Investigation of the Surface Tension of Atmospheric Liquids by Molecular Dynamics Simulations

Abstract

The surface tension of liquids is important for many processes in the atmosphere. However, a lot of uncertainties exist over its influencing factors. For example, people are still debating whether the “second inflection point (SIP)” exists in the temperature dependence of the surface tension of water in the supercooled regime. Controversies can also be found in the discussions about the concentration-of-solutes dependence and size dependence in the nano-regime. In this PhD project, these uncertainties are clarified by using molecular dynamics (MD) simulations. In the first part of this work, we use MD simulations of the SPC/E water model to study the surface tension of water (σ_w) as a function of temperature down to 198.15 K, and find a minimum point of surface excess entropy per unit area around ~240-250 K. Hence, we predict a second inflection point (SIP) of σ_w roughly in this region, at the boundary where the “no man’s land” happens. Furthermore, we find that σ_w has a near-linear correlation with the interfacial width, which can be well explained by the capillary wave theory. Deep in the supercooled regime, a compact water layer in the interface is detected in our simulations. The second part is about the surface tension of aqueous NaCl solution (σ_(NaCl,sol)) and the concentration dependence. We show that the linear approximation of concentration dependence of σ_(NaCl,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 (x_NaCl) of ~0.39). After that, the simulated σ_(NaCl,sol) remains almost unchanged until x_NaCl of ~0.47 (near the concentration upon efflorescence). After a “second inflection point” at x_NaCl of ~0.47, the σ_(NaCl,sol) gradually regains a strong concentration dependence with a tendency to approach the surface tension of molten NaCl (~175.58 mN m-1 at 298.15 K). Finally, the vapor pressure and surface tension of water nano-droplets are studied by MD simulations. We found the value of ln⁡(P_sat (r))/P_(sat_0) increases from 0.46 to 2.1 when the radius decreases from ~1.92 nm to ~0.4 nm. The size dependence of the surface tension was analyzed by using a modified form of Tolman equation. The Tolman length is conjectured to be -0.12 nm. In conclusion, several anomalies exist in the temperature dependence, concentration dependence and size dependence of the surface tension, and these anomalies are analyzed from the perspective of energy and structure.