Precision-engineered nanomaterials: unlocking the potential of water-in-oil microemulsions for the controlled synthesis, morphology design, and future innovations

Abstract Nanomaterials with precisely controlled size and shape have attracted considerable attention over the past two decades due to their diverse applications in pharmaceuticals, oil recovery, energy production and storage, wastewater treatment, and advanced magnetic devices. Among the various synthesis techniques, water-in-oil (W/O) microemulsions have emerged as a superior strategy for the preparation of nanoparticles with well-defined morphology and minimal aggregation. These self-assembled colloidal dispersions act as nanoscale reactors, offering exceptional control over nanoparticle growth and structural evolution. This review critically examines the role of W/O microemulsions in tailoring nanoparticle size, geometry, and crystallinity by modulating key physicochemical parameters such as surfactant architecture, cosurfactant-to-surfactant molar ratio, and water-to-surfactant molar ratio. The influence of surfactant head group charge, hydrophobic tail length, and cosurfactant selection on nanoparticle synthesis is systematically analyzed, highlighting the versatility of anionic, cationic, and nonionic surfactants. Despite significant progress, critical gaps remain, including the underutilization of ionic liquid-based microemulsions and the limited exploration of amines as cosurfactants. Additionally, the potential of biosurfactants as sustainable alternatives for nanoparticle synthesis remains largely untapped. By consolidating extensive research findings, this review provides a comprehensive framework for optimizing W/O microemulsion systems and offer valuable insights for future advances in nanomaterial synthesis and applications.