XPS study of oriented organic molecules. Vesicles of azobenzene-containing alkyl ammonium amphiphiles

Highly ordered molecular membranes have been playing important roles in both basic studies and applications. Among the molecules that form such molecular membranes are synthetic amphiphiles. We have observed the N 1s spectral change by aggregation of azobenzene-containing ammonium amphiphiles C₁₂AzoC_MN⁺Br^- (M = 2, 10) in water by utilizing the freezing method. By comparing the calculated intensity ratio of azo-group nitrogen to ammonium nitrogen with that experimentally observed, the N 1s spectral change has been successfully correlated with various aggregate's model structures of the amphiphiles, such as the lamella, tube, and globule. With the assumption that the N 1s photoelectron (about 850 eV in kinetic energy) mean free path is about 30 Å, the bilayer lamella model has proved to be most probable for both M = 2 and M = 10 compounds. This paper has successfully demonstrated that XPS (X-ray photoelectron spectroscopy) has the potential for use in analyzing molecular orientation in addition to conventional elemental and functional group analyses.