This work is focused on the use of supercritical carbon dioxide (ScCO2) as an environmentally friend process to improve the surface functionalization of silica nanoparticles with amino- and perfluoro-silanes. Here, we combine the use of a phosphonium ionic liquid which plays a role of an activator of silanol groups in ScCO2 to increase the grafting density of silane agents on silica. Then, diglycidyl ether of bisphenol A (DGEBA)-based epoxy nanocomposites filled with different amount of new functionalized silicas (1, 2.5 and 3.5 phr) are prepared and characterized by dynamic-mechanical analysis, transmission electron microscopy and small-angle X-ray scattering. Finally, the treated silica/epoxy systems exhibit better mechanical properties (+30%) when compared to neat epoxy. (C) 2013 Elsevier B.V. All rights reserved.