pH-responsive foams based on a transition between a bola surfactant and a traditional surfactant

Abstract pH-responsive aqueous foams were prepared by using a totally rigid surfactant, sodium N-azobenzenemaleimidepimaric carboxylate (Na-MPA-AZO-Na), derived from rosin. When the pH of aqueous solution decreases from 12.35 to 9.31, a Bola surfactant converts into a traditional surfactant containing a hydrophilic group and a hydrophobic group (Na-MPA-AZO), which severely affects the foaming ability and foam stability. And such changes of molecular configuration were rare reported. When the pH of aqueous solution is 9.31, the Na-MPA-AZO molecule tends to be upright at the air/water interface, which results in a close surfactant arrangement. The original volume of foams at 2 mM is 72 mL, and its half-life is up to 2145 min, which are far larger than that of sodium dodecyl sulfate (SDS) at the same concentration as a reference. When the pH of aqueous solution is 12.35, the Bola surfactant Na-MPA-AZO-Na tends to lie at the air/water interface in a V configuration, which leads Na-MPA-AZO-Na easily to desorb from the interface. The original volume of foams at 2 mM is only 9.5 mL, and its half-life is only 2 min, the foams will completely disappear in 6 min. The stable and unstable foams can be reversed at least 3 times by alternating the pH between 9.31 and 12.35. Besides, because of the fixed positions of carbon atoms in the totally rigid skeleton, the pH-responsive foams show highly pH-responsive efficiency.