Forces between functionalized silica nanoparticles in solution

TL;DR

This study uses atomistic molecular dynamics simulations to analyze how poly(ethylene oxide) coatings influence silica nanoparticle interactions and hydrodynamic forces in water, revealing predominantly repulsive forces without oscillations.

Contribution

It provides detailed insights into the effects of surfactant coatings on nanoparticle interactions and validates fluid theory predictions at the nanoscale.

Findings

Hydrodynamic drag predicted well by fluid theory.

Repulsive solvent-mediated and lubrication forces observed.

No force oscillations for coated nanoparticles.

Abstract

To prevent the flocculation and phase separation of nanoparticles in solution, nanoparticles are often functionalized with short chain surfactants. Here we present fully-atomistic molecular dynamics simulations which characterize how these functional coatings affect the interactions between nanoparticles and with the surrounding solvent. For 5 nm diameter silica nanoparticles coated with poly(ethylene oxide) (PEO) oligomers in water, we determined the hydrodynamic drag on two approaching nanoparticles moving through solvent and on a single nanoparticle as it approaches a planar surface. In most circumstances, acroscale fluid theory accurately predicts the drag on these nano-scale particles. Good agreement is seen with Brenner's analytical solutions for wall separations larger than the soft nanoparticle radius. For two approaching coated nanoparticles, the solvent-mediated (velocity-independent) and lubrication (velocity-dependent) forces are purely repulsive and do not exhibit force oscillations that are typical of uncoated rigid spheres.