Microtribological properties of a spin-coated thin film of 1-butyl-3-(propyltrimethoxysilane)imidazolium bis(mandelato)borate ionic liquid

A thin film of 1-butyl-3-(propyltrimethoxysilane)imidazolium bis(mandelato)borate ionic liquid prepared by spin coating on silicon surface exhibited significantly low friction and excellent wear-resistivity.

Addition of low concentrations of an ionic liquid to a base oil reduces friction over multiple length scales: a combined nano- and macrotribology investigation

The efficacy of ionic liquids (ILs) as lubricant additives to a model base oil has been probed at the nanoscale and macroscale as a function of IL concentration using the same materials.

Effect of the environmental humidity on the bulk, interfacial and nanoconfined properties of an ionic liquid

Structural and dynamical properties of ILs are altered by the weakening of ion–ion correlations in the presence of water.

Weighing the surface charge of an ionic liquid

Electrochemical quartz crystal microbalance has been used to measure changes in the composition of the capacitive electrical double layer for 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)-trifluorophosphate, an ionic liquid, in contact with a gold electrode surface as a function of potential. The mass difference between the cation and anion means that the technique can effectively "weigh" the surface charge accurately with high temporal resolution. This reveals quantitatively how changing the potential alters the ratio of cations and anions associated with the electrode surface, and thus the charge per unit area, as well as the kinetics associated with these interfacial processes. The measurements reveal that it is diffusion of co-ions into the interfacial region rather than expulsion of counterions that controls the relaxation. The measured potential dependent double layer capacitance experimentally validates recent theoretical predictions for counterion overscreening (low potentials) and crowding (high potentials) at electrode surfaces. This new capacity to quantitatively measure ion composition is critical for ionic liquid applications ranging from batteries, capacitors and electrodeposition through to boundary layer structure in tribology, and more broadly provides new insight into interfacial processes in concentrated electrolyte solutions.

Irreversible structural change of a dry ionic liquid under nanoconfinement

Studies of 1-hexyl-3-methyl-imidazolium ethylsulfate ([HMIM] EtSO4) using an extended surface forces apparatus show, for the first time, an ordered structure within the nanoconfined ionic liquid (IL) between mica surfaces that extends up to ∼60 nm from the surface. Our measurements show the growth of this ordered IL-film upon successive nanoconfinements-the structural changes being irreversible upon removal of the confinement-and the response of the structure to shear. The compressibility of this system is lower than that typically measured for ILs, while creep takes place during shear, both findings supporting a long-range liquid-to-solid transition. AFM (sharp-tip) studies of [HMIM] EtSO4 on mica only reveal ∼2 surface IL-layers, with order extending only ∼3 nm from the surface, indicating that confinement is required for the long-range IL-solidification to occur. WAXS studies of the bulk IL show a more pronounced ordered structure than is the case for [HMIM] with bis(trifluoromethylsulfonyl)imide as anion, but no long-range order is detected, consistent with the results obtained with the sharp AFM tip. These are the first force measurements of nanoconfinement-induced long-range solidification of an IL.

Influence of electric potentials on friction of sliding contacts lubricated by an ionic liquid

Tribological investigations on the macroscopic scale revealed that friction can be influenced in situ by applying electric potentials, if electrically conductive fluid such as an ionic liquid is used as a lubricant. Enrichment of charged ions at a steel interface occurs by applying electric surface potentials in a three-electrode setup. As a consequence, the lubrication conditions change. It is supposed that electrically influenced surface adsorption and electrokinetic effects are the main mechanisms by which friction is varied.

