Potential-Dependent Superlubricity of Ionic Liquids on a Graphite Surface

Abstract

The lubricities of four quaternary phosphonium ionic liquidstrihexyl­tetradecyl­phosphonium bis­(2,4,4-trimethylpentyl)­phosphinate ([P6,6,6,14]­[(iC8)2PO2]), trihexyltetradecyl­phosphonium bis­(2-ethylhexyl)­phosphate ([P6,6,6,14]­[BEHP]), trihexyltetradecyl­phosphonium bis­(trifluoromethyl­sulfonyl)­imide ([P6,6,6,14]­[TFSI]), and tributylmethyl­phosphonium bis­(trifluoro­menthyl­sulfonyl)­imide ([P4,4,4,1]­[TFSI])were measured as a function of potential on highly oriented pyrolytic graphite (HOPG) by using atomic force microscopy (AFM). The shear strength and surface contact radius values fitted from JKR model indicate the AFM probe slides on an IL boundary layer rather than bare HOPG. Frictions change as the compositions of the boundary layers switch from cation enriched to anion enriched when the potential changes from negative to positive. Superlubricity, which refers to near zero increase in friction with load, is achieved for the [P6,6,6,14]+ cation at −1.0 V and the [TFSI]− anion at +1.0 V. The lubricities of ILs are mainly influenced by three factors: the alkyl chain length, chemical composition, and ion sizes.

MAS-NMR of [Pyr13][Tf2N] and [Pyr16][Tf2N] Ionic Liquids Confined to Carbon Black: Insights and Pitfalls

“…It has been demonstrated that a certain lateral ordering of an IL develops, which, e.g., enhances lubricity by forming crystal-like structures on ionophilic surfaces, depending on the polarity of the matrix [44][45][46]. This has also been shown for carbon surfaces under an applied potential [40,47,48].…”

Electrical Double Layer Structure in Ionic Liquids and Its Importance for Supercapacitor, Battery, Sensing, and Lubrication Applications

“…Superlubricity, which refers to a near-zero increase in friction with load, is achieved for (i) a [P 6,6,6,14 ] + -enriched Stern layer at −1.0 V, where long alkyl chains associated with [P 6,6,6,14 ] + cations assist the formation of robust boundary layers via enhancing lateral cohesive forces and (ii) a [TFSI] − -enriched Stern layer at +1.0 V, where the bulky fluorinated ligands reduce geometric contact corrugations and form a smooth and flat boundary layer. 93 One key barrier to the commercialization of pure ILs as lubricants on a large scale is high cost. However, their tunable nature means that ILs can be customized to be oil-miscible by introducing long alkyl chains to both cations and anions.…”

“…If one of the sliding surfaces is an electrode, the ion composition of the boundary layer can be controlled by applying a potential and, through this, the lubricity. This makes trobotronics (the active external control of friction) achievable with ILs. , As shown in Figure , the lateral force (friction) changes as the compositions of the Stern layers switch from cation-enriched to anion-enriched when the potential changes from negative to positive. Superlubricity, which refers to a near-zero increase in friction with load, is achieved for (i) a [P 6,6,6,14 ] + -enriched Stern layer at −1.0 V, where long alkyl chains associated with [P 6,6,6,14 ] + cations assist the formation of robust boundary layers via enhancing lateral cohesive forces and (ii) a [TFSI] − -enriched Stern layer at +1.0 V, where the bulky fluorinated ligands reduce geometric contact corrugations and form a smooth and flat boundary layer …”

“…Lateral force vs normal load of four ILs at the open-circuit potential (OCP), −1.0 V, and +1.0 V on highly oriented pyrolytic graphite: (a) trihexyltetradecylphosphonium bis­(2-ethylhexyl)­phosphate ([P 6,6,6,14 ]­[BEHP]), (b) trihexyltetradecylphosphonium bis­(trifluoromethylsulfonyl)­imide ([P 6,6,6,14 ]­[TFSI]). Adapted from ref with permission. Copyright 2021 American Chemical Society.…”

Vacuum Interfacial Structure and X-ray Reflectivity of Imidazolium-Based Ionic Liquids with Perfluorinated Anions from a Theory and Simulations Perspective

“…Our group has also investigated ILs under confinement ,− using MD simulations, and in ref we developed the peaks and antipeaks analysis for reflectivity; the ideas introduced in ref were later expanded and applied to the study of diamond-confined molten alkali chloride salts . The literature on the structure of ILs using other surface sensitive techniques is vast, and a full set of citations is beyond the scope of this work, but particularly important has been the pioneering work by Baldelli et al using sum frequency generation − as well as Perkin et al , and Atkin et al using force microscopy. − …”