Langmuir monolayer behavior of an ion pair amphiphile with a double-tailed cationic surfactant

The spread or Langmuir monolayer behavior of an ion pair amphiphile (IPA), hexadecyltrimethylammonium-dodecylsulfate (HTMA–DS), with a double-tailed cationic surfactant, dihexadecyldimethylammonium bromide (DHDAB), at the air/water interface was analyzed with surface pressure–area isotherms, area relaxation curves, and Brewster angle microscope (BAM) images. The surface pressure–area isotherms showed that with increasing the DHDAB molar ratio, XDHDAB, spread monolayers of HTMA–DS with DHDAB became rigid. In addition, unreasonably small limiting areas per alkyl chain of the molecules in the monolayers were found, especially at XDHDAB = 0.5, implying the molecular loss from the monolayers at the interface. For spread HTMA–DS/DHDAB monolayers at the interface, a new IPA, DHDA–DS, was proposed to form through the displacement of HTMA+ from HTMA–DS by DHDA+, leaving HTMA+ dissociated. The formation of DHDA–DS and the desorption of dissociated HTMA+ upon the interface compression were supported by the results obtained from designed monolayer experiments with BAM observations, and were discussed by considering the hydrophilicity, packing efficiency, and headgroup charge characteristic of the species. Moreover, the area relaxation curves of spread HTMA–DS/DHDAB monolayers suggested that the formation of DHDA–DS was strongly related to the improved monolayer stability at the interface, which may have implications for the DHDAB-enhanced physical stability of catanionic vesicles composed of HTMA–DS.