Molecular dynamics simulation of the surface tension of aqueous sodium chloride: from dilute to highly supersaturated solutions and molten salt

Wang, Xiaoxiang; Chen, Chuchu; Binder, Kurt; Kuhn, Uwe; Pöschl, Ulrich; Su, Hang; Cheng, Yafang

Sodium chloride (NaCl) is one of the key components of atmospheric aerosols. The surface tension of aqueous NaCl solution (inline-formulaσNaCl,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 inline-formulaσNaCl,sol are mostly limited up to subsaturated solutions. In this study, the surface tension of aqueous NaCl is investigated by molecular dynamics (MD) simulations and the pressure tensor method from dilute to highly supersaturated solutions. We show that the linear approximation of concentration dependence of inline-formulaσNaCl,sol at molality scale can be extended to the supersaturated NaCl solution until a molality of inline-formula∼10.7inline-formulamol kg−1 (i.e., solute mass fraction (inline-formulaxNaCl) of inline-formula∼0.39). Energetic analyses show that this monotonic increase in surface tension is driven by the increase in excess surface enthalpy (inline-formulaΔH) as the solution becomes concentrated. After that, the simulated inline-formulaσNaCl,sol remains almost unchanged until inline-formulaxNaCl of inline-formula∼0.47 (near the concentration upon efflorescence). The existence of the “inflection point” at inline-formulaxNaCl of inline-formula∼0.39 and the stable surface tension of inline-formulaxNaCl between inline-formula∼0.39 and inline-formula∼0.47 can be attributed to the nearly unchanged excess surface entropy term (inline-formulaT⋅ΔS) and the excess surface enthalpy term (inline-formulaΔH). After a “second inflection point” at inline-formulaxNaCl of inline-formula∼0.47, the simulated inline-formulaσNaCl,sol gradually regains the growing momentum with a tendency to approach the surface tension of molten NaCl (inline-formula∼175.58inline-formulamN m−1 at 298.15 inline-formulaK, MD simulation-based extrapolation). This fast increase in inline-formulaσNaCl,sol at inline-formulaxNaCl>0.47 is a process driven by excess surface enthalpy and excess surface entropy. Our results reveal different regimes of concentration dependence of the surface tension of aqueous NaCl at 298.15 inline-formulaK: a water-dominated regime (inline-formulaxNaCl from 0 to inline-formula∼0.39), a transition regime (inline-formulaxNaCl from inline-formula∼0.39 to inline-formula∼0.47) and a molten NaCl-dominated regime (inline-formulaxNaCl from inline-formula∼0.47 to 1).

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Wang, Xiaoxiang / Chen, Chuchu / Binder, Kurt / et al: Molecular dynamics simulation of the surface tension of aqueous sodium chloride: from dilute to highly supersaturated solutions and molten salt. 2018. Copernicus Publications.

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