Emulsions Stabilized with an Electrostatic Complex of Quaternized Cellulose Nanofiber and Octanoyl Gelatin and the Effect of pH Value on Their Stability

Oil/water (O/W) emulsions were prepared using the complex of quarternized cellulose nanofiber (QCNF) and octanoyl gelatin (OC−Gel) as an emulsifier, and the effect of pH value on their stability was investigated. OC−Gel was prepared through a condensation reaction, confirmed by ¹H Nuclear Magnetic Resonance (NMR) and Fourier Transform Infrared (FT−IR) spectroscopy. It reduced air/water interfacial tension more effectively than unmodified gelatin. The complexation degree of OC−Gel and QCNF, measured by optical density, showed its maximum at a QCNF/OC−Gel mass ratio of 1/8 when the pH value of the medium was 7.4, and it increased in a saturated manner with increasing pH value. The signals of QCNF were found in the FT−IR and X−ray diffraction spectra of the complex, suggesting that the positively charged CNF were included in the OC−Gel−based complex. The complex formed a rough surface with smooth debris because the surface roughness of the complex aggregation reflected that of both QCNF and OC−Gel aggregation. QCNF could stabilize oil droplets to form a Pickering O/W emulsion. The complex of QCNF/OC−Gel was also a good emulsifier. QCNF and the complex were as potent as OC−Gel in emulsifying mineral oil in water. Most of the droplets fell within 5–25 µm, regardless of what the emulsifier was. The emulsion stabilized with OC−Gel increased in its oil droplet size more than two times in 20 days at all the pH vales tested (pH 3, 5, 7.4, 9), whereas the emulsion stabilized with QCNF remained almost constant in size during the same period regardless of the pH values. The droplet size of emulsion stabilized with the QCNF/OC−Gel complex did not change appreciably when the pH value was 5, 7.4, and 9. The complex seemed to act as a capsule wall and prevent the coalescence of the droplets. However, it increased dramatically due to the coalescence at pH 3, possibly because the complex could be dissolved under a strong acidic condition.