Nanostructure of the Ionic Liquid-Graphite Stern Layer

Ionic liquids (ILs) are attractive solvents for devices such as lithium ion batteries and capacitors, but their uptake is limited, partially because their Stern layer nanostructure is poorly understood compared to molecular solvents. Here, in situ amplitude-modulated atomic force microscopy has been used to reveal the Stern layer nanostructure of the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIm TFSI)-HOPG (highly ordered pyrolytic graphite) interface with molecular resolution. The effect of applied surface potential and added 0.1 wt/wt % Li TFSI or EMIm Cl on ion arrangements is probed between ±1 V. For pure EMIm TFSI at open-circuit potential, well-defined rows are present on the surface formed by an anion-cation-cation-anion (A-C-C-A) unit cell adsorbed with like ions adjacent. As the surface potential is changed, the relative concentrations of cations and anions in the Stern layer respond, and markedly different lateral ion arrangements ensue. The changes in Stern layer structure at positive and negative potentials are not symmetrical due to the different surface affinities and packing constraints of cations and anions. For potentials outside ±0.4 V, images are featureless because the compositional variation within the layer is too small for the AFM tip to detect. This suggests that the Stern layer is highly enriched in either cations or anions (depending on the potential) oriented upright to the surface plane. When Li(+) or Cl(-) is present, some Stern layer ionic liquid cations or anions (respectively) are displaced, producing starkly different structures. The Stern layer structures elucidated here significantly enhance our understanding of the ionic liquid electrical double layer.

Amplitude-modulated atomic force microscopy reveals the near surface nanostructure of surfactant sponge (L(3)) and lamellar (L(α)) phases

Amplitude-modulated atomic force microscopy (AM-AFM) has been used to study the nanostructure of cetylpyridinium chloride (CPCl)-hexanol-0.2 M NaCl sponge (L3) and lamellar (Lα) phases near a mica surface. For both phases, membrane volume fractions of 22, 27, and 32 vol % were investigated, with the L3 or Lα phase selected by adjusting the co-surfactant/surfactant ratio (hexanol/CPCl). For the L3 phase, the presence of the surface flattens the three-dimensional bulk structure. AM-AFM clearly resolves the membrane and solvent passages in the near surface layer. Increasing the membrane volume fraction decreases the size of the image features because of the lower solvent content. Within error, the average passage sizes in the near surface layer are the same as those in the bulk at the same concentration. Images of the Lα phase reveal undulating near surface sheets. At the highest membrane concentration, the image is very smooth, because the lamellar sheet is confined between the surface and the next near surface layer, which is in close proximity as a result of the low solvent content. As the membrane concentration is reduced, the space between layers is increased and undulations appear in the near surface lamellar structure. Undulations are more pronounced at the lowest membrane volume fraction.

Restructuring of the electrical double layer in ionic liquids upon charging

We have investigated the electrical double layer (EDL) structure at an interface between ionic liquid (IL) and charged surface using molecular dynamics simulations. We show that for three different models of ILs the EDL restructuring, driven by surface charging, can be rationalized by the use of two parameters--renormalized surface charge (κ) and charge excess in the interfacial layers (λ). Analysis of the relationship between the λ and κ parameters provides new insights into mechanisms of over-screening and charge-driven structural transitions in the EDL in ionic liquids. We show that the restructuring of the EDL upon charging in all three studied systems has two characteristic regimes: (1) transition from the bulk-like (κ(Ion) = 0) to the multilayer structure (κ(Ion) ≈ 0.5) through the formation of an ionic bilayer of counter- and co-ions; and (2) transition from the multilayer (κ(Ion) ≈ 0.5) to the crowded (κ(Ion) > 1) structure through the formation of a monolayer of counter-ions at κ(Ion) = 1.

Electrotunable lubricity with ionic liquid nanoscale films

One of the main challenges in tribology is finding the way for an in situ control of friction without changing the lubricant. One of the ways for such control is via the application of electric fields. In this respect a promising new class of lubricants is ionic liquids, which are solvent-free electrolytes, and their properties should be most strongly affected by applied voltage. Based on a minimal physical model, our study elucidates the connection between the voltage effect on the structure of the ionic liquid layers and their lubricating properties. It reveals two mechanisms of variation of the friction force with the surface charge density, consistent with recent AFM measurements, namely via the (i) charge effect on normal and in-plane ordering in the film and (ii) swapping between anion and cation layers at the surfaces. We formulate conditions that would warrant low friction coefficients and prevent wear by resisting "squeezing-out" of the liquid under compression. These results give a background for controllable variation of friction.

Near surface properties of mixtures of propylammonium nitrate with n-alkanols 2. Nanotribology and fluid dynamics

Colloid probe friction force microscopy (FFM) has been used to study the lubricity of propylammonium nitrate (PAN) mixed with n-alkanols confined between sliding silica and mica surfaces.

