(355i) Dendrimers for Oil Dispersion: Atomistic and Coarse-Grained Molecular Dynamics Investigations of Dendrimer–Hydrocarbon Interactions

Dendrimers are a class of branched macromolecules with generational growth that have been explored for applications in fields including water purification, drug delivery and light harvesting. Dendrimers offer desirable properties, such as precise control of the size, shape, and chemical functionality of the molecule. Applications such as oil dispersion and water purification extensively involve dendrimers interactions with aromatic and linear hydrocarbons. A detailed description of the interactions between dendrimers and hydrocarbons can assist researchers in selecting and tuning dendrimer properties for specific applications.

We used a combination of all-atom and coarse-grained (CG) molecular dynamics (MD) simulations to investigate the interactions between dendrimers and hydrocarbons. All of our studies focused on polyamidoamine (PAMAM) dendrimers. All-atom MD simulations with explicit water were used to investigate the mechanistic details of association of a model aromatic hydrocarbon guest molecule, naphthalene (NPH), with 3^rd – 6^thgeneration (G3–G6) dendrimers. We find that the mechanism of NPH association with the dendrimers is similar for G3–G6 dendrimers. For all dendrimer generations, we observe NPH–NPH interactions which appear to promote NPH association with the dendrimers. NPH associates at a distance from the dendrimer center that is accessible to the water solvent. However, we observe dendrimer branches form local pocket-like structures around NPH molecules associated with the dendrimers. These pockets provide a local environment around NPH that is protected from water. When we studied the effect of increasing the NPH concentration, we found no clear saturation point, but instead observed a transition to NPH molecules aggregating together on the dendrimers.

Motivated by our observations of NPH aggregation on the dendrimers and experiments that have shown large aggregates of dendrimers when dendrimers are in solution with linear hydrocarbons, we performed CG simulations of two dendrimers in solution with octane. The two dendrimers appeared to aggregate via the hydrophobic interactions between droplets of octane inside each of the dendrimers. We calculate the dendrimer–dendrimer potential of mean force (PMF) for two dendrimers in water both with and without octane molecules to show the extent to which the dendrimer aggregation is driven by the hydrophobic interactions of the octane molecules. Since the dendrimer aggregates observed in experiments are much larger than a few dendrimers, we performed simulations of the dendrimers at the octane–water interface.

Our results highlight the importance of considering both dendrimer–guest and guest–guest interactions when designing dendrimers for a variety of applications. We report results from atomistic and CG simulations of dendrimers, examining the mechanistic details of dendrimer–hydrocarbon interactions through the behavior of dendrimers at water–hydrocarbon interfaces.