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Thermodiffusion of citrate-coated γ-Fe 2 O 3 nanoparticles in aqueous dispersions with tuned counter-ions – anisotropy of the Soret coefficient under a magnetic field

Abstract : Under a temperature gradient, the direction of thermodiffusion of charged g Fe 2 O 3 nanoparticles (NPs) depends on the nature of the counter-ions present in the dispersion, resulting in either 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 S T is measured in stationary conditions together with the mass-diffusion coefficient D m using a Forced Rayleigh Scattering method. The strong interparticle repulsion, determined by SAXS, is also attested by the increase of D m with NP's volume fraction F. The F-dependence of S T 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 transferŜ NP) 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 S T > 0 and with S T < 0 are then probed under an applied fieldH, an anisotropy of D m and of S T is induced while the in-field system remains monophasic. Whatever theH-direction (parallel or perpendicular to the gradients-T and-F), 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.
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Submitted on : Monday, September 2, 2019 - 10:55:53 AM
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M. Kouyaté, C. Filomeno, G. Demouchy, G. Mériguet, S. Nakamae, et al.. Thermodiffusion of citrate-coated γ-Fe 2 O 3 nanoparticles in aqueous dispersions with tuned counter-ions – anisotropy of the Soret coefficient under a magnetic field. Physical Chemistry Chemical Physics, Royal Society of Chemistry, 2019, 21 (4), pp.1895-1903. ⟨10.1039/C8CP06858E⟩. ⟨hal-02275983⟩

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