Explicit solvent (water) molecular dynamics simulations were undertaken containing three pairs of amorphous silica nanoparticles, having diameters of 2.0 nm, 2.4 nm and 2.8 nm, respectively. Mean forces acting between the silica nanoparticles were calculated in a background electrolyte, i.e., NaCl at four different concentrations. Dependence of the inter-particle potential of mean force on the center of mass separation, silicon to sodium ratio (Si:Na+), background electrolyte concentration, number of hydrogen bonds directly linking pairs of silica nanoparticles and the density of charged surface sites, are calculated. The pH was indirectly accounted for via the ratio of silicon to sodium used in the simulations. The close relationship between the variation of the number of hydrogen bonds between the pairs of silica nanoparticles and the inter-particle potential of mean force indicates that the degree of inter-particle hydrogen bonding quantifies, for a given size of nanoparticle, the degree of nanoparticle 'stickiness'. Simulations also show that the number of hydrogen bonds between the charged surface (O-) sites and the surrounding water molecules increases with increase in charged sites, in agreement with the interaction behavior of silica nanoparticles Usually seen in experiments. (C) 2009 Elsevier Inc. All rights reserved.