Switchable Pickering foams stabilized by mesoporous nanosilica hydrophobized in situ with a Gemini surfactant

Abstract

• MSNPs and a pH-insert Gemini surfactant were used as foam stabilizers. • Stable base/acid responsive Pickering foams were prepared. • Pickering foams defoamed promptly upon adding NaOH and refoamd after adding HCl. • Defoaming-foaming progress was repeated 10 times, with increase foam volume. • The Pickering foam system showed high salt tolerance. Switchable Pickering foams which can be transformed between stable and unstable states under an external stimulus are widely applied in many fields and of great interest. Here, base/acid switchable Pickering foams, which were stabilized by mesoporous silica nanoparticles (MSNPs) in combination with a pH-insensitive cationic Gemini surfactant (GS), were prepared. These Pickering foams showed favorable stability with a half-life time t 1/2 > 1000 min, at GS concentrations of 0.1–0.6 mmol·L −1 . After adding a tiny amount of NaOH solutions, the Pickering foams rapidly disappeared within 10 s without shaking, showing an instant response to the added NaOH. Stable foams can be re-formed by further adding an equimolar amount of HCl solution to NaOH by hand shaking method. The foaming-defoaming progress can be cycled at least 10 times, with an increase of foam volume. The Pickering foams also showed high salt resistance. The foam volume was almost the same with that of the initial foams, at NaCl concentration of 40 mmol·L −1 . The unusual switching behavior and high salt resistant property were attributed to the presence of MSNPs with rich silica hydroxyl groups distributed at the inner and outer surfaces. The silica hydroxyl group and its anionic form can be remotely switched by alternately adding NaOH and HCl, leading to the great discrepancy in the absorption ability of MSNPs and thus the switching behavior of Pickering foams. The present work provided a new pathway of preparing responsive Pickering foams and was inspirable to the system design of smart materials.