Exploring dipole-dipole interactions and interfacial thermal conductance of coconut shell bio-nanocarbon particles dispersed in polar and non-polar liquid lubricants

This study investigates the influence of dipole–dipole interactions on enhancing the thermal conductivity of solid–liquid lubricant systems, a relatively underexplored mechanism, particularly in bio-based interfaces. A comparative analysis was conducted using two types of lubricants: a polar covalent biolubricant (Bombax ceiba oil, BCO) and a nonpolar covalent lubricant (SAE 20W40). Each was blended with coconut shell-derived nanocarbon (CSC) particles, with and without antioxidant (AO) modification, at concentrations ranging from 0.05 to 0.3 wt.%. Four distinct solid–liquid mixtures were prepared and evaluated: AO-modified CSC + BCO, unmodified CSC + BCO, AO-modified CSC + SAE20W40, and unmodified CSC + SAE20W40. Among these, the unmodified CSC + BCO mixture exhibited the highest enhancement in thermal conductivity, achieving an 8.69% increase at 80°C. This enhancement is attributed to efficient acoustic phonon propagation facilitated by strong dipole–dipole interactions between the short intermolecular distances of the charged species (C (-, +), H ⁺ , and O ⁻ ) at the solid–liquid interface. The results highlight the potential of unmodified biocarbon nanoparticles to improve heat transfer in polar covalent biolubricants, underscoring their promise for use in sustainable lubrication systems operating at medium to high temperatures.