Silicone Oil/Water Pickering Emulsions with Surface-Hydrophobized Cellulose Nanofibers Formed via In Situ Homogenization with Amino-Silicone

The preparation of stable silicone oil/water emulsions is highly challenging because of the large difference in surface tension between the two liquids. Stable Pickering silicone oil/water emulsions were prepared by high-pressure homogenization of silicone oil/cellulose nanofibers/amino-silicone/water mixtures. TEMPO-oxidized cellulose nanofibers (TEMPO–CNFs) (TEMPO: 2,2,6,6-tetramethylpiperidine-1-oxyl radical) with protonated carboxy groups, TEMPO–CNF-COOH, nanodispersed in water and amine groups in amino-silicone molecules were ionically linked in situ during homogenization treatment of the mixtures to form partly surface-hydrophobized TEMPO–CNFs by neutralization on the silicone oil droplet surfaces. More than 99% of the TEMPO–CNFs present in the systems were adsorbed on the silicone oil droplet surfaces in combination with amino-silicone and existed at the silicone oil/water interfaces. As a result, stable o/w Pickering emulsions with small and homogeneous volume-mean diameters (0.3–0.6 μm) were obtained, maintaining the droplet surfaces to have highly negative charges (−50 mV) owing to the dissociated carboxy groups of TEMPO–CNFs in water. When the as-prepared silicone oil Pickering emulsion was coated on glass plates and dried, transparent, water/moisture-resistant, and strongly water-repellent films were obtained. Thus, the Pickering emulsions prepared in this study are possibly used as coating or painting materials to continuously keep windows, glassware, and mirrors in a clean state.