Micelle formation by the anionic amino acidbased surfactant undecyl L-phenylalaninate (und-Phe) was investigated as a function of pH in solutions containing either Na + , L-arginine, L-lysine, or L-ornithine counterions. In each mixture, the surfactant's critical micelle concentration (CMC) was the lowest at low pH and increased as solutions became more basic. Below pH 9, surfactant solutions containing L-arginine and L-lysine had lower CMC than the corresponding solutions with Na + counterions. Nuclear magnetic resonance (NMR) diffusometry and dynamic light scattering studies revealed that und-Phe micelles with Na + counterions had hydrodynamic radii of approximately 15 Å throughout the investigated pH range. Furthermore, L-arginine, L-lysine, and L-ornithine were found to bind most strongly to the micelles below pH 9 when the counterions were cationic. Above pH 9, the counterions became zwitterionic and dissociated from the micelle surface. In und-Phe/L-arginine solution, counterion dissociation was accompanied by a decrease in the hydrodynamic radius of the micelle. However, in experiments with L-lysine and L-ornithine, micelle radii remained the same at low pH when counterions were bound and at high pH when they were not. This result suggested that Larginine is attached perpendicular to the micelle surface through its guanidinium functional group with the remainder of the molecule extending into solution. Contrastingly, L-lysine and L-ornithine likely bind parallel to the micelle surface with their two amine functional groups interacting with different surfactant monomers. This model was consistent with the results from two-dimensional ROESY (rotating frame Overhauser enhancement spectroscopy) NMR experiments. Two-dimensional NMR also showed that in und-Phe micelles, the aromatic rings on the phenylalanine headgroups were rotated toward the hydrocarbon core of micelle. KeywordsAmino acid surfactant Á Micelle Á Counterion Á NMR J Surfact Deterg (2018) 21: 139-153. Abbreviations CE capillary electrophoresis CMC critical micelle concentration DLS dynamic light scattering HPLC high-performance liquid chromatography NMR nuclear magnetic resonance NOESY nuclear Overhauser effect spectroscopy ROESY rotating frame Overhauser enhancement spectroscopy TMS tetramethylsilane und-Leu undecylenyl L-leucine und-Phe L-undecyl phenylalaninateFig. 1 Structures of (a) L-undecyl phenylalaninate, (b) L-arginine, (c) L-lysine, and (d) L-ornithine. Atom labels are used to assign the twodimensional NMR spectra. Cationic forms of the amino acid counterions found in solution below pH 9.0 are shown 140 J Surfact Deterg J Surfact Deterg (2018) 21: 139-153 141 J Surfact Deterg J Surfact Deterg (2018) 21: 139-153