  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.
