Mesomorphic phases of the electrostatic complexes of amphiphilic surfactants with low-generation dendrimers

Through the microphase separation between the polar and nonpolar components, the complexation of macromolecule with an amphiphilic surfactant has the potential to construct a broad spectrum of mesomorphic phase with long-range order. This study explores the self-assembly behavior of the electrostatic complexes formed between poly(amidoamine) (PAMAM) dendrimers of generation two (G2) and generation three (G3) with two surfactants: dodecylbenzenesulfonic acid (DBSA) and sodium dodecyl sulfate (SDS). Long-range ordered lamellar structures formed in the complexes of DBSA with both PAMAM G2 and G3 dendrimers via acid-base interaction. A detailed analysis of the lamellar structure revealed that the thickness of the polar layer containing the dendrimer was virtually independent of the dendrimer generation number. This phenomenon even extended to the complexes of DBSA with PAMAM G4 and G6 dendrimers, indicating that the dendrimer molecules were in general highly compressed along the lamellar interface due to the electrostatic attraction. The complexations of protonated dendrimer systems with SDS driven by the entropic gain from counterion release yielded a higher-curvature structure, where the SDS formed cylindrical micelles packed in a 2D hexagonal lattice, as the original spherical SDS micelles merged into cylinders to enhance the charge matching with the dendrimers. The subtle variation of the relative intensities of the diffraction peaks in the small angle X-ray scattering pattern suggested that, as the binding density of the surfactant to the dendrimer increased, the cross section of the cylindrical micelle progressively deformed from nearly circular to elliptical shape to facilitate the charge matching under the hexagonal packing symmetry.