Local hydration structures at the solid–liquid interface around boundary edges on heterostructures are key to an atomic-level understanding of various physical, chemical and biological processes. Recently, we succeeded in visualising atomic-scale three-dimensional hydration structures by using ultra-low noise frequency-modulation atomic force microscopy. However, the time-consuming three-dimensional-map measurements on uneven heterogeneous surfaces have not been achieved due to experimental difficulties, to the best of our knowledge. Here, we report the local hydration structures formed on a heterogeneously charged phyllosilicate surface using a recently established fast and nondestructive acquisition protocol. We discover intermediate regions formed at step edges of the charged surface. By combining with molecular dynamics simulations, we reveal that the distinct structural hydrations are hard to observe in these regions, unlike the charged surface regions, possibly due to the depletion of ions at the edges. Our methodology and findings could be crucial for the exploration of further functionalities.

Hydration structures at crystal surfaces play important roles in crystal growth or dissolution processes in liquid environments. Recently developed two-dimensional (2D) and three-dimensional (3D) force mapping techniques using frequency-modulation atomic force microscopy (FM-AFM) allow us to visualize the hydration structures at the solid–liquid interfaces at angstrom-scale resolution in real space. Up to now, the experimental and theoretical studies on local hydration structures have mainly focused on those on the terrace, but little work has looked at step edges, usually the key areas in dissolution and growth. In this study, we measured local hydration structures on water-soluble alkali halide crystal surfaces by 2D force mapping FM-AFM. The atomic-scale hydration structures observed on the terraces agree well with molecular-dynamics (MD) simulations. We also measured the hydration structures at the step edge of the NaCl(001) surface, which was constantly dissolving and growing, leading to the clear observation of atomic fluctuations. We found, with the support of MD simulations, that the hydration structures measured by FM-AFM at a time scale of a minute can be interpreted as the time-average of the hydration structures on the upper terrace and those on the lower terrace.

Water molecules at solid surfaces typically arrange in layers. The physical origin of the hydration layers is usually explained by two different reasons: (1) the attraction between the surface and water and (2) the water confinement due to the surface. While the attraction is specific to the particular solid, the confinement is a general property of surfaces; a differentiation between the two effects is, therefore, critical for research on interactions at aqueous interfaces. Here, we investigate the graphite-water interface, which is a widely used model system where the solid-water attraction is often considered to be negligible. Similar to previous studies, we observe hydration layers using three-dimensional atomic force microscopy at the graphite-water interface. We explain why the confinement could cause the formation of hydration layers even in the absence of attraction between surface and water by employing Monte Carlo simulations. Using additional molecular dynamics simulations, we continue to show that at ambient conditions, however, the confinement alone does not cause the formation of layers at the graphite-water interface. We thereby demonstrate that there is a significant graphite-water attraction.

Calcite and magnesite are important mineral constituents of the earth’s crust. In aqueous environments, these carbonates typically expose their most stable cleavage plane, the (10.4) surface. It is known that these surfaces interact with a large variety of organic molecules, which can result in surface restructuring. This process is decisive for the formation of biominerals. With the development of 3D atomic force microscopy (AFM) it is now possible to image solid–liquid interfaces with unprecedented molecular resolution. However, the majority of 3D AFM studies have been focused on the arrangement of water at carbonate surfaces. Here, we present an analysis of the assembly of ethanol – an organic molecule with a single hydroxy group – at the calcite and magnesite (10.4) surfaces by using high-resolution 3D AFM and molecular dynamics (MD) simulations. Within a single AFM data set we are able to resolve both the first laterally ordered solvation layer of ethanol on the calcite surface as well as the following solvation layers that show no lateral order. Our experimental results are in excellent agreement with MD simulations. The qualitative difference in the lateral order can be understood by the differing chemical environment: While the first layer adopts specific binding positions on the ionic carbonate surface, the second layer resides on top of the organic ethyl layer. A comparison of calcite and magnesite reveals a qualitatively similar ethanol arrangement on both carbonates, indicating the general nature of this finding.

The original version of the Supplementary Information associated with this Article contained an error in Supplementary Figure 9e,f in which the y-axes were incorrectly labelled from '−40' to '40', rather than the correct '-400' to '400'. The HTML has been updated to include a corrected version of the Supplementary Information.

Local hydration structures around boundary edges on heterostructures are essential for development of novel functional materials and devices. Recently we succeeded in visualizing atomic-scale local 3D hydration structures by using ultra-low noise frequency-modulation atomic force microscopy. However, a time-consuming 3D measurement on uneven heterogeneous surfaces has never been reported due to experimental difficulties. In this work, we investigate the local hydration structures around boundary edge between oppositely charged surfaces of a phyllosilicate by the previously established fast and nondestructive protocol and molecular dynamics simulation.