Structure and Transport Properties of Water and Hydrated Ions in Nano‐Confined Channels

Abstract

Nano-confined flow is ubiquitous in modern engineering and biomedical applications. As the peculiar phenomena of nano-confined flow were continuously reported and traditional theories failed to describe them accurately, simulation efforts have been made to gain deeper insight. The objective of this paper is to provide an overview of the recent progress on nano-confined flow, including water flow and hydrated ions transport. This report starts with the water behavior under nano-confinement, summarizing the water structures, flow properties and their dependence on wall wettability, channel size, etc. The report also presents the efforts made to modify the existing theories to describe nano-confined flows. Then, the report goes to the hydrated ions. The dehydration process of hydrated ions and ions sieving are discussed in detail, and the effects of membrane surface charge, grafting functional groups, and external field on ions transport are reviewed. In this Progress Report, the transport properties of water and hydrated ions in nano-confined channels are highlighted, which is critical to the design and application of nanofluidic devices, such as nanofiltration membranes, biosensors, and other related fields.

Water in Nanopores and Biological Channels: A Molecular Simulation Perspective

“…These effects have also been discussed in detail in the context of CNTs and related pores by Wu et al 203 (see section 5.1 ) and in a recent review. 204 …”

“…Nonbiological nanopores, especially carbon nanotubes (CNTs), are an active area of research on the behavior of nanoconfined water in pores ( Figure 8 ). A recent review, 204 mainly focused on CNTs, compares MD and continuum approaches to water flow in such nanopores. In this section we will first discuss more recent simulation studies of water in CNTs, before examining a range of other nonbiological nanopores.…”

Fast Permeation of Small Ions in Carbon Nanotubes

“…Several simulations have indeed predicted enhanced self‐diffusivities for large ions inside CNT channels, and a few self‐diffusion experiments seem to support these claims. [ 20–26 ] In addition, our previous reports have shown water vapor permeances under a relative humidity gradient that are 24–100 times larger than Knudsen theory estimates, [ 14,27 ] and others have measured proton diffusivity in very narrow CNTs exceeding bulk transport by ten times. [ 28 ] Unfortunately, conclusive experimental validation of enhanced diffusion in liquid phases has remained elusive thus far.…”

Intercalated water in aqueous batteries

“…The hydration of ions and the transport dynamics are widely studied in areas such as desalination and biological channels 19 . Figure 1 depicts the hydration structure of a cation.…”

“…Certain number of water molecules are closely coordinated with the cation through bonding with oxygen, forming an inner hydration sphere. Conventionally, the hydration number was determined by means of atomic force microscopy, 20 X‐ray absorption spectroscopy (XAS), 21 infrared photodissociation spectroscopy, 22 Raman spectroscopy, 23 luminescence, 24 nuclear magnetic resonance, 25 neutron scattering, 26 and so forth in combination with molecular dynamics simulation 19 . Free water molecules farther away from the inner hydration shell are loosely bonded through hydrogen bond and form an outer hydration sphere.…”