In-situ characterization of surface restructuring and molecular self-assembly at the calcite-water interface
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Abstract
Organic molecules at mineral-water interfaces are known to alter the morphology and composition of the interface in various ways. Hence, the presence of organic additives exhibits a decisive influence on dynamic processes on the surface, such as: dissolution, growth, surface restructuring, mineral replacement and molecular self-assembly. Especially for calcite, the most thermodynamically stable polymorph of calcium carbonate (CaCO3), the impact of organic additives in biomineralization, scale inhibition and decalcification has been studied extensively with experimental, as well as with computational techniques. Nevertheless, very little is known about the detailed mode of action of organic molecules at interfaces and a comprehensive molecular understanding of surface processes (e.g. adsorption and molecular self-assembly) is still missing. Therefore, in this thesis, interfacial interactions of organic molecules at the nanoscale are characterized to contribute to a molecular-scale knowledge of the composition of mineral-water interfaces. Using in-situ high-resolution dynamic atomic force microscopy the impact of organophosphonates and organic azo dyes is elucidated on two dynamic interfacial processes at the calcite (10.4)-water interface: molecule-induced surface restructuring of the dissolving calcite (10.4) surface and molecule adsorption of the organic additives onto the calcite (10.4) substrate and subsequent their self-assembly into ordered structures. The mode of action of the twelve additive molecules is studied systematically through the variation of different functional groups and structural patterns, as well as the molecule concentration, solution pH and calcium ion concentration. The results indicate that the interplay of surface restructuring and molecular self-assembly in the complex environment of a mineral-water interface is the result of the subtle balance between molecule-molecule, molecule-surface, molecule-water, surface-water and water-water interactions. It is shown that that the surface restructuring mechanism seems to be very robust, whereas molecular self-assembly requires a more rigorous control of molecular structure, conformation and protonation and deprotonation state. For the first time, three possibilities to control systematically the adsorption of organic azo dyes and their subsequent self-assembly into highly ordered molecular islands on the calcite (10.4) surface are presented. Moreover, results from molecular dynamic simulations highlight the decisive impact of the hydration structure for the adsorption behavior of organic molecules at the calcite (10.4)-water interface. In summary, this thesis provides high-resolution insights into dissolution, surface restructuring and molecular self-assembly at the calcite (10.4)-water interface in the presence of organic molecules. Furthermore, it highlights that when aiming for a rational understanding, modelling and predicting of interfacial processes, a molecular-scale knowledge of the composition of the entire interface is essential. This includes the mineral surface, the hydration structure, as well as the behavior of the additive molecules in solution and at the interface.