Cutoff Ostwald ripening stability of alkane-in-water emulsion loaded with eugenol

This study demonstrates that eugenol molecules in the aqueous phase of emulsions play a key role in determining both their antimicrobial activity and physical stability. Firstly, the antimicrobial activity of 5% (v/v) tetradecane-in-water emulsions (0.5% (v/v) Tween 20, pH 7) containing 1, 20 and 50% eugenol of the oil phase were assessed at a fixed 0.025% eugenol concentration against Staphylococcus carnosus. The results showed that the higher the ratio of eugenol in tetradecane emulsions was, the higher the antimicrobial activity. Secondly, the impact of eugenol loading (0–50%) in short chain hydrocarbon oil-in-water emulsions (5% (v/v) oil, 0.5% (v/v) Tween 20, pH 7) was investigated for physical stability. The short chain hydrocarbons used were n-decane, 1-dodecene, n-dodecane, 1-tetradecene, and n-tetradecane. When increasing the eugenol ratio, the Ostwald ripening rate (ω) showed a cutoff pattern with a threshold at 20% of eugenol in all the short chain hydrocarbon emulsions. Ostwald ripening was the main instability mechanism at low concentrations of eugenol (1–20%), however, this rate was reduced by compositional ripening. By contrast, the emulsions coalesced at higher concentrations of eugenol (>20%). This study is a good example to help one understand the application of essential oil in the emulsion system in a balance between antimicrobial property and physical stability.