1 research outputs found
Specific Anion Effects on Na<sup>+</sup> Adsorption at the Aqueous Solution–Air Interface: MD Simulations, SESSA Calculations, and Photoelectron Spectroscopy Experiments
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<sup>+</sup>) 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 (<i>Chem. Phys. Lett.</i> <b>2009</b>, <i>479</i>, 173–183) accurately
predicts the local concentration and distribution of Na<sup>+</sup> 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<sup>+</sup> 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<sup>+</sup>–H<sub>2</sub>O 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