Deviation from the Unimolecular Micelle Paradigm of PAMAM Dendrimers Induced by Strong Interligand Interactions

PAMAM (polyamidoamine) dendrimers have been recently exploited as efficient and biocompatible unimolecular micelles for oil spill remediation utilizing their robust encapsulation capability. However, experimental evidence suggested that contrasting dispersion mechanisms of PAMAM exist toward different types of hydrocarbon ligands, including linear and polyaromatic oil molecules. Specifically, the dispersion of linear hydrocarbons by PAMAM was found to violate the unimolecular micelle convention by forming molecular complexes orders of magnitude larger than a single PAMAM. It is, therefore, essential to re-examine the dispersion mechanisms of PAMAM toward different types of ligands in order to facilitate dendrimer applications in environmental remediation, catalysis, and nanomedicine. Here, we applied atomistic discrete molecular dynamics simulations to study generation-four (G4) PAMAM dendrimers dispersing hexadecane (C16) and phenanthrene (PN), two representative linear and polyaromatic hydrocarbons in crude oil. We observed a strong cooperativity in the binding of both C16 and PN to PAMAM dendrimers, especially with C16. Simulations of multiple PAMAM molecules interacting with many hydrocarbons illustrated that phenanthrene bound to individual dendrimers to render a unimolecular micelle, while multiple C16 molecules formed a large droplet enclosed and stabilized by multiple PAMAM dendrimers to assemble into a multimolecular micelle. Our analysis revealed that such deviation of the PAMAM ligand architecture from the conventional unimolecular micelle paradigm arose from strong interligand interactions between linear hydrocarbons.