Molecular dynamics simulation of nanocolloidal amorphous silica particles: Part II

TL;DR

This study uses molecular dynamics simulations to analyze the interactions, water structuring, and aggregation behavior of amorphous silica nanoparticles in electrolyte solutions, considering factors like electrolyte concentration and surface charge.

Contribution

It provides detailed insights into the inter-particle forces, ion interactions, and water ordering around silica nanoparticles, highlighting effects of electrolyte concentration and surface chemistry.

Findings

Inter-particle potential of mean force varies with electrolyte concentration and surface charge.

Water ordering around nanoparticles is influenced by sodium ion concentration.

Differences in water trapping are linked to electric dipole moments of nanoparticles.

Abstract

Explicit molecular dynamics simulations were applied to a pair of amorphous silica nanoparticles of diameter 3.2 nm immersed in a background electrolyte. Mean forces acting between the pair of silica nanoparticles were extracted at four different background electrolyte concentrations. Dependence of the inter-particle potential of mean force on the separation and the silicon to sodium ratio, as well as on the background electrolyte concentration, are demonstrated. The pH was indirectly accounted for via the ratio of silicon to sodium used in the simulations. The nature of the interaction of the counter-ions with charged silica surface sites (deprotonated silanols) was also investigated. The effect of the sodium double layer on the water ordering was investigated for three Si:Na+ ratios. The number of water molecules trapped inside the nanoparticles was investigated as the Si:Na+ ratio was varied. Differences in this number between the two nanoparticles in the simulations are attributed to differences in the calculated electric dipole moment. The implications of the form of the potentials for aggregation are also discussed.