Molecular Resolution of the Water Interface at an Alkali Halide with Terraces and Steps

Abstract

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.

Atomic-resolution three-dimensional hydration structures on a heterogeneously charged surface

“…In order to better understand the nature of the B II region, we first focus in detail on the hydration structures found on the B I and T regions. Previous studies have already shown that the force profiles or maps obtained by AFM gave good qualitative agreement with the density distributions of the water molecules determined by the X-ray reflectivity or predicted by the MD simulations 13 , 14 , 17 , 23 , 31 – 35 . Specifically, it has been reported in some papers that AFM force maps were well reproduced by free-energy calculations 17 , 32 .…”

“…We consider that the ions were moving around during the millisecond timescale of the AFM measurements, and thus that the experimentally observed hydration forces and their contrast patterns in the 2D maps mainly originate from the water density distribution. This is supported by repeated observations of the common similarities between experimental force and simulated water/force on several other surfaces 13 , 14 , 17 , 21 , 23 , 31 – 35 . A recent paper on FM-AFM measurements of the hydration structures of calcium fluorite in ultra-pure water and supersaturated solution has shown that the adsorbed ions could give a slight modulation in the oscillatory force profiles, but they did not change the overall features 33 .…”

“…Recently, we have opened the door to atomic-scale three-dimensional (3D) mapping of the local hydration structures by ultra-low noise frequency-modulation atomic force microscopy (FM-AFM) 12 , 13 , which has been applied to various homogeneous samples 12 – 18 . Moreover, with the support from theoretical molecular dynamics (MD) simulations, deeper molecular detail has been achieved 17 , 19 – 23 . However, the time-consuming atomic-scale 3D hydration measurements on the uneven heterogeneous surfaces have never been achieved, to the best of our knowledge, because these experiments are extremely difficult without any deformation of the surface structures by the tip.…”

Stability of Two-Dimensional Ionic Clusters at Solid–Liquid Interfaces

“…Interfaces between solids and liquids are typical examples to explore microstructures in complex environments. , Among various interfaces, the interface between ionic crystals and aqueous solutions (ICASs) is particularly interesting and complicated. ICASs are ubiquitous in daily life and nature and intimately related to many phenomena and processes, such as wetting, electrochemistry, − flotation, , and nanorheology. , Especially, due to the existence of water molecules, the structure, − thermodynamics, , and kinetics , properties of ICASs may be very different from simple solid–liquid interfaces, which has aroused great scientific interest in past years.…”

Investigation of Local Hydration Structures of Alkanethiol Self-Assembled Monolayers with Different Molecular Structures by FM-AFM

“…We applied this technique to study the local hydration structures on various alkali halides surfaces and found that the in-plane hydration structures depend on the atomic periodicity of the surface with respect to the size of a single water molecule. 13 More recently, we applied this technique to study the local hydration structures on various silicate surfaces, such as albite and apophyllite, and reported that the local hydration structures depend on the arrangements of the silicon−oxygen tetrahedral at the surface. 14 In this study, we investigated the relationship between the local hydration structures and the molecular arrangements of self-assembled monolayers (SAMs) in order to gain more insight into the local hydration structures at the solid−liquid interfaces and their effects on the surface phenomena.…”