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

Abstract

Thermo-osmosis refers to fluid migration due to the temperature gradient. The mechanistic understanding of thermo-osmosis in charged nano-porous media is still incomplete, while it is important for several environmental and energy applications, such as low-grade waste heat recovery, wastewater recovery, fuel cells, and nuclear waste storage. This paper presents results from a series of molecular dynamics simulations of thermo-osmosis in charged silica nanochannels that advance the understanding of the phenomenon. Simulations with pure water and water with dissolved NaCl are considered. First, the effect of surface charge on the sign and magnitude of the thermo-osmotic coefficient is quantified. This effect was found to be mainly linked to the structural modifications of an aqueous electrical double layer (EDL) caused by the nanoconfinement and surface charges. In addition, the results illustrate that the surface charges reduce the self-diffusivity and thermo-osmosis of interfacial liquid. The thermo-osmosis was found to change direction when the surface charge density exceeds −0.03C · m–2. It was found that the thermo-osmotic flow and self-diffusivity increase with the concentration of NaCl. The fluxes of solvent and solute are decoupled by considering the Ludwig–Soret effect of NaCl ions to identify the main mechanisms controlling the behavior. In addition to the advance in microscopic quantification and mechanistic understanding of thermo-osmosis, the work provides approaches to investigate a broader category of coupled heat and mass transfer problems in nanoscale space.

Complex coupling between surface charge and thermo-osmotic phenomena

“…41 Secondly, for an electrically neutral surface, there is a change of sign of the enthalpy excess, indicating a change of direction of the thermo-osmotic flow, again being consistent with a previous study that reports a change of direction of the flow when changing the wetting of the system. 26 On silica-based materials, which have hydrophilic surfaces, it has been shown that the surface charge density can affect the thermo-osmotic flow direction, this effect being attributed to a change of sign of the enthalpy excess, 23,36 our calculations tend to confirm qualitatively the results of these studies.…”

“…There is an increasing effort to better understand thermo-osmotic flows, through experimental characterization 23,24 and modeling. [25][26][27][28][29][30][31][32][33][34][35][36][37] As detailed in the Theory section, for liquids, Derjaguin and collaborators developed a standard theoretical framework, 13,[38][39][40] which relates the thermo-osmotic flow velocity to the interfacial enthalpy excess, stemming from the interactions of the liquid with the solid. This framework has been extended recently to take into account liquid-solid slippage arising on low-friction surfaces, 13,26,41 which can boost the flow.…”

“…Fig.7Slip length and resulting transport coefficients M to and K to as a function of the surface charge density S. The slip length is relatively small for this surface, and decreases rapidly with the surface charge density, in particular in the heterogeneous and defective cases; the thickness of the interaction layer l h is shown with a dotted line for comparison (a). Corresponding thermo-osmotic coefficient (eqn (7)), along with the standard electrostatic prediction (eqn(35)) plotted in light green (b), and theoretical thermo-osmotic conductance (eqn (11)), along with the standard electrostatic prediction (eqn(36)) plotted in light green (c).…”