Phosphonium-organophosphate ionic liquids as lubricant additives: Effects of cation structure on physicochemical and tribological characteristics

William C. Barnhill, Jun Qu, Huimin Luo, Harry M. Meyer, Cheng Ma, Miaofang Chi, Brian L. Papke

Research output: Contribution to journalArticlepeer-review

163 Scopus citations

Abstract

Our previous work suggested 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 ILs 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. Characterizations on the tribofilm formed by the best-performing symmetric-cation IL revealed the film thickness, nanostructure, and chemical composition. Results here provide fundamental insights for future molecular design in developing oil-soluble ILs as lubricant additives.

Original languageEnglish
Pages (from-to)22585-22593
Number of pages9
JournalACS Applied Materials and Interfaces
Volume6
Issue number24
DOIs
StatePublished - Dec 24 2014

Bibliographical note

Publisher Copyright:
© 2014 American Chemical Society.

Keywords

  • gas-to-liquid oil
  • ionic liquid
  • lubricant antiwear additive
  • oil solubility
  • organophosphate
  • phosphonium

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