Abstract
Under a temperature gradient, the direction of thermodiffusion of charged γ-Fe2O3 nanoparticles (NPs) depends on the nature of the counter-ions present in the dispersion, resulting in either a positive or negative Soret coefficient. Various counter-ions are probed in finely tuned and well characterized dispersions of citrate-coated NPs at comparable concentrations of free ionic species. The Soret coefficient ST is measured in stationary conditions together with the mass-diffusion coefficient Dm using a forced Rayleigh scattering method. The strong interparticle repulsion, determined by SAXS, is also attested by the increase of Dm with NP volume fraction Φ. The Φ-dependence of ST is analyzed in terms of thermophoretic and thermoelectric contributions of the various ionic species. The obtained single-particle thermophoretic contribution of the NPs (the Eastman entropy of transfer ŝNP) varies linearly with the entropy of transfer of the counter-ions. This is understood in terms of electrostatic contribution and of hydration of the ionic shell surrounding the NPs. Two aqueous dispersions, respectively, with ST > 0 and with ST < 0 are then probed under an applied field H[combining right harpoon above], and an anisotropy of Dm and of ST is induced while the in-field system remains monophasic. Whatever the H[combining right harpoon above]-direction (parallel or perpendicular to the gradients and ), the Soret coefficient is modulated keeping the same sign as in zero applied field. In-field experimental determinations are well described using a mean field model of the interparticle magnetic interaction.
Highlights
It is easy to see that controlling both the magnitude and the direction of thermodiffusion, either towards cold regions (ST > 0) or towards hot regions (ST < 0), can be of paramount importance for thermoelectric applications [22,23,24,25]
The electrophoretic mobility measurements are performed at F ⇠ 0.01% with a NanoZS from Malvern and the mass diffusion coefficient is obtained by Quasi-Elastic Light Scattering (QELS) at F ⇠ 0.2% with a Vasco DLS particle analyzer from Cordouan Technologies dedicated to dark media, as in 42
To measure the diffusion coefficient Dm at finite concentration and the Soret coefficient ST, we use here the home-made Forced Rayleigh Scattering (FRS) † setup which is extensively described in Ref. 34
Summary
It is easy to see that controlling both the magnitude and the direction of thermodiffusion, either towards cold regions (ST > 0) or towards hot regions (ST < 0), can be of paramount importance for thermoelectric applications [22,23,24,25]. In 26,27, we have shown that the sign of Soret coefficient is controlled by the nature of the interface between the charged NPs and the solvent in which they are dispersed. It is related 1) to the nature of the solvent, 2) to that of the NP-coating and 3) to the kind of counter-ions used to stabilize the dispersion. Sign of the Soret coefficient is controlled by the nature of the counter-ions, together with an applied magnetic field H~ capable of modulating the ST value whatever its sign. The field-dependence of ST is here adjusted with a mean field model 45, similar to the one previously developed to describe the in-field Dm-anisotropy [46,47]
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