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

Maria Ricci,Clodomiro Cafolla,William Trewby,Kislon Voïtchovsky

doi:10.1038/srep43234

Maria Ricci, Clodomiro Cafolla + Show 2 more

Open Access

https://doi.org/10.1038/srep43234

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Journal: Scientific Reports	Publication Date: Feb 23, 2017
Citations: 47	License type: open-access

Affiliation: University of Cambridge, Durham University

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 H3O+ 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 τ1 = 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 τ2 = 610 ± 30 ms depending on H3O+ 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.

Highlights

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
The ‘dynamic Stern model’, which describes electro-osmotic flow parallel to the surface, assumes that water molecules are immobile but ions can move within the Stern layer[22,23,24]
Studies based on Scanning Electrochemical Microscopy[26] or Polarisation Force Microscopy[27] conducted in 95% controlled humidity suggest relaxation times between 20 and 30 ms for ions at the surface of calcite

Summary

Introduction

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. 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. Increasing the temperature appears to affect the behaviour of the hydration water at the surface of mica, but not the respective ions’ dynamics These timescales, remarkably slow for individual monovalent ions, have important implications for many interfacial processes, for example in electrochemical and electrokinetic systems[20,33,34], in self-assembly processes[35,36], and at biointerfaces where ions modulate charge transport[6,7,8,9,10], mechanical properties[37,38,39,40], and function[6]

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