Hydration of Spherical PEO-Grafted Gold Nanoparticles: Curvature and Grafting Density Effect

Nanoparticle modification by water-soluble polymers, such as poly­(ethylene oxide) (PEO), relies on polymer hydration to ensure nanoparticle solubility, dispersion, and protection from undesirable interactions. The state of polymer hydration in grafted polymer layers is not easily assessible experimentally but can be ascertained from computer simulations. Using atomistic molecular dynamics simulations, we studied the equilibrium and dynamic properties of spherical PEO brushes grafted to gold nanoparticles of different radii (1, 2, and 3 nm). We obtained the volume fraction of PEO (Φ­(r)) as a function of radial distance r (counted from micelle core center) which is found to follow the Daoud–Cotton model, Φ­(r) ∼ r^–4/3, except for low grafting density when PEO adsorption onto the gold surface is observed, in agreement with experimental observations. With an increase of grafting density σ (up to 4.17 nm^–2), the PEO chains become more stretched and oriented along the radial direction leading to an increase of the polymer brush height and a decrease in PEO hydration. We analyzed hydrogen bonding between PEO and water and found that similarly to a planar PEO brush, it depends only on the local polymer (water) content and can be predicted based on PEO solution behavior. While at low grafting densities and small nanoparticle size PEO chains are fully hydrated (except for the immediate gold surface vicinity), with an increase of grafting density and/or decrease of nanoparticle radius of curvature the dehydrated or low hydration zone substantially expands up to a few nanometers, in agreement with recent experimental data. Furthermore, we found that hydrogen-bonded water remains rather stable in the inner region of the spherical brush, implying slow water exchange with the surrounding solution which in turn may affect chain dynamics and the susceptibility of the PEO brush to protein adsorption.