pH-responsive and thermoreversible hydrogels of N-(2-hydroxyalkyl)-L-valine amphiphiles.

Gelation behavior of a series of low-molecular-weight-hydrogelators, N-(2-hydroxydodecyl)-L-amino acid, was studied in aqueous phosphate buffer (pH 12). The effect of head-group structure and chirality and hydrocarbon chain length on the gelation efficiency was investigated. Only N-(2-hydroxyalkyl)-L-valine (L-C(n)HVal, n = 10, 12, 14, and 16) derivatives were found to form gel in aqueous buffer at pH 12. The increase of the chain length of the hydrocarbon tail enhances the ability to gelate buffered water up to C14 chain length. The L-C16HVal amphiphile was found to have gelation ability lower than L-C14HVal. The gelation number, mechanical strength, thermal stability, and morphology of the supramolecular aggregates were studied. The effect of salt concentrations on the gelation was investigated. Addition of NaCl increased gelation number but decreased melting temperature of the hydrogels slightly. The morphology of the hydrogels was characterized by electron microscopy and small-angle X-ray diffraction techniques. Rheology measurements were performed to examine the mechanical strength of the hydrogels. Both hydrogen-bonding and hydrophobic interactions were shown to be the driving forces for supramolecular aggregate formation.