  We study the Soret effect of charged polystyrene particles as a function of
temperature and electrolyte composition. As a main result we find that the
Soret coefficient is determined by charge effects, and that non-ionic
contributions are small. In view of the well-kown electric-double layer
interactions, our thermal field-flow fractionation data lead us to the
conclusion that the Soret effect originates to a large extent from
diffusiophoresis in the salt gradient and from the electrolyte Seebeck effect,
both of which show strong specific-ion effects. Moreover, we find that
thermophoresis of polystyrene beads is fundamentally different from proteins
and aqueous polymer solutions, which show a strong non-ionic contribution.
