Impact of dissolved ions on hydration layers at the solid-liquid interface of carbonates

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Interfacial water plays a key role in a large range of fields, including geochemistry and environmental science as well as corrosion protection and catalysis. The detailed knowledge of the surface composition and the local water structure is decisive for understanding and predicting interfacial chemistry processes.While it is well established, e.g., from surface-specific spectroscopy studies that the local hydration structure at an interface is commonly different from the bulk structure, the detailed arrangement remains a topic of intense research. Only very recently, atomic force microscopy (AFM) instrumentation has been further advanced to provide three-dimensional (3D), molecular-level information of the local ordering at the solid-liquid interface in direct space. This technique now offers the unique capability for directly mapping the hydration layers at the interface.In this project, we will investigate the hydration layers on four different surfaces using a state-of-the-art 3D hydration layer mapping AFM. The focus will be on three different carbonate surfaces, namely the most stable cleavage planes of calcite, dolomite and magnesite. Besides being highly relevant for natural and technical processes, these surfaces provide an ideal system to systematically study the influence of changing the unit cell lattice dimensions on the hydration layer structure. As a fourth substrate, chemically and structurally different calcium fluoride will be studied as a prominent system that can be benchmarked against existing nonlinear vibrational spectroscopy studies.It is generally accepted that the presence of dissolved ions can alter the hydration layer structure, however, the details of this perturbation are still largely unknown. In this project, we will elucidate the impact of dissolved ions on the hydration layer structure at the solid-liquid interface. We will investigate the influence of both concentration (ionic strength) and chemical nature (mono- and bivalent ions, different ionic radii and polarizabilities) of ions on the structure of the hydration layers at the interface.This project will provide detailed, molecular-scale knowledge of the composition of the interface, including the solid surface, hydration layers and additional ions. These experimental insights, combined with molecular dynamics simulations, will constitute an essential step forward for understanding and predicting fundamental interfacial processes.