Schematic illustration of the simulation system. Two chambers (aqueous solution with 0.5 M NaCl on the left and pure water bath on the right) are separated by a ZIF-8 membrane. Two graphene plates in the two chambers are exerted under hydraulic pressures P left and P right, respectively. Color codes of ZIF-8: Zn, red; N, blue; C, cyan; H, white; graphene, yellow; Na+, blue; Cl−, orange; H of H2O, white; O of H2O, magenta (left chamber) and green (right chamber).
ZIF-8 membrane. As shown in the xy plane, there exist 36 geometrically identical pores along the z-axis. The pore morphologies are illustrated in the yz plane, with large cavities in yellow and small apertures in green.
(a) A fragmental cluster of ZIF-8 used for charge calculation. (b) Atomic charges in ZIF-8. Color codes: Zn, red; N, blue; C, grey; and H, white.
Simulation snapshots at t = 2, 7, and 12 ns, respectively. The pressure difference is 90 MPa. The dashed lines indicate the positions of two graphene plates after reaching a steady state at approximately t = 2 ns.
Number distributions N d(z) of Na+ and Cl− ions along the z-axis at t = 2, 7, and 12 ns, respectively. The pressure difference is 90 MPa. ZIF-8 membrane is between the two dotted lines. The pores (a, b, c, and d) are morphologically identical, but located in different positions at the z-axis.
Number of net transferred water molecules N w(t) from NaCl solution to pure water bath as a function of time.
Water flux J w as a function of pressure difference ΔP. The dotted line is to guide the eye.
Water flux as a function of inverse temperature (1000/T) at ΔP = 90 MPa. The dashed line fits to the Arrhenius equation.
Autocorrelation function c(t) of hydrogen-bonding in ZIF-8 membrane at ΔP = 90 MPa and in bulk phase, respectively.
Force field parameters for ZIF-8, water, ions, and graphene.
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