Cholesterol ester derivatives as oil-based lubricant additives: mesogenic and tribological properties

Abstract

The aim of this work is to investigate the tribological behavior of a series of newly synthesized cholesterol liquid crystals used as oil-based lubricant additives and the relationship with their chemical structures and mesogenic phases. Totally five cholesterol ester type liquid crystals with flexible chain lengths of different carbon numbers were synthesized and all of them exhibited relatively wide chiral mesogenic phase temperature ranges. The chemical structures and purities of the liquid crystals were confirmed by different analytical methods; the mesophase behaviors were also characterized. UMT-tribolab and four ball tester were applied to test the anti-friction abilities of the liquid crystal additives in 150 N and PAO 4 base oils, respectively; 3D optical profiler and atomic force microscope (AFM) were used to observe the generated wear scars and molecular layout of the synthesized additives; reflection-absorption fourier transform infrared spectrometry (RA-FT-IR) was employed to explain the possible changes in molecular orientation of liquid crystal additives and produced tribological effect. The results showed the tribological properties of the liquid crystal additives were closely related to their chemical structures and mesogenic behaviors. The liquid crystals with wider mesophase temperature ranges presented better tribological behaviors, and the ester groups with long alky tail lengths in their structures also played a crucial role. Among the five liquid crystal additives, LC-8 showed the widest mesogenic phase temperature range and the best tribological performance. Most of the series of cholesterol liquid crystals possessed the potential to be used as friction modifiers in some temperature sensitive applications.