Thermal conductivity studies on magnetite nanofluids coated with short-chain and long-chain fatty acid surfactants

The effect of the length of surfactant molecules on the surface of the nanoparticles on the thermal conductivity of nanofluids is studied. Magnetite ( $$\hbox {Fe}₃\hbox {O}₄$$ ) nanoparticles of comparable sizes are stabilized with short-chain capric acid ( $$\hbox {C}₉\hbox {H}₁₉\hbox {COOH}$$ ) and long-chain stearic acid ( $$\hbox {C}₁₇\hbox {H}₃₅\hbox {COOH}$$ ) molecules. Thermal conductivity of the two surfactant-coated magnetite nanoparticles dispersed in toluene is measured as a function of the concentration of the particles in the fluids and in the presence of a magnetic field. Studies showed that the critical concentration for thermal conductivity enhancement is lower for stearic-acid-coated fluid as compared with the capric-acid-coated fluid. Comparable enhancement in the thermal conductivity is observed at higher concentrations of the particles. Relatively larger enhancement in the thermal conductivity is observed for the capric-acid-coated fluid in a magnetic field. The difference in the enhancement in the thermal conductivity, depending on the chain length of the surfactant, is explained in terms of the inter-particle magnetic interactions and formation of clusters.