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

WQ Chen and AP Jivkov and M Sedighi, ACS APPLIED MATERIALS & INTERFACES, 15, 34159-34171 (2023).

DOI: 10.1021/acsami.3c02559

Thermo-osmosis refersto fluid migration due to the temperaturegradient. The mechanistic understanding of thermo-osmosis in chargednano-porous media is still incomplete, while it is important for severalenvironmental and energy applications, such as low-grade waste heatrecovery, wastewater recovery, fuel cells, and nuclear waste storage.This paper presents results from a series of molecular dynamics simulationsof thermo-osmosis in charged silica nanochannels that advance theunderstanding of the phenomenon. Simulations with pure water and waterwith dissolved NaCl are considered. First, the effect of surface chargeon the sign and magnitude of the thermo-osmotic coefficient is quantified.This effect was found to be mainly linked to the structural modificationsof an aqueous electrical double layer (EDL) caused by the nanoconfinementand surface charges. In addition, the results illustrate that thesurface charges reduce the self-diffusivity and thermo-osmosis ofinterfacial liquid. The thermo-osmosis was found to change directionwhen the surface charge density exceeds -0.03C & BULL; m(-2). It was found that the thermo-osmotic flow and self-diffusivityincrease with the concentration of NaCl. The fluxes of solvent andsolute are decoupled by considering the Ludwig-Soret effectof NaCl ions to identify the main mechanisms controlling the behavior.In addition to the advance in microscopic quantification and mechanisticunderstanding of thermo- osmosis, the work provides approaches to investigatea broader category of coupled heat and mass transfer problems in nanoscalespace.

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