Surface-active ionic liquids as lubricant additives to hexadecane and diethyl succinate

The effectiveness of surface-active ionic liquids (SAILs) as lubricant additives in both polar and apolar base liquids has been investigated on stainless steel surfaces at both macroscale and nanoscale. The SAILs studied are composed of trihexyltetradecylphosphonium ([P6,6,6,14]+) cations paired with either dioctyl sulfosuccinate ([AOT]-) or dodecyl sulphate ([DS]-) anions. The polar and apolar base liquids used to dilute the SAILs are diethyl succinate ((CH2CO2Et)2) and hexadecane (HD), respectively. The friction coefficients of pure [P6,6,6,14] [AOT] and [P6,6,6,14] [DS] are up to ∼80 % lower than those of (CH2CO2Et)2 and HD. Remarkably, diluted solutions containing as little as 5 wt% SAIL in either (CH2CO2Et)2 or HD exhibit lubricity comparable to that of pure SAILs, revealing their excellent potential as lubricant additives. The high lubricity of the SAIL solutions in both (CH2CO2Et)2 and HD is attributed to the adsorption of anions onto the positively charged stainless steel surface, resulting in the formation of a robust boundary layer that reduces surface contact and facilitates smooth sliding. These insights aid the development of advanced, cost-effective lubricant additives, with potential applications across various industrial sectors.