Colloidal stability mechanism of copper nanomaterials modified by bis(2-ethylhexyl) phosphate dispersed in polyalphaolefin oil as green nanolubricants.

Nanomaterials stabilization in lube oils poses an acute challenge in nanolubricants/nanofluids formulation. This study aims to improve the dispersion stability of copper (Cu) nanomaterials in polyalphaolefin-6 (PAO6) oil to overcome the agglomeration/sedimentation problem. Here, we modified the surface of Cu nanomaterials using bis(2-ethylhexyl) phosphate (IL) to enhance the electrostatic repulsion force in Cu nanomaterials. We evaluated the dispersion behavior of Cu nanolubricants by visual observation, ultraviolet-visible spectroscopy, dynamic light scattering, and zeta potential measurements. Furthermore, we determined the rheological and thermo-oxidation behavior of Cu nanolubricants using Brookfield viscometer, thermogravimetric, and Fourier transform infrared. Our experiments showed that dispersion stability depends on Cu concentration and settling time. IL demonstrated effective miscibility when blended with PAO6 oil and displayed non-Newtonian behavior. The results suggest that Cu modified by IL provides superior dispersion in PAO6 oil without sedimentation for 60 days, compared to unmodified Cu. Moreover, the hydrodynamic diameter of the modified Cu did not exceed 240 nm even after 60 days of preparation. The excellent dispersion behavior can be ascribed to the domination of the electrostatic repulsion forces over the inter-nanomaterials van der Waals interactions, which is related to the formation of the electrical adsorption layer on the Cu surface. The obtained colloidal dispersions have the potential to be utilized as green nanolubricants for lubricating tribological systems.