Theoretical and Computational Chemistry

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



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 species. In all cases, we find that average 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 larger at the interface than it is in bulk water, by an amount that is greater than the standard deviation of the fluctuations. The Cl– ion also has a slightly larger CT energy at the interface but NO3–(aq) does not; these two species are borderline cases where consensus is lacking regarding their surface activity. However, CT stabilization amounts to < 20% of the total induction energy, for all of the ions considered here, and CT-free polarization energies are systematically larger in bulk water, again in all cases, so the role of these effects in soft anion surface activity remains unclear. 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.

Version notes

Added new references and clarified the discussion of charge transfer.


Thumbnail image of anion-EDA-awi-REV1.pdf

Supplementary material

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AMOEBA parameter file
Parameter file for the AMOEBA force field, for the aqueous ions studied in this work.
Thumbnail image of monov-ions-awi.tar.gz
Coordinate files
Cartesian coordinates of all structures used in this work