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Issue 4, 2019
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Thermodiffusion of citrate-coated γ-Fe2O3 nanoparticles in aqueous dispersions with tuned counter-ions – anisotropy of the Soret coefficient under a magnetic field

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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 with combining right harpoon above (vector)], and an anisotropy of Dm and of ST is induced while the in-field system remains monophasic. Whatever the [H with combining right harpoon above (vector)]-direction (parallel or perpendicular to the gradients Image ID:c8cp06858e-t1.gif and Image ID:c8cp06858e-t2.gif), 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.

Graphical abstract: Thermodiffusion of citrate-coated γ-Fe2O3 nanoparticles in aqueous dispersions with tuned counter-ions – anisotropy of the Soret coefficient under a magnetic field

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Publication details

The article was received on 05 Nov 2018, accepted on 28 Dec 2018 and first published on 28 Dec 2018


Article type: Paper
DOI: 10.1039/C8CP06858E
Citation: Phys. Chem. Chem. Phys., 2019,21, 1895-1903

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    Thermodiffusion of citrate-coated γ-Fe2O3 nanoparticles in aqueous dispersions with tuned counter-ions – anisotropy of the Soret coefficient under a magnetic field

    M. Kouyaté, C. L. Filomeno, G. Demouchy, G. Mériguet, S. Nakamae, V. Peyre, M. Roger, A. Cēbers, J. Depeyrot, E. Dubois and R. Perzynski, Phys. Chem. Chem. Phys., 2019, 21, 1895
    DOI: 10.1039/C8CP06858E

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