Ion structure controls ionic liquid near-surface and interfacial nanostructure

A unique, but unifying, feature of ionic liquids (ILs) is that they are nanostructured on the length scale of the ions; in many ILs well-defined polar and apolar domains exist and may percolate through the liquid. Near a surface the isotropic symmetry of the bulk structure is broken, resulting in different nanostructures which, until now, have only been studied indirectly. In this paper, in situ amplitude modulated atomic force microscopy (AM-AFM) has been used to resolve the 3-dimensional nanostructure of five protic ILs at and near the surface of mica. The surface and near surface structures are distinct and remarkably well-defined, but are very different from previously accepted descriptions. Interfacial nanostructure is strongly influenced by the registry between cations and the mica surface charge sites, whereas near surface nanostructure is sensitive to both cation and anion structure. Together these ILs reveal how interfacial nanostructure can be tuned through ion structure, informing "bottom-up" design and optimisation of ILs for diverse technologies including heterogeneous catalysis, lubrication, electrochemical processes, and nanofluids.

Combined friction force microscopy and quantum chemical investigation of the tribotronic response at the propylammonium nitrate–graphite interface

The energetic origins of the variation in friction with potential at the propylammonium nitrate–graphite interface are revealed using friction force microscopy (FFM) in combination with quantum chemical simulations.

Conformation of poly(ethylene oxide) dissolved in the solvate ionic liquid [Li(G4)]TFSI

PEO dissolves in [Li(G4)]TFSI via coordination with Li⁺.

Ionic Liquid Adsorption and Nanotribology at the Silica-Oil Interface: Hundred-Fold Dilution in Oil Lubricates as Effectively as the Pure Ionic Liquid

The remarkable physical properties of ionic liquids (ILs) make them potentially excellent lubricants. One of the challenges for using ILs as lubricants is their high cost. In this article, atomic force microscopy (AFM) nanotribology measurements reveal that a 1 mol % solution of IL dissolved in an oil lubricates the silica surface as effectively as the pure IL. The adsorption isotherm shows that the IL surface excess need only be approximately half of the saturation value to prevent surface contact and effectively lubricate the sliding surfaces. Using ILs in this way makes them viable for large-scale applications.

Water in ionic liquids at electrified interfaces: the anatomy of electrosorption

Complete removal of water from room-temperature ionic liquids is nearly impossible. For the electrochemical applications of ionic liquids, how water is distributed in the electrical double layers when the bulk liquids are not perfectly dry can potentially determine whether key advantages of ionic liquids, such as a wide electrochemical window, can be harnessed in practical systems. In this paper, we study the adsorption of water on electrode surfaces in contact with humid, imidazolium-based ionic liquids using molecular dynamics simulations. The results revealed that water molecules tend to accumulate within sub-nanometer distance from charged electrodes. At low amount of water in the bulk, the distributions of ions and of electrostatic potential in the double layer are affected weakly by the presence of water, but the spatial distribution of water molecules is strongly dependent on both. The preferential positions of water molecules in double layers are determined by the balance of several factors: the tendency to follow the positions of the maximal absolute value of the electrical field, the association with their ionic surroundings, and the propensity to settle at positions where more free space is available. The balance between these factors changes with charging the electrode, but the adsorption of water generally increases with voltage. The ion specificity of water electrosorption is manifested in the stronger presence of water near positive electrodes (where anions are the counterions) than near negative electrodes (where cations are counterions). These predictions await experimental verification.

Molecular Friction Mechanisms Across Nanofilms of a Bilayer-Forming Ionic Liquid

The prevailing paradigm in boundary lubrication asserts, in essence, that surfaces coated in amphiphiles slide past each other by way of the "slippery" exposed alkyl chains while the polar head group remains anchored at the surface. Here we show, for ionic liquid boundary lubricants, that the molecular mechanism of shearing is more subtle; while a monolayer on each surface gives rise to alkyl plane shearing, a bilayer on each surface shears at the ionic (nonalkyl) interface. The incorporation of water from the environment dramatically alters the shear at ionic interfaces but leaves alkyl plane shearing unaffected. Our experiments involve shearing two identical and atomically smooth surfaces past one another with films of an ionic liquid between, with subnanometer control of the film thickness and ultrasensitive shear stress resolution. With this, we uncover molecular mechanistic details relevant to boundary lubrication in general and the development of ionic liquid lubricants in particular.

