FORMATION OF FIBROUS MOLECULAR ASSEMBLIES BY AMINO-ACID SURFACTANTS IN WATER

Gel-like solutions are obtained at medium pH and low temperature for aqueous solutions of N-acyl-L-aspartic acids (C(n)Asp, n = 12-18). Electron microscopic observation reveals that helical, fibrous molecular assemblies are formed in the gel-like solutions. The rheological behavior reflects the formation of swarms or bundles of fibers. The hydrogen bonding of amide groups and the moderate degree of ionization of carboxyl groups are confirmed from the infrared absorption bands, consistent with the pH titration result. The possible models of helical, fibrous assemblies are the double stranded or the superhelical structure of helical bilayer strands or the twisted ribbon of a planar bilayer sheet. While N-dodecanoyl-L-glutamic acid (C-12Glu) molecules cannot be associated into fibrous assemblies, N-dodecanoyl-beta-alanine (C-12Ala) molecules form cylindrical fibers which have no helical structure.