Correlation lengths in nanoconfined water and transport properties

Abstract

We report the existence of disparate static and dynamic correlation lengths that could describe the influence of confinement on nanoconfined water (NCW). Various aspects of viscous properties, such as anisotropy, viscoelasticity among others, of NCW are studied by computer simulations by varying the separation 'd' between the two confining hydrophobic plates. The mean square stress exhibits a slow spatial decay (measured from the surface) beyond ~1.8 nm. The static correlation length obtained from fitting the exponential decay of the mean-square stress with d is 0.75 nm, while the decay time of stress-stress time correlation function gives a dynamic correlation length of only 0.35 nm. The shortness of the latter seems to arise from the low sensitivity of orientational relaxation to confinement. In the frequency dependent viscosity, we observe a new peak at about 50 cm-1 that is not present in the bulk. This new peak is prominent even at 3nm separations. The peak is absent in the bulk, although it is close to the intermolecular -O-O-O- bending mode well known in liquid water. We further explore relation between diffusion and viscosity in NCW by varying 'd', and find disagreement with an existing theory.

What Is the Viscosity of Liquid Water Confined in a Hydrophobic Nanotube? Estimation Using a Novel Approach

“…This model also assumes that hydrodynamics applies in such highly confined systems. Previous studies already showed the applicability of the hydrodynamics in similar confined systems. ,,,, Finally, the CSE relation (eq ) assumes that the hydrodynamic diameter of bulk water is the same as the water inside CNTs. In other words, the extent of breakdown of the SE relation in bulk water is the same as the confined water.…”

“…However, this is not always true. Some of the circumstances when the SE relation is not completely obeyed include water in confinement, , water in a supercooled state, ,,− water under high pressure, and the presence of another solvent. − Therefore, some modification(s) of the existing CSE approach are necessary for a more robust estimation of viscosity of water in different scenarios.…”

Anomalous Concentration Dependence of Viscosity: Hidden Role of Cross-Correlations in Aqueous Electrolyte Solutions

“…In the expression for shear viscosity (eq ), the stress tensor σ αβ consists of five independent components: σ xy , σ yz , σ zx , 1 2 ( σ italicxx − σ italicyy ) , and 1 2 ( σ italicyy − σ italiczz ) . For isotropic systems, all of these five components are equivalent. , …”

“…For isotropic systems, all of these five components are equivalent. 64,67 First, we calculated the shear viscosity of the SPC/E and the TIP4P/2005 water models and obtained values of 0.73 and 0.86 mPa s, respectively. The results are in good agreement with the previous studies.…”

An off-center endohedrally confined hydrogen molecule

“…In recent years, there has been increasing interest in the study of confined systems and their applications in several fields owing to modifications in their chemical and physical properties. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Some examples of these applications are the use of molecular cages as nanocontainers, 15 the design of materials with specific optical properties, [16][17][18][19][20] the development of rechargeable ion batteries, 21 the fabrication of gas sensors to detect pollutant gases, 22 the study of quantum-chemical models of enzyme active sites, 23 the use of nanocarriers as drug delivery systems for tumor treatments, 24 the improvement of nanoelectronic devices, 25 and the understanding of certain modifications in the properties of atoms and ions in a plasma environment. 26,27 In particular, the storage and transport of hydrogen has attracted a great deal of attention due to its use as a clean source of energy, 28,29 and several theoretical and experimental works have focused on this subject under different confining situations.…”