Tris(dialkylamino)cyclopropenium dialkylphosphate ionic liquids as lubricants

Six new tris(dialkylamino)cyclopropenium dialkylphosphate ionic liquids (ILs), [C-3(NR2)(3)]BEHP (NR2 = NEt2, NBuMe, NPr2, NBu2, NHex(2), NDec(2); 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 [C-3(NEt2)(3)]BEHP would not be suitable due to its water solubility and hexane immiscibility. [C-3(NDec(2))(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, [C-3(NHex(2))(3)]BEHP was found to exhibit significantly less wear for pin-on-disk test conditions than the standard phosphonium [P-6,P-6,P-6,P-14]BEHP IL. The amount of wear was found to generally decrease with increasing molecular weight.