Thermal diffusion of ionic species in charged nanochannels

Abstract

Diffusion of ions due to temperature gradients (known as thermal diffusion) in charged nanochannels is of interest in several engineering fields, including energy recovery and environmental protection. This paper presents...

Thermo-Osmosis in Charged Nanochannels: Effects of Surface Charge and Ionic Strength

“…Therefore, the thermo-osmotic coefficient quantified in Figure 9b intrinsically includes the Ludwig−Soret effect of ionic species. Based on the thermoosmotic coefficient measured in this study and the thermal diffusion coefficient obtained from a previous one, 15 the fluxes of solvent and solute are de-coupled to assess the extent to which the thermo-osmosis and thermal diffusion co-exist in the nanoconfined fluid mixture when a temperature gradient is imposed. Figure 11 shows that thermo-osmosis is the creep fluid flow of the interfacial liquid propelled by the pressure gradient induced by the temperature gradient in the boundary layers and this phenomenon will not occur under bulk conditions because thermal gradients do not lead to pressure gradients in the bulk liquid.…”

“…Equations 11 and 12 show that the solute flux and solvent flux can be determined if all other data are known. Taking our case with the NaCl concentration of 4.04 mol/kg, the surface charge density of 0.00C • m −2 , and a unit temperature gradient as an example, the corresponding data from this study and the previous one on the thermal diffusion 15 in the same nanofluidic system is presented in Table 1. The obtained J Solvent and J Solute indicate that the thermally induced migration of water molecules and dissolved NaCl ions are in opposite directions due to the thermophilic thermal diffusion of NaCl ions, i.e., D T T < 0.…”

“…The silica substrates with surface charge densities of 0.00, −0.03, −0.06, and −0.12 C • m −2 were built following our previous work. 15 The partial charges on atoms were determined based on the study of Kroutil et al 31 The interatomic interactions were computed by the ClayFF force field. 32 3.2.…”

“…A relaxation stage composed of a 0.1 ns run in an NVT ensemble and a 1 ns run in the NPT ensemble, using a Nose−Hoover thermostat and a barostat, was then carried out to achieve a system pressure of P = 600 bar with a coupling time of 3 ps and a system temperature of T = 300 K with a coupling time of 0.3 ps. The adopted thermodynamic conditions are consistent with our previous study 15 on the thermal diffusion of aqueous NaCl solutions occurring at the same nanofluidic system. This enabled to directly use the thermal diffusion coefficient obtained in that study and further decouple the fluxes of solvent and solute (see details in Discussion).…”

“…Thermo-osmosis in such systems is predominantly controlled by the pore size (nano-confinement effect), the charge of the solid surface (surface charge effect), and the salinity of pore fluid (ionic strength effect). Quantification of the nano-confinement effects on thermo-osmosis have progressed in literature. , However, understanding the surface charge effects and ionic strength effects on thermo-osmosis is still limited, in contrast to the understanding of these two effects on other transport phenomena such as thermal diffusivity and Fickian diffusivity. − This paper aims to provide new insights into the surface charge effects and ionic strength effects on thermo-osmosis and the mechanisms controlling the phenomenon by isolating the coupled processes of thermo-osmosis and thermal diffusion (or Soret–Ludwig effect).…”