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

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 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 σNaCl,sol at molality scale can be extended to the supersaturated NaCl solution until a molality of ∼ 10.7 mol kg-1 (i.e., solute mass fraction (xNaCl) of ∼ 0.39). Energetic analyses show that this monotonic increase in surface tension is driven by the increase in excess surface enthalpy (ΔH) 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 ∼ 0.39 and the stable surface tension of xNaCl between ∼ 0.39 and ∼ 0.47 can be attributed to the nearly unchanged excess surface entropy term (T · ΔS) and the excess surface enthalpy term (ΔH). 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 ( ∼ 175.58 mN m-1 at 298.15 K, MD simulation-based extrapolation). This fast increase in σNaCl,sol at xNaCl > 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 K: a water-dominated regime (xNaCl from 0 to ∼ 0.39), a transition regime (xNaCl from ∼ 0.39 to ∼ 0.47) and a molten NaCl-dominated regime (xNaCl from ∼ 0.47 to 1).