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

Abstract

Soret coefficient is modulated in sign by the counterion nature and in magnitude by volume fraction and applied magnetic field; single-particle and thermoelectric contributions are sorted out.

Inversion of thermodiffusive properties of ionic colloidal dispersions in water-DMSO mixtures probed by forced Rayleigh scattering

“…The values of ⇠ 0 and @⇠ 0 /@T for the various mixtures probed here are obtained by a linear interpolation of x W between the values in pure water and in pure DMSO. As in [16,17], this e↵ective charge ⇠ 0 is of the same sign but much smaller than the structural charge of the NPs, which is due the hydroxyl species H + present at the maghemite surface (⇠ 2 charges per nm 2 , leading to a structural charge number ⇠ 450). This di↵erence has to be related here to a large amount of ClO 4 species condensed on the surface.…”

“…It is adjusted with a log-normal distribution of diameter, of median diameter d 0 = 7.5 nm and The dynamic electrophoretic charge number ⇠ 0 of the NPs and its derivative @⇠ 0 /@T are deduced (as in Ref. [16,17]) in water and in DMSO from D DLS and from measurements of the NP's electrophoretic mobility µ el at ⇠ 0.01% with a NanoZS from Malvern (with a dip cell) for 25  T  40 C, using:…”

“…1a presents the evolution of in water as a function of the NP volume fraction . As the interparticle repulsion is strong in these colloidal dispersions stabilized electrostatically, ( ) is here adjusted (as in [16][17][18][19]44]) by the Carnahan-Starling expression for e↵ective hard spheres [45,46] of volume fraction e↵ (and e↵ective diameter taking in account the screening length  1 ) as:…”

“…Their Seebeck coe cient S St e is given by E = S St e rT and it is connected to S T , througĥ S NP , the Eastman entropy of transfer of the nanoparticles in the solvent 1 [2,[12][13][14][15]]. This single-particle contribu-tionŜ NP characterizes the a nity of the ionic NPs with the polar solvent in which they are dispersed [12,[15][16][17]. S T depends on the interparticle interaction (thus on the NP's concentration and that of counterions), but also on the nature of these counterions and on the nature of the solvent [17][18][19].…”

“…This single-particle contribu-tionŜ NP characterizes the a nity of the ionic NPs with the polar solvent in which they are dispersed [12,[15][16][17]. S T depends on the interparticle interaction (thus on the NP's concentration and that of counterions), but also on the nature of these counterions and on the nature of the solvent [17][18][19].…”