Abnormal behaviors of 3-nonylphenoxy-2-hydroxyl-propyl-trimethyl-ammonium bromide surfactant at the interface of air–alkaline aqueous solution

The surface behaviors of 3-nonylphenoxy-2-hydroxyl-propyl-trimethyl-ammonium bromide (RTAB), a cationic surfactant, are examined by dynamic surface tensions (DSTs), π–A isotherms and molecular dynamics simulations as a function of pH. While the DST remains almost constant when the pH of the solutions changes from 7.0 to 3.0, which decreases remarkably in alkaline solutions. The results of π–A isotherms indicate that RTAB molecule has a much larger cross-sectional area at the air–water interface in alkaline solutions than that in acidic or neutral solutions. Molecular dynamics (MD) simulation studies revealed that the hydrocarbon chains of the surfactant molecules adsorbed at the air–water interface become conformationally more complex under alkaline solutions than in neutral or acidic conditions. These abnormal surface behaviors of RTAB in alkaline solutions are explained by assuming the binding of a hydroxide ion to the hydroxyl group in RTAB molecule.