Direct observation of the dynamics of single metal ions at the interface with solids in aqueous solutions

Abstract

The dynamics of ions adsorbed at the surface of immersed charged solids plays a central role in countless natural and industrial processes such as crystal growth, heterogeneous catalysis, electrochemistry, or biological function. Electrokinetic measurements typically distinguish between a so-called Stern layer of ions and water molecules directly adsorbed on to the solid's surface, and a diffuse layer of ions further away from the surface. Dynamics within the Stern layer remain poorly understood, largely owing to a lack of in-situ atomic-level insights. Here we follow the dynamics of single Rb + and H 3 O + ions at the surface of mica in water using high-resolution atomic force microscopy with 25 ms resolution. Our results suggest that single hydrated Rb + ions reside τ 1 = 104 ± 5 ms at a given location, but this is dependent on the hydration state of the surface which evolves on a slower timescale of τ 2 = 610 ± 30 ms depending on H 3 O + adsorption. Increasing the liquid's temperature from 5 °C to 65 °C predictably decreases the apparent glassiness of the interfacial water, but no clear effect on the ions' dynamics was observed, indicating a diffusion-dominated process. These timescales are remarkably slow for individual monovalent ions and could have important implications for interfacial processes in electrolytes.Most solids are charged in aqueous solutions. The ions dissolved in the liquid tend to accumulate near the solid's surface to ensure charge neutrality, forming a so-called electrical double layer. Countless natural and industrial processes rely on the organization and dynamics of ions in this layer, from crystal growth 1-3 , to heterogeneous catalysis 4 , electrochemistry 5 , or biological function [6][7][8][9][10] . Ionic distribution within the electrical double layer is typically described by the Gouy-Chapman-Stern model 11 that comprises a Stern layer of largely static ions and water molecules adsorbed to the solid's surface, and a diffuse layer of ions further away from the surface in the liquid. The diffuse layer region is generally well understood, but the Stern layer is more difficult to model, partly because continuous assumptions have to be made about an intrinsically discrete object where the size and nature of the adsorbed ions play an important role. Significantly, existing models assume a homogenous charge distribution within the Stern layer, which is unlikely to hold at the nanoscale. Recently, a handful of studies [12][13][14][15][16] have shown that it is possible to observe in-situ single ions within the Stern layer using atomic force microscopy (AFM). However these investigations were conducted at equilibrium and no information about the ion's dynamics could be derived. The motion of single ions at interfaces remains a central question in interfacial processes with significant implications in nanotechnology, for example in the development of novel power-generating devices 17,18 . Existing theories require corrections to explain experimental observations [19][20][21] . The 'dynami...

Lubricating properties of single metal ions at interfaces

“…Numerous studies 2,6,7,[12][13][14][15]17,[20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] have examined the behaviour of nano-confined aqueous solutions in various systems and under different circumstances. Experimentally, the two main approaches are based on the surface force apparatus (SFA) and atomic force microscopy (AFM) with each family of methods offering different modes of operation.…”

“…33 Ions do not necessarily arrange randomly at the surface of solids, but instead can form organised nanoscale clusters through water-mediated correlations, 24 and with millisecond dynamics at the single ion level. 23,53 While only a handful of reports evidence mesoscale order (1-100 nm), 24,25 its origin would suggest mesoscale effects to be the rule rather than the exception. In nano-shearing experiments, mesoscale order could have significant consequences for the dynamics and lubrication properties of the system depending on the scale considered.…”

Environment‐Dependent Adhesion Energy of Mica Nanolayers Determined by a Nanomanipulation‐Based Bridging Method

“…The contribution of capillary force would be attenuated, even vanished when the temperature of the interface of MNL and mica substrate was increased. The previous studies using scanning thermal microscopy demonstrated that the evaporation of adsorbed water decreased the capillary force between the tip and hydrophilic surface at relatively high temperatures . Nevertheless, the contaminant blocking effect might be the dominant mechanism responsible for the decreased adhesion energy at elevated temperatures.…”

“…However, as MNLs were prepared via mechanical exfoliation, so the distribution of K ions on the surface of each nanobridge could vary. After the cleavage K ions form rows and small domains on the surface to minimize the energy, leading to further change in surface condition . The difference can be aggravated by water and carbonates adsorbed on a MNL surface.…”

Self-assembly of small molecules at hydrophobic interfaces using group effect

“…High-resolution amplitude modulation atomic force microscopy (AFM) in liquid 23 is used to explore in situ the sub-nanometre details of the resulting interfacial molecular assemblies. When operated in solution and at small amplitude, the technique can routinely provide molecular-level details of the interface, 24,25 including of solvation structures 26,27 and stable supramolecular assemblies. 21 The AFM results include simultaneously acquired topographic and phase images of the interface.…”