Effect of the headgroup structure on the aggregation behavior and stability of self-assemblies of sodium N-[4-(n-dodecyloxy)benzoyl]-l-aminoacidates in water

Three amino acid-derived chiral surfactants, sodium N-4-(n-dodecyloxy)benzoyl]-L-leucinate (SDBL), sodium N-4-(n-dodecyloxy)benzoyl]-L-isoleucinate (SDBIL), and sodium N-4-(n-dodecyloxy)benzoyl]-L-threoninate (SDBT), were synthesized, and their aggregation behavior was studied in aqueous solution. Surface tension, fluorescence probe, dynamic light scattering, nuclear magnetic resonance (NMR), gel permeation chromatography, circular dichroism, and optical as well as transmission electron microscopic techniques were utilized to characterize the self-assemblies formed by the amphiphiles. Results of these studies reveal that the surfactants have a very low critical aggregation concentration (cac) and they form spherical vesicles spontaneously in dilute aqueous solution. The mean diameters of the vesicles were measured to be in the range of 130-190 nm. 1H NMR spectra indicated hydrogen bonding between the amide groups near the surfactant headgroup, which is one of the driving forces for vesicle formation. The vesicle formation is more favored at a pH of about 7.0. The amphiphiles also form chiral helical aggregates at relatively higher concentrations as indicated by circular dichroism spectra. The stability of the vesicles was also evaluated with respect to the surfactant concentration, pH, temperature, and aging. The vesicles have a tendency to transform into elongated vesicles (closed tubules) or rodlike micelles with an increase of the surfactant concentration and/or pH. On the basis of the results obtained from different studies, phase diagrams for all three water/amphiphile systems have been constructed. The studies have further shown that the stereogenic center at the amino acid side chain has a significant effect on the aggregation properties of the amphiphiles and on the stability of the self-assemblies.