  The depletion theory of nanoparticles immersed in a semidilute polymer
solution is reinterpreted in terms of depleted chains of polymer segments.
Limitations and extensions of mean-field theory are discussed. An explicit
expression for the interaction between two small spheres is derived. The
depletion free energy for a particle of general shape is given in terms of the
capacitance or effective Stokes radius. This affords a close to quantitative
explanation for the effect of polymer on protein precipitation.
