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

Abstract

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.

Dynamic response of a thermoelectric cell induced by ion thermodiffusion

“…In the case of nanoparticles dispersed in acidic solutions, a total Seebeck coefficient S ≈ -1.5 mV/K (similar to the values obtained in the present study) was measured and used to predict the negative Soret coefficient for the positively charged nanoparticles, as observed experimentally [30]. In another publication, the experimentally obtained salt Soret coefficient of hydroxides [32], which is peoportional to α + +α − , was used to predict the positive Soret coefficients of negatively charged nanoparticles in TMAOH and TBAOH, as also observed experimentally [36]. Thus, our present results are consistent with those obtained in previous studies, suggesting that the opposite S total sign predicted by equation 2 for acids and hydroxides, which does not fit the overall consistency from the many experimental results, may come from a still existing problem in modelling the Seebeck response from thermodiffusion effects.…”

“…In the case when a negatively charged nanoparticle is dispersed only by an ionic solution of NaOH, three different systems in literature [4,36,37] show that particles move to the hot side while our experimental results predicts the steady-state thermoelectric field should push them to the cold, like in the case of particles in TMAOH and TBAOH [36]. This may be related to salt specific effects, which is still an open question to be solved in thermodiffusion.…”

A Molecular Dynamics Approach to Calculate the Thermodiffusion (Soret and Seebeck) Coefficients of Salts in Aqueous Solutions

“…MD/TN values were obtained by using ionic force field parameters with best experimental/theoretical agreement ( Soret coefficient S T and the salt thermal diffusion coefficient D T . 18,19,[23][24][25]33,35,42 Using the relation between D T and α i (Equation 5), the temperature dependence of the salt thermal diffusion coefficient D T (T ) was analyzed. Note that the calculated single-ion Soret coefficients α i for all salts were obtained as constant and positive at temperatures ranging from 293 to 353 K. Then, we used the calculated effective mass diffusion coefficient D (showed in Figure 3) obtained for different temperatures to calculate D T (T ).…”

“…The nature of ions [16][17][18] such as the hydrophilic/hydrophobic degree has an important role in driving solute migration towards the cold or hot side of the solution in temperature gradients. 19,20 As regards electrolytes, differences in α i 21 or Q * i 7 and in D i 22 of anions and cations are predicted to generate a thermoelectric field, which leads to thermophoresis of charged nanoparticles 18,[23][24][25] and it has been increasingly investigated so that it can be applied in energy harvesting. 1,2,26,27 Some theoretical concepts of thermodiffusion in a given solution have been developed…”

Molecular Dynamics Approach to Calculate the Thermodiffusion (Soret and Seebeck) Coefficients of Salts in Aqueous Solutions

“…The nature of ions [16][17][18] such as the hydrophilic/hydrophobic degree has an important role in driving solute migration towards the cold or hot side of the solution in temperature gradients. 19,20 As regards electrolytes, differences in α i 21 or Q * i 7 and in D i 22 of anions and cations are predicted to generate a thermoelectric field, which leads to thermophoresis of charged nanoparticles 18,[23][24][25] and it has been increasingly investigated so that it can be applied in energy harvesting. 1,2,26,27 Some theoretical concepts of thermodiffusion in a given solution have been developed…”