title: pH-Responsive Pickering Emulsions Stabilized by Silica Nanoparticles in Combination with a Conventional Zwitterionic Surfactant.
abstract: pH-responsive oil-in-water Pickering emulsions were prepared simply by using negatively charged silica nanoparticles in combination with a trace amount of a zwitterionic carboxyl betaine surfactant as stabilizer. Emulsions are stable to coalescence at pH ≤ 5 but phase separate completely at pH > 8.5. In acidic solution, the carboxyl betaine molecules become cationic, allowing them to adsorb on silica nanoparticles via electrostatic interactions, thus hydrophobizing and flocculating them and enhancing their surface activity. Upon increasing the pH, surfactant molecules are converted to zwitterionic form and significantly desorb from particles' surfaces, triggering dehydrophobization and coalescence of oil droplets within the emulsion. The pH-responsive emulsion can be cycled between stable and unstable many times upon alternating the pH of the aqueous phase. The average droplet size in restabilized emulsions at low pH, however, increases gradually after four cycles due to the accumulation of NaCl. Experimental evidence including adsorption isotherms, zeta potentials, microscopy, and three-phase contact angles is given to support the postulated mechanisms.
authors.affiliation: The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University , 1800 Lihu Road, Wuxi, Jiangsu 214122, P. R. China.
authors.forename: Kaihong
authors.initials: K
authors.lastname: Liu
authors.affiliation: The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University , 1800 Lihu Road, Wuxi, Jiangsu 214122, P. R. China.
authors.forename: Jianzhong
authors.identifier: 0000-0002-3984-6658
authors.initials: J
authors.lastname: Jiang
authors.affiliation: The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University , 1800 Lihu Road, Wuxi, Jiangsu 214122, P. R. China.
authors.forename: Zhenggang
authors.identifier: 0000-0002-0006-9636
authors.initials: Z
authors.lastname: Cui
authors.affiliation: School of Mathematics and Physical Sciences, University of Hull , Hull HU6 7RX, U.K.
authors.forename: Bernard P
authors.initials: BP
authors.lastname: Binks