Thermodiffusion of Monovalent Organic Salts in Water

Abstract

The ionic Soret effect induced by temperature gradients is investigated in organic electrolytes (tetramethylammonium and tetrabutylammonium hydroxides) dispersed in water using a holographic grating experiment. We report the influences of temperature and salt concentrations on the Soret, diffusion, and thermal diffusion coefficients. Experimental results to the thermal diffusion coefficient are compared with a theoretical description for thermodiffusion of Brownian particles in liquids based in the thermal expansion of the liquid solution. It is observed that the obtained thermal diffusion coefficients for the organic electrolytes present a similar temperature dependence as the theoretical prediction. Comparing the experimental results for the organic and common inorganic salts it is proposed an additional physical mechanism as the cause to the different thermal diffusion coefficients in both types of salt. We propose that the temperature dependence of hydration free energy gives rise to a force term that also leads to ion migration in a temperature gradient. We describe the thermal diffusion results as a competition between thermal expansion and hydration effects. The specific structure each type of ion cause in water molecules is considered in the heat of transport theory to describe thermal diffusion of electrolytes. A qualitative agreement is seen between our results and the classical heat of transport theory.

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

“…The present result has also to be compared with the FRS measurements of Ref. 21 on pure electrolytes, showing that the slope ∂ S T /∂ T of (TBuA + ,OH ) aqueous solutions is negative, while it is positive for (TMA + ,OH ) solutions, as usually observed for alkali halide salt solutions under thermal gradients 18 . TBuA + counter-ions thus appears much more hydrophobic than TMA + k i.e.…”

“…The Seebeck effect 7,13 induces a local electric fieldẼ = S e-T in the dispersion (S e being the Seebeck coefficient) and the Ludwig-Soret effect 14,15 gradient of volume fraction-F = FS T-T (S T being the Soret coefficient). These two effects are intricately connected via the Eastman entropies of transferŜ i ⇤ of the charged species {i} in the system [16][17][18][19][20][21] .…”

Thermo-osmosis in hydrophilic nanochannels: mechanism and size effect

“…which is equivalent to that of Derjaguin et al, 37 based on Onsager's linear non-equilibrium thermodynamics. 45,46 We note eqn (13) indicates that the phenomenological coefficients are inversely proportional to the fluid viscosity, which is also found in recent rigorous theoretical derivations based on mixture coupling theory and irreversible thermodynamics. 14,15 The calculation of the excess enthalpy density in the nano-channel requires atomistic analysis to which we turn in the next sub-section.…”

“…In both sets, periodic boundary conditions are considered in y and z-direction. The MD simulations provide further the excess enthalpy density, required for the calculation of the macroscopic coefficients by eqn (12) or (13). The thermo-osmotic system is adopted only to demonstrate the thermo-osmosis phenomenon and validate the Onsager reciprocity relations, while the majority of the results are obtained with the mechano-caloric system.…”

“…shale) and clay soils is essential for underpinning the design and safety assessments of many geotechnical infrastructures such as nuclear waste repositories and groundsource heat or energy foundations. 6,7 Compared to the theoretical studies of thermophoresis, in particular thermal diffusion, [9][10][11][12][13] the microscopic mechanistic understanding of thermo-osmosis is rather incomplete, although its macroscopic thermodynamical description has been widely discussed and used both in modelling multiscalemultiphase couplings problems 14,15 and in experimental works. 16,17 In fact, there is a debate between two approaches for describing the underlying mechanism for thermo-osmosis, namely the interfacial approach and the energetic approach.…”