  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 a fundamental
investigation of the thermal diffusion of sodium chloride in charged silica
nanochannels performed by molecular dynamics (MD). The results reveal the
effects of nanoconfinement and surface charges on the sign and magnitude of the
Soret coefficient. It is shown that the sign and magnitude of the Soret
coefficient are controlled by the structural modifications of the interfacial
solutions. These modifications include the ionic solvation and hydrogen bond
structure induced by the nanoconfinement and surface charges. The results show
that both nanoconfinement and surface charges can make the solutions more
thermophilic. Furthermore, the thermal diffusion of solutions in boundary
layers is significantly different from that of solutions in bulk fluid,
contributing to the overall difference between the thermal diffusivity of pore
fluid and that associated with bulk fluid. The findings provide further
understanding of thermal diffusion in nano-porous systems. The proposed MD
simulation methodology is applicable to a wider category of coupled heat and
mass transfer problems in nanoscale spaces.
