The peculiar size and temperature dependence of water diffusion in carbon nanotubes studied with 2D NMR diffusion–relaxation D –T2eff spectroscopy

Abstract

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.

Effect of Size and Temperature on Water Dynamics inside Carbon Nano-Tubes Studied by Molecular Dynamics Simulation

“…For large CNTs, a bulk-like density profile is observed near the nano-tube center in agreement with previous studies [ 8 , 34 ]. For the 3.0 nm-CNT, we observed two additional water layers consistent with the NMR experiments [ 26 ]. Density maps corresponding to the oxygen occurrence during MD simulation in xy direction are shown in Figure S1 (Supporting Information (SI)) .…”

“…On the other hand, inside 3.0 nm, 3.5 nm, and 10 nm, it was observed that water diffuses faster in the central sheets, compared to others. These results were experimentally verified by our group using NMR spectroscopy [ 26 ]. Structural and vibrational studies [ 39 , 40 ] have also shown that water confined inside a CNT of size 3.0 nm acquires ice-like clusters, showing cooperative motion with high diffusion.…”

“…Therefore, the enhancement of water diffusion in this particular confinement size cannot be totally attributed to change in the water orientation or structure. Recently, Gkoura et al [ 26 ] also observed the enhancement in water diffusion for CNT with length 3.5 nm using the two-dimensional nuclear magnetic resonance diffusion-relaxation ( D - ) spectroscopy method. The observed diffusion coefficient measurements agree qualitatively with our MD simulation results.…”

“…We have examined the fragility of water molecules inside different CNT sizes and various temperatures. Figure 8 shows the temperature dependence of the inverse of the self diffusion coefficient 1/ D versus 1000/T of the water confined in different CNT sizes (from 2.0 nm to 10 nm) in the temperature range between 260 K to 320 K. Experimental data for bulk water and their fitted curve was taken from our previous paper [ 26 ]. The bulk data was fitted to the Speedy–Angell power law (shown in blue color), i.e., D = , where = 1.635 × , = 215 K, and = −2.063.…”

“…Recently [ 26 ], we addressed these issues using two-dimensional NMR diffusion- relaxation spectroscopy and we found that the water molecules inside the nano-tube resolve in two or more tubular components acquiring different-self diffusion constants ( D ) for a specific CNT diameter. Moreover, for the first time [ 27 ], we reported experimental results on the stratified water diffusion in CNTs and showed that water dynamics at the center of CNT exhibiting non-Arrhenius behavior, characterized by ultrafast diffusion and extraordinary fragility.…”