Design Considerations for Essential Oil Formulations: Emulsification, Encapsulation and Stabilisation Strategies

Essential oils are widely recognised for their bioactive properties and therapeutic effects. Despite their potential, their practical application is often limited by intrinsic physicochemical constraints, including volatility, chemical instability, hydrophobicity and, most of all, compositional complexity. The development of effective delivery systems is therefore key to stabilise essential oil constituents, improve bioavailability and enable controlled release. Here we provide a comprehensive analysis of formulation strategies for essential oils, focusing on emulsification and encapsulation techniques. Emulsion-based delivery systems, including nanoemulsions and microemulsions, are discussed in relation to their interfacial behaviour, surfactant interactions and preparation methods. In parallel, encapsulation systems, such as vesicular systems, lipid-based carriers, polymeric nanoparticles and inclusion complexes, are evaluated according to their structural characteristics, protective capabilities, encapsulation efficiencies and release profiles. Collectively, these approaches are examined in the context of their influence on the stability, functionality, and overall performance of essential oil formulations. Emphasis is placed on essential oil-specific challenges that arise when preparing a formulation, especially the heterogenous nature of their chemical composition and its impact on physicochemical properties, partitioning, loading capacity, stability and safety. To contextualise these principles, representative examples of essential oil-based formulations (e.g., thyme, clove, rosemary and mint), formulated singular metabolites (e.g., carvacrol, eucalyptol, limonene and eugenol) and essential oil patent applications (>20,000) and products are outlined to illustrate behaviour and performance within the respective delivery system. Overall, the fundamental design considerations that guide the rational selection, preparation and evaluation of formulations for essential oil-based applications are discussed. Considerations for future directions are also outlined.