Continuous Water Filling in a Graphene Nanochannel: A Molecular Dynamics Study

Abstract

Low dimensional materials especially carbon materials hold high promise in the fields of water purification, mineral separation, energy harvesting/conversion, and so on. The fluidic devices fabricated by direct synthesis, lithography, or self-assembly of low dimensional materials provide opportunities for exploring the novel properties and applications of nanoconfined transport. Here, continuous filling of water and acetone molecules into a graphene nanochannel is investigated. A stairlike nonlinear dependence of the number of filling water molecules on interlayer distance d is found when d < 1 nm due to the existence of out-plane layered and in-plane ordered monolayer structure, while near-linear dependence is found for acetone because of the freely rotating configurations along with varying d during the filling process. The entropy, potential energy, and free energy of the confined system during the continuous filling are analyzed to understand the structural evolution of water. The energy-costs are discussed depending on the structure evolution of water during the filling, which is crucial to understanding the swelling and capillary condensation widely existing in the angstrom/nanometer-scale separation membranes.

Reduced Ionic Conductivity but Enhanced Local Ionic Conductivity in Nanochannels

“…17,25 It is known that water intercalated in the graphene channel with h ≈ 0.68 nm features as a solidlike monolayer (1L) structure at room temperature. 17,26,45,46 The D of Na + and K + embedded in the solidlike water is on the order of 0.01 ∼ 0.001 × 10 −9 m 2 /s, which is about 100 ∼ 1000-fold lower than the value in bulk water. 36 Moreover, recent experimental and simulation results indicate that an extremely high barrier (≳ 10 k B T) should be overcome when ions enter the monolayer water.…”

“…The nanochannel is made by graphene flakes with an atomically flat, charge-neutral, and chemically inert surface (Figure , see details in Models and Methods), which can be fabricated by the van der Waals (vdW) assemble method and served as an ideal model material to study confined mass transport. The width h of a nanochannel (defined as the atom-to-atom distance of the channel, not the thickness of water ) is critical for the molecular structures of confined solution. , It is known that water intercalated in the graphene channel with h ≈ 0.68 nm features as a solidlike monolayer (1L) structure at room temperature. ,,, The D of Na + and K + embedded in the solidlike water is on the order of 0.01 ∼ 0.001 × 10 –9 m 2 /s, which is about 100 ∼ 1000-fold lower than the value in bulk water . Moreover, recent experimental and simulation results indicate that an extremely high barrier (≳ 10 k B T ) should be overcome when ions enter the monolayer water. ,, Thus, in this work, we focus on the result of the channel with bilayer (2L) and multilayer (ML) water ( h ranges from 1.0 to 3.0 nm) that the D of ions or water is comparable to the results of the bulk condition. , …”

“…It seems that we can estimate the lowest σ h when h = 0.68 nm ( h w = 0.34 nm, which can only intercalate 1L water) using eq . However, this estimation is illegal because the structure of 1L water is much different from ML water. , …”

“…However, this estimation is illegal because the structure of 1L water is much different from ML water. 17,46 Spatial Heterogeneity and Enhanced Local Electric Conductivity. In order to probe the local ionic conductivity and spatial heterogeneity of the confined ions, we use nonequilibrium MD simulation (NEMD) by driving the ionic current using a small uniform electric field with strength E (see details in Models and Methods).…”

Atomistic Simulations of the Permeability and Dynamic Transportation Characteristics of Diamond Nanochannels

“…Due to structural similarity, fluid transportation characteristics within two-dimensional (2D) graphene slits [ 14 , 42 , 43 , 44 , 45 , 46 ] have also been extensively discussed. The interlayer distance, or the channel height, determines the degree of confinement on the solution, and a wide range of factors affect the transportation behavior of the confined solutions, such as atomic structure, surface curvature, surface charge [ 18 ], chirality [ 47 ], pressure [ 48 ], and in-plane strain [ 49 ].…”