Abstract
Soft anions exhibit surface activity at the air/water interface that can be probed using surface-sensitive vibrational spectroscopy, but the structural implications of this surface activity remain 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−, , , and (), 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 same quantity computed in the interfacial environment) is well within the magnitude of the instantaneous fluctuations. Specifically for the surface-active species Br−(aq), I−(aq), (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−(aq) ion has a slightly larger CT energy at the interface, but (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 each of the ions considered here, and CT-free polarization energies are systematically larger in bulk water in all cases. As such, the role of these effects in the surface activity of soft anions 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.
Highlights
One of the earliest results of surface-sensitive vibrational sum-frequency generation (VSFG) experiments [1,2] was the observation that soft anions impact the vibrational lineshape in the O–H stretching region, but that hard anions do not [3,4,5,6]
Detailed analysis of anion–water clusters extracted from Molecular dynamics (MD) simulations reveals that the total ion–water interaction energy is systematically larger for a given ion in bulk water than it is for the same ion near the air/water interface
The same is true for the CT-free polarization component of the total interaction energy, which is interesting given that polarization is often assumed to play a central role in surface activity [13], this contention is disputed [23,24]
Summary
One of the earliest results of surface-sensitive vibrational sum-frequency generation (VSFG) experiments [1,2] was the observation that soft anions impact the vibrational lineshape in the O–H stretching region, but that hard anions do not [3,4,5,6]. More recent work suggests that these concentration enhancements were exaggerated by the force fields in use at the time [23,24,25,26,27,28,29], which aligns with the interpretation of some of the early experiments [3]. According to this point of view, surface activity may reflect the absence of depletion of soft anions at the interface [30,31], rather than a concentration enhancement
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