  Thermo-osmosis refers to fluid migration due to 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 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 $-0.03 C/m^2$. It was found that the
thermo-osmotic flow and self-diffusivity increases 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.
