Tris(dialkylamino)cyclopropenium dialkylphosphate ionic liquids as lubricants

Abstract

Six new tris(dialkylamino)cyclopropenium dialkylphosphate ionic liquids (ILs), [C₃(NR₂)₃]BEHP (NR₂ = NEt₂, NBuMe, NPr₂, NBu₂, NHex₂, NDec₂; BEHP = bis(2-ethylhexyl)phosphate), were prepared and characterised as potential lubricants. Thermophysical and thermochemical properties of these ILs were investigated, namely: viscosity, density, conductivity, miscibility, thermal stability and phase transitions. Miscibility studies indicated that [C₃(NEt₂)₃]BEHP would not be suitable due to its water solubility and hexane immiscibility. [C₃(NDec₂)₃]BEHP was not investigated as a lubricant due to its low purity (the chloride salt of this cation is also hexane miscible). Of the other four, [C₃(NHex₂)₃]BEHP was found to exhibit significantly less wear for pin-on-disk test conditions than the standard phosphonium [P_6,6,6,14]BEHP IL. The amount of wear was found to generally decrease with increasing molecular weight.