Potential-dependent superlubricity of stainless steel and Au(111) using a water-in-surface-active ionic liquid mixture

HYPOTHESIS

The friction and interfacial nanostructure of a water-in-surface-active ionic liquid mixture, 1.6 M 1-butyl-3-methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate ([BMIm][AOT]), can be tuned by applying potential on Au(111) and stainless steel.

EXPERIMENTAL

Atomic force microscopy (AFM) was used to examine the friction and interfacial nanostructure of 1.6 M [BMIm][AOT] on Au(111) and stainless steel at different potentials.

FINDINGS

Superlubricity (vanishing friction) is observed for both surfaces at OCP+1.0 V up to a surface-dependent critical normal force due to [AOT]- bilayers adsorbing strongly to the positively charged surface thus allowing AFM tip to slide over solution-facing hydrated anion charged groups. High-resolution AFM imaging reveals ripple-like features within near-surface layers, with the smallest amplitudes at OCP+1 V, indicating the highest structural stability and resistance to thermal fluctuations due to highly ordered boundary [AOT]- bilayers templating robust near-surface layers. Exceeding the critical normal force at OCP+1.0 V causes the AFM tip to penetrate the hydrated [AOT]- layer and slide over alkyl chains, increasing friction. At OCP and OCP-1.0 V, higher friction correlates with more pronounced ripples, attributed to the rougher templating [BMIm]+ boundary layer. Kinetic experiments show that switching from OCP-1.0 V to OCP+1.0 V achieves superlubricity within 15 s, enabling real-time friction control.

Molecular-Resolution Imaging of Ionic Liquid/Alkali Halide Interfaces with Varied Surface Charge Densities via Atomic Force Microscopy

When in contact with charged solid surfaces, ionic liquids (ILs) are known to form solvation structures consisting of alternating cation and anion layers. This phenomenon is considered to originate from the adsorption layer of counterions overcompensating the surface charge, so-called overscreening. However, the response of these layers to surfaces with near-zero or extremely high surface charge density (σ) remains inadequately understood. Here, we probe the solvation structure of ILs on alkali halide surfaces with varied surface orientations: nearly zero-charged RbI(100) and highly charged RbI(111), by employing frequency modulation atomic force microscopy with atomic resolution. Two commonly used ILs are examined in this study: 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C3mpyr][NTf2]) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]). On RbI(100) surfaces with near zero σ, we observe alternating cation and anion layers, diverging from the previously proposed monolayer model for IL/alkali halide(100) interfaces. These results support the argument that overscreening occurs under low σ, even approaching zero, and reconcile conflicting experimental conclusions about low σ systems. On RbI(111) surfaces with high σ, we identify solvation structures consisting of two consecutive counterion layers. This structure aligns with the theoretically predicted crowding; a phenomenon rarely observed in commonly used ILs due to typically unreachable σ in electrochemical IL/electrode systems. Our findings indicate that alkali halide(111) surfaces are potentially valuable for exploring the crowding phenomenon in ILs, addressing the current scarcity of experimental observations.

Ionic Liquid-Based Extraction Strategy for the Efficient and Selective Recovery of Scandium

The recovery of scandium (Sc) from highly acidic industrial effluents is currently hindered by the use of large quantities of flammable and toxic organic solvents. This study developed an extraction system using ionic liquids (ILs) and phenylphosphinic acid (PPAH) as diluents and an extractant, respectively, to selectively recover Sc from the aqueous phase. The effect of IL chemical structure, aqueous pH and temperature on the extraction of Sc was systematically investigated and the findings revealed that ILs with longer alkyl side chains had reduced Sc extraction ability due to the presence of continuous nonpolar domains formed by the self-aggregation of the IL alkyl side chain. The IL/PPAH system maintained high extraction ability toward Sc across a wide temperature range (288 K to 318 K) and the extraction efficiency of Sc could be improved significantly by increasing the aqueous pH. The extraction process involved proton exchange, resulting in the formation of a metal-ligand complex (Sc(PPA)3).

