Pickering emulsions synergistic-stabilized by amphoteric lignin and SiO2 nanoparticles: Stability and pH-responsive mechanism

The pH-responsive O/W Pickering emulsions were stabilized by amphoteric lignin (AML) absorbed with SiO2 nanoparticles via electrostatic interaction. AML was gained by quaternary ammonium modification of alkali lignin, and its structure was characterized by H-1 NMR. Fourier transform infrared spectroscopies and Zeta potential measurements were performed to investigate the interaction between AML and SiO2. The formed AML@ SiO2 compounds were utilized to prepare Pickering emulsions. The influences of AML concentration and pH on emulsions were studied by rheological measurements and Turbiscan Lab Expert stability index system. The pH-responsive mechanism of Pickering emulsions was investigated by Zeta potential, quartz crystal microbalance with dissipation, atomic force microscopy and three-phase contact angle. The results indicated that at the pH range of 3-4, AML was absorbed on SiO2 to realize the in-situ hydrophobization of SiO2, forming stable Pickering emulsions. When pH was above 4, only a little amount of AML was absorbed on SiO2 due to the strong electrostatic repulsion between them, resulting in unstable emulsions. The emulsions could be cycled at least 5 times between emulsification and demulsification as well as exhibited excellent salt tolerance.