J. Yan, F. Mangolini
The University of Texas at Austin,
United States
Keywords: lubrication, additives, ionic liquids
Summary:
While ionic liquids (ILs) have attracted much attention as potential next-generation lubricant additives owing to their unique physico-chemical properties and promising friction-reducing and/or anti-wear behavior, their implementation in oil formulations has partly been hindered by their limited solubility in hydrocarbon fluids. Additionally, even though ILs exhibit high chemical tunability due to the number of cations and anions available, the rational design of task-specific ILs for tribological applications is still hampered by our limited understanding of the mechanisms by which ILs reduce friction and/or wear. Here, we encapsulate a class of oil-insoluble, halogen-free ILs, namely tetraalkylammonium orthoborate ILs, within poly(ethylene glycol dimethacrylate-butyl methacrylate copolymer) (poly(EGDM-c-BMA)) microshells using a mini-emulsion polymerization process. The synthesized poly(EGDM-c-BMA)-encapsulated IL microparticles are dispersible in a non-polar, synthetic oil (i.e., poly-alpha-olefin). Tribological experiments indicated that the microcapsules act as an additive reservoir that reduces friction by releasing the encapsulated IL at the sliding interface following the mechanical rupture of the polymer shell. The friction-reducing ability of polymer-encapsulated tetraalkylammonium orthoborate ILs was found to depend on the IL molecular structure as increasing the length of the alkyl chains attached to ammonium cations led to an improvement of the lubricating properties. X-ray photoelectron spectroscopy (XPS) analyses allowed for gaining insights into the origin of the dependence of the friction-reducing ability of these ILs on the cation architecture. In the case of tetraalkylammonium orthoborate ILs with ammonium cations containing a long alkyl chain, no sacrificial tribofilms were formed on steel surfaces, thus suggesting that the friction-reducing ability of these ILs derives from their pressure-induced morphological change at sliding interfaces, which leads to the generation of a lubricious, solid-like interfacial layered structure difficult to squeeze out from the contact. Conversely, the higher friction response observed in the case of tribological tests performed in the presence of tetraalkylammonium orthoborate ILs with ammonium cations containing shorter alkyl chains is proposed to originate from the inability of this IL to create a transient interfacial layer due to the reduced Van der Waals interactions between the cationic alkyl chains. The resulting hard/hard contact between the sliding surfaces is proposed to lead to the cleavage of boron-oxygen bonds in orthoborate anions in the presence of water, which results in the formation of trivalent borate esters and oxalic acid that adsorb onto the steel surface as well as hydroxides that can trigger the degradation of alkylammonium cations and form tertiary amine able to absorb on steel. The results of this work do not only provide evidence for the dependence of the lubrication mechanism of ILs on the IL chemical structure, but also establish a new, broadly-applicable framework based on polymer encapsulation for utilizing ILs or other compounds with limited solubility as additives for oil formulations.