Interfacial Assembly of Cellulose Nanofibers at Oil/Water Interface via Tailored Electrostatic Attraction to Establish Surfactant-Free Emulsion Stabilization

The pervasive utilization of petroleum-based cationic surfactants has led to significant pollution of aquatic environments, emphasizing the urgent need to explore sustainable alternatives. Aligned with this concern, this study introduces a novel bio-friendly natural cellulose nanofiber system for surfactant-free emulsion stabilization. To achieve this, glycidyltrimethylammonium chloride was incorporated into bacterial cellulose fibers to enable cationization and nanofibrillation. The newly developed ammonium-functionalized bacterial cellulose (ABC) exhibited electrostatic adsorption at the oil/water interface, facilitated by the spontaneous negative charge present on interfacial hydrocarbon oil drops. Through theoretical considerations that employed the Young's equation and contact angle measurements of three-phase sessile droplets, we observed that current ABC-based solid interface displayed approximately 10-fold higher adsorption energy compared to hydroxylated solid interface. This unique interfacial adsorption behavior of ABC not only reduced interfacial tension but also promoted the formation of a robust interfacial membrane. Importantly, these interfacial properties are corresponding to the high aspect ratio of nanofibers, which provide a larger surface area and, consequently, an increased adsorption energy. Furthermore, our rheological studies revealed that the presence of a fibril network formed by the adsobed ABC nanofibers facilitated the association of droplets and gelling of the continuous phase, thereby improving the durability of emulsion drops against applied shear stress. Collectively, our findings highlight that our innovative surfactant-free emulsion system would pave the way for development of environment-friendly complex fluids.