Transition from nanotubes to micelles with increasing concentration in dilute aqueous solution of potassium N-acyl phenylalaninate

An aggregation behavior of potassium N-acyl phenylalaninate in dilute aqueous solution was investigated. It was found that this surfactant formed large aggregates at lower concentrations, which were then transformed to micelles at higher concentrations. Fluorescence intensity measurements using a probe were used to examine the effects of alkali concentration, acyl chain length, and solvent isotope on the aggregation behavior. The influence of the alkali concentration suggested that formation of an acid-soap dimer brought about the construction of the large particles at very dilute concentrations. Increases in both the acyl chain length and replacement of H2O with D2O resulted in stronger hydrophobic interactions; consequently, the large aggregate formation was enhanced. This aggregation behavior has not been observed when racemic modification of N-acyl phenylalaninate has taken place. By using cryo-transmission electron microscopy (TEM) with a Zernike differential contrast phase plate, it was found that the large aggregates were tubes with bilayer structures, which were then transformed into spherical micelles via threadlike micelles with increasing concentration due to a drastic increase in the concentration of ionic species in the aggregate.