Ribbon Phase of Dendrimer-Surfactant Complexes

The cylindrical micelles formed by amphiphilic surfactants in aqueous media generally organize in two-dimensional (2D) hexagonal lattice because of their largely circular cross section. Here we demonstrate that the packing habit of the cylindrical micelles of anionic sodium dodecyl sulfate (SDS) was perturbed by the electrostatic binding with a positively charged poly(amidoamine) (PAMAM) G4 dendrimer via cooperative deformation for charge matching, leading to the ribbon phase with nonhexagonal 2D lattices. The cylindrical micelles with elliptic cross section packed in the centered rectangular lattice with orthorhombic cmm symmetry over a broad range of surfactant-to-dendrimer amine group molar ratio. The packing structure transformed to the oblique lattice with monoclinic p2 symmetry at the stoichiometric composition. Detailed analysis of the geometry of the dendrimers accommodated in the interstitial tunnels surrounded by the SDS cylinders revealed that the dendrimer molecules were elongated into prolate ellipsoids along the long axis of the cylinder for enhancing charge matching. The degree of deformation was governed by the interplay among the electrostatic free energies and the elastic free energy associated with dendrimer deformation. The presence of monovalent salt was found to relax the dendrimer deformation due to electrostatic screening, while retaining the packing symmetry of the ribbon phase.