Room-temperature ionic liquids: a novel versatile lubricant

Resonance shear measurement of nanoconfined ionic liquids

Two types of imidazolium-based ionic liquid (IL), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C(4)mim][NTF(2)]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([C(4)mim][BF(4)]), confined between silica surfaces were investigated by surface force apparatus (SFA)-based resonance shear measurements together with surface force measurements. The surface force profiles in the ILs showed oscillatory solvation forces below the characteristic surface separations: 10.0 nm for [C(4)mim][NTf(2)] and 6.9 nm for [C(4)mim][BF(4)]. The more pronounced solvation force found in [C(4)mim][NTf(2)] suggests that the crystal-forming ability of the IL contributes to the stronger layering of the ILs adjacent to the surface. The resonance shear measurement and the physical model analysis revealed that the viscosities of the confined ILs were 1-3 orders of magnitude higher than that of the bulk IL. This paper also focused on the correlation between the resonance shear behaviour and the lubrication property of the ILs, and the suspension rheology in the ILs. An understanding of the solid-IL interface and of ILs confined in nanospace will facilitate the further development of novel applications employing ILs.

Thermal and transport properties of ionic liquids based on benzyl-substituted phosphonium cations

The physicochemical properties of two novel ionic liquids based on benzyltriethylphosphonium and benzyltributylphosphonium cations are described in this report. It was found that both benzyl-substituted phosphonium cations gave low-melting salts in combination with a bis(trifluoromethylsulfonyl)amide anion. The thermogravimetric analysis suggested that the benzyl-substituted phosphonium ionic liquids showed higher thermal stability than those of not only the alkyl-substituted phosphonium ILs but also the corresponding benzyl-substituted ammonium compounds. The benzyl-substituted phosphonium ionic liquids also exhibited relatively high conductivities when compared to those of the corresponding ammonium compounds. These results indicate an improving effect of introducing a benzyl group into the phosphonium cations on both the thermal stability and the conductivity.

Solidification of Ionic Liquids: Theory and Techniques

Ionic liquids (ILs) have become an important class of solvents and soft materials over the past decades. Despite being salts built by discrete cations and anions, many of them are liquid at room temperature and below. They have been used in a wide variety of applications such as electrochemistry, separation science, chemical synthesis and catalysis, for breaking azeotropes, as thermal fluids, lubricants and additives, for gas storage, for cellulose processing, and photovoltaics. It has been realized that the true advantage of ILs is their modular character. Each specific cation–anion combination is characterized by a unique, characteristic set of chemical and physical properties. Although ILs have been known for roughly a century, they are still a novel class of compounds to exploit due to the vast number of possible ion combinations and one fundamental question remains still inadequately answered: why do certain salts like ILs have such a low melting point and do not crystallize readily? This Review aims to give an insight into the liquid–solid phase transition of ILs from the viewpoint of a solid-state chemist and hopes to contribute to a better understanding of this intriguing class of compounds. It will introduce the fundamental theories of liquid–solid-phase transition and crystallization from melt and solution. Aside form the formation of ideal crystals the development of solid phases with disorder and of lower order like plastic crystals and liquid crystals by ionic liquid compounds are addressed. The formation of ionic liquid glasses is discussed and finally practical techniques, strategies and methods for crystallization of ionic liquids are given.

Layering and shear properties of an ionic liquid, 1-ethyl-3-methylimidazolium ethylsulfate, confined to nano-films between mica surfaces

We report high-resolution measurements of the forces between two atomically smooth solid surfaces across a film of 1-ethyl-3-methylimidazolium ethylsulfate ionic liquid, for film thickness down to a single ion diameter. For films thinner than approximately 2 nm oscillatory structural forces are observed as the surface separation decreases and pairs of ion layers are squeezed out of the film. Strikingly, measurements of the shear stress of the ionic liquid film reveal low friction coefficients which are 1-2 orders of magnitude smaller than for analogous films of non-polar molecular liquids, including standard hydrocarbon lubricants, up to ca. 1 MPa pressure. We attribute this to the geometric and charge characteristics of the ionic liquid: the irregular shapes of the ions lead to a low shear stress, while the strong coulombic interactions between the ions and the charged confining surfaces lead to a robust film which is maintained between the shearing surfaces when pressure is applied across the film.

