Molecular dynamics simulation of nanocolloidal amorphous silicaparticles: Part Ill

摘要

Explicit-solvent molecular dynamics simulations were applied to four pairs of amorphous silicananoparticles, two pairs having a diameter of 2.0 nm and two pairs with diameter 3.2 nm. The silicananoparticles were immersed in a background electrolyte consisting of Ca~(2+)andC1ions and waterand mean forces acting between the pair of silica nanoparticles were extracted at four differentbackground electrolyte concentrations. The pH was indirectly accounted for via the ratio of siliconto sodium used in the simulations. Dependence of the interparticle potential of mean force on thecenter-of-mass separation and the silicon to sodium ratio (5:1 and 20:1) is demonstrated. A Si: M.+ratio of 5:1 gave more repulsive interparticle potentials and lower numbers of internanoparticle or"bridging" hydrogen bonds. Conversely a Si: Ma~+ratio of 20:1 yielded more attractive potentials andhigher numbers of bridging hydrogen bonds. The nature of the interaction of the counterions withcharged silica surface sites (deprotonated silanols) was also investigated. The effect of the sodiumdouble layer on water ordering was observed. The number of water molecules trapped inside thenanoparticles was investigated, and at the highest background ionic concentrations were found toconsistently behave in accordance with there being an osmotic pressure. This study highlights theeffect of divalent (Ca~(2+)) background ions on the interparticle potentials compared with previouswork using- monovalent (Na~+) background ions. Mechanisms of coagulation and the stability ofsilica nanocolloids found from this work appear to be in agreement with findings from experimentsdescribed in the literature.