Charge-driven interfacial gelation of cellulose nanofibrils across the water/oil interface

Understanding the assembly of cellulose nanoparticles at water/oil (W/O) interfaces, underpins the use of these easily prepared biobased particles in formulated products. In this study we investigate the spontaneous adsorption of oxidised cellulose nanofibrils (OCNF) across planar W/O interfaces under conditions where bulk contributions are negligible, allowing exploration of
the interfacial phenomena alone. Spontaneous adsorption of the water-dispersible OCNF across the W/O interface was induced by employing an oppositely-charged oil-soluble surfactant. This approach relies on the use of two immiscible liquids as scaffolds, delivering two oppositely-charged species and directing a charge-driven complexation across the W/O interface. Interfacial
rheology experiments showed that, upon complexation of OCNF and the oppositely charged surfactant, interfacial gels were produced, and different interfacial properties obtained by modulating the ζ-potential of the OCNF (by electrolyte addition). Spontaneous OCNF adsorption at the W/O interface occurred for particles with ζ-potential more negative than -30 mV, resulting
in the formation of interfacial gels; whilst for particles with ζ-potential of ca. -30 mV, spontaneous adsorption occurred coupled with augmented interfibrillar interactions, yielding stronger and tougher interfacial gels. Contrarily, charge neutralisation of OCNF (ζ-potential values more positive than -30 mV) did not allow spontaneous adsorption of OCNF at the W/O interface. In the case of favourable OCNF adsorption, the interfacial gel was found to embed oil-rich droplets - a spontaneous emulsification process (Figure 1). The development of interfacial gels has implications for capsule formation and tuning in O/W emulsions – systems where interfacial rheology studies are extremely challenging, yet where tuneability is key to utility.