Microemulsions and Nanoemulsions: concepts review

Abstract

Nanotechnology has played a central role in the development of innovative delivery systems for active compounds, with particular emphasis on microemulsions (MEs) and nanoemulsions (NEs). Although both are colloidal dispersions composed of oil, water, and surfactants, they present conceptual, structural, and stability-related differences that directly affect their pharmaceutical and cosmetic applications. This narrative review aimed to discuss the fundamental aspects, physicochemical properties, preparation methods, and potential applications of MEs and NEs. A literature search was conducted in indexed databases (PubMed, Web of Science, Scopus, and ScienceDirect), including original articles, review papers, and regulatory documents. The results indicate that MEs are characterized by high surfactant/co-surfactant concentrations, spontaneous formation, thermodynamic stability, and a translucent appearance, whereas NEs are obtained using high- or low-energy methods, require lower surfactant concentrations, and exhibit only kinetic stability. Differences in droplet size, free energy of formation, and responsiveness to external conditions (pH, temperature, and dilution) explain the selection of each system for specific purposes. Both platforms have been widely investigated to improve the solubility, bioavailability, and therapeutic efficacy of bioactive compounds, thereby expanding their applicability in pharmaceutical products. Studies of this nature, particularly in the national literature and published in Portuguese, are essential to facilitate a clearer understanding of the concepts underlying these two systems, thereby preventing misclassification and errors in the development of nanomaterials.