Neem oil emulsions stabilized by natural and synthetic emulsifiers: a study on physical stability and antifungal activity

Neem oil (NO) is a plant-derived bioactive compound with known antimicrobial properties. Incorporating NO into oil-in-water (O/W) emulsions can improve its stability, bioavailability, and allow controlled release. This study developed and characterized O/W emulsions using NO as the dispersed phase and active agent. Emulsions were formulated with natural emulsifiers (soy lecithin - SL, pea protein - PP), a synthetic emulsifier (Tween 80 - T80), and their combinations. Over 21 days, formulations were analyzed for chemical interactions, droplet size (D_4,3), polydispersity (Span), zeta potential (ZP), microstructure, color, kinetic stability, and antifungal activity (AFA) against Alternaria alternata and Penicillium expansum. All emulsions showed stable chemical interactions and ZP values above |38 mV|. D_4,3 ranged from 0.59 to 2.01 μm, influenced by emulsifier type. Thermally treated PP in alkaline conditions produced stable, slightly polydisperse emulsions for 14 days, while SL led to larger droplets and instability. PP:T80 and T80:SL combinations showed synergistic effects and good stability, unlike PP:SL, which resulted in unstable emulsions. The PP-only emulsion had the highest AFA, inhibiting P. expansum (85.06 %) and A. alternata (79.97 %). These findings highlight the critical role of emulsifier selection in enhancing the stability, controlled release, and antifungal efficacy of NO in emulsified systems.