  The structure and interactions of coated silica nanoparticles have been
studied in water using molecular dynamics simulations. For 5 nm diameter
amorphous silica nanoparticles we studied the effects of varying the chain
length and grafting density of polyethylene oxide (PEO) on the nanoparticle
coating's shape and on nanoparticle-nanoparticle effective forces. For short
ligands of length $n=6$ and $n=20$ repeat units, the coatings are radially
symmetric while for longer chains ($n=100$) the coatings are highly
anisotropic. This anisotropy appears to be governed primarily by chain length,
with coverage playing a secondary role. For the largest chain lengths
considered, the strongly anisotropic shape makes fitting to a simple radial
force model impossible. For shorter ligands, where the coatings are isotropic,
we found that the force between pairs of nanoparticles is purely repulsive and
can be fit to the form $(R/2r_\text{core}-1)^{-b}$ where $R$ is the separation
between the center of the nanoparticles, $r_\text{core}$ is the radius of the
silica core, and $b$ is measured to be between 2.3 and 4.1.
