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Title: Phosphonium-Organophosphate Ionic Liquids as Lubricant Additives: Effects of Cation Structure on Physicochemical and Tribological Characteristics

In our previous work we suggest great potential for a phosphonium-organophosphate ionic liquid (IL) as an antiwear lubricant additive. In this study, a set of five ILs were carefully designed and synthesized, with identical organophosphate anions but dissimilar phosphonium cations, to allow systematic investigation of the effects of cation alkyl chain length and symmetry on physicochemical and tribological properties. Symmetric cations with shorter alkyl chains seem to increase the density and thermal stability due to closer packing. On the other hand, either higher cation symmetry or longer alkyl moieties induce a higher viscosity, though the viscosity index is dependent more on molecular mass than on symmetry. While a larger cation size generally increases an IL’s solubility in nonpolar hydrocarbon oils, six-carbon seems to be the critical minimum alkyl chain length for high oil miscibility. Both the two ILs, that are mutually oil miscible, have demonstrated promising lubricating performance at 1.04% treat rate, though the symmetric-cation IL moderately outperformed the asymmetric-cation IL. Moreover, characterizations on the tribofilm formed by the best-performing symmetric-cation IL revealed the film thickness, nanostructure, and chemical composition. Our results provide fundamental insights for future molecular design in developing oil-soluble ILs as lubricant additives.
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  1. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
  2. Shell Projects and Technology (US), Houston, TX (United States)
Publication Date:
OSTI Identifier:
Grant/Contract Number:
Accepted Manuscript
Journal Name:
ACS Applied Materials and Interfaces
Additional Journal Information:
Journal Volume: 6; Journal Issue: 24; Journal ID: ISSN 1944-8244
American Chemical Society
Research Org:
Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States). Center for Nanophase Materials Sciences (CNMS)
Sponsoring Org:
USDOE Office of Energy Efficiency and Renewable Energy (EERE), Vehicle Technologies Office (EE-3V); USDOE Office of Science (SC), Workforce Development for Teachers and Scientists (WDTS) (SC-27)
Country of Publication:
United States
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY gas-to-liquid oil; ionic liquid; lubricant antiwear additive; oil solubility; organophosphate; phosphonium