Anion Architecture Controls Structure and Electroresponsivity of Anhalogenous Ionic Liquids in a Sustainable Fluid

Three nonhalogenated ionic liquids (ILs) dissolved in 2-ethylhexyl laurate (2-EHL), a biodegradable oil, are investigated in terms of their bulk and electro-interfacial nanoscale structures using small-angle neutron scattering (SANS) and neutron reflectivity (NR). The ILs share the same trihexyl(tetradecyl)phosphonium ([P6,6,6,14]+) cation paired with different anions, bis(mandelato)borate ([BMB]-), bis(oxalato)borate ([BOB]-), and bis(salicylato)borate ([BScB]-). SANS shows a high aspect ratio tubular self-assembly structure characterized by an IL core of alternating cations and anions with a 2-EHL-rich shell or corona in the bulk, the geometry of which depends upon the anion structure and concentration. NR also reveals a solvent-rich interfacial corona layer. Their electro-responsive behavior, pertaining to the structuring and composition of the interfacial layers, is also influenced by the anion identity. [P6,6,6,14][BOB] exhibits distinct electroresponsiveness to applied potentials, suggesting an ion exchange behavior from cation-dominated to anion-rich. Conversely, [P6,6,6,14][BMB] and [P6,6,6,14][BScB] demonstrate minimal electroresponses across all studied potentials, related to their different dissociative and diffusive behavior. A mixed system is dominated by the least soluble IL but exhibits an increase in disorder. This work reveals the subtlety of anion architecture in tuning bulk and electro-interfacial properties, offering valuable molecular insights for deploying nonhalogenated ILs as additives in biodegradable lubricants and supercapacitors.

Ordering of ionic liquids at a charged sapphire interface: Evolution with cationic chain length

HYPOTHESIS

Room Temperature Ionic Liquids (RTILs) bulk's molecular layering dominates their structure also at the RTIL/sapphire interface, increasing the layer spacing with the cationic alkyl chain length n. However, the negatively-charged sapphire surface compresses the layers, increases the layering range, and affects the intra-layer structure in yet unknown ways.

EXPERIMENTS

X-ray reflectivity (XR) off the RTIL/sapphire interface, for a broad homologous RTIL series 1-alkyl-3-methylimidazolium bis(trifluoromethansulfonyl)imide, hitherto unavailable for any RTIL.

FINDINGS

RTIL layers against the sapphire, exhibit two spacings: da and db. da is n-varying, follows the behavior of the bulk spacing but exhibits a downshift, thus showing significant layer compression, and over twofold polar slab thinning. The latter suggests exclusion of anions from the interfacial region due to the negative sapphire charging by x-ray-released electrons. The layering range is larger than the bulk's. db is short and near n-independent, suggesting polar moieties' layering, the coexistence mode of which with the da-spaced layering is unclear. Comparing the present layering with the bulk's and the RTIL/air interface's provides insight into the Coulomb and dispersion interaction balance dominating the RTIL's structure and the impact thereon of the presence of a charged solid interface.

Unprotected Organic Cations─The Dilemma of Highly Li-Concentrated Ionic Liquid Electrolytes

Highly Li-concentrated electrolytes have been widely studied to harness their uniquely varying bulk and interface properties that arise from their distinctive physicochemical properties and coordination structures. Similar strategies have been applied in the realm of ionic liquid electrolytes to exploit their improved functionalities. Despite these prospects, the impact of organic cation behavior on interfacial processes remains largely underexplored compared to the widely studied anion behavior. The present study demonstrates that the weakened interactions between cations and anions engender "unprotected" organic cations in highly Li-concentrated ionic liquid electrolytes, leading to the decomposition of electrolytes during the initial charge. This decomposition behavior is manifested by the substantial irreversible capacities and inferior initial Coulombic efficiencies observed during the initial charging of graphite negative electrodes, resulting in considerable electrolyte consumption and diminished energy densities in full-cell configurations. The innate cation behavior is ascertained by examining the coordination environment of ionic liquid electrolytes with varied Li concentrations, where intricate ionic interactions between organic cations and anions are unveiled. In addition, anionic species with high Lewis basicity were introduced to reinforce the ionic interactions involving organic cations and improve the initial Coulombic efficiency. This study verifies the role of unprotected organic cations while highlighting the significance of the coordination environment in the performance of ionic liquid electrolytes.

