Probing the reversible self-assembly of pH-responsive switchable surfactants using SFG spectroscopy

Stimuli-responsive amphiphiles, such as monoalkyl diamines, have garnered significant interest for their capacity to modulate the properties of oil-water systems through structural changes driven by external stimuli. This work presents a molecular-level examination of N-dodecylpropane-1,3-diamine (DPDA) as a pH-sensitive switchable surfactant, probed at air-water and dodecane-water interfaces. The study encompasses an analysis of adsorption, self-assembly, molecular organization, and surface activities of water, dodecane, and DPDA at different pH levels. Surface tension measurements and density functional theory-based free energy calculations complemented spectral data obtained using SFG spectroscopy. At neutral pH, uncharged surfactants from the oil phase migrated to the oil-water interface, undergoing protonation upon interaction with neighboring water molecules to form a charged surfactant layer. By adjusting the water pH, we fine-tuned the charge of adsorbed surfactants, their self-assembly, interfacial molecular conformation, and interface stability, leveraging the unique structure of DPDA with diamine. The formation of dicationic surfactant at pH < 7 resulted in a lower intensity ratio of the terminal CH₃ symmetric stretch (~ 2880 cm^-1) and CH₂ symmetric stretch (~ 2850 cm^-1). This observation indicates the presence of defects in alkyl chain conformations due to variations in the conformation of the propyl chain, which separated the two cationic centers of DPDA under acidic conditions. These findings suggest potentially enhancing the utilization of DPDA and similar switchable surfactants as effective chemical demulsifiers for oil-water emulsion separation.