Molecular dynamics simulation of nanocolloidal amorphous silica particles: Part I

TL;DR

This study uses molecular dynamics simulations to analyze the interactions, solvation forces, and counter-ion behavior of amorphous silica nanoparticles in water, revealing how electrolyte concentration affects interparticle forces and water ordering.

Contribution

It provides detailed molecular insights into silica nanoparticle interactions, including counter-ion adsorption patterns and water structuring, which were not previously characterized at this level.

Findings

Interparticle forces depend on separation and electrolyte concentration.

Counter-ions form a 'patchy' double layer on silica surfaces.

Water ordering at nanoparticle surfaces is disrupted by silica surface 'hairiness'.

Abstract

Explicit molecular dynamics simulations were applied to a pair of amorphous silica nanoparticles in aqueous solution, of diameter 4.4 nm with four different background electrolyte concentrations, to extract the mean force acting between the pair of silica nanoparticles. Dependences of the interparticle forces with separation and the background electrolyte concentration were demonstrated. The nature of the interaction of the counter-ions with charged silica surface sites (deprotonated silanols) was investigated. A 'patchy' double layer of adsorbed sodium counter-ions. was observed. Dependences of the interparticle potential of mean force with separation and the background electrolyte concentration were demonstrated. Direct evidence of the solvation forces is presented in terms of changes of the water ordering at the surfaces of the isolated and double nanoparticles. The nature of the interaction of the counter-ions with charged silica surface sites (deprotonated silanols) was investigated in terms of quantifying the effects of the number of water molecules separately inside each of the pair of nanoparticles by defining an impermeability measure. A direct correlation was found between impermeability (related to the silica surface 'hairiness') and the disruption of water ordering. Differences in the impermeability between the two nanoparticles are attributed to differences in the calculated electric dipole moment.