A relationship between three-dimensional surface hydration structures and force distribution measured by atomic force microscopy

Abstract

Hydration plays important roles in various solid-liquid interfacial phenomena. Very recently, three-dimensional scanning force microscopy (3D-SFM) has been proposed as a tool to visualise solvated surfaces and their hydration structures with lateral and vertical (sub) molecular resolution. However, the relationship between the 3D force map obtained and the equilibrium water density, ρ(r), distribution above the surface remains an open question. Here, we investigate this relationship at an interface of an inorganic mineral, fluorite, and water. The force maps measured in pure water are directly compared to force maps generated using the solvent tip approximation (STA) model and from explicit molecular dynamics simulations. The results show that the simulated STA force map describes the major features of the experimentally obtained force image. The agreement between the STA data and the experiment establishes the correspondence between the water density used as an input to the STA model and the experimental hydration structure and thus provides a tool to bridge the experimental force data and atomistic solvation structures. Further applications of this method should improve the accuracy and reliability of both interpretation of 3D-SFM force maps and atomistic simulations in a wide range of solid-liquid interfacial phenomena.

Influence of ions on two-dimensional and three-dimensional atomic force microscopy at fluorite–water interfaces

“…We previously investigated this influence by detailed comparison between the experiments and atomistic MD simulation [19] and explained why we can visualize the intrinsic 3D hydration structures even with a nanoscale tip. In addition, we also previously reported the detailed comparison between the force images obtained by the STA model and experiment [23], discussed the role of the tip hydration shell and explained that it rather helps us to probe the true hydration structure of the sample. Therefore, here we assume that the measured force distributions largely represent the true interfacial structures.…”

“…σ denotes the degree of supersaturation and defined by σ = (c − c eq )/c eq , where c and c eq are the concentrations of the solution used for the experiment and saturated solution, respectively. For the 3D force images obtained in pure water, we previously reported detailed comparison with the simulated 3D images [23]. In contrast, 3D force images obtained in a supersaturated solution have not been reported.…”

“…Thus, we used special experimental procedures and the data processing methods previously reported in detail in Ref. [23]. Here, we only describe them in short.…”

“…We applied a correlation averaging filter to the ρ image. We converted the filtered ρ image into the force image (F STA ) using the STA model [23][24][25]. In this model, the relationship between F STA and ρ is described by…”

“…The force image measured on a calcite (CaCO 3 ) (1014) surface in pure water was compared with the one obtained by an atomistic MD simulation [22], which clarified the central issues underpinning the mechanism of 3D hydration structure measurements [19]. The small cantilever was also used for imaging the 3D force distribution on a fluorite (CaF 2 ) (111) surface in pure water [23]. The obtained image was compared with the force image calculated by the solvation tip approximation (STA) model [24,25], where a tip is approximated by a single water molecule.…”