Tuneable interphase transitions in ionic liquid/carrier systems via voltage control

The structure and interaction of ionic liquids (ILs) influence their interfacial composition, and their arrangement (i.e., electric double-layer (EDL) structure), can be controlled by an electric field. Here, we employed a quartz crystal microbalance (QCM) to study the electrical response of two non-halogenated phosphonium orthoborate ILs, dissolved in a polar solvent at the interface. The response is influenced by the applied voltage, the structure of the ions, and the solvent polarizability. One IL showed anomalous electro-responsivity, suggesting a self-assembly bilayer structure of the IL cation at the gold interface, which transitions to a typical EDL structure at higher positive potential. Neutron reflectivity (NR) confirmed this interfacial structuring and compositional changes at the electrified gold surface. A cation-dominated self-assembly structure is observed for negative and neutral voltages, which abruptly transitions to an anion-rich interfacial layer at positive voltages. An interphase transition explains the electro-responsive behaviour of self-assembling IL/carrier systems, pertinent for ILs in advanced tribological and electrochemical contexts.

Effect of Surface Chemistry on the Electrical Double Layer in a Long-Chain Ionic Liquid

Room temperature ionic liquids (ILs) can create a strong accumulation of charges at solid interfaces by forming a very thin and dense electrical double layer (EDL). The structure of this EDL has important consequences in numerous applications involving ILs, for example, in supercapacitors, sensors, and lubricants, by impacting the interfacial capacitance, the charge carrier density of semiconductors, as well as the frictional properties of the interfaces. We have studied the interfacial structure of a long chain imidazolium-based IL (1-octyl-3-methylimidazolium dicyanamide) on several substrates: mica, silica, silicon, and molybdenum disulfide (MoS2), using atomic force microscopy (AFM) experiments and molecular dynamics (MD) simulations. We have observed 3 types of interfacial structures for the same IL, depending on the chemistry of the substrate and the water content, showing that the EDL structure is not an intrinsic property of the IL. We evidenced that at a low water content, neutral and apolar (thus hydrophobic) substrates promote a thin layer structure, where the ions are oriented parallel to the substrate and cations and anions are mixed in each layer. In contrast, a strongly charged (thus hydrophilic) substrate yields an extended structuration into several bilayers, while a heterogeneous layering with loose bilayer regions was observed on an intermediate polar and weakly charged substrate and on an apolar one at a high bulk water content. In the latter case, water contamination favors the formation of bilayer patches by promoting the segregation of the long chain IL into polar and apolar domains.

In Situ Measurement of Effective Gate Bias Voltage in Ionic Liquid-Gated Organic Field-Effect Transistors: Exploring Intrinsic Performance and Trap Density of States

Electric double layer (EDL)-mediated transistors with ionic liquid (IL) gating have garnered substantial interest due to their exceptional properties, such as high transconductance and low-voltage operation, positioning them as promising candidates for organic electronics. In this study, we present an in situ measurement of effective gate bias voltage (VGS,eff) in IL-gated organic field-effect transistors (IL-OFETs) using a modified current-voltage measurement configuration. The results reveal a significant deviation between VGS,eff and the applied gate bias (VGS,app), indicating that the EDL at the gate/IL interface screens the applied voltage. It is observed that the screening effect varies depending on the specific cation and anion present in the IL. The evaluation of VGS,eff plays a pivotal role in understanding the intrinsic behavior of IL-OFETs and addresses the challenges associated with accurate performance assessment. Inherently, IL-OFETs demonstrate high transconductance, achieving values of approximately 9 mS while operating at a low threshold voltage of around 0.55 V. Through the acquisition of VGS,eff, we have successfully addressed the limitations impeding the numerical estimation of the trap density of states (trap DOS) in IL-OFETs. Remarkably, our calculations reveal an exceptionally low density of deep traps, which serves as a crucial factor contributing to the near-ideal subthreshold swing (61-68 mV dec-1) observed in IL-OFETs. Further investigations unveil the neutral electrical nature of the IL bulk during OFET operation, confirming the hypothesis that the applied gate bias voltage in electrolyte-gated OFETs drops across the EDLs formed at the interfaces. The impedance spectroscopic (IS) analysis confirms the low contact resistance (≈1 Ω·m) of IL-OFETs calculated using the transition voltage method. The IS analysis also reveals the low-transmissive nature of the IL/organic semiconductor interface. The knowledge gained from this study holds significant implications for realizing high-performance electrolyte-gated OFETs in various applications including digital electronics, energy storage, and sensing. By unraveling the factors influencing the device performance, such as VGS,eff and trap DOS, this research contributes to the advancement of organic electronics and paves the way for future developments in the field.

