PH-responsive, salt-resistant, and highly stable foam based on a silicone-containing dynamic imine surfactant

Abstract

The preparation of a responsive foam, which can be destroyed on-demand and re-formed in a short time upon the imposition of an external environmental stimulus, using a dynamic imine surfactant (DIS) alone, remains a major challenge due to the presence of unreacted oil-soluble precursor. We report herein the design and synthesis of a novel silicone-containing DIS based on commercial 1,3-bis(3-aminopropyl)tetramethyldisiloxane, BAPTS, as an amino-containing precursor, and 3-(4-formylphenoxy)-N,N,N-trimethylpropan-1-aminium, as an aldehyde-containing precursor. 1H NMR spectrometry and FT-IR spectroscopy have been employed to confirm the formation of dynamic covalent bonds and to calculate the conversion of aldehyde to imine. Surface activity and foam properties have been systematically investigated. The results demonstrated that a traditional one-head-one-tail silicone-containing DIS was firstly formed at pH > 7.0 and then transformed into Bola-type DIS at pH > 9.0. Due to the low surface tension resulting from the introduction of silicone group and complete consumption of BAPTS at pH > 9.0, the as-prepared DIS surfactant showed excellent foam performance. Indeed, the foam could be reversibly switched on and off by alternately changing the pH, irrespective of the electrolyte concentration. The foaming performance was only minimally influenced by different electrolytes (NaCl, CaCl2, and MgSO4), but foam stability was sensitive to these electrolytes. Despite the foam becoming highly unstable at low electrolyte concentration (t1/2 < 0.5 min), its stability was significantly enhanced at high electrolyte concentration (t1/2 = 0.5–20 h) due to a microstructure transition from low viscosity spherical micelles to high viscosity rodlike micelles. Such a system may potentially be applied in mineral floatation or foam flood in oil production.