Molecular dynamics study of nanoconfined TIP4P/2005 water: how confinement and temperature affect diffusion and viscosity

Abstract

In the last decades a large effort has been devoted to the study of water confined in hydrophobic geometries at the nanoscale (tubes, slit pores), because of the multiple technological applications of such systems, ranging from drugs delivery to water desalinization devices. To our knowledge, neither numerical/theoretical nor experimental approaches have so far reached a consensual un-derstanding of structural and transport properties of water under these conditions. In this work, we present molecular dynamics simulations of TIP4P/2005 water under different hydrophobic nanoconfinements (slit pores or nanotubes, with two degrees of hydrophobicity) within a wide temperature range. On the one side, water is more structured near the hydrophobic walls, inde-pendently on the confining geometries. On the other side, we show that the combined effect of confinement and curvature leads to an enhanced diffusion coefficient of water in hydrophobic nan-otubes. Finally, we propose a confined Stokes-Einstein relation to extract viscosity from diffusivity, whose result strongly differs from the Green-Kubo expression that has been used in previous work. We discuss the shortcomings of both approaches, which could explain this discrepancy.

Water in Nanopores and Biological Channels: A Molecular Simulation Perspective

“…Switching to more complex water models, Zaragoza et al have explored TIP4P/2005 water in nanoslits and nanopores (graphene and CNTs). 201 As with simpler fixed point-charge models, enhanced diffusion of water is seen in hydrophobic nanotubes. These results and those discussed above agree with a CFD model 202 in which the apparent viscosity of water decreases with an increasing contact angle (i.e., hydrophobicity), and tends to bulk viscosity with an increase of pore dimension.…”

Bulk viscosity of liquid noble gases

“…Finite size effects were minimized by placing N = 4096 particles in cubic volumes with periodic boundary conditions and choosing a sufficiently large cutoff radius rc ≥ 5.5σ. [140][141][142] The employed particle number is well chosen, as simulations containing N = 12 000 particles yielded virtually identical results, cf. supplementary material.…”

Fast increase of nanofluidic slip in supercooled water: the key role of dynamics

“…[55][56][57][58][59] Confined water has also been explored from an experimental and theoretical point of view, with an special interest in the novel 2D materials such as graphene. [60][61][62][63][64][65][66][67] In particular, the temperature evolution of supercooled water under confinement has been the subject of intensive experimental research. 56,57,[68][69][70][71] Broadband dielectric spectroscopy, nuclear magnetic resonance, as well as neutron scattering experiments have successfully probed water confined in pores with sub-nm radii at temperatures as low as about 130 K, in order to connect the dynamical behavior of supercooled confined water to that of bulk water in the so-called no-man's land (150 K to 230 K).…”