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

• Published in: Physical chemistry chemical physics : PCCP 2019-01, Vol.21 (4), p.1895-1903
• Main Authors: Kouyaté, M, Filomeno, C L, Demouchy, G, Mériguet, G, Nakamae, S, Peyre, V, Roger, M, Cēbers, A, Depeyrot, J, Dubois, E, Perzynski, R
• Format: Article
• Language: English
• Subjects: Chemical Physics; Physics
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/C8CP06858E

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.