Moving beyond the Solvent-Tip Approximation to Determine Site-Specific Variations of Interfacial Water Structure through 3D Force Microscopy

Abstract

Although interfacial solution structure impacts environmental, biological, and technological phenomena, including colloidal stability, protein assembly, heterogeneous nucleation, and water desalination, its molecular details remain poorly understood. Here, we visualize the three-dimensional (3D) hydration structure at the boehmite(010)−water interface using fast force mapping (FFM). Using a self-consistent scheme to decouple long-range tip-sample interactions from short-range solvation forces, we obtain the solution structure with lattice resolution. The results are benchmarked against molecular dynamics simulations that explicitly include the effects of the tip with different levels of approximation and systematically account for tip size, chemistry, and confinement effects. We find four laterally structured water layers within 1 nm of the surface, with the highest water densities at sites adjacent to hydroxyl groups. The key features beyond the first two layers can only be predicted using a full-scale simulation of the boehmite−water−silica system. Our findings further reveal a complex relationship between site-specific chemistry, water density, and long-range particle interactions; and present important advances toward quantitative data interpretation in 3D FFM.

Does the Structural Water within Gypsum Remain Crystalline at the Aqueous Interface?

“…It is important to note that the STA constitutes a simple model that only provides qualitative interpretation and has been shown to be incapable of capturing some important features at the solid−liquid interface, e.g., tip chemistry and confinement. 46 However, in the present work we do not rely on a quantitative comparison: The only assumption made is that a single feature in AFM represents a single water molecule. As can be seen in Figure 4a, a layered water structure with several layers can be identified in the slice.…”

Water Structure in the Electrical Double Layer and the Contributions to the Total Interfacial Potential at Different Surface Charge Densities

“…[34][35] Other methods include atomic force microscopy (AFM), [36][37] X-ray spectroscopies like X-ray standing wave and X-ray reflectivity measurements, 26,[38][39] and vibrational sum frequency generation spectroscopy. [40][41][42] Employing ultraflat boehmite, AFM has recently been used to report directly on the interfacial water structure 43 that constitutes the greatest component of the EDL. X-ray reflectivity and spectroscopy measurements have yielded the position of ions and hydration layers 34,44 in the EDL for a number of different mineral oxides including mica, [45][46][47][48] titania, 34,49 alumina, 44 and quartz.…”

Predicting hydration layers on surfaces using deep learning

“…The measured force between an AFM tip and the surface hydration layers in the solution comprises tipsurface-solvent interactions and entropic effects, and linking this to the image-contrast mechanism requires an intensive modelling approach for reliable understanding. [15][16][17][18] The demonstration of a direct relationship between experimental AFM force data and water densities opened an easier route to interpret the images through simulations, 19,20 which were further advanced by inclusion of the inuence of the tip's hydration structure in the forces 21 and an analysis of the role of tip radius. 17 However, these studies still require detailed molecular dynamics (MD) simulation of water over various estimates of surface structures to describe the hydration structure formed at the mineral-liquid interfacethis is computationally expensive and requires complex parameterisation of classical force elds.…”

“…[15][16][17][18] The demonstration of a direct relationship between experimental AFM force data and water densities opened an easier route to interpret the images through simulations, 19,20 which were further advanced by inclusion of the inuence of the tip's hydration structure in the forces 21 and an analysis of the role of tip radius. 17 However, these studies still require detailed molecular dynamics (MD) simulation of water over various estimates of surface structures to describe the hydration structure formed at the mineral-liquid interfacethis is computationally expensive and requires complex parameterisation of classical force elds. These technical challenges encumber nanoscale characterisation of solid-liquid interfaces through AFM and is in stark contrast to the breakthroughs in molecular characterization offered by low-temperature functional-tip AFM in ultra-high vacuum.…”