Supercritical carbon dioxide was used as a solvent to modify a nanofiller using small amounts of dispersing additives and a silane coupling agent to obtain modified, non-agglomerated dry nanoparticles. Two types of silica (mono-dispersed, non-agglomerated silica and agglomerated silica) were used in this research, with the silica particles modified by three kinds of dispersing additives and two kinds of silane coupling agent. The particle dispersion and size of the obtained silica particles were evaluated by small angle X-ray scattering (SAXS). An extensive physicochemical evaluation of the obtained modified silica particles was also conducted using Fourier transform infrared (FTIR) analysis. It was found that the silica particles possess mono-dispersed and non-agglomerated properties, and that the surface silanol group of the silica particles reacts with hydrolysable silane alkoxy groups.