Heat capacity-driven inverse Soret effect of colloidal nanoparticles

We discuss the thermophoresis of colloidal suspensions and derive their Soret coefficient in terms of known thermodynamic functions. For solid nanoparticles, S-T is determined by the ratio of the heat capacities of the solute and the solvent, and is in general much larger than the ideal gas value S-T = 1/T. For aqueous suspensions of most solid materials, we obtain a strong inverse Soret effect (S-T < 0), whereas a positive sign occurs for common organic solvents such as toluene and cyclohexane. These findings provide an explanation for recent experiments on ferro fluids.