Speaker
Description
Understanding aqueous interfaces at electrified metal surfaces remains a fundamental challenge, as continuum descriptions treat the solvent as a homogeneous medium and thus neglect its molecular nature. In particular, the neglect of hydrogen bonding and density fluctuations highlights the need to go beyond classical mean-field approaches.
A striking example of this breakdown is provided by molecular dynamics simulations at constant applied potential at the gold-water interface, representing a prototypical “soft meets hard” system. [1] While the first interfacial water layer forms a highly ordered, two-dimensional hydrogen-bond network, the adjacent layer exhibits markedly reduced connectivity. This leads to a situation where water effectively “repels” water, resulting in the formation of cavities at the interface that preferentially host small molecules such as CO or N₂.
To experimentally probe such effects, we employ terahertz (THz) spectroscopy under externally controlled electric fields, providing direct access to collective intermolecular vibrations at charged interfaces. [2] To the best of our knowledge, this represents the first setup capable of probing such dynamics under an applied potential. As an initial application, this setup enables the direct observation of a hydrophobic cation-rich film, which profoundly reshapes the electric double layer. [3]
Building on this approach, we extend our investigation to different tetraalkylammonium-based species and aqueous electrolytes. By systematically varying the cation hydrophobicity and tuning the anion size, we reveal how ion-specific interactions drive the formation of interfacial films. We further combine THz spectra with molecular dynamics simulations to gain molecular-level insight into the underlying mechanisms.
[1] A. Serva, et al., PNAS, 118, e2023867118 (2021).
[2] S.R. Alfarano, et al., PNAS 118, e2108568118 (2021).
[3] N.S. Wichmann, et al., JACS, 148, 13, 13550-13560 (2026).