Double‐Tailed Single‐Head Amino Acid‐Based Chiral Cationic Amphiphilic Molecules: Synthesis, Characterization, and Physicochemical Properties

Two double-tailed single-head amino acid-based chiral cationic amphiphilic molecules, l-aspartic acid hydrochloride dihexylester (L-ADHE, C-6-C-6) and l-aspartic acid hydrochloride dioctylester (L-ADOE, C-8-C-8), were synthesized. H-1 NMR, C-13 NMR, and Fourier transform infrared (FT-IR) were used to characterize the structures of the esters. Micellization, surface, and structural properties of L-ADHE and L-ADOE in aqueous solution were studied by means of a number of techniques including conductivity, pH, surface tension, density, zeta potential, and dynamic light scattering (DLS). The critical micelle concentration (CMC) of L-ADHE and L-ADOE were found to be very low compared with those of corresponding single-tailed amino acid-based amphiphilic molecules. The pK(a) values of L-ADHE and L-ADOE in aqueous solutions were determined via pH measurements, and it was observed that they had acidic properties giving low pK(a) values. The excellent adsorption behaviors at the air/water interface of L-ADHE and L-ADOE in aqueous medium were obtained, i.e. they efficiently lowered the surface tension of water at CMC down approximately to 24.3 and 23.4 mN m(-1), respectively. From the density measurements, the volume change due to the micellization process for L-ADOE was found to be quite high compared to L-ADHE, i.e. about three times more. This result and the data obtained from the DLS revealed that the increase in aggregated structures of L-ADOE may be due to vesicle formation. The dye solubilization study was performed by UV-vis absorption spectroscopy for L-ADHE. Besides, the CMC value of L-ADHE was also successfully determined without using any dye through UV-vis spectroscopy.