Molecular-weight dependent thermal diffusion in dilute polymer solutions.

Thermal diffusion of high polymers in a continuous medium is independent of the molecular weight Mw. Accounting for the solvent molecular structure and the Brownian motion of the solute, we derive an additional contribution of opposite sign which is significant for short chains but vanishes as Mw-->infinity. Our findings explain the dependence on Mw observed recently for polystyrene, and its inverse Soret effect at very small Mw. Moreover, they bridge the gap between the macroscopic hydrodynamics description for large solutes and the enthalpy-of-transport picture for small molecules.