Three-dimensional imaging of the solid-liquid interface with high-resolution atomic force microscopy
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Abstract
The solid-liquid interface plays a key role in a large range of fields, including geochemistry and environmental science. The detailed knowledge of the surface composition and the local solvent structure is decisive for understanding and predicting interfacial processes. It is well-known that the presence of a surface induces an ordered arrangement of the interfacial solvent molecules. Only very recently, atomic force microscopy (AFM) instrumentation has been further advanced to provide three-dimensional (3D), molecular-level information of the local ordering of the solvent molecules at the solid-liquid interface in direct space. In this thesis, the successful implementation of a state-of-the art 3D mapping routine that combines the easy-to-use amplitude modulation AFM technique with reliable high-speed data acquisition is demonstrated.
The main purpose of this study is to investigate the arrangement of different solvent molecules, such as water and different alcohols, at pristine surfaces, for instance lithium niobate and the natural carbonates, calcite, dolomite and magnesite. The 3D mapping routine has been further used in an investigation of the complex interactions of the Alizarin Red S (ARS) molecule with the calcite (10.4) surface in water.
When considering interfacial processes, natural carbonates are of major importance since they are highly abundant in nature. Among them, calcite is the most thoroughly studied mineral, which is relevant in numerous fields, such as biomineralisation and oil recovery. The related minerals dolomite and magnesite exhibit the same crystal structure as calcite, but differ in their cation composition. This results in slightly different lattice constants of the unit cell on their most stable cleavage plane, providing an ideal model system to systematically study the influence of the lattice constant on the arrangement of interfacial water molecules. High-resolution 3D maps are presented for each of the carbonates, and three hydration layers with a periodic structure commensurate to the underlying surface are observed.
Most studies in the field of hydration layer mapping have only focused on pristine surfaces in water, and comparatively little is known about the arrangement of other solvent molecules. In this thesis, the arrangement of other simple solvent molecules on the (10.4) surface of calcite is examined by means of 3D mapping. Three linear alcohols, namely, methanol, ethanol and propan-1-ol are chosen as model systems to study the interaction of organic/bio- molecules, which often contain hydroxyl groups (-OH) and hydrophobic functionalities. The obtained experimental data reveal one solvation layer for methanol but a distinct vertical and lateral order in the solvation structure. In sharp contrast, for ethanol and propan-1-ol a layer-like structure, with only vertical order, is found.
In conclusion, the high-resolution capabilities of 3D AM-AFM mapping is demonstrated and high quality 3D maps are obtained on different relevant surfaces. The obtained experimental AFM data in this thesis reflect the intrinsic solvation structure at the solid-liquid interface. These insights are a fundamental prerequisite for understanding and tailoring surface reactivity. The AFM mapping technique has proven to be irreplaceable for studying the solid-liquid interface with atomic-scale site-specificity and is ideally suited to investigate the knowledge gaps in this field of research. Here, an essential further step, when aiming for an accurate description of interfacial processes, is the investigation of the impact of ions on the structure and formation of hydration.