Interaction Energy Analysis of Monovalent Inorganic Anions in Bulk Water Versus Air/Water Interface

13 October 2021, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Soft anions exhibit surface activity at the air/water interface that can be probed using surface-sensitive vibrational spectroscopy, yet the statistical mechanics behind this surface activity remains a matter of debate. Here, we examine the nature of anion--water interactions at the air/water interface using a combination of molecular dynamics simulations and quantum-mechanical energy decomposition analysis based on symmetry-adapted perturbation theory. Results are presented for a set of monovalent anions including Cl–, Br–, I–, CN–, OCN–, SCN–, NO2–, NO3–, and ClOn– (n = 1, 2, 3, 4), several of which are archetypal examples of surface-active ions. In all cases, we find that anion–water interaction energies are systematically larger in bulk water although the difference (with respect to the interaction energy in the interfacial environment) is well within the magnitude of the instantaneous fluctuations. Specifically for the surface-active species Br–(aq), I–(aq), ClO4–(aq), and SCN–(aq), and also for ClO–(aq), the charge-transfer (CT) energy is found to be slightly larger at the interface than it is in bulk water, but in all cases the CT stabilization amounts to < 20% of the total induction energy. CT-free polarization energies are systematically larger in bulk water, for all of the ions. This analysis complements our recent work suggesting that the short-range solvation structure around these ions is scarcely different at the air/water interface from what it is in bulk water. Together, these observations suggest that changes in first-shell hydration structure around soft anions cannot explain observed surface activities.

Keywords

air-water interface
Hofmeister series
hydrogen bonding
noncovalent interactions
charge transfer
symmetry-adapted perturbation theory

Supplementary materials

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Description
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AMOEBA parameter file
Description
Parameter file for the AMOEBA force field, for the aqueous ions studied in this work.
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Coordinate files
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Cartesian coordinates of all structures used in this work
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