Molecular Architecture Effects on Bulk Nanostructure in Bis(Orthoborate) Ionic Liquids

A series of 19 ionic liquids (ILs) based on phosphonium and imidazolium cations of varying alkyl-chain lengths with the orthoborate anions bis(oxalato)borate [BOB]- , bis(mandelato)borate, [BMB]- and bis(salicylato)borate, [BScB]- , are synthesized and studied using small-angle neutron scattering (SANS). All measured systems display nanostructuring, with 1-methyl-3-n-alkyl imidazolium-orthoborates forming clearly bicontinuous L3 spongelike phases when the alkyl chains are longer than C6 (hexyl). L3 phases are fitted using the Teubner and Strey model, and diffusely-nanostructured systems are primarily fitted using the Ornstein-Zernicke correlation length model. Strongly-nanostructured systems have a strong dependence on the cation, with molecular architecture variation explored to determine the driving forces for self-assembly. The ability to form well-defined complex phases is effectively extinguished in several ways: methylation of the most acidic imidazolium ring proton, replacing the imidazolium 3-methyl group with a longer hydrocarbon chain, substitution of [BOB]- by [BMB]- , or exchanging the imidazolium for phosphonium systems, irrespective of phosphonium architecture. The results suggest there is only a small window of opportunity, in terms of molecular amphiphilicity and cation:anion volume matching, for the formation of stable extensive bicontinuous domains in pure bulk orthoborate-based ILs. Particularly important for self-assembly processes appear to be the ability to form H-bonding networks, which offer additional versatility in imidazolium systems.

Electrotunable friction with ionic liquid lubricants

Room-temperature ionic liquids and their mixtures with organic solvents as lubricants open a route to control lubricity at the nanoscale via electrical polarization of the sliding surfaces. Electronanotribology is an emerging field that has a potential to realize in situ control of friction-that is, turning the friction on and off on demand. However, fulfilling its promise needs more research. Here we provide an overview of this emerging research area, from its birth to the current state, reviewing the main achievements in non-equilibrium molecular dynamics simulations and experiments using atomic force microscopes and surface force apparatus. We also present a discussion of the challenges that need to be solved for future applications of electrotunable friction.

Nanostructure, electrochemistry and potential-dependent lubricity of the catanionic surface-active ionic liquid [P6,6,6,14] [AOT]

HYPOTHESIS

A catanionic surface-active ionic liquid (SAIL) trihexyltetradecylphosphonium 1,4-bis(2-ethylhexoxy)-1,4-dioxobutane-2-sulfonate ([P_6,6,6,14] [AOT]) is nanostructured in the bulk and at the interface. The interfacial nanostructure and lubricity may be changed by applying a potential.

EXPERIMENTS

The bulk structure and viscosity have been investigated using small angle X-ray scattering (SAXS) and rheometry. The interfacial structure and lubricity as a function of potential have been investigated using atomic force microscopy (AFM). The electrochemistry has been investigated using cyclic voltammetry.

FINDINGS

[P_6,6,6,14] [AOT] shows sponge-like bulk nanostructure with distinct interdigitation of cation-anion alkyl chains. Shear-thinning occurs at 293 K and below, but becomes less obvious on heating up to 313 K. Voltammetric analysis reveals that the electrochemical window of [P_6,6,6,14] [AOT] on a gold micro disk electrode exceeds the potential range of the AFM experiments and that negligible redox activity occurs in this range. The interfacial layered structure of [P_6,6,6,14] [AOT] is weaker than conventional ILs and SAILs, whereas lubricity is better, confirming the inverse correlation between the near-surface structure and lubricity. The adhesive forces of [P_6,6,6,14] [AOT] are lower at -1.0 V than at open circuit potential and +1.0 V, likely due to reduced electrostatic interactions caused by shielding of charge centres via long alkyl chains.

