Direct Determination of Apparent Particle Density and Porosity in Silica Nanoparticles by Size-Resolved Mass-Volume...

Porous silica nanoparticles are widely used in catalysis, separations, and nanomaterial applications, yet their internal structure is typically inferred from ensemble measurements that assume structural uniformity across particle populations. Here, we determine the apparent particle density and porosity of silica nanoparticles on a fraction-resolved basis by independently measuring nanoparticle mass and geometric volume. Particle effective mass is obtained from centrifugal field-flow fractionation (CFFF) retention behavior, while particle volume is determined from transmission electron microscopy measurements of narrowly fractionated particle populations. The analysis was applied to silica nanoparticles spanning 100-700 nm, including mesoporous particles, nominally nonporous nanoparticles, and NIST-traceable size standards. Apparent densities range from approximately 1210 to 2150 kg m^-3, corresponding to porosities from near-dense silica to highly void-rich frameworks. Nominally monodisperse 160 and 200 nm samples resolve into distinct buoyant-mass-defined subpopulations that are internally monodisperse yet apparently structurally distinct. These results demonstrate that external particle diameter does not uniquely define internal architecture and that significant structural heterogeneity can exist within nominally monodisperse silica nanoparticle populations. The mass-volume approach provides a direct method for determining the apparent particle density and porosity of nanoparticles without relying on ensemble assumptions and enables fraction-resolved characterization of porous nanomaterials.