What Have We Learnt About the Mechanisms of Rapid Water Transport, Ion Rejection and Selectivity in Nanopores from Molecular Simulation?

Abstract

Nanopores have demonstrated an extraordinary ability to allow water molecules to pass through their interiors at rates far exceeding expectations based on continuum theory. Moreover, simulation studies suggest that particular nanoscale pores have the potential to discriminate between water and salts as well as to distinguish between a range of different ion types. Some of the unusual features of transport in these nanopores have been elucidated with molecular dynamics simulation, specifically the spontaneous filling and rapid transport of water, the rejection of ions and the selection between ions. The main focus of this review, however, is the physical mechanisms which act to produce such remarkable behaviour at this scale, drawing on the many studies that have been conducted in the last decade. Since molecular dynamics simulations allow the motion of individual atoms to be followed over time, they have the potential to provide fundamental insight into the reasons why transport in nanoscale pores differs from expectations based on macroscopic theory. Gaining an understanding of the mechanisms of transport in these tiny pores should guide future experiments in this area aimed at developing novel technologies and improving existing membrane separation techniques.

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“…Thus, the water flux across nanoporous graphene depends on the chemistry, size and geometry of the nanopores. 12 Thomas et al 33 proposed that there are six main mechanisms for salt rejection by monolayer graphene membranes, including:…”

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“…Falk et al (24,25) later identified the crucial role of wall curvature in reducing water friction at the wall of CNTs. Joseph and Aluru investigated the pressure--driven flow of liquid water through (16,16) CNTs of diameter 2.17 nm (26). They found a ~2000 times enhancement of mass flow rate with respect to that predicted by continuum theory, a consequence of the velocity the other hand, the ice-like water inside the 1.1-and 1.2-nm loses on average 7.4 and 4.1 eu (1 eu ¼ 1 J · mol…”

“…gain further insight into the nature of these confined s by expressing the total entropy of water as a sum of transal, rotational, and internal vibrational components (16): S trans þ S rot þ S vib . Due to the rigid water model used in tudy, S vib ¼ 0.…”

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“…Membrane-based technology is a potential method for controlling the emission of greenhouse gases and has seen substantial advancement during the past few decades [5]. Besides the traditional silica-based membranes, other novel porous materials, from zeolite, metal-organic frameworks (MOFs) to carbon materials, CONTACT Linghong Lu linghonglu@njtech.edu.cn; Xiaohua Lu xhlu@njtech.edu.cn including carbon nanotubes (CNTs) and graphene-based structures, have received an increasing research focus in the past few years [6][7][8]. It is worth noting that for carbon materials, the production cost is quite low compared with other materials, and that their structural and chemical properties can be easily tuned.…”