Physicochemical Characterization and Tube-like Structure Formation of a Novel Amino Acid-Based Zwitterionic Amphiphile<i>N</i>-(2-Hydroxydodecyl)-<scp>l</scp>-valine in Water

Abstract

Surface activity and aggregation behavior of an amino acid-based zwitterionic amphiphile N-(2-hydroxydodecyl)-L-valine were studied in aqueous solutions (pH 13). The self-assembly formation was investigated by use of a number of techniques including surface tension, conductivity, viscosity, fluorescence spectroscopy, dynamic light scattering, and transmission electron microscopy. The amphiphile exhibits two breaks in the surface tension vs concentration plot indicating stepwise aggregate formation and thus results in two values of critical aggregation concentration. The amphiphile was found to be very surface active compared to fatty acid soaps. The average hydrodynamic diameter and size distribution of the aggregates were obtained from DLS measurements. Conductivity measurements suggested formation of vesicles or closed tubules. TEM pictures revealed the existence of spherical vesicles, separated tubules, and tubules with multiple Y-type junctions in going from dilute to moderately concentrated solution. However, in concentrated solution, the junctions break to form separate tubular structures which upon further increase of concentration are converted to rod-like micelles. The mechanism of branched tubule formation is discussed in light of the experimental observations.