Toward green and sustainable dielectric nanofluids: surfactant impacts on stability, properties, and regulations

Transformer fluids face major limitations in the dielectric strength and thermal conductivity of fluids, which hinders effective application in high-voltage applications. Adding nanoparticles holds potential for enhancement, but it is challenging to accomplish and maintain a stable dispersion because of a tendency toward agglomeration. Surfactants are found to act as critical stabilizing agents that aid in the dispersion of nanoparticles and temporal stability through steric and electrostatic interactions at the oil-nanoparticle interface. This review closely evaluates the preparation methodologies of dielectric nanofluids, paying specific attention to the functionality of surfactants and temporal stability effects. Surfactant-treated nanofluids showed temporal stability from a few weeks to a few months along with significant enhancements: breakdown voltage (up to 93.17%), dielectric constant (up to 47.4%), decrease in dissipation factor (up to 97.3%), increase in resistivity (up to 917.93%), and enhancement in thermal conductivity (up to 216.2%). These enhancements are necessarily connected to interfacial alterations that control charge trapping, polarization, and phonon conduction. Gaps between theoretical models of dielectric constant (like Maxwell-Garnett and Loyang models and others) and experimental findings are resolved to recommend enhancement considering surfactant-induced interfacial effects. Also, sustainability aspects such as biodegradability, toxicity, recyclability, and regulatory compliance are discussed. This review's uniqueness lies in a detailed discussion of interfacial mechanisms, model enhancements, and eco-friendly surfactant design, and suggestions for entering into molecular dynamics and interfacial modelling to rationally design environmentally sustainable, high-performance dielectric nanofluids.