Hydration Structures at Charged Surfaces

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In this project, we aim for investigating the influence of an applied potential on the interfacial hydration structure of a range of metal and insulator surfaces. An emphasis will be on elucidating the competition between the alignment of the water dipole in the electrostatic field and specific binding towards the surface, e.g., via hydrogen bonds. Furthermore, as water forms an extended hydrogen bonded network, also cooperative effects will govern the resulting hydration structure. Moreover, added ions might or might not concentrate at the interface as a function of the applied potential. In the classical picture, an electrical double layer forms, with ion concentrations that can be calculated based on continuum theory. While the presence of the ions will impact the hydration structure, a clear understanding of the effect of specific ions in a molecular-scale picture is still lacking. Here, we will develop a setup that allows for high-resolution hydration layer mapping under potential control. This development will build upon the existing modified atomic force microscopes in the group that have been proven to allow for three-dimensional hydration layer mapping. For this project, we will add an electrochemical cell with potential control to an existing instrument. As a first benchmark for testing the new setup, we will investigate inert metal surfaces such as gold (111) and platinum (111) in de-ionized water, to validate our results against existing theoretical calculations and experimental findings. In a next step, we will benefit from the fact that atomic force microscopy is not limited to electrical conducting samples. Therefore, we will investigate well-studied insulator surfaces such as calcium fluoride (111) and silica as well as calcite (10.4). These studies will allow for elucidating the interplay between specific binding of water molecules to the surface, e.g., via hydrogen bonds, and the impact of the electrostatic potential on the molecule’s electric dipole. Due to the fact that these samples are transparent, they are accessible to vibrational sum frequency generation spectroscopy. This is extremely helpful as it enables the comparison of the spatially resolved structural data from the hydration layer mapping with the averaged information on the orientation from the sum frequency generation spectroscopy. Finally, we will investigate the impact of added ions on the hydration structure under potential control. Here, we will systematically investigate the effect of ions as a function of both ionic charge and ion size on the resulting hydration structure in presence of an applied electric potential. The project will, therefore, provide detailed insights into the driving forces that govern the hydration structure formation at charged interfaces.