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

Thermodiffusion of different ferrite nanoparticles (NPs), ∼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 Π, pH and ionic strength. It is then possible to map Π and the NPs' osmotic compressibility χ 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 ∼ 2 or 7, leading to a surface charge, either positive or negative. Their Ludwig-Soret ST coefficient together with their mass diffusion Dm coefficient are determined experimentally by forced Rayleigh scattering. All probed NPs display a thermophilic behavior (ST < 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 sNP is interpreted through the electrostatic and hydration contributions of the ionic shell surrounding the NPs.

Thermoelectricity and thermodiffusion in charged colloids.

The Seebeck and Soret coefficients of ionically stabilized suspension of maghemite nanoparticles in dimethyl sulfoxide are experimentally studied as a function of nanoparticle volume fraction. In the presence of a temperature gradient, the charged colloidal nanoparticles experience both thermal drift due to their interactions with the solvent and electric forces proportional to the internal thermoelectric field. The resulting thermodiffusion of nanoparticles is observed through forced Rayleigh scattering measurements, while the thermoelectric field is accessed through voltage measurements in a thermocell. Both techniques provide independent estimates of nanoparticle's entropy of transfer as high as 82 meV K(-1). Such a property may be used to improve the thermoelectric coefficients in liquid thermocells.