Molecular Insight into the Coalescence Mechanism of Droplets Stabilized by Amphiphilic Polymer-Grafted Nanoparticles

Recent progress in developing amphiphilic polymer-grafted nanoparticles (PGNPs) has led to extensive research on efficiently preparing emulsion droplets stabilized by these PGNPs. However, the molecular-level mechanism of droplet coalescence remains unclear. This study examines the relationship between the interfacial structures of PGNPs and the resistance force of emulsion droplets during coalescence, focusing on various grafting architectures and graft densities. A hybrid simulation approach combining dissipative particle dynamics and steered molecular dynamics is used to investigate this process. We observed various coalescence mechanisms at the molecular level based on the graft density. At low graft densities, the monolayered structure of the PGNP core between two oil droplets significantly contributes to the resistance against droplet-droplet coalescence due to the insufficient number of grafted polymers. Thus, diblock PGNPs with inner hydrophilic blocks are promising candidates for stabilizing emulsions, as they are pushed out from the droplet surface by the block immersed in the oil phase. Conversely, at higher graft density, this graft design causes the formation of the "sticky" point, promoting coalescence. On the other hand, diblock PGNPs with outer hydrophilic blocks exhibited a larger resistance force, accompanied by (multiple) layered structures between the two droplets during collision. More interestingly, the layer of Janus PGNPs had insufficient structural robustness, even at a high graft density, due to the penetration of the grafted homopolymers. These results improve the understanding of emulsion droplet coalescence and offer a theoretical guideline for designing optimal PGNPs for specific grafting architectures and graft densities.