Key parameters were identified to obtain long-term stable ionic liquid-based colloids.

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 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 Ŝ_NP is interpreted through the electrostatic and hydration contributions of the ionic shell surrounding the NPs.

Present environmental issues force the research to explore radically new concepts in sustainable and renewable energy production. In the present work, a functional fluid consisting of a stable colloidal suspension of maghemite magnetic nanoparticles in water was characterized from the points of view of thermoelectrical and optical properties, to evaluate its potential for direct electricity generation from thermoelectric effect enabled by the absorption of sunlight. These nanoparticles were found to be an excellent solar radiation absorber and simultaneously a thermoelectric power-output enhancer with only a very small volume fraction when the fluid was heated from the top. These findings demonstrate the investigated nanofluid’s high promise as a heat transfer fluid for co-generating heat and power in brand new hybrid flat-plate solar thermal collectors where top-heating geometry is imposed.