Water self-diffusivity confined in graphene nanogap using molecular dynamics simulations

Abstract

Fundamental understanding of water confined in graphene is crucial to optimally design and operate sustainable energy, water desalination, and bio-medical systems. However, the current understanding predominantly remains in the static properties near the graphene surfaces. In this paper, a key water transport property, i.e., self-diffusivity, is examined under confinement by various graphene nanogap sizes (L z ¼ 0.7-4.17 nm), using molecular dynamics simulations with various graphene-water interatomic potentials (Simple Point Charge (SPC/E) and TIP3P water models). It is found that the water self-diffusivity nearly linearly decreases as the graphene-water interatomic potential energy increases at a given nanogap size. It also decreases as the graphene nanogap size decreases down to L z ¼ 1.34 nm; however, it shows the peak water self-diffusivity at L z ¼ 0.8 nm and then continues to decrease. The peak water self-diffusivity is related to the significant change of the overlapping surface force, and associated, nonlinear local water density distribution. The in-plane water selfdiffusivity is higher up to nearly an order of magnitude than that of the out-of-plane due to the geometrical confinement effect by the graphene nanogap. The obtained results provide a roadmap to fundamentally understand the water transport properties in the graphene geometries and surface interactions.

Nanoconfined Water Dynamics in Multilayer Graphene Nanopores

“…The high diffusivity in the two systems was also similar to results from previous studies . It was found that 2D diffusivity of water molecules confined in graphene nanogap was significantly higher than that in bulk water . Interestingly, there seems to be a similar critical pore diameter (1.36–1.63 nm) for water diffusion in carbon nanotubes (CNTs).…”

“…36 It was found that 2D diffusivity of water molecules confined in graphene nanogap was significantly higher than in bulk water. 53 Interestingly, there seems to be a similar critical pore diameter (1.36 ~1.63 nm) for water diffusion in carbon nanotubes (CNTs). By comparing results from water diffusion in CNTs with the same diameters and at the same temperature (results of CNTs from Farimani et al 42 , Zheng et al 43 , Köhler et al 54 , Won et al 55 ), the D z of water molecules was also close to bulk water in the CNTs with diameter larger than 1.36 nm, otherwise significantly lower in smaller CNTs (Figure 3a, data points from references).…”

Investigation into the microscopic mechanisms influencing convective heat transfer of water flow in graphene nanochannels

“…Density profiles for corresponding ε c-o values in Fig. 8, which are similar to that of another report [34], displays this increasing water molecule density near the interfacial regions that enable for enhanced heat transfer at higher interaction strengths. As system size increases with channel height from H = 4.0 to 8.0 nm, convection performance displays enhancement with an increase towards that of macroscale behavior from Nu D,fd = 0.…”

“…Water molecules are kept rigid, maintaining bond length (O-H) and angle (H-O-H) constant using the SHAKE algorithm [33]. Liquid water molecules corresponding to a density of ρ = 1000 kg/m 3 are placed in-between the two nanochannel walls; an additional 0.25 nm of spacing between the block of liquid water molecules and each nanochannel surface is added so to account for the water unoccupied volume [34][35][36].…”

Enhancing the Dynamics of Water Confined between Graphene Oxide Surfaces with Janus Interfaces: A Molecular Dynamics Study

“…The OG structure is similar to a Janus pore or interface, , where the fluid is confined between two surfaces, one hydrophilic and the other hydrophobic. Deposition rates during vacuum filtration have been shown to influence the interlayer stacking, with slower rates giving rise to structures assembled with a larger number of G–G and O–O interfaces, enabling higher water permeability through the G–G regions . For a given interlayer spacing, the extent of occurrences of these different interfaces is expected to determine the water transport in these systems.…”