Thermodiffusion of repulsive charged nanoparticles - the interplay between single-particle and thermoelectric contributions

Thermodiffusion of different ferrite nanoparticles (NPs), similar to 10 nm in diameter, is explored in tailor-made aqueous dispersions stabilized by electrostatic interparticle interactions. In the dispersions, electrosteric repulsion is the dominant force, which is tuned by an osmotic-stress technique, i. e. controlling of osmotic pressure Pi, pH and ionic strength. It is then possible to map Pi and the NPs' osmotic compressibility chi in the dispersion with a Carnahan-Starling formalism of effective hard spheres (larger than the NPs' core). The NPs are here dispersed with two different surface ionic species, either at pH similar to 2 or 7, leading to a surface charge, either positive or negative. Their Ludwig-Soret S-T coefficient together with their mass diffusion D-m coefficient are determined experimentally by forced Rayleigh scattering. All probed NPs display a thermophilic behavior (S-T < 0) regardless of the ionic species used to cover the surface. We determine the NPs' Eastman entropy of transfer and the Seebeck (thermoelectric) contribution to the measured Ludwig-Soret coefficient in these ionic dispersions. The NPs' Eastman entropy of transfer <(S)over cap>(NP) is interpreted through the electrostatic and hydration contributions of the ionic shell surrounding the NPs.