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 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 pass each; the bond dynamics and the strength of the interaction at the slip plane 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, layers of ions slide past each (“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.

Interfacial Structure and Boundary Lubrication of a Dicationic Ionic Liquid

“…13 A recent colloid probe study on [C 2 C 1 Im][NTf 2 ] reported an increase of friction for the liquid equilibrated with 44% RH compared to the dry liquid. 35 Molecular dynamics simulations also showed an increase in friction with added water. 42 In those cases, the films were often determined to be solid-like; i.e.…”

“…13 A recent colloid probe study on [C 2 C 1 Im][NTf 2 ] reported an increase of friction for the liquid equilibrated with 44% RH compared to the dry liquid. 35 Molecular dynamics simulations also showed an increase in friction with added water. 42 In those cases, the films were often determined to be solidlike; i.e., with some elastic characteristics; shearing the film is then an activated process, involving either translation of layers across one another or shear-melt.…”

“…The wavelength of oscillations is found to be 0.8±0.1 nm, which is very similar to that previously reported for imidazolium-based 1:1 ionic liquids with the same [NTf 2 ] ion and short alkyl-chains: For [C n C 1 (Im)][NTf 2 ] with n = 2 or 4, wavelengths or repeat-distances for the structural force have been reported in the range 0.71-0.8 nm. 5,10,34,35 These values are similar to the dimension of an ion pair (or, equivalently, to the ++ or --correlation length in the bulk fluid), and so have been interpreted as the repeat-distance for a layered film consisting of (on average) alternating layers of cations and anions which are expelled as ion-pair-layers when the surfaces approach. 5 As long as the alkyl chain is sufficiently small, the value of n was found not to alter the wavelength, indicating that the alkyl chain is oriented in the plane of the film.…”

Electroviscous Retardation of the Squeeze Out of Nanoconfined Ionic Liquids

“…46,47 A similar collective motion of the IL ions could be possible during the squeeze out of ion layers. In fact, recent friction-force measurements with an atomic force microscope conducted in our laboratory 52 show the increase in the activation volume of [C 2 C 1 Im][TFSI] and hence the collective motion of the nanoconfined ions. In these experiments, a silica colloid slides over a mica surface at high compressive stresses.…”

Engineering encapsulated ionic liquids for next-generation applications

“…In the case of studies exploring the potential use of ILs as electrolytes for energy storage devices, adsorbed layers on solid catalysts, or lubricants, signicant efforts have been spent in evaluating the structure and dynamical evolution of solid/IL interfaces as they determine the functional behavior (e.g., charge storage of supercapacitors, friction reduction). [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] The connement of ILs in nanoscale geometries creates radically different conditions than the ones in the bulk phase, and promotes a strong interaction with the boundaries of the conning matrix, which affects the behavior of ILs (e.g., phase transition, wetting, and ion mobility). 19,21,22 In the case of studies aiming to use ILs as separation or reaction media, the high viscosity and surface tension of these uids compared to conventional organic solvents can lead to unfavorable transport properties.…”