Molecular Self-Assembly Versus Surface Restructuring During Calcite Dissolution.

Organic additives are known to alter the mineral-water interface in various ways. On the one hand, organic molecules can self-assemble into ordered structures wetting the surface. On the other hand, their presence can affect the interfacial morphology, referred to as surface restructuring. Here, we investigate the impact of a class of calcium-complexing azo dyes on the dissolution of calcite (10.4) using high-resolution atomic force microscopy operated in aqueous solution, with a focus on the two constitutional isomers Eriochrome Black T and Eriochrome Black A. A very pronounced surface restructuring is observed in the presence of the dye solution, irrespective of the specific dye used and independent of the pH. This surface restructuring is obtained by the stabilization of both the nonpolar acute and the polar [010] step edges, resulting in a greatly altered, characteristic interface morphology. In sharp contrast to the prevalence of the surface restructuring, an ordered molecular structure on the crystal terraces is observed only under very specific conditions. This formation of an ordered stripe-like molecular structure is obtained from Eriochrome Black A only and limited to a very narrow pH window at a pH value of around 3.6. Our results indicate that such molecular self-assembly requires a rather precise adjustment of the molecular properties including control of the conformation and deprotonation state. This is in sharp contrast to the additive-induced surface restructuring, which appears to be far more robust against both pH changes and variations in the molecular conformation.

Stabilization of Polar Step Edges on Calcite (10.4) by the Adsorption of Congo Red.

In this work, we present the stabilization of polar step edges along the [010] direction of calcite (10.4) by the presence of a water-soluble organic molecule, namely Congo Red. While characteristic etch pits are observed on the surface in the absence of the additive, no etch pits can be found in the presence of the additive. Using atomic force microscopy, we can directly follow the restructuring of the surface. Upon addition of Congo Red, the charge-neutral step edges confining the characteristic etch pits vanish, while polar step edges along the [010] direction appear on the surface, which are entirely decorated by well-ordered molecular islands of the additive. After the restructuring has taken place, the surface exclusively exhibits these polar step edges. Our results give direct evidence of the fact that these polar step edges become thermodynamically favored when Congo Red is present.

Three-dimensional atomic force microscopy mapping at the solid-liquid interface with fast and flexible data acquisition.

We present the implementation of a three-dimensional mapping routine for probing solid-liquid interfaces using frequency modulation atomic force microscopy. Our implementation enables fast and flexible data acquisition of up to 20 channels simultaneously. The acquired data can be directly synchronized with commercial atomic force microscope controllers, making our routine easily extendable for related techniques that require additional data channels, e.g., Kelvin probe force microscopy. Moreover, the closest approach of the tip to the sample is limited by a user-defined threshold, providing the possibility to prevent potential damage to the tip. The performance of our setup is demonstrated by visualizing the hydration structure above the calcite (10.4) surface in water.