Thermodiffusion of citrate-coated γ-Fe2O3 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 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 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 volume fraction Φ. The Φ-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 transfer Ŝ_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 field , and an anisotropy of D_m and of S_T is induced while the in-field system remains monophasic. Whatever the -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.