PH-Responsive Pickering foams stabilized by silica nanoparticles in combination with trace amount of dodecyl dimethyl carboxyl betaine

Abstract

pH-Responsive Pickering foams were prepared by using negatively charged silica nanoparticles in combination with trace amount of dodecyl dimethyl carboxyl betaine as stabilizer. The foams are stable at pH ≤ 4.3 but unstable at pH ≥ 10 and can then be cycled between stable and unstable for many times by alternating the pH of the aqueous phase. It is shown that in acidic aqueous media the carboxyl betaine molecules are turned to cationic form which can adsorb at surfaces of the negatively charged silica nanoparticles with head-on configuration via electrostatic interaction, rendering particles surface activity by in situ hydrophobization, and the particles can then adsorb at air/water interface to stabilize the Pickering foams; whereas in neutral and alkaline aqueous media, the carboxyl betaine molecules are in zwitterionic form, which tend to desorb from particle surface due to weakening or removing of electrostatic interaction, triggering de-hydrophobization of the particles and defoaming of the systems. This principle makes it possible to construct stimuli-responsive aqueous foaming systems using commercial inorganic nanoparticles in combination with trace amount of conventional surfactants avoiding synthesis or preparation of complicated stimuli-responsive colloid particles.