  Integral equation of pure liquids, combined with a new "scaling
approximation" based on a corresponding states treatment of pair correlation
functions, is used to evaluate approximate structure factors for colloidal
fluids constituted of uncharged particles with polydispersity in size and
energy parameters. Both hard spheres and Lennard-Jones interactions are
considered. For polydisperse hard spheres, the scaling approximation is
compared to theories utilized by small angle scattering experimentalists
(decoupling approximation, local monodisperse approximation)and to the van der
Waals one-fluid theory. The results are tested against predictions from
analytical expressions, exact within the Percus-Yevick approximation. For
polydisperse Lennard-Jones particles, the scaling approximation combined with a
"modified hypernetted chain" integral equation, is tested against molecular
dynamics data generated for the present work. Despite ist simplicity, the
scaling approximation exhibits a satisfactory performance for both potentials
and represents a considerable improvement over the above mentioned theories.
Shortcomings of the proposed theory, its applicability to the analysis of
experimental scattering data, and its possible extensions to different
potentials are finally discussed.
