Components interactions and stability factors in terpenes oil-in-water emulsions stabilized with hydrolyzed soybean lecithin

Abstract

R-(+)-limonene, eucalyptol, and linalool are natural compounds with promising antifungal activity but limited practical applicability due to high volatility, low water solubility, and photodegradability. This study investigates the formulation of stable oil-in-water emulsions using a low-energy phase inversion method and exclusively naturally derived components. The influence of the surfactant concentration (0.5–1.1 wt% hydrolyzed soybean lecithin), the addition of a co-surfactant (2.6 wt% isoeugenol), thickener concentration (0.05–0.50 wt% guar gum), continuous-phase pH (3, 5, & 7), and active compound physicochemical properties was systematically evaluated. Emulsions containing 11 wt% terpene were prepared by water titration. Stable formulations were obtained at neutral pH using 1.1 wt% lecithin, 2.6 wt% isoeugenol and 0.5 wt% guar gum. Acidification reduced stability, associated with pH-dependent variations in surface charge and intermolecular interactions affecting both interfacial organization and bulk rheology. Differences in droplet size distribution and viscosity among terpenes highlight the role of active compound polar character in governing emulsion microstructure. Overall, this work demonstrates that stable, fully natural terpene-based emulsions can be rationally designed using low- energy methods, providing a formulation framework relevant to sustainable phytosanitary applications.