J. Gale
HCl是地壳中最重要的挥发物之一。众所周知，氯化物活性和酸度（pH）在控制金属在含水热液流体中的溶解度方面起重要作用。因此，在很宽的温度和压力范围内量化HCl在水溶液中的解离对于理解热液流体中的元素迁移率和数值模拟至关重要。在这里，我们进行了从头分子动力学（MD）模拟，以研究HCl（aq）解离的机制，并计算解离反应在25-700 °C，1 bar至60 kbar（即包括地质上重要的高温和高压条件）下的热力学性质，但很难通过实验进行研究。我们的结果预测，HCl（aq）倾向于随着温度的升高而结合，并随着压力的升高而解离。特别地，HCl（aq）在极高的压力下，甚至在高温下（例如，60 kbar，600-700 °C）。在25 °C下，计算的logKd值（6.79 ± 0.81）接近IUPAC（国际纯粹与应用化学联合会）和一些先前的实验和理论研究（Simonson等人，1990; Sulpizi和Sprik，2008，2010）。MD模拟表明HCl在低温下完全解离;相反，一些实验被解释为假设即使在室温下在高HCl浓度（≥1 m HCltot）下也存在显著缔合（logKd ≥ 0.7;例如，Ruaya和Seward，1987年; Sretenskaya，1992年; Tagirov等人的综述，1997年）。这种差异很可能是由于在环境条件下难以通过实验测定相关HCl（aq）的次要（如果有的话）浓度，因此反映了用于解释实验的活性模型的差异。随着温度的升高，我们的MD结果和以前的实验研究之间的差异，以及不同的研究之间的差异，随着HCl缔合程度的增加而变小。MD模拟和可用的实验研究显示在热液条件下（300-700 °C，高达5 kbar）的一致结果。基于MD结果的新热力学性质提供了对HCl（aq）解离常数的独立检查，以及超出可用实验条件的高P-T流体（高达60 kbar，700 °C）中HCl解离的第一个数据集。我们的研究结果将使预测的作用，盐酸在控制元素的流动性在地球深部热液系统，包括流体与超高压交代在俯冲带。
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 conductedab initiomolecular 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 logKdvalues (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.