Rheology and thermal stability of pH-dependent hydrogels of N-acyl-L-carnosine amphiphiles: effect of the alkoxy tail length

A series of L-carnosine derived organogelators, N-(4-n-alkyloxybenzoyl)-L-carnosine (CnOBC, where n = 6-16 denotes the number of carbon atoms in the alkoxy tail), were synthesized to elucidate the effect of hydrophobic interaction on their gelation abilities in water and gel-sol melting temperatures. The variation of pH-responsiveness, morphology, and mechanical strength of the hydrogels with the change of the alkoxy tail length has been studied. The thermal stability and gelation ability were observed to increase linearly with the alkoxy tail length. Both thermal stability and mechanical strength were observed to be highest with the C16OBC gelator. Although thermal stability of the hydrogels of any gelator was highest at pH 2, the yield stress of the corresponding gel was highest at pH 7. The mechanical strengths of the hydrogels were observed to decrease sharply with the increase of pH above 7. All the gelators self-assembled to form ribbon-like aggregates consisting of interdigitated bilayers, which suggest that van der Waals interaction between hydrocarbon chains is the dominant driving force for aggregate formation.