Nature of Excess Hydrated Proton at the Water-Air Interface.

Sudipta Das,Yuki Nagata,Ellen H G Backus,Mischa Bonn,Shumei Sun,Sho Imoto

doi:10.1021/jacs.9b10807

Abstract

Understanding the interfacial molecular structure of acidic aqueous solutions is important in the context of, e.g., atmospheric chemistry, biophysics, and electrochemistry. The hydration of the interfacial proton is necessarily different from that in the bulk, given the lower effective density of water at the interface, but has not yet been elucidated. Here, using surface-specific vibrational spectroscopy, we probe the response of interfacial protons at the water–air interface and reveal the interfacial proton continuum. Combined with spectral calculations based on ab initio molecular dynamics simulations, the proton at the water–air interface is shown to be well-hydrated, despite the limited availability of hydration water, with both Eigen and Zundel structures coexisting at the interface. Notwithstanding the interfacial hydrated proton exhibiting bulk-like structures, a substantial interfacial stabilization by −1.3 ± 0.2 kcal/mol is observed experimentally, in good agreement with our free energy calculations. The surface propensity of the proton can be attributed to the interaction between the hydrated proton and its counterion.

Highlights

The proton in water is as ubiquitous as water itself, given that the proton is a product of the autoionization of water (2H2O ⇌ H3O+ + OH−)
Combined with spectral calculations based on ab initio molecular dynamics simulations, the proton at the water−air interface is shown to be wellhydrated, despite the limited availability of hydration water, with both Eigen and Zundel structures coexisting at the interface
An Eigen moiety is a proton as a part of a hydronium (H3O+) ion, which is solvated by three additional water molecules to produce H9O4+

Summary

■ INTRODUCTION

The proton in water is as ubiquitous as water itself, given that the proton is a product of the autoionization of water (2H2O ⇌ H3O+ + OH−). For hydrated protons in bulk, two limiting structures, namely Zundel and Eigen, have been proposed.[1] An Eigen moiety is a proton as a part of a hydronium (H3O+) ion, which is solvated by three additional water molecules to produce H9O4+. In order to address the structure of the interfacial hydrated proton, we use surface-specific vibrational spectroscopy, i.e., conventional and phase-sensitive (PS-) sum-frequency generation (SFG) spectroscopy, at the water−air interface in the presence of HCl. From our experiments, we find that the protons adsorb at the surface and produce a “proton continuum” response reminiscent of that observed in bulk infrared spectroscopy. We quantify the adsorption free energy of the proton at the surface to be ∼1.3 kcal/mol, substantially higher than kBT of ∼0.6 kcal/mol

■ RESULTS AND DISCUSSION

■ CONCLUSION

■ EXPERIMENTAL METHODS

■ ACKNOWLEDGMENTS

■ REFERENCES