Micelle formation by the amino acid‐based surfactant undecylenyl l‐leucine was investigated as a function of solution pH with NMR, dynamic light scattering, and fluorescence spectroscopy. NMR and dynamic light scattering showed that 50 mM undecylenyl l‐leucine and 50 mM NaHCO3 solutions contained micelles approximately 20 Å in diameter and that micelle radius and the mole fraction of surfactant molecules associated with micelles changed very little with solution pH. The binding of the amino acids arginine and lysine to the anionic micelles was also investigated from pH 7.0 to 11.5. Below pH 9.0, the mole fraction of arginine cations bound to the micelles was approximately 0.4. Above pH 9.0, the arginine counterions became zwitterionic, and the mole fraction of bound arginine molecules decreased steadily to less than 0.1 at pH 11. When arginine dissociated from the micelles, their radii decreased from 14 to 10 Å. Similar behavior was observed with lysine; however, when lysine dissociated from the micelle surface, little change in micelle radius was observed. Two‐dimensional NMR experiments suggested that below pH 9.0, l‐arginine bound perpendicular to the micelle surface primarily though its side chain amine while l‐lysine bound parallel to the surface through both of its amine functional groups. Finally, the rate at which the amide protons on the surfactant headgoup exchanged with solvent was investigated with NMR spectroscopy. The exchange reaction was faster in solutions containing only surfactant monomers and slower when the surfactants were in micellar form and the headgoup amide protons were less exposed to solvent.

The binding of linear diamine counterions with different methylene chain lengths to the amino-acid-based surfactants undecanoic L-isoleucine (und-IL) and undecanoic L-norleucine (und-NL) was investigated with NMR spectroscopy. The counterions studied were 1,2-ethylenediamine, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, and 1,6-diaminohexane. These counterions were all linear diamines with varying spacer chain lengths between the two amine functional groups. The sodium counterion was studied as well. Results showed that when the length of the counterion methylene chain was increased, the surfactants’ critical micelle concentrations (CMC) decreased. This decrease was attributed to diamines with longer methylene chains binding to multiple surfactant monomers below the CMC and thus acting as templating agents for the formation of micelles. The entropic hydrophobic effect and differences in diamine counterion charge also contributed to the size of the micelles and the surfactants’ CMCs in the solution. NMR diffusion measurements showed that the micelles formed by both surfactants were largest when 1,4-diaminobutane counterions were present in the solution. This amine also had the largest mole fraction of micelle-bound counterions. Finally, the und-NL micelles were larger than the und-IL micelles when 1,4-diaminobutane counterions were bound to the micelle surface. A model was proposed in which this surfactant formed non-spherical aggregates with both the surfactant molecules’ hydrocarbon chains and n-butyl amino acid side chains pointing toward the micelle core. The und-IL micelles, in contrast, were smaller and likely spherically shaped.