Long-term colloidal stability of cobalt ferrite nanoparticles in magnetic fluids: A nine-year study

The need for sustainable and efficient heat transfer methods, as part of the drive to mitigate global warming, has incentivized research concerning nanofluids that can assist in reducing carbon emissions and improving global energy efficiency. Due to their superior thermal conductivity, the use of nanofluids can enhance the performance of heat exchangers employed in industrial processes and renewable energy systems. Nanofluids can contribute to reducing emissions by optimizing cooling techniques, thus playing an important role in addressing the issues of energy sustainability and climate change. This study investigates the long-term stability of a ferrofluid based on cobalt ferrite magnetic nanoparticles (MNPs). Surface modification of the nanoparticles with oleic acid (OA) was used to enhance the energy barrier and stability of MNPs dispersed in 1-octadecene (OCT), aiming to overcome the difficulty of ensuring long-term colloidal stability, which is crucial for the practical viability of nanofluids. The MNPs were synthesized using the co-precipitation method, at different temperatures, resulting in crystallite sizes ranging from 2 to 7 nm. Structural variations were observed that did not affect the colloidal stability of the uncoated MNPs, since all the samples formed mass fractal aggregates in aqueous solution, regardless of primary particle size and zeta potential prior to coating. Rheological measurements of the magnetic fluids (MFs) prepared by dispersing MNPs@OA in OCT indicated Newtonian behavior for all the volume fractions tested (0.2–0.002 vol%). Measurements over time using dynamic light scattering (DLS) revealed that the colloidal stability was highly dependent on the coating thickness and the volumetric particle fraction. By optimizing these parameters, it was possible to maintain stability over a long period (200 days to nine years), avoiding depletion mechanisms that typically destabilize MFs. An important contribution to the colloidal stability field was the observed accelerate destabilization of the colloids caused by dilution, with was associated to the dynamic equilibrium between the adsorption/desorption and the depletion mechanisms.