Carbon dioxide adsorption and activation on ionic liquid decorated Au(111) surface: A DFT study

We use first principle approaches to study the adsorption and catalytic activation mechanism of CO₂ on ionic liquids (ILs, [C_nMIm]⁺[Cl]^- (n = 0-6)) attached to a Au(111) surface. The adsorption of CO₂ at this liquid-solid model interface occurs via either (i) parallel π-stacking mode or (ii) CO₂ oxygen lone pair (lp)···π interaction. These CO₂ physisorption modes, which depend on the CO₂ landing angle at this interface, are identified as an efficient way to activate CO₂ and its further conversion into value-added products. For illustration, we discuss the conversion of CO₂ into formic acid where the ILs@Au(111) decorated interface allows reduction of the activation energy for the CO₂ + H₂ → HCOOH reaction. In sum, our electrode/electrolyte based interface model provides valuable information to design novel heterogeneous catalysts for CO₂ conversion. Indeed, our work establishes that a suitable interface material is enough to activate CO₂.

The effect of anion architecture on the lubrication chemistry of phosphonium orthoborate ionic liquids

Phosphonium ionic liquids with orthoborate anions have been studied in terms of their interfacial film formation, both physisorbed and sacrificial from chemical breakdown, in sheared contacts of varying harshness. The halogen-free anion architecture was varied through (i) the heteronuclear ring size, (ii) the hybridisation of the constituent atoms, and (iii) the addition of aryl functionalities. Time of Flight-Secondary Ion Mass Spectrometry analysis revealed the extent of sacrificial tribofilm formation allowing the relative stability of the ionic liquids under tribological conditions to be determined and their breakdown mechanisms to be compared to simple thermal decomposition. Overall, ionic liquids outperformed reference oils as lubricants; in some cases, sacrificial films were formed (with anion breakdown a necessary precursor to phosphonium cation decomposition) while in other cases, a protective, self-assembly lubricant layer or hybrid film was formed. The salicylate-based anion was the most chemically stable and decomposed only slightly even under the harshest conditions. It was further found that surface topography influenced the degree of breakdown through enhanced material transport and replenishment. This work thus unveils the relationship between ionic liquid composition and structure, and the ensuing inter- and intra-molecular interactions and chemical stability, and demonstrates the intrinsic tuneability of an ionic liquid lubrication technology.

Observation of the Pockels Effect in Ionic Liquids and Insights into the Length Scale of Potential-Induced Ordering

Ionic liquids (ILs) under electric fields play essential roles in the electrochemical utilization of ILs. Recently, long-range organization of ILs in the vicinity of charged (and even neutral) surfaces has been revealed, but experimental evidence for such an ordering is still limited and its spatial length scale remains controversial. Here, we use confocal Raman microspectroscopy to investigate the effect of an applied electric potential on the IL 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and its analogues in a space-resolved manner. Much to our surprise, the observed Raman difference spectra of the ILs obtained with and without an applied potential exhibit uniform intensity changes independent of vibrational modes of cations and anions, a finding in sharp contrast with the electric field effects on molecular liquids that we have previously observed. We interpret this unexpected finding in terms of the Pockels effect that occurs as a result of a potential-induced ordering of the IL near an IL-electrode interface. The refractive index changes due to the applied potential are estimated using the experimental Raman intensity changes. The results allow us to deduce that the length scale of the ordering in the ILs is tens to hundreds of nanometers, extending more than would be expected for the electrical double layer but not as far as a micrometer scale.

Rapid relaxation NMR measurements to predict rate coefficients in ionic liquid mixtures. An examination of reaction outcome changes in a homologous series of ionic liquids

A series of ionic liquids based on the 1-alkyl-3-methylimidazolium cations were examined as components of the solvent mixture for a bimolecular substitution process. The effects on both the rate coefficient of the process and the NMR spin-spin relaxation of the solvent components of changing either the alkyl chain length or the amount of ionic liquid in the reaction mixture were determined. At a constant mole fraction, a shorter alkyl chain length resulted in a greater rate coefficient enhancement and a longer relaxation time, with the opposite effects for a longer alkyl chain length. For a given ionic liquid, increasing the proportion of salt in the reaction mixture resulted in a greater rate coefficient and a shorter relaxation time. The microscopic origins of the rate coefficient enhancement were determined and a step change found in the activation parameters on increasing the alkyl chain length from hexyl to octyl, suggesting notable structuring in solution. Across a range of ionic liquids and solvent compositions, the relaxation time from NMR measurements was shown to relate to the reaction rate coefficient. The approach of using fast and simple NMR relaxation measurements to predict reaction outcomes was exemplified using a morpholinium-based ionic liquid.

