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

Sodium chloride (NaCl) is one of the key components of atmospheric aerosols. The surface tension of aqueous NaCl solution (sigma(NaCl, 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 sigma(NaCl, 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 sigma(NaCl, sol) at molality scale can be extended to the supersaturated NaCl solution until a molality of similar to 10.7 mol kg(-1) (i.e., solute mass fraction (x(NaCl)) of similar to 0.39). Energetic analyses show that this monotonic increase in surface tension is driven by the increase in excess surface enthalpy (Delta H) as the solution becomes concentrated. After that, the simulated sigma(NaCl, sol) remains almost unchanged until (NaCl)-Na-x of similar to 0.47 (near the concentration upon efflorescence). The existence of the "inflection point" at x(NaCl) of similar to 0.39 and the stable surface tension of x(NaCl) between similar to 0.39 and similar to 0.47 can be attributed to the nearly unchanged excess surface entropy term (T.Delta S) and the excess surface enthalpy term (Delta H). After a "second inflection point" at x(NaCl) of similar to 0.47, the simulated sigma(NaCl,sol) gradually regains the growing momentum with a tendency to approach the surface tension of molten NaCl (similar to 175.58 mNm(-1) at 298.15 K, MD simulation-based extrapolation). This fast increase in sigma(NaCl, sol) at x(NaCl) > 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 (x(NaCl) from 0 to similar to 0.39), a transition regime (x(NaCl) from similar to 0.39 to similar to 0.47) and a molten NaCl-dominated regime (x(NaCl) from similar to 0.47 to 1).