Influence of alkyl chain length and anion species on ionic liquid structure at the graphite interface as a function of applied potential

Atomic force microscopy (AFM) force measurements elucidate the effect of cation alkyl chain length and the anion species on ionic liquid (IL) interfacial structure at highly ordered pyrolytic graphite (HOPG) surfaces as a function of potential. Three ILs are examined: 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([HMIM] FAP), 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([EMIM] FAP), and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM] TFSA). The step-wise force-distance profiles indicate the ILs adopt a multilayered morphology near the surface. When the surface is biased positively or negatively versus Pt quasireference electrode, both the number of steps, and the force required to rupture each step increase, indicating stronger interfacial structure. At all potentials, push-through forces for [HMIM] FAP are the highest, because the long alkyl chain results in strong cohesive interactions between cations, leading to well-formed layers that resist the AFM tip. The most layers are observed for [EMIM] FAP, because the C2 chains are relatively rigid and the dimensions of the cation and anion are similar, facilitating neat packing. [EMIM] TFSA has the smallest push-through forces and fewest layers, and thus the weakest interfacial structure. Surface-tip attractive forces are measured for all ILs. At the same potential, the attractions are the strongest for [EMIM] TFSA and the weakest for [HMIM] FAP because the interfacial layers are better formed for the longer alkyl chain cation. This means interfacial forces are stronger, which masks the weak attractive forces.

Effect of dissolved LiCl on the ionic liquid-Au(111) interface: an in situ STM study

The structure of the electrolyte/electrode interface plays a significant role in electrochemical processes. To date, most studies are focusing on understanding the interfacial structure in pure ionic liquids. In this paper in situ scanning tunnelling microscopy (STM) has been employed to elucidate the structure of the charged Au(111)-ionic liquid (1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate, [Py1,4]FAP) interface in the presence of 0.1 M LiCl. The addition of the Li salt to the ionic liquid has a strong influence on the interfacial structure. In the first STM scan in situ measurements reveal that Au(111) undergoes the (22 x √3) 'herringbone' reconstruction in a certain potential regime, and there is strong evidence that the gold surface dissolves at negative electrode potentials in [Py1,4]FAP containing LiCl. Bulk deposition of Li is obtained at -2.9 V in the second STM scan.

Force microscopy of layering and friction in an ionic liquid

The mechanical properties of the ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate ([Py1,4][FAP]) in confinement between a SiOx and a Au(1 1 1) surface are investigated by means of atomic force microscopy (AFM) under electrochemical control. Up to 12 layers of ion pairs can be detected through force measurements while approaching the tip of the AFM to the surface. The particular shape of the force versus distance curve is explained by a model for the interaction between tip, gold surface and ionic liquid, which assumes an exponentially decaying oscillatory force originating from bulk liquid density correlations. Jumps in the tip-sample distance upon approach correspond to jumps of the compliant force sensor between branches of the oscillatory force curve. Frictional force between the laterally moving tip and the surface is detected only after partial penetration of the last double layer between tip and surface.

Effect of molecular orientation angle of imidazolium ring on frictional properties of imidazolium-based ionic liquid

Ionic liquids have significant potential as lubricants, and it is known that ionic liquids exhibit characteristic behavior at solid-liquid interfaces. Although it is believed that the structure of ionic liquids at the interface contributes to the tribological properties in the region of boundary-mixed lubrication, this contribution has not been clarified because such analysis is difficult. In this research, we clarify the lubrication mechanism of an imidazolium-based ionic liquid by comparing the results of friction tests with interfacial molecular orientation analysis using sum frequency generation spectroscopy. Consequently, we clarify that the tilt angle of the imidazolium ring affects the friction coefficient of the ionic liquid; that is, the larger tilt angle, the lower the friction coefficient.

Microslips to "Avalanches" in Confined, Molecular Layers of Ionic Liquids

We have measured forces between mica surfaces across two hydrophobic ionic liquids with a surface forces apparatus. Both surface-adsorbed water and alkyl-chain length on the imidazolium cation influence the structure of the nanoconfined film and the dynamics of film-thickness transitions. Friction shows accumulative microslips as precursors to collective "avalanches" that abruptly reduce friction momentarily. This behavior is interpreted as a consequence of interlayer ion correlations within the 1 to 2 nm thick film; we identify this to be analogous to the friction response of crackling noise systems over a broad range of sizes.