Ionic liquids as advanced lubricant fluids

Ionic liquids (ILs) are finding technological applications as chemical reaction media and engineering fluids. Some emerging fields are those of lubrication, surface engineering and nanotechnology. ILs are thermally stable, non-flammable highly polar fluids with negligible volatility, these characteristics make them ideal candidates for new lubricants under severe conditions, were conventional oils and greases or solid lubricants fail. Such conditions include ultra-high vacuum and extreme temperatures. Other very promising areas which depend on the interaction between IL molecules and material surfaces are the use of ILs in the lubrication of microelectromechanic and nanoelectromechanic systems (MEMS and NEMS), the friction and wear reduction of reactive light alloys and the modification of nanophases.

Ionic liquids in tribology

Current research on room-temperature ionic liquids as lubricants is described. Ionic liquids possess excellent properties such as non-volatility, non-flammability, and thermo-oxidative stability. The potential use of ionic liquids as lubricants was first proposed in 2001 and approximately 70 articles pertaining to fundamental research on ionic liquids have been published through May 2009. A large majority of the cations examined in this area are derived from 1,3-dialkylimidazolium, with a higher alkyl group on the imidazolium cation being beneficial for good lubrication, while it reduces the thermo-oxidative stability. Hydrophobic anions provide both good lubricity and significant thermo-oxidative stability. The anions decompose through a tribochemical reaction to generate metal fluoride on the rubbed surface. Additive technology to improve lubricity is also explained. An introduction to tribology as an interdisciplinary field of lubrication is also provided.

Solvation in gas-phase reactions of sulfonic group containing ionic liquids in electrospray ionization quadrupole ion trap mass spectrometry

In electrospray ionization (ESI) quadrupole ion trap mass spectrometry, certain fragment ions generated in the ion trap from a series of sulfonic group containing ionic liquids (ILs) are found to undergo ion-molecule reactions with ESI solvent molecules (water, acetonitrile and methanol) to form adduct species. These unexpected solvated fragment ions severely complicate the interpretation of mass spectrometric data. Multi-stage fragmentation mass spectra were used to ascertain the chemical structures of these adduct species. It is important that solvation must be taken into account in order to prevent erroneous data interpretation for sulfonic group containing ILs.

Application of static charge transfer within an ionic-liquid force field and its effect on structure and dynamics

The effects of linear scaling of the atomic charges of a reference potential on the structure, dynamics, and energetics of the ionic liquid 1,3-dimethylimidazolium chloride are investigated. Diffusion coefficients that span over four orders of magnitude are observed between the original model and a scaled model in which the ionic charges are +/-0.5 e. While the three-dimensional structure of the liquid is less affected, the partial radial distribution functions change markedly--with the positive result that for ionic charges of +/-0.7 e, an excellent agreement is observed with ab initio molecular dynamics data. Cohesive energy densities calculated from these partial-charge models are also in better agreement with those calculated from the ab initio data. We postulate that ionic-liquid models in which the ionic charges are assumed to be +/-1 e overestimate the intermolecular attractions between ions, which results in overstructuring, slow dynamics, and increased cohesive energy densities. The use of scaled-charge sets may be of benefit in the simulation of these systems--especially when looking at properties beyond liquid structure--thus providing an alternative to computationally expensive polarisable force fields.

AFM-based nanotribological and electrical characterization of ultrathin wear-resistant ionic liquid films

Ionic liquids (ILs) are considered as lubricants for micro/nanoelectromechanical systems (MEMS/NEMS) due to their excellent thermal and electrical conductivity. So far, only macroscale friction and wear tests have been conducted on these materials. Evaluating the nanoscale tribological performance of ILs when applied as a few nanometers-thick film on a substrate is a crucial step to understand how these novel materials can efficiently lubricate MEMS/NEMS devices. To this end, the adhesion, friction and wear properties of two ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) and 1-butyl-3-methylimidazolium octyl sulfate (BMIM-OctSO4), applied on Si(100), are investigated for the first time using atomic force microscopy (AFM). Data is compared to the perfluoropolyether lubricant Z-TETRAOL, which has high thermal stability and extremely low vapor pressure. Wear at ultralow loads was simulated and the lubricant removal mechanism was investigated using AFM-based surface potential and contact resistance techniques. Thermally treated coatings containing a mobile lubricant fraction (i.e., partially bonded) were better able to protect the Si substrate from wear compared to the fully bonded coatings, and this enhanced protection is attributed to lubricant replenishment.

