Self-Diffusion in Confined Water: A Comparison between the Dynamics of Supercooled Water in Hydrophobic Carbon Nanotubes and Hydrophilic Porous Silica

Abstract

Confined liquids are model systems for the study of the metastable supercooled state, especially for bulk water, in which the onset of crystallization below 230 K hinders the application of experimental techniques. Nevertheless, in addition to suppressing crystallization, confinement at the nanoscale drastically alters the properties of water. Evidently, the behavior of confined water depends critically on the nature of the confining environment and the interactions of confined water molecules with the confining matrix. A comparative study of the dynamics of water under hydrophobic and hydrophilic confinement could therefore help to clarify the underlying interactions. As we demonstrate in this work using a few representative results from the relevant literature, the accurate assessment of the translational mobility of water molecules, especially in the supercooled state, can unmistakably distinguish between the hydrophilic and hydrophobic nature of the confining environments. Among the numerous experimental methods currently available, we selected nuclear magnetic resonance (NMR) in a field gradient, which directly measures the macroscopic translational self-diffusion coefficient, and quasi-elastic neutron scattering (QENS), which can determine the microscopic translational dynamics of the water molecules. Dielectric relaxation, which probes the re-orientational degrees of freedom, are also discussed.

Investigation of Dynamic Behavior of Confined Ionic Liquid [BMIM]+[TCM]− in Silica Material SBA-15 Using NMR

“…Under this context, a great number of experimental and theoretical efforts have been employed in order to examine guest–host molecular systems [ 15 , 16 , 17 ], including structural relaxation times and transport coefficients of confined glass-forming materials [ 18 ]. From the experimental point of view, the dynamical properties of many ILs have been investigated with several techniques, such as dielectric spectroscopy [ 19 , 20 ], neutron scattering [ 21 , 22 , 23 , 24 ], light scattering [ 19 ], and NMR spectroscopy [ 17 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ]. Theoretical studies using Molecular Dynamics (MD) simulations also reveal complex phase transitions of nano-confined ILs [ 32 , 33 , 34 ].…”