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Discussion papers | Copyright
© Author(s) 2017. This work is distributed under
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Research article 08 Nov 2017

Research article | 08 Nov 2017

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Molecular Dynamics Simulation of the Surface Tension of Aqueous Sodium Chloride: from Dilute to Highly Supersaturated Solutions and Molten Salt

Xiaoxiang Wang1, Chuchu Chen1, Kurt Binder2, Uwe Kuhn1, Ulrich Pöschl1, Hang Su3,1, and Yafang Cheng1,3 Xiaoxiang Wang et al.
  • 1Max Planck Institute for Chemistry, Multiphase Chemistry Department, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
  • 2Institut für Physik, Johannes Gutenberg-Universität, Staudinger Weg 7, 55128 Mainz, Germany
  • 3Institute for Environmental and Climate Research, Jinan University, 510632 Guangzhou, China

Abstract. Sodium chloride (NaCl) is one of the key components of atmospheric aerosols. The surface tension of aqueous NaCl solution (σNaCl,sol) and its concentration dependence are essential to determine the equilibrium water vapor pressure of aqueous NaCl droplets. Supersaturated NaCl solution droplets are observed in laboratory experiments and under atmospheric conditions, but the experimental data for σNaCl,sol are mostly limited up to sub-saturated solutions. In this study, the surface tension of aqueous NaCl is investigated by molecular dynamics (MD) simulations and pressure tensor method from dilute to highly supersaturated solutions. We show that the linear approximation of concentration dependence of σNaCl,sol at molality scale can be extended to the supersaturated NaCl solution until a molality of ~9.6molkg−1 (i.e., solute mass fraction (xNaCl) of ~0.36). Energetic analyses show that this monotonic increase of surface tension is driven by the increase of excessive surface enthalpy (∆H) as the solution becomes concentrated. After that, the simulated σNaCl,sol remains almost unchanged until xNaCl of ~0.47 (near the concentration upon efflorescence). The existence of the inflection point at xNaCl of ~0.36 and the stable surface tension of xNaCl between ~0.36 and ~0.47 can be attributed to a competitive growth of excessive surface entropy term (T·∆S) and the excessive surface enthalpy term (∆H). After a second inflection point at xNaCl of ~0.47, the simulated σNaCl,sol gradually regains the growing momentum with a tendency to approach the surface tension of molten NaCl (~148.4mNm−1 at 298.15K, MD simulation based extrapolation). This fast increase of σNaCl,sol at xNaCl>0.47 is primarily still an excessive surface enthalpy-driving process, although contribution from concurrent fluctuation of excessive surface entropy is expected but in a relatively smaller scale. Our results reveal different regimes of concentration dependence of the surface tension of aqueous NaCl at 298.15K: a water-dominated regime (xNaCl from 0 to ~0.36), a transition regime (xNaCl from ~0.36 to ~0.47) and a molten NaCl-dominated regime (xNaCl from ~0.47 to 1).

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