SAXS studies on silica nanoparticle aggregation in a humid atmosphere

The influence of varying moisture conditions on the properties of amorphous silica nanoparticles was investigated. 10 nm silica particles in powder form were measured by small-angle X-ray scattering at different relative humidities ranging from 20 to 84 % at 45 °C. The data were analyzed using the generalized inverse Fourier transform method which enables separation of intra- and interparticle contribution to the scattering intensities. The former accounts for shape, size, homogeneity, and polydispersity of scattered nanoscaled objects, the latter describes the forces acting between the particles in the system. Formation of a water layer, of a thickness depending on the water content, covering the silica spheres has been detected. The pair correlation function derived from the structure factor shows a higher degree of structural order with increasing relative humidity. A model of ordering of silica nanoparticles is proposed.