The energetics of formation and hydration of functionalized silica nanoparticles were studied using a combination of first-principles calculations based on density functional theory with van der Waals dispersion correction and molecular dynamics. The energetics and effects of group density were evaluated in both; hydrophilic (ethylene-glycol) and hydrophobic (sulfonic) organosilane functional groups, and the optimum group density were obtained in vacuum and aqueous environment. The functional group bounded in a geminal silanol site was found to be more stable than silanol one, by similar to 1.30 and similar to 1.32 eV for hydrophilic and hydrophobic groups, respectively. In vacuum, an optimum graft density of 4.2 and 4.5 groups/nm(2) was obtained for hydrophobic and hydrophilic coverage, based on molecular dynamics calculations. Interestingly, a double well energy profile is obtained when functionalized nanoparticles are placed within aqueous media, and those minima for hydrophilic groups appear at lower coverage compared to hydrophobic one. The double energy minima is explained by the H2O molecules arrangement as function of the group density on nanoparticles surface. At low coverage, H2O molecules surround the groups while at high coverage, the functional groups shield the molecules to penetrate within the groups and the size effect of the functional group studied here was found to be negligible on the stability. (C) 2013 Elsevier B.V. All rights reserved.