Co-hydrogelation of Dendritic Surfactant and Amino Acids in Their Common Naturally-occurring Forms: A Study of Morphology and Mechanisms

Abstract

Sixteen different amino acids (AAs) presenting different side-chains as neutral nonpolar (Gly, Ala, Val, Leu, Ile, and Phe), neutral polar (Ser, Thr, Cys, Asn and Gln), acidic (Asp and Glu) and basic (Lys, Arg and His) could induce co-hydrogelation in 1% solutions of the dendritic surfactant, 3,3′-(octadecylazanediyl) dipropionic acid, at various concentration (1.6% ≤ C_co-hgel ≤ 20%). The amino acids Asp and Glu showed a strong favourable intercation with the surfactant, and, together with Leu and Gln, were identified as low-molecular-weight co-hydrogelators (C_co-hgel < 2%). Interpretation of XRD data indicated that all of the hydrogels exhibited lamellar structures with an occasional presence of AA crystallites. By using scanning electron microscopy, we found that the morphology of hydrogels comprises various particles (cloddy, platy, lath-/needle-like and amyloid-like fibrillar ones) and 3D structures (sheet-like and tubular ones). Furthermore, two mechanisms were identified for AA co-hydrogelation: (1) the interpenetration of AA crystallites and AA/surfactant lamellae, and (2) the formation of 3D solvent-trapping, mostly percolating structures.