Supramolecular Gels Formed by Amphiphilic Low-Molecular-Weight Gelators of Nα,Nε-Diacyl-L-Lysine Derivatives



Simple L-lysine derivatives, Nα-hexanoyl-Nε-lauroyl-L-lysine (1), its alkali metal salts (24), and two-component compounds that consist of 1 with 2 to 4, were synthesized and their hydrogelation and organogelation properties were studied. Addition of hydrochloric acid to an aqueous solution of the alkali metal salt at room temperature produced a translucent hydrogel. This hydrogelation occurred as a result of a change in nanostructure from micelle-like aggregates to nanofibers, which was induced by partial protonation of the carboxylate to form a carboxylic acid. On the other hand, two-component low-molecular-weight gelators exhibited amphiphilic gelation behavior and functioned as not only hydrogelators, but also as organogelators. FTIR studies revealed that lateral ionic interactions between the carboxylate, alkali metal cation, carboxylic acid, and protons, in addition to hydrogen-bonding and van der Waals interactions play a very important role in hydrogelation. Furthermore, it was found that the water-insoluble carboxylic acid compound underwent a precipitation–dissolution transition with a thermally reversible sol–gel transition in the two-component gelator systems.