Issue 25, 2018

Lubricating properties of single metal ions at interfaces


The behaviour of ionic solutions confined in nanoscale gaps is central to countless processes, from biomolecular function to electrochemistry, energy storage and lubrication. However, no clear link exists between the molecular-level behaviour of the liquid and macroscopic observations. The problem mainly comes from the difficulty to interrogate a small number of liquid molecules. Here, we use atomic force microscopy to investigate the viscoelastic behaviour of pure water and ionic solutions down to the single ion level. The results show a glassy-like behaviour for pure water, with single metal ions acting as lubricants by reducing the elasticity of the nano-confined solution and the magnitude of the hydrodynamic friction. At small ionic concentration (<20 mM) the results can be quantitatively explained by the ions moving via a thermally-activated process resisted by the ion's hydration water (Prandtl–Tomlinson model). The model breaks down at higher salt concentrations due to ion-ion interaction effects that can no longer be neglected. The correlations are confirmed by direct sub-nanometre imaging of the interface at equilibrium. The results provide a molecular-level basis for explaining the tribological properties of aqueous solutions and suggest that ion-ion interactions create mesoscale effects that prevent a direct link between nanoscale and macroscopic measurements.

Graphical abstract: Lubricating properties of single metal ions at interfaces

Supplementary files

Article information

Article type
09 Apr 2018
31 May 2018
First published
19 Jun 2018
This article is Open Access
Creative Commons BY license

Nanoscale, 2018,10, 11831-11840

Lubricating properties of single metal ions at interfaces

C. Cafolla and K. Voïtchovsky, Nanoscale, 2018, 10, 11831 DOI: 10.1039/C8NR02859A

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