thermal Diffusion Coefficient or Thermodiffusion

Hi all

I am trying to solve a problem of a binary system for the moment using Comsol version 4.3 to study thermal diffusion or thermodiffusion. It consist of solving beside the fluid flow and heat transfer equations, the mass transfer equation and themass flux contain usually two terms one for the molecular diffusion multiplied by the concentration gradient and the second term is thermal diffusion coefficient multiplied by the temperature gradient. Surpsingly in comsol there is a third term relation to mass fraction gradient!I have no clue where this term come from.My question did anyone know what is this term and whether someone has an input file for me to read to see how this part is handled.

Thank you for replying

Regards