A novel pH-responsive emulsion system stabilized by nanocelluloses modified with a rosin-based charge-reversible...

Nanocelluloses as a natural polysaccharide nanoparticle are widely used in preparing Pickering emulsions, but less involved in stimuli-responsive Pickering emulsions, due to complicated covalent modification. In this study, a novel pH-responsive emulsion system was prepared using nanocellulose hydrophobized in situ with a unique pH-responsive surfactant (MPAA) derived from rosin. The headgroup charge of MPAA could be reversibly switched between a cationic form (MPAAH) and an anionic form (MPAANa) via adjusting pH, both of which had excellent water solubility. In acidic condition (pH 4.0), the negatively charged nanocellulose could be hydrophobized in situ by absorbing the cationic MPAAH, and stable and high-viscosity Pickering emulsion gels were obtained. In alkaline condition (pH 10.0), the nanocellulose dispersed in aqueous phase and formed thick aqueous lamellae with negative charge, preventing the flocculation and coalescence of the negatively charged droplets, and oil-in-dispersion emulsions with smaller droplet size and low viscosity were formed. The corresponding properties, such as droplet size, stability, and viscosity could be easily controlled by changing pH. Importantly, MPAA and nanocellulose could be separated and reused multiple times. The work proposed an effective method to achieve multiple recycling and reuse of emulsifiers, showing good economic benefits and potential sustainable applications.