  A novel concept of membrane process in thermal-driven system is proposed for
water desalination. By means of molecular dynamics simulations, we show fast
water transport through graphene galleries at a temperature gradient. Water
molecules are driven to migrate through nanometer-wide graphene channels from
cold reservoir to hot reservoir by the effect of thermal creep flow. Reducing
the interlayer spacing to 6.5 {\AA}, an abrupt escalation occurs in water
permeation between angstrom-distance graphene slabs. The change from disordered
bulklike water to quasi-square structure have been found under this extremely
confined condition. This leads to a transition to subcontinuum transport. Water
molecules perform collective diffusion behaviors inside graphene channels. The
special transport processes with structure change convert thermal energy into
motion without dissipation, resulting in unexpected high water permeability.
The thermal-driven system reaches maximum flowrate at temperature variance of
80 K, corresponding to the quantity at pressure difference up to 10^5 bar in
commercial reverse osmosis processes and 230 bar in pressure-driven slip flow.
Our results also reveal the movement of saline ions influenced by
thermophoretic effect, which complement the geometry limitation at greater
layer spacing, enhancing the blockage of ions. This finding aims to provide an
innovational idea of developing a high-efficiency desalination technology able
to utilize various forms of energy.
