A multi-scale molecular dynamics study of the assembly of micron-size supraparticles from 30 nm alkyl-coated nanoparticles.

Atomistic and meso scale computer simulations of nanoparticle aggregation are combined to describe the self-assembly of supraparticles in bulk and on surfaces under vacuum conditions. At the nano scale, atomic resolution molecular dynamics simulations provide the structures of 30 nm-diameter nanoparticles bound to each other and to coated hydrophobic surfaces, through the physical contacting of their alkyl coats. This "molecular velcro" has been recently exploited in experiments to direct the aggregation of coated nanoparticles into stable assemblies on electronics platforms. Interaction potentials are extracted from the nano scale simulations and transferred to coarse grained Brownian dynamics simulations that describe multi-nanoparticle aggregation and surface deposition. The simulation results show that the large interaction area between 30 nm nanoparticles provides a strong driving force for assembly of strongly-welded, porous supraparticles under vacuum conditions. Interaction forces are significantly larger than those found in earlier simulations of the aggregation of smaller nanoparticles, indicating that supraparticle assembly using large 30 nm nanoparticles may be kinetically controlled. The porosity programmed into kinetic assembly may potentially benefit emerging applications of nanoparticle assemblies in medicine, in particular the development of nanostructured drug-eluting stent coatings. Future work will involve potential of mean force calculations in a variety of solvents to estimate the porosity obtainable for specific applications.