DYNAmics at ionic Water-air INterfaces: Synergy between SFG experiments and DFTMD simulations

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Physical and chemical processes occurring at aqueous interfaces have been shown to play a prominent role in a variety of fields ranging from chemistry of atmospheric aerosols and heterogeneous catalysis to biophysics and biochemistry. This proposal aims at obtaining 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 is 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 will be unraveled by the present research program. A unique combination of experimental and theoretical approaches will provide new information about these important interfaces. Questions to be answered include: how does the presence of ions affect the structural relaxation dynamics at the interface? Do ions, by interacting with water molecules, induce heterogeneity amongst the interfacial water? How fast is energy transferred at the interface? We will focus on aqueous solutions containing halide salts and on acidic solutions. The German team will experimentally study the surface using surface-sensitive sum-frequency generation (SFG) spectroscopy, which probes the first few monolayers of the interface. Recent extensions of this technique by the German team including time-resolved, and two dimensional SFG allow obtaining the dynamics of interfacial water molecules with sub-picosecond time resolution. Although powerful, these experiments require theoretical guidance and support to achieve detailed understanding of the physics and chemistry occurring at the interface. To this end, state of the art first-principles molecular dynamics simulations will be performed in the French team. Simulations will be used to calculate trajectories that provide the structure, dynamics, vibrational energy relaxation, and chemical reactivity, at interfaces. The essential input provided by the French partner is the use of first principle MD to calculate SFG spectra, which can be directly compared to the experimental measured spectra. With this combined theoretical and experimental approach with state-of-the-art techniques in both fields we aim to get a full understanding of the surface structure and the dynamics of the aqueous interfaces under study. We believe the results will be essential for understanding reactions occurring on aerosols relevant for heterogeneous chemistry.