Studying the properties of water confined in carbon nanotubes (CNTs) have gained a lot of interest in recent years due to the vast potential applications of systems in nanoscale liquid transport as well as biology functions. This article presents a comprehensive review of recent experimental and theoretical results using nuclear magnetic resonance (NMR) and molecular dynamics (MD) simulations. Different NMR methods including

Water, when confined at the nanoscale acquires extraordinary transport properties, and yet, there is no direct experimental evidence of these properties at nanoscale resolution. Here, by using two-dimensional NMR diffusion–relaxation (D–T 2) and spin–lattice – spin–spin relaxation (T 1–T 2) spectroscopy, we succeeded to resolve at the nanoscale water diffusion in single- and double-walled carbon nanotubes (SWCNTs/DWCNTs). In SWCNTs, the spectra display the characteristic shape of uniform water diffusion restricted in one dimension. Remarkably, in DWCNTs, water is shown to split into two axial components with the inner one acquiring unusual flow properties: high fragility, ultrafast self-diffusion coefficient, and “rigid” molecular environment, revealing a stratified cooperative motion mechanism to underlie fast diffusion in water-saturated CNTs.

It is well known that water inside hydrophobic nano-channels diffuses faster than bulk water. Recent theoretical studies have shown that this enhancement depends on the size of the hydrophobic nanochannels. However, experimental evidence of this dependence is lacking. Here, by combining two-dimensional nuclear magnetic resonance diffusion–relaxation (D–T2eff) spectroscopy in the stray field of a superconducting magnet and molecular dynamics simulations, we analyze the size dependence of water dynamics inside Carbon Nanotubes (CNTs) of different diameters (1.1–6.0 nm), in the temperature range of 265–305 K. Depending on the CNT diameter, the nanotube water is shown to resolve in two or more tubular components acquiring different self-diffusion coefficients. Most notably, a favorable CNT diameter range (3.0–4.5 nm) is experimentally verified for the first time, in which water molecule dynamics at the center of the CNTs exhibits distinctly non-Arrhenius behavior, characterized by ultrafast diffusion and extraordinary fragility, a result of significant importance in the efforts to understand water behavior in hydrophobic nanochannels.

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