Specific Anion Effects on Na⁺ Adsorption at the Aqueous Solution-Air Interface: MD Simulations, SESSA Calculations,...

Specific ion effects of the large halide anions have been shown to moderate anion adsorption to the air-water interface (AWI), but little quantitative attention has been paid to the behavior of alkali cations. Here we investigate the concentration and local distribution of sodium (Na⁺) at the AWI in dilute (<1 M) aqueous solutions of NaCl, NaBr, and NaI using a combination of molecular dynamics (MD) and SESSA simulations, and liquid jet ambient pressure photoelectron spectroscopy measurements. We use SESSA to simulate Na 2p photoelectron intensities on the basis of the atom density profiles obtained from MD simulations, and we compare the simulation results with photoelectron spectroscopy experiments to evaluate the performance of a nonpolarizable force field model versus that of an induced dipole polarizable one. Our results show that the nonpolarizable force model developed by Horinek and co-workers (Chem. Phys. Lett. 2009, 479, 173-183) accurately predicts the local concentration and distribution of Na⁺ near the AWI for all three electrolytes, whereas the polarizable model does not. To our knowledge, this is the first interface-specific spectroscopic validation of a MD force field. The molecular origins of the unique Na⁺ distributions for the three electrolytes are analyzed on the basis of electrostatic arguments, and shown to arise from an indirect anion effect wherein the identity of the anion affects the strength of the attractive Na⁺-H₂O electrostatic interaction. Finally, we use the photoelectron spectroscopy results to constrain the range of inelastic mean free paths (IMFPs) for the three electrolyte solutions used in the SESSA simulations that are able to reproduce the experimental intensities. Our results suggest that earlier estimates of IMFPs for aqueous solutions are likely too high.