Interfacial Polarization and Ionic Structure at the Ionic Liquid-Metal Interface Studied by Vibrational Spectroscopy and Molecular Dynamics Simulations

Ionic liquids (ILs) have both fundamental and practical value in interfacial science and electrochemistry. However, understanding their behavior near a surface is challenging because of strong Coulomb interactions and large and irregular ionic sizes, which affect both their structure and energetics. To understand this problem, we present a combined experimental and computational study using a vibrational probe molecule, 4-mercaptobenzonitrile, inserted at the junction between a metal and a variety of ILs. The vibrational frequency of the nitrile in the probe molecule reports on the local solvation environment and the electrostatic field at this junction. Within the ethylmethyl imidazolium (EMIM⁺) cation family of ILs, we varied the anions over a range of sizes and types. Complementing our surface spectroscopy, we also ran molecular dynamics simulations of these interfaces to better understand the ionic structures that produced the measured fields. The magnitude of the frequency shifts, and thereby fields, shows a general correlation with the size of anions, with larger anions corresponding to smaller fields. We find that the source of this correlation is partial intercalation of smaller anions into the probe monolayer, resulting in tighter packing of ionic layers near the surface. Larger anions reduce the overall lateral ion packing density near the surface, which reduces the net charge per unit area and explains the smaller observed fields. The insight from this work is important for developing a fundamental picture of concentrated electrolytes near interfaces and can help with designing ILs to create tailored electric fields near an electrode.

Effect of water on the electroresponsive structuring and friction in dilute and concentrated ionic liquid lubricant mixtures

The effect of water on the electroactive structuring of a tribologically relevant ionic liquid (IL) when dispersed in a polar solvent has been investigated at a gold electrode interface using neutron reflectivity (NR). For all solutions studied, the addition of small amounts of water led to clear changes in electroactive structuring of the IL at the electrode interface, which was largely determined by the bulk IL concentration. At a dilute IL concentration, the presence of water gave rise to a swollen interfacial structuring, which exhibited a greater degree of electroresponsivity with applied potential compared to an equivalent dry solution. Conversely, for a concentrated IL solution, the presence of water led to an overall thinning of the interfacial region and a crowding-like structuring, within which the composition of the inner layer IL layers varied systematically with applied potential. Complementary nanotribotronic atomic force microscopy (AFM) measurements performed for the same IL concentration, in dry and ambient conditions, show that the presence of water reduces the lubricity of the IL boundary layers. However, consistent with the observed changes in the IL layers observed by NR, reversible and systematic control of the friction coefficient with applied potential was still achievable. Combined, these measurements provide valuable insight into the implications of water on the interfacial properties of ILs at electrified interfaces, which inevitably will determine their applicability in tribotronic and electrochemical contexts.

Electroresponsive structuring and friction of a non-halogenated ionic liquid in a polar solvent: effect of concentration

Neutron reflectivity (NR) measurements have been employed to study the interfacial structuring and composition of electroresponsive boundary layers formed by an ionic liquid (IL) lubricant at an electrified gold interface when dispersed in a polar solvent. The results reveal that both the composition and extent of the IL boundary layers intricately depend on the bulk IL concentration and the applied surface potential. At the lowest concentration (5% w/w), a preferential adsorption of the IL cation at the gold electrode is observed, which hinders the ability to electro-induce changes in the boundary layers. In contrast, at higher IL bulk concentrations (10 and 20% w/w), the NR results reveal a significantly larger concentration of the IL ions at the gold interface that exhibit significantly greater electroresponsivity, with clear changes in the layer composition and layer thickness observed for different potentials. In complementary atomic force microscopy (AFM) measurements on an electrified gold surface, such IL boundary layers are demonstrated to provide excellent friction reduction and electroactive friction (known as tribotronics). In agreement with the NR results obtained, clear concentration effects are also observed. Together such results provide valuable molecular insight into the electroactive structuring of ILs in solvent mixtures, as well as provide mechanistic understanding of their tribotronic behaviours.