Resistance of water transport in carbon nanotube membranes

Carbon nanotube (CNT) membranes have long been considered as next-generation membranes due to superfast water transport inside tubes. However, a large pressure loss occurs at the pore mouth, and consequently water transport through the whole tubes is significantly retarded. To find out the reason behind this, we conduct systematic non-equilibrium molecular dynamics (NEMD) simulations on water transport through CNT membranes with various tube diameters and lengths. The whole transport resistance is contributed by the interfacial and interior parts, and the interfacial contribution plays a dominating role in short tubes and only can be ignored when the tube length reaches a scale of several micrometers. With regard to the origin of the interfacial resistance, the hydrogen bonding rearrangement (HBR) effect accounts for at least 45%, and the rest is attributed to the geometrical or steric crowding of water molecules near the pore mouth. To reduce the dominant interfacial resistance, we change the shape of the pore mouth from plate to hourglass by mimicking the aquaporin water channels. The interfacial resistance is thus decreased by >27%. It is also found that the reduction is originated from the optimized HBR rather than the subdued steric crowding of water molecules near the pore mouth.