Andrea Arcifa

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.

Layering of ionic liquids on rough surfaces.

Understanding the behavior of ionic liquids (ILs) either confined between rough surfaces or in rough nanoscale pores is of great relevance to extend studies performed on ideally flat surfaces to real applications. In this work we have performed an extensive investigation of the structural forces between two surfaces with well-defined roughness (<9 nm RMS) in 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide by atomic force microscopy. Statistical studies of the measured layer thicknesses, layering force, and layering frequency reveal the ordered structure of the rough IL-solid interface. Our work shows that the equilibrium structure of the interfacial IL strongly depends on the topography of the contact.

Influence of Environmental Humidity on the Wear and Friction of a Silica/Silicon Tribopair Lubricated with a Hydrophilic Ionic Liquid

In this study, the tribological behavior of silica/silicon surfaces lubricated with the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate ([EMIM] EtSO4) was investigated. Tests were carried out in the presence of either humid air (45-55% relative humidity) or in a nitrogen atmosphere, and the results were compared with those obtained using pure water as a lubricant. The cross-sectional analysis of the contact area performed by focused-ion-beam scanning electron microscopy indicated the presence of cracks in the subsurface region, showing that brittle fracture contributed to wear. Sliding promoted the formation of a third body, the presence of which was indicated by optical and secondary electron microscopy. X-ray photoelectron spectroscopy showed that the third body was mostly composed of silicon oxides. The accumulation of the debris was controlled by the presence of water: in the presence of a nitrogen atmosphere, particles were trapped between the sliding surfaces, whereas in the case of humid air, the debris was progressively removed from the contact. Notably, the presence of trapped particles was associated with higher values of wear coefficients of both disks and pins. In addition, a lower roughness was observed along the direction of sliding in the case of water-containing ionic liquid. The observed trends in wear and the combined results of the various techniques, as well as the comparison with tests carried out in the presence of pure water, all point to the characteristic tribochemical reactions of water with silicon-based materials, namely, the formation of a sacrificial layer of hydrated oxide and the dissociative adsorption of water at crack tips of SiO2. In the absence of water, the lack of a tribochemical mechanism forming a sacrificial layer leads to a microfracture-dominated wear mechanism over the entire duration of the test, thus leading to more severe wear. The possible occurrence of stress-induced phase transformation of silicon during sliding is also discussed.

Ionic Liquids at Interfaces and Their Tribological Behavior

Ionic liquids display novel self-assembling behavior at the solid–liquid interface, where they remain firmly surface-adsorbed under high normal and shear stress. Such appealing property has inspired increasing research efforts in the field of tribology of ionic liquids during the past 15 years. This article summarizes our current knowledge of the mechanisms of ionic-liquid-mediated lubrication. We first review the studies that have revealed the composition of the confined thin films as a function of surface properties, environmental conditions, and chemical structures of the ionic liquid. This knowledge is then correlated with the proposed lubrication mechanisms for smooth surfaces and mild tribological conditions on the nanoscale, which are, for example, of relevance in the context of lubrication of micro- and nanoelectromechanical systems. We further address the additional effects of surface roughness, contamination, wear, and the formation of tribolayers under high pressures and temperatures, which are convoluted in the tribological response of the ionic liquid lubricant subjected to high stress. Although many works have demonstrated the lubricious properties of several ionic liquids, generalizations are not possible to date, as the molecular structure and chemical composition of the ionic liquid, as well as the substrate’s surface chemistry and topography, affect the properties of the adsorbed films, and in turn, their tribochemical reactivity. Future directions for research in the field of ionic liquid tribology are proposed at the end of this article.