  Many low-density nanoparticles show a radial inner structure. This work
proposes a novel approach to contrast variation with SAXS based on the
constitution of a solvent density gradient in a glass capillary in order to
resolve this internal morphology. Scattering curves of a polymeric core-shell
colloid were recorded at different suspending medium contrasts at the
four-crystal monochromator beamline of PTB at the synchrotron radiation
facility BESSY II. The mean size and size distribution of the particles as well
as an insight into the colloid electron density composition were determined
using the position of the isoscattering points in the Fourier region of the
scattering curves and by examining the Guinier region in detail. These results
were corroborated with a model fit to the experimental data, which provided
complementary information about the inner electron density distribution of the
suspended nanoparticles.
