Complexation between amine- and hydroxyl-terminated PAMAM dendrimers and sodium dodecyl sulfate

Abstract

Isothermal titration calorimetry (ITC), dynamic light scattering (DLS), electrophoretic mobility and transmission electronic microscopy (TEM) were employed to study the supramolecular complexation of amine- (G3[EDA] PAMAM-NH 2) and hydroxyl-terminated (G3[EDA]PAMAM-OH) PAMAM dendrimers induced by the binding of anionic surfactant, sodium dodecyl sulfate (SDS). The binding was driven by the electrostatic interaction between protonated amines on the dendrimer and SDS at pH ≤ 2. The amine-terminated PAMAM dendrimer was able to host more SDS molecules because it possessed more binding sites compared to the hydroxyl-terminated system. The stoichiometry of binding suggested that SDS only binds to the amine groups on the outer rim of the dendrimers, i.e. the 3rd generation and the outmost layer of the 2nd generation, the 32 amine groups within the inner layer of the 2nd generation cannot electrostatically host SDS molecules owing to the strong steric hindrance. The binding induced the dendrimer/SDS supramolecular complexation via hydrophobic association between bound SDS molecules resulting in the formation of insoluble precipitates. The insoluble complex of amine-terminated PAMAM dendrimer persisted with further addition of SDS, whereas those of hydroxyl-terminated dendrimer resolubilized when SDS concentration exceeded 10 mM. With increasing SDS concentration, the dendrimer/SDS complex self-assembled into spherical aggregates that transformed into a highly ordered, hyper-branched conformation, where the branched structure appeared to be similar to the dendritic structure of individual PAMAM dendrimer. This demonstrates the possibility of generating dendritic supramolecular architecture by self-assembly .