Ultrafast stratified diffusion of water inside carbon nanotubes. Direct experimental evidence with 2D (D-T2) NMR spectroscopy

Abstract: 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 2D NMR diffusion-relaxation (D-T2) and spin-lattice - spin-spin relaxation (T1-T2) spectroscopy, we succeeded to resolve at the nanoscale water diffusion in single and double-walled carbon nanotubes (SWCNT/DWCNT). In SWCNTs, 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.