Diffusiophoresis as ruling effect: Influence of organic salts on thermodiffusion of iron oxide nanoparticles

Colloidal particles, including ferrofluidic nanoparticles (NP), move in a temperature gradient due to thermodiffusion. Organic salts, which are often added to disperse the NP in aqueous solution, also move in the temperature gradient. This can have a strong influence on the behavior of the NP, which not only respond to the temperature gradient but also to the concentration gradient of the dispersive salt, an effect termed diffusiophoresis. In this work we present experimental results on thermodiffusion of iron oxide nanoparticles dispersed in aqueous solutions of organic hydroxides, which illustrate the possibility to manipulate the thermodiffusion of NP through the addition of organic salts. Our experiments investigate the temperature dependence of the particles' Soret coefficient, a recurring question on thermodiffusion of water-dispersed particles. Existing theoretical approaches are compared and we relate the Soret coefficient of the NP with two main physical parameters ruling particle motion: the NP's electrostatic potential and the Soret coefficient of the dispersing ions. These parameters are also experimentally determined. At the order of magnitude of the NP's Soret coefficient good agreement between experiments and theory is achieved by including the experimental data on the Soret coefficient of the dispersing ions and therefore the NP's displacement due to the ion concentration gradient. Taking into account the temperature dependence of such previously unknown parameters is a relevant step to describe the temperature dependence of the NP's Soret coefficient.