Due to ever increasing demands on lubricants, such as increased service intervals, reduced volumes and reduced emissions, there is a need to develop new lubricants and improved wear additives. Ionic liquids (ILs) are room temperature molten salts that have recently been shown to offer many advantages in this area. The application of ILs as lubricants in a diverse range of systems has found that these materials can show remarkable protection against wear and significantly reduce friction in the neat state. Recently, some researchers have shown that a small family of ILs can also be incorporated into non-polar base oils, replacing traditional anti-wear additives, with excellent performance of the neat IL being maintained. ILs consist of large asymmetrical ions that may readily adsorb onto a metal surface and produce a thin, protective film under boundary lubrication conditions. Under extreme pressure conditions, certain IL compounds can also react to form a protective tribofilm, in particular when fluorine, phosphorus or boron atoms are present in the constituent ions.

We report on the synthesis, characterisation, and physical and tribological properties of halogen-free ionic liquids based on various chelated orthoborate anions with different phosphonium cations, both without halogen atoms in their structure. Important physical properties of the ILs including glass transition temperatures, density, viscosity and ionic conductivity were measured and are reported here. All of these new halogen-free orthoborate ionic liquids (hf-BILs) are hydrophobic and hydrolytically stable liquids at room temperature. As lubricants, these hf-BILs exhibit considerably better antiwear and friction reducing properties under boundary lubrication conditions for steel-aluminium contacts as compared with fully formulated (15W-50 grade) engine oil. Being halogen free these hf-BILs offer a more environmentally benign alternative to ILs being currently developed for lubricant applications.

The use of ionic liquids as additives to base oil for the lubrication of steel on aluminum was investigated. The miscibility and wear performance of various phosphonium, imidazolium, and pyrrolidinium ionic liquids in a range of polar and nonpolar base oils was determined. The structure and ion pairing of the ionic liquids was found to be important in determining their miscibility in the base oils. In wear tests, some of the miscible base oil/IL blends reduced the aluminum wear depth when compared to that found with the base oil alone. The nonpolar base oil/IL blends were able to withstand higher wear-test loads than the polar base oil/IL blends. At 10 N, as little as 0.01 mol/kg of IL, or 0.7-0.9 wt %, in the nonpolar base oils was enough to drastically reduce the wear depth on the aluminum. XPS analysis of the wear surfaces suggested that the adsorbing of the IL to the surface, where it can form low-shear layers and also react to form tribofilms, is important in reducing friction and wear. The largest reductions in wear at the highest load tested were found for a mineral oil/P6,6,6,14 (i)(C8)2PO2 blend.

In the field of ionic liquids (ILs) it has long been of fundamental interest to examine the transition from salt-in-solvent behaviour to pure liquid-salt behaviour, in terms of structures and properties. At the same time, a variety of applications have beneficially employed IL-solvent mixtures as media that offer an optimal set of properties. Their properties in many cases can be other than as expected on the basis of simple mixing concepts. Instead, they can reflect the distinct structural and interaction changes that occur as the mixture passes through the various stages from pure coulombic medium, to "plasticised" coulombic medium, into a meso-region where distinct molecular and ionic domains can co-exist. Such domains can persist to quite a high dilution into the salt-in-solvent regime and their presence manifests itself in a number of important synergistic interaction effects in diverse areas such as membrane transport and corrosion protection. Similarly, the use of ionic liquids in synthetic processes where there is a significant volume fraction of molecular species present can produce a variety of distinct and unexpected effects. The range of these salt-solvent mixtures is considerably broader than just those based on ionic liquids, since there is only minor value in the pure salt being a liquid at the outset. In other words, the extensive families of organic and metal salts become candidates for study and use. Our perspective then is of an evolution of ionic liquids into a broader field of fundamental phenomena and applications. This can draw on an even larger family of tuneable salts that exhibit an exciting combination of properties when mixed with molecular liquids.

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.

Mild steel infrastructure is constantly under corrosive attack in most environmental and industrial conditions. There is an ongoing search for environmentally friendly, highly effective inhibitor compounds that can provide a protective action in situations ranging from the marine environment to oil and gas pipelines. In this work an organic salt comprising a protic imidazolinium cation and a 4-hydroxycinnamate anion has been shown to produce a synergistic corrosion inhibition effect for mild steel in 0.01 M NaCl aqueous solutions under acidic, neutral, and basic conditions; an important and unusual phenomenon for one compound to support inhibition across a range of pH conditions. Significantly, the individual components of this compound do not inhibit as effectively at equivalent concentrations, particularly at pH 2. Immersion studies show the efficacy of these inhibitors in stifling corrosion as observed from optical, SEM, and profilometry experiments. The mechanism of inhibition appears to be dominated by anodic behavior where dissolution of the steel, and in particular the pitting process, is stifled. FTIR spectroscopy provides confirmation of a protective interfacial layer, with the observation of interactions between the steel surface and 4-hydroxycinnamate.

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Ionic liquids have been shown to be highly effective lubricants for a steel on aluminium system. This work shows that the chemistry of the anion and cation are critical in achieving maximum wear protection. The performance of the ILs containing a diphenylphosphate (DPP) anion all showed low wear, as did some of the tris(pentafluoroethyl)trifluorophosphate (FAP) and bis(trifluoromethanesulfonyl)amide (NTf 2 ) anion containing ILs. However, in the case of the FAP and NTf 2 based systems, a cation dependence was observed, with relatively poor wear resistance obtained in the case of an imidazolium FAP and two pyrrolidinium NTf 2 salts, probably due to tribocorrosion caused by the fluorine reaction with the aluminium substrate. The systems exhibiting poor performance generally had a lower viscosity, which also impacts on their tribological properties. Those ILs that exhibited low wear were shown to have formed protective tribofilms on the aluminium alloy surface.