Lubricating properties of single metal ions at interfaces

Abstract

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.

Lubricated friction around nanodefects

“…The relaxation dynamics of the lubricant molecules has a marked impact on the lubricant's effective viscosity  eff when nanoconfined, as to be expected from previous studies (9,15,18). Here, we find an increase of at least three orders of magnitude for  eff at 308 K when compared with the bulk viscosity of squalane, regardless of the applied pressure ( fig.…”

“…An accurate cleaning procedure was performed so as to remove any possible sources of contamination (see the Supplementary Materials) (9). The absence of contaminants was confirmed by subnanometric imaging of the area investigated (9,57). Imaging The AFM was operated in amplitude modulation, fully immersing the cantilever tip in liquid.…”

Effect of Fluid Chemistry on the Interfacial Composition, Adhesion, and Frictional Response of Calcite Single Crystals—Implications for Injection‐Induced Seismicity

“…As in the case of Ω, the thermal activation energy exhibits two different trends in the HS‐regime. Although using different methods and experimental conditions to ours, Cafolla and Voitchovsky inferred from their friction measurements on mica that the lubricious properties of ions originate from the induced distortion of the structure (hydrogen bond network) of the hydration layers of mica upon shear (Cafolla & Voitchovsky, ). They associated this distortion to an empiric activation energy (~1.3 k B T ), which is about the half of the energy needed to dehydrate the mica surface per unit cell.…”

Impact of water on the lubricating properties of hexadecane at the nanoscale

“…They allow the parts to slide easily and smoothly past each other by confining the shearing-induced molecular rearrangements to the lubricant layer. 7,8 The efficiency of a lubricant depends on many different parameters such as the lubricant's molecular structure and intrinsic properties, the local topography and chemical characteristics of the sliding surfaces, 9 and the operating parameters such as sliding speed, 10,11 contact pressure, 12 and environmental factors such as temperature 10 and humidity. 9 While many of these parameters can be kept to a particular operating regime, certain environmental parameters are significantly harder to control in real-life applications.…”

“…14,[25][26][27] Mica is a hydrophilic aluminosilicate that acts here as a model metal oxide surface. 12 Mica has been extensively used as a model substrate to study the fundamental aspects of lubricated friction, including real-life applications. Studies have explored lubricants as diverse as commercial additives, 31 hydrated metal ions, 12 ionic liquids 32 and liquid crystals intended as the new generation of lubricants.…”