Supramolecular Assembly of Poly(propyleneimine) Dendrimers Driven By Simple Monovalent Counterions

The self-assembly of semiglobular, positively charged poly(propyleneimine) (PPI) dendrimers with small monovalent counterions (e.g., Cl(-)) in water/acetone mixtures was investigated. We showed that PPI dendrimers can assemble into hollow, spherical, single-layered blackberry-type structures mediated by the presence of monovalent counterions. The effects on the assembly of changing the solvent polarity and adjusting the pH were further investigated to confirm the presence of electrostatic interactions and hydrogen bonding as the driving forces. Results showed that PPI dendrimers form stable, hollow spheres in 5-20% v/v acetone/water and that the size of the spheres decreases monotonically as the solvent polarity and/or the charge on the dendrimers (i.e., lower solution pH) increases. This is the first example to show that small monovalent counterions can trigger attraction among PPI dendrimers (or broadly defined polyelectrolytes) that is strong enough to bring them together to form large, stable supramolecular assemblies, which indicates that these organic macroions have similar solution behavior to more-well-defined inorganic molecular macroions.