Coating thickness and coverage effects on the forces between silica nanoparticles in water

TL;DR

This study uses molecular dynamics simulations to explore how coating thickness and coverage influence the shape and effective forces between silica nanoparticles in water, revealing chain length-dependent anisotropy and force behavior.

Contribution

It provides new insights into how polyethylene oxide chain length and coverage affect nanoparticle coating shape and interparticle forces in aqueous environments.

Findings

Short chains produce radially symmetric coatings.

Longer chains lead to highly anisotropic coatings.

For short, isotropic coatings, forces are purely repulsive and follow a specific power-law form.

Abstract

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.