Molecular Dynamics Simulation Study of Organically Modified Nanoparticle Dispersibility in Nonpolar Organic Solvents

High Resolution Image Download MS PowerPoint Slide Nanoparticles made up of metal or metal oxides have garnered much attention owing to their unique optical, electronic, and catalytic properties. To maintain these nanoparticles in a dispersed state within a solvent, their surfaces are often modified with organic compounds. However, the prediction of the dispersibility of organically modified nanoparticles (OMNPs) based on solubility parameters may be inaccurate which is often one of the barriers to mass production of OMNPs. Previous experiments have demonstrated that OMNPs disperse well in hexane but show reduced dispersibility in decane, despite the similar solubility parameters of these solvents. However, the underlying mechanism remains unclear. In this study, we perform molecular dynamics simulations of OMNPs in both hexane and decane to investigate the cause of their reduced dispersibility in decane, which cannot be explained by the conventional theory of solubility parameters. In simulations involving one OMNP, we find hexane molecules orient randomly relative to the modifiers, whereas decane molecules align parallel to the modifiers. Moreover, a remarkable structure that could facilitate the aggregation of OMNPs is found in simulations involving two OMNPs in decane. When two OMNPs are close to each other, the decane molecules penetrate the modifier shells of both OMNPs in an orientation parallel to the modifiers and form bridge structures that bind the two OMNPs. Energy analysis indicates that these bridge structures bind the OMNPs more strongly to decane than to hexane. Consequently, we conclude that the longer decane molecules bridge the two OMNPs and promote their aggregation because of their preference for forming ordered structures with the ligands. Finally, we propose that not only the solubility parameter but also the nanoscale OMNP/solvent interface structure, which depends on the solvent molecule shape strongly, affects the dispersibility of OMNPs.