Micelle formation by the anionic amino acid-based 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 l-arginine 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.
Keywords: Amino acid surfactant; Counterion; Micelle; NMR.