Viscosity and Structure of Water and Ethanol within GO Nanochannels: A Molecular Simulation Study

Abstract

The behavior of liquids in two-dimensional (2-D) graphene oxide (GO) nanopores is important for developing GObased nanoscience and nanofluidics. Herein, molecular dynamics simulation was carried out to study the equilibrium structures and shear viscosity for water and ethanol confined within 2-D GO nanochannels. It was observed that both species obviously exhibit structured features near GO surfaces. The confined viscosities are anisotropic with axial shear viscosity larger than vertical viscosity. The axial shear viscosities of water and ethanol are greatly enhanced for the 2-D GO nanochannels, wherein the viscosity features a decreased pattern with the pore width. Compared with water molecules, the confinement of GO channels has more effect on the viscosity of ethanol molecules. The confined shear viscosity can be described by combining contributions of the interfacial layer viscosity and the bulk-like viscosity. The influences of oxidation degrees and pore widths on the structure and transport properties have been systematically investigated, in which the interlayer viscosity is the critical determining factor. The confined structures and surface interaction were applied to interpret the transport properties of confined liquids. The enhanced interfacial layer viscosity can be attributed to the surface hydrogen-bonding interaction arising from the oxygen-containing functional groups.

Insight into the Structure and Dynamics of Ethanol–Water Binary Mixture Confined in Nanochannel by Mica and Graphene

“…It has been reported that the self-diffusion coefficient of ethanol in bulk mixture decreases with an increase in ethanol concentration and passes through a minimum around x EtOH = 0.2–0.25 in the blends. − However, results show that the diffusion coefficient of confined ethanol and water molecules reduces with increasing the mole fraction of ethanol (i.e., up to x EtOH = 0.5) in the mixtures, which is akin to the behavior found in previous MD studies of confined ethanol–water in the hydrophilic or hydrophobic surfaces. ,, This might be because water and ethanol molecules strongly adsorb near the hydrophilic mica and hydrophobic graphene sheets, respectively. The diffusivities of ethanol in nanopores at different concentrations and temperatures vary from 10 –11 to 10 –9 which is in line with the previously reported simulation value of the diffusion coefficient of ethanol confined in different nanopores. ,, However, the lateral diffusion coefficient of confined ethanol in the experiment , ranges from 10 –15 to 10 –14 , which is approximately 4–5 orders of magnitude lower than the simulation value. This difference in the diffusion coefficient of ethanol is due to ethanol molecules forming 2D islands in between an ice-like water layer and flexible hydrophobic graphene.…”

“…The selectivity of ethanol molecules is greater in the hexagonal boron nitride slit nanopore than graphene pore . A recent study shows a higher shear viscosity of the ethanol–water mixture confined in graphene oxide (GO) nanopores in an axial direction compared to the perpendicular direction of pore walls . It has been found that confinement significantly affects the viscosity of ethanol molecules compared to water molecules, and the viscosity decrease with the nanopore width …”

“…42 A recent study shows a higher shear viscosity of the ethanol−water mixture confined in graphene oxide (GO) nanopores in an axial direction compared to the perpendicular direction of pore walls. 43 It has been found that confinement significantly affects the viscosity of ethanol molecules compared to water molecules, and the viscosity decrease with the nanopore width. 43 Although several studies focused on the separation of water from aqueous ethanol solutions using the diverse nature of the pore walls, comprehensive knowledge of the structure and dynamics properties of fluid mixture confined between a hydrophilic and hydrophobic nature of the pore walls is still lacking.…”

“…43 It has been found that confinement significantly affects the viscosity of ethanol molecules compared to water molecules, and the viscosity decrease with the nanopore width. 43 Although several studies focused on the separation of water from aqueous ethanol solutions using the diverse nature of the pore walls, comprehensive knowledge of the structure and dynamics properties of fluid mixture confined between a hydrophilic and hydrophobic nature of the pore walls is still lacking. The dependency on the concentrations of the ethanol, pore size, and more specifically, confined surfaces with hydrophilic and hydrophobic interaction with fluid is yet to be explored.…”

Molecular-Level Insights into Unique Behavior of Water Molecules Confined in the Heterojunction between One- and Two-Dimensional Nanochannels

“…However, larger interlayer distance can lead to a larger distance between two secondary ones. Similar multilayer phenomena have been observed for water molecules within other 2-D confinements. ,,− ,− As water molecules enter into subregion 2 from −40 to −35 Å, most of their structural distributions are identical to those in subregion 2 except for the HCG6 system by comparing Figure b with Figure a. In the HCG6 system, two secondary ones show a certain integration toward each CNT orifice although the subregion is still far away from the CNT orifice.…”