Ionic Liquids for Soft Functional Materials with Carbon Nanotubes

A serendipitous finding that ionic liquids gel with carbon nanotubes has opened a new possibility of ionic liquids as modifiers for carbon nanotubes. Upon being ground into ionic liquids, carbon nanotube bundles are untangled, and the resultant fine bundles form a network structure. This is due to the possible specific interaction between the imidazolium ion component and the pi-electronic nanotube surface. The resultant gelatinous materials, consisting of highly electroconductive nanowires and fluid electrolytes, can be utilized for a wide variety of electrochemical applications, such as sensors, capacitors, and actuators. Ionic liquids allow for noncovalent and covalent modifications of carbon nanotubes and fabrication of polymer composites with enhanced physical properties. The processing of carbon nanotubes with ionic liquids is not accompanied by the disruption of the pi-conjugated nanotube structure and does not require solvents; therefore it can readily be scaled up. This article focuses on new aspects of ionic liquids for designer soft materials based on carbon nanotubes.

Physicochemical properties of nitrile-functionalized ionic liquids

A series of ionic liquids (ILs) based on nitrile-functionalized imidazolium, pyridinium, and quaternary ammonium as cations and chlorides and tetrafluoroborate, hexafluorophosphate, dicyanamide, and bis(trifluoromethanesulfonyl)imide as anions have been prepared and characterized. The physicochemical properties such as spectroscopic, thermal, solubility, surface, electrochemical, tribological, and toxic properties were comparatively studied. The results showed that the incorporation of a CN group to cations could result in remarkable changes in these properties. The reason resulting in such remarkable differences in the properties may be attributed to the conformational changes in the imidazolium groups caused by the interaction between the CN group with other neighboring cations or anions and the enhancement in hydrogen-bonding interactions due to the incorporation of a CN group.

From Dysfunction to Bis-function: On the Design and Applications of Functionalised Ionic Liquids

Some of the recent developments concerning the synthesis, properties and applications of functionalised ionic liquids are highlighted. Various strategies are presented, including functionalisation of the cation, anion or both cation and anion in the same ionic liquid, leading to what has been termed dual-functionalised ionic liquids. Particular attention is given to the application of functionalised ionic liquids as reaction media, to stabilise nanoparticles/modify surfaces and to generate porous materials.

Electrospray mass spectral fragmentation study of N,N'-disubstituted imidazolium ionic liquids

The tandem positive electrospray mass spectrometry (ESMS(n)) fragmentation of ionic liquids incorporating the 1-methyl-imidazolium ring substituted on N(II) with an alkyl chain functionalized with an alcohol, carboxylic acid, or an iodobenzyl or iodobenzoyl ester is presented for the first time. The influence of chain length and function is studied. Esterified structures led to intense CID fragments lacking the imidazolium ring allowing full characterization of the ester moiety. Fragment ion compositions for this interesting and newly important class of compounds are established through accurate mass data and deuterium labeling. The presence of the cationic ring system produces intense even electron molecular cations in electrospray that undergo multiple stages of CID to yield fragments which often are radical cations. Unusual losses of methyl and hydrogen radicals are frequently noted.

Syntheses of very dense halogenated liquids

A family of halogenated liquids with densities ranging from 1.95 to 2.80 g cm(-3) was readily synthesized by a one-pot procedure. These liquids exhibit characteristics of ionic liquids with melting/transition points lower than room temperature, long liquid ranges, and marked hydrolytic and thermal stabilities.

Self-Assembled Structure in Room-Temperature Ionic Liquids

Self-assembled vesicles, structurally equivalent to some hydrotropes, have been obtained from a Zn2+-fluorous surfactant or in the mixture of Zn2+-fluorous surfactant/zwitterionic surfactant in room-temperature ionic liquids (RTILs). The existence of bilayers arranged in vesicles in RTILs would be very exciting, open several new possibilities as reaction media, and increase our understanding of the physical and chemical factors for self-assembling systems in RTILs.