ABSTRACT:
Microemulsions (MEs) are thermodynamically stable, optically isotropic, nanosized (10–200 nm) dispersions of oil and water stabilised by an interfacial film of surfactant, often in combination with a co-surfactant. Microemulsions have emerged as a versatile drug-delivery platform capable of enhancing the solubility, bioavailability, and stability of both hydrophilic and lipophilic therapeutics. Unlike conventional emulsions, MEs form spontaneously upon gentle agitation, exhibit ultralow interfacial tension, and remain stable without the need for external energy input. This review provides a detailed appraisal of the physicochemical principles governing ME formation, the selection of excipients (oils, surfactants, co-surfactants, and co-solvents), pseudoternary phase diagram construction, and the full spectrum of in vitro, ex vivo, and in vivo evaluation techniques. Special attention is given to contemporary characterisation tools such as dynamic light scattering (DLS), transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and rheological analysis. The therapeutic applications of MEs across oral, transdermal, ocular, nasal, and parenteral routes are critically discussed, together with regulatory considerations, scale-up challenges, and future perspectives.