Molecular Mechanisms Underlying Lubrication by Ionic Liquids: Activated Slip and Flow

Abstract

The present study provides molecular insight into the mechanisms underlying energy dissipation and lubrication of a smooth contact lubricated by an ionic liquid. We have performed normal and lateral force measurements with a surface forces apparatus and by colloidal probe atomic force microscopy on the following model systems: 1-ethyl-3-methyl imidazolium bis-(trifluoro-methylsulfonyl) imide, in dry state and in equilibrium with ambient (humid) air; the surface was either bare mica or functionalized with a polymer brush. The velocity-dependence of the friction force reveals two different regimes of lubrication, boundary-film lubrication, with distinct characteristics for each model system, and fluid-film lubrication above a transition velocity V∗. The underlying mechanisms of energy dissipation are evaluated with molecular models for stress-activated slip and flow, respectively. The stress-activated slip assumes that two boundary layers (composed of ions/water strongly adsorbed to the surface) slide past each other; the dynamics of interionic interactions at the slip plane and the strength of the interaction dictate the change in friction -decreasing, increasing or remaining constant- with velocity in the boundary-film lubrication regime. Above a transition velocity V∗, friction monotonically increases with velocity in the three model systems. Here, multiple layers of ions slide past each other (“flow”) under a shear stress and friction depends on a shear-activation volume that is significantly affected by confinement. The proposed friction model provides a molecular perspective of the lubrication of smooth contacts by ionic liquids and allows identifying the physical parameters that control friction.

Effects of Nanoscale Roughness on the Lubricious Behavior of an Ionic Liquid

“…[ 16 ] In some replica experiments, however, the low‐friction regime was not observed and only a single slope was detected (see empty circles, Figure 3a), which reflects that the composition of the confined IL film remained constant upon an increase in load. Our extensive studies [ 15,17 ] of the influence of traces of water on the interfacial structure and on the friction force at mica‐silica contacts have demonstrated that a higher water content at the interface can reduce the prominence of the low friction regime or even eliminate it, as observed here.…”

“…The investigation of the dependence of friction on velocity in smooth contacts revealed an intricate lubrication behavior. [ 17 ] Depending on the load, friction can increase, decrease, or remain constant with sliding velocity. This was explained through the strength and the dynamics of the interionic interactions at the slip plane via a stress‐assisted thermally activated slip model.…”

A review of recent advances in tribology

“…An et al [86] found that the ionic liquid, glycol ether mixtures, at titanium interfaces had a negative friction load dependence, i.e., the friction force decreased with the increase of normal load. Han et al [87] discussed the activated slip and flow of ionic liquid lubricating molecules.…”

Lubrication Mechanism of Phosphonium Phosphate Ionic Liquid in Nanoscale Single‐Asperity Sliding Contacts

“…[28] Studies at the nanoscale have ascribed the lubrication mechanism of ILs to their adsorption and confinement between smooth surfaces, in absence of any mechano-chemical reactions. [4,29,30] The confined ions resist being "squeezed out" when surfaces are compressed, thus resulting in a film that prevents direct contact between the two surfaces. The properties and dynamic behavior of the adsorbed films are dependent on surface chemistry, charges and applied potentials, molecular structures of the ILs, as well as environmental conditions.…”