Monolayers of Octadecyl Hydroxypropyl Sulfobetaine Surfactant: Performance Characterization and Multifactor...

The Langmuir monolayer properties of betaine-based surfactant octadecyl hydroxypropyl sulfobetaine (HSB18), as well as the effects of sodium chloride (NaCl) concentration, pH, and ion valence in the subphase on its monolayer properties were investigated. The results indicated that HSB18 formed stable Langmuir monolayer. With the increase of the surface pressure, the Langmuir monolayer underwent the transition from gaseous to liquid expanding transition and liquid expanding state. The Langmuir monolayer of HSB18 exhibited the greatest expansion when the NaCl concentration was 0.8 M, while at 0.6 M NaCl, it demonstrated the strongest adaptability to external conditions, along with optimal resistance to deformation and compressibility. With the increase of pH, the lift-off area, limiting area and collapse area increased first and then decreased, while the collapse pressure decreased first and then gradually stabilized. With the increase of the valence of metal ions, the monolayer expanded gradually, and both the limiting area and collapse pressure increased obviously. The lift-off area of the HSB18 monolayer in the CaCl₂ system was the smallest, only was 120 Ų, indicating stronger intermolecular attractive forces and the most closely packed molecular arrangement. Therefore, HSB18 molecules possess the ability to form stable and adjustable Langmuir monolayers, and properties can be optimized via adjusting environmental factors, which provides important insights for the construction of Langmuir monolayers in practical applications.