Effects of Size, Shape, and Surface Geometry on the Nanoparticle Diffusion at/Across Water/Oil Interfaces Via Molecular Dynamics Simulations

Effects of Size, Shape, and Surface Geometry on the Nanoparticle Diffusion at/across Water/Oil Interfaces via Molecular Dynamics Simulations

ABSTRACT: We have employed molecular dynamic simulations to systematically investigate the effects of nanoparticles’ structural and chemical parameters on their self-assembly and diffusion behaviors at/across water-benzene interfaces. Four different nanoparticles were studied: modified hydrocarbon nanoparticles with mean diameter of approximately 1.2 nm (1.2HCP), modified hydrocarbon nanoparticles with mean diameter of about 0.6 nm (0.6HCP), singled-walled-carbon nanotube (SWCNT) and buckyball. We found that the diffusion coefficient of 0.6 HCP and 1.2 HCP were larger than that predicted by Stokes-Einstein (SE) equations. We attributed this deviation to two reasons: first, the sizes of the nanoparticles were extra small thus SE equation overestimated the effects of viscosity; second, the anisotropy of the simulated system may also lead the deviation of the diffusion coefficients and we hypothesize that the differentiation of the Potential of Mean Force may serve as a driving force in the z direction. We also found that nanoparticle with isotropic shape and uniform surface such as buckyballs tended to have smaller diffusion coefficients than that of nanoparticles with comparable dimensions but have directional shape and non-uniform surface e.g, SWCNT and 0.6HCP. We suggest that the “perfect” isotropic shape and uniform surface of buckyball would lead to a better defined solvation shell which made the “effective radius” of the buckyball larger than its own radius, thus resulted in a decrease in the diffusion coefficient.