DYNAmics at ionic Water-air INterfaces: Synergy between SFG experiments and DFTMD simulations
Final Report Abstract
Physical and chemical processes occurring at aqueous interfaces are playing a prominent role in a variety of fields ranging from chemistry of atmospheric aerosols and heterogeneous catalysis to biophysics and biochemistry. The aim of the proposal was to obtain molecular-level information on the structure and dynamics of water at specifically the interface between air and ionic solutions. Such insights are highly relevant for atmospheric chemistry where, for instance, on the surface of aerosols heterogeneous ozone chemistry occurs. Despite of the apparent importance, surprisingly little was known about the dynamics of these interfaces. Truly understanding the reactivity of such an interface requires knowledge of both the structural and energy flow dynamics, properties that were unraveled by the research program. To study the structure and dynamics of just the interfacial layer, the surface-sensitive spectroscopic method sum-frequency generation (SFG) has been used. In the conventional SFG method basically the vibrational spectrum of just the interfacial molecules is obtained. The frequency of the stretch vibration of water molecules provides information about the interfacial structure. Phase-resolved SFG enlightens the orientation of the water molecules. Energy transfer processes are studied by time-resolved SFG spectroscopy in which the system is perturbed with an infrared pulse. With this methodology we studied the aqueous solution with a variety of salt focusing on environmentally relevant ions. We found that sulfate and carbonate ions change the structure of the water network at the interface as the SFG spectrum changes. However, the vibrational dynamics is the same for pure water and water with 1.8 M of these ions dissolved. As such we conclude that although the structure is changed, the vibrational dynamics is not affected. Moreover, by comparing NaCl and HCl solutions at the water-air interface and covered with a monolayer of negatively charged surfactants, we could demonstrate that the surface propensity of ions is a function of both the nature of the ion and the nature of the surface. As autoionization products of water both the hydrated proton and hydronium are present in bulk water and at the surface potentially determining surface processes. As such, we studied the surface affinity of these ions and studied in more depth the structure and dynamics of the interfacial hydrated proton. We found that the onset of perturbation of the water surface starts for the proton at roughly two orders of magnitude smaller bulk concentration that hydronium. 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. Although the interfacial hydrated proton exhibits bulk-like structures, a substantial interfacial stabilization by -1.3±0.2 kcal/mol is observed. Time-resolved SFG experiments show a slow transient signal for a 1 M solution of protons, which is attributed to the formation of a new equilibrium between interfacial and bulk protons, following the quasi-instantaneous temperature jump following vibrational relaxation of the interfacial water molecules. In summary, the interfacial behavior of ions is dependent on the type of interface and on the nature of the ion. Our work unraveled certain aspect of ionic solutions, but opened up new research questions for example in the direction of proton dynamics at the interface. Moreover, not only the water-air interface is relevant. We observed as well that specific ion effects are observed for the nucleation of ice at micawater interfaces.
Publications
- Surface Potential of a Planar Charged Lipid−Water Interface. What Do Vibrating Plate Methods, Second Harmonic and Sum Frequency Measure? J. Phys. Chem. Lett. 9 (2018) 5685
L. B. Dreier, C. Bernhard, G. Gonella, E. H. G. Backus, and M. Bonn
(See online at https://doi.org/10.1021/acs.jpclett.8b02093) - Electrolytes change the structure, but not the vibrational dynamics, of interfacial water. J. Phys. Chem. B 123 (2019) 8610
M. Deiseroth, M. Bonn, and E.H.G. Backus
(See online at https://doi.org/10.1021/acs.jpcb.9b08131) - Hydration and Orientation of Carbonyl Groups in Oppositely Charged Lipid Monolayers on Water. J. Phys. Chem. B 123 (2019) 1085
L. Dreier, M. Bonn, and E.H.G. Backus
(See online at https://doi.org/10.1021/acs.jpcb.8b12297) - Molecular hydrophobicity at a macroscopically hydrophilic surface. Proc. Nat. Acad. Sci. USA 116 (2019) 1520
J.D. Cyran, M.A. Donovan, D. Vollmer, F. Siro Brigiano, S. Pezzotti, D.R. Galimberti, M.-P. Gaigeot, M. Bonn, and E.H.G. Backus
(See online at https://doi.org/10.1073/pnas.1819000116) - The surface activity of the hydrated proton is substantially higher than that of hydroxide. Angew. Chem. Int. Ed. 58 (2019) 15636
S. Das, M. Bonn, and E.H.G. Backus
(See online at https://doi.org/10.1002/anie.201908420) - The surface affinity of cations depends on both the cations and the nature of the surface. J. Chem. Phys. 150 (2019) 044706
S. Das, M. Bonn, and E.H.G. Backus
(See online at https://doi.org/10.1063/1.5065075)
