Interfacial Stabilization Mechanism in One-Step W/O/W Multiple Emulsions: The Role of Cellulose Nanofibrils and Oleic Acid

The multiphase structure and large interfacial area of multiple emulsions have driven increasing interest in their use in the pharmaceutical, cosmetics, and food industries. However, the presence of two thermodynamically unstable interfaces remains a challenge that hinders their broader application. Typically, the production of multiple emulsions requires large amounts of synthetic emulsifiers and a two-step preparation process. Here, we developed novel almond oil W 1 /O/W 2 multiple emulsions in one step by combining cellulose nanofibrils (CNFs) and oleic acid as bio-based emulsifiers. Cationic or anionic cellulose nanofibrils (CNFs), with average widths of about 2 nm, acted as Pickering stabilizers at the O/W 2 interface, producing oil droplets with diameters in the tens of micrometers. The internal W 1 /O emulsion, composed of water droplets a few micrometers in size, was stabilized by the naturally occurring oleic acid in the oil phase. Molecular dynamics simulations showed that increasing oleic acid content reduces the interfacial tension from 28 to 19 mN·m -1 . Emulsions prepared with mineral oil as an alternative oil phase did not result in multiple emulsion formation, confirming the essential role of oleic acid in stabilizing the W 1 /O interface. Additionally, the stability of multiple emulsions can be tailored by the type of CNF, as electrostatic repulsion between the negatively charged CNFs and oleic acid enhances the migration of oleic acid to the W 1 /O interface, promoting stability for over 60 days. These results demonstrate the capability of functionalized CNFs and oleic acid in the design of multiple emulsions using a practical method, with possible use in food and cosmetic formulations. • A W 1 /O/W 2 multiple emulsion was successfully produced in a single step. • The O/W 2 interface was stabilized by either cationic or anionic CNFs. • The W 1 /O interface was stabilized by oleic acid naturally present in the oil phase. • Multiple emulsions prepared with anionic CNFs were more stable.