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

Thermodiffusion properties at room temperature of colloidal dispersions of hydroxyl-coated nanoparticles (NPs) are probed in water, in dimethyl sulfoxide (DMSO) and in mixtures of water and DMSO at various proportions of water, [Formula: see text]. In these polar solvents, the positive NPs superficial charge imparts the systems with a strong electrostatic interparticle repulsion, slightly decreasing from water to DMSO, which is here probed by Small Angle Neutron Scattering and Dynamic Light Scattering. However if submitted to a gradient of temperature, the NPs dispersed in water with ClO4- counterions present a thermophilic behavior, the same NPs dispersed in DMSO with the same counterions present a thermophobic behavior. Mass diffusion coefficient [Formula: see text] and Ludwig-Soret coefficient [Formula: see text] are measured as a function of NP volume fraction [Formula: see text] at various [Formula: see text]. The [Formula: see text]-dependence of [Formula: see text] is analyzed in terms of thermoelectric and thermophoretic contributions as a function of [Formula: see text]. Using two different models for evaluating the Eastman entropy of transfer of the co- and counterions in the mixtures, the single-particle thermophoretic contribution (the NP's Eastman entropy of transfer) is deduced. It is found to evolve from negative in water to positive in DMSO. It is close to zero on a large range of [Formula: see text] values, meaning that in this [Formula: see text]-range [Formula: see text] largely depends on the thermoelectric effect of free co- and counterions.