The permselectivity of H2/O2, H2/N2, H2/CO, and H2/CH4 mixtures passing a graphdiyne membrane is studied by molecular dynamics simulations. At pressure range of 0.047-4.5 GPa, H2 can pass the graphdiyen membrane quickly, while all the O2, N2, CO, and CH4 molecules are blocked. At pressure of 47 kPa, the hydrogen flow is 7 mol/m^2s. With increase of pressure, the hydrogen flow goes up, and reaches maximum of 6×10^5 mol/m^2s at 1.5 GPa. Compared to other known membranes, graphdiyne can be used for means of hydrogen purification with the best balance of high selectivity and high permeance.
Different structures of graphite oxide （GO） with and without water are optimized by density functional theory. Without H20 in interlayer space, the optimized interlayer distances are about 6A, smaller than the experimental values of 6.5-7A. On the other hand, the interlayer distances of hydrated graphite oxide structures are in good agreement with experimental observations. Based on the optimized GO structures, we then simulate the immersion of GO in water or methanol by molecular dynamics. For the dry GO, water and methanol molecules do not enter the nanopore. While for the hydrated GO, the liquid molecules enter the interlayer space and enlarge the interlayer distance, semi-quantitatively reproducing the experimental phenomena.
We perform molecular dynamics simulations for water confined between two smooth hydrophobic walls and observe two crystalline structures with one being first reported. Both of these structures obey the ice rule. The novel ice phase is a flat hexagonal-rhombic trilayer ice, obtained under 1 GPa load at wall separation of 1.0 nm. In this structure, the water molecules in the two layers next to one of the walls （outer layers） and in the middle layer form hexagonal rings and rhombic rings, respectively. For a molecule in the outer layers, three of its four hydrogen bonds are in the same layer, and the other one hydrogen bond connects to the middle layer. For a molecule in the middle layer, only two of its four hydrogen-bonds are located in the same layer, and the other two connect to two different outer layers. Despite their different motifs, the area densities of the three layers are almost equal. The other structure is a flat hexagonal bilayer ice produced at wall separation of 0.8 nm under lateral pressure of 100 MPa, analogous to a system demonstrated by Koga et al [Phys. Rev. Lett. 79, 5262 （1997）]. Both first-order and continuous phase transitions take place in these simulations.
Molecular dynamics （MD） simulations are performed to study the structure and adsorption of ethanol/water mixture within carbon nanotubes （CNTs）. Inside the （6,6） and （10,10） CNTs, there are always almost full of ethanol molecules and hardly water molecules. Inside wider CNTs, there are some water molecules, while the ethanol mass fractions inside the CNTs are still much higher than the corresponding bulk values. A series of structural analysis for the molecules inside and outside the CNTs are performed, including the distributions of radial, axial, angular density, orientation, and the number of hydrogen bonds. The angular density distribution of the molecules in the first solvation shell outside the CNTs indicates that the methyl groups of ethanol molecules have the strongest interaction with the carbon wall, and are pinned to the centers of the hexagons of the CNTs. Based on the understanding of the microscopic mechanism of these phenomena, we propose that the CNTs prefer to contain ethanol rather than methanol.
