Amphiphilic dendrimers as building blocks in supramolecular assemblies

The self-assembly of amphiphilic dendrimers based on poly(propylene imine) dendrimers of five different generations with up to 64 end groups modified with long hydrophobic chains has been studied. At the air-water interface stable monolayers form in which the dendritic surfactants presumably adopt a cylindrical shape; all the chains are aligned perpendicular to the water surface, and the dendritic poly(propylene imine) core faces the aqueous phase. Electron microscopy and dynamic light-scattering measurements performed on aqueous solutions of the amphiphiles at pH = 1 showed the formation of small spherical aggregates with diameters varying between 20 and 200 nm. X-ray diffraction of cast films of these aggregates revealed bilayer thicknesses of 48-54 Angstrom. A phase transition was detected by measuring fluorescence anisotropy. The theoretical volumes of the dendritic amphiphiles were in good agreement with those calculated from the monolayer experiments and X-ray diffraction data. Hence, the amphiphilic dendrimers within the aggregates in solution have the same highly asymmetric conformation as that proposed at the air-water interface. Calculations showed that the shape of the dendritic poly(propylene imine) core in the aggregates is distorted and that the axial ratio (r(b):r(a)) ranges from 1:2.5 for the first generation to approximately 1:8 for the three highest generation of dendrimer. This behavior illustrates the high flexibility of the poly(propylene imine) dendrimers.