Design of Alkyl Sulfate Ionic Liquids for Lubricants

Abstract

Tribological properties of alkyl sulfate ionic liquids have been evaluated; hydrophobicity of ionic liquids significantly influenced the tribological characteristics and combination of phosphonium cation with hydrophobic octafluoropentyl sulfate anion afforded good ionic liquid that showed excellent tribological performance.Ionic liquids (ILs) are a new class of fluids and interest has been growing in their application in diverse fields of science and technology.1,2 The tribological properties of ionic liquids have attracted increased attention in recent years since Liu and coworkers 3a first reported the possible use of these compounds as a lubricant, because of their low volatility and flammability. [3][4][5][6][7][8][9][10][11][12] ILs generally show outstanding thermo-oxidative stability, this property being particularly important for lubricants that are used under extreme conditions. 13Alkyl sulfate ionic liquids are very economical and have broad potential in their molecular design.2c However, little attention has been given to their application as lubricant to date. We recently reported the successful design of hydrophobic alkyl sulfate ionic liquids using fluorine chemistry; 2e the hydrophobicity of alkyl sulfate imidazolium salts was dependent on the content ratio of the fluorine to the alkyl sulfate anion and 2,2,3,3,4,4,5,5-octafluoropentyl sulfate (C5F8) imidazolium salt showed perfect hydrophobic properties, 2e while non-fluorinated pentyl sulfate (C5F0) salt showed a strong hydrophilic nature. 2c,2e In this paper, we report the results of evaluation of alkyl sulfate ionic liquids as lubricants.Tribological properties for our three types of original ionic liquids, i.e. 1-butyl-3-methylimidazolium pentyl sulfate was 25 Â 7 mm made of SUJ2 steel (JIS). Friction force was monitored throughout the test and the data were converted to friction coefficient. Two tests were run for each set of test parameters, and differences in the results were less than 15%.Both imidazolium salts showed good lubricant properties that were lower than those from perfluoropolyether (PFPE), 13 which is widely used as a lubricant for head/disk interfaces or micromachines. However, a significant weak point was foundwas not constant during the experiments, it initially increased and then decreased with rubbing ( Figure 1). It was assumed that the friction became constant after 1000 s. owing to the reduced contact pressure by wear.On the contrary, no fluctuation of the friction coefficient was observed for [bmim][C5F8] (Figure 2). Since [bmim][C5F8] has perfect hydrophobic properties, no rust was found on the surface of the ball and disk; this provides a constant friction coefficient. The results suggest that hydrophobic ionic liquid is appropriate as a lubricant.It has been pointed out that reaction of iron metal with phosphorous forms a stable boundary film composed of iron phosphate and this generally contributes to an improvement in tribological properties.4 Therefore, we next tested ionic liquid2g as a lubricant under the sa...

New Frontiers in Materials Science Opened by Ionic Liquids

“…[47] In addition, phosphonium cation-based ILs with appropriate anions show excellent tribological properties more than imidazolium-based ILs. [50][51][52] As expected, adsorption species contained phosphoric compound. Also a new IL was synthesized by combining imidazolium cation and phosphate anion.…”

Study of tribological performance and mechanism of phosphonate ionic liquids for steel/aluminum contact

“…So it can be concluded that the lubricant with phosphonate anion is suitable for the steel-aluminium contact than that with the hexafluorophosphate anion. As is known that anion plays an important role in the tribological performance of ionic liquids [23,24]. It can not only make the viscosity change but also have an impact on the formation of boundary lubricating film (adsorption and chemical reaction film) [9].…”

Polarizability versus mobility: atomistic force field for ionic liquids

“…From the practical point of view, RTILs can boast a wide variety of potential applications 7,8 , not limited to components of robust electrolytes for electrochemical devices 10,11 , reaction medias 12 , lubricants 13 , solvents for synthetic and catalytic schemes 14,15 . The prominent properties of RTILs, as salts with very low melting points, are determined by the structure and inter-ionic interactions.…”