Effect of Adsorbed Alcohol Layers on the Behavior of Water Molecules Confined in a Graphene Nanoslit: A Molecular Dynamics Study

Abstract

With the rapid development of a two-dimensional (2D) nanomaterial, the confined liquid binary mixture has attracted increasing attention, which has significant potential in membrane separation. Alcohol/water is one of the most common systems in liquid-liquid separation. As one of the most focused systems, recent studies have found that ethanol molecules were preferentially adsorbed on the inner surface of the pore wall and formed an adsorbed ethanol layer under 2D nanoconfinement. To evaluate the effect of the alcohol adsorption layer on the mobility of water molecules, molecular simulations were performed to investigate four types of alcohol/water binary mixtures confined under a 20 Å graphene slit. Residence times of the water molecules covering the alcohol layer were in the order of methanol/water < ethanol/water < 1-propanol/water < 1-butanol/water. Detailed microstructural analysis of the hydrogen bonding (H-bond) network elucidated the underlying mechanism on the molecular scale in which a small average number of H-bonds between the preferentially adsorbed alcohol molecules and the surrounding water molecules could induce a small degree of damage to the H-bond network of the water molecules covering the alcohol layer, resulting in the long residence time of the water molecules.

Adsorption of water, methanol, and their mixtures in slit graphite pores

“…These results are consistent with another recent simulation study in which the adsorption of alcohol/water mixtures at a mole fraction of 5% in slit graphene pores 20 Å-wide is modeled using SPC/E water and OPLS-AA alcohol. 21 In that instance, an alcohol concentration close to 91% for the adsorbed methanol-water mixture was reported, in agreement with our findings. Note, however, that the study of Gao et al 21 is not directly comparable to ours since in their case the confined fluid is not in equilibrium with a liquid solution mediated by the corresponding vapor phase, i.e., it is not the result of an equilibrium adsorption process.…”

“…4 On the other hand, studies addressing the adsorption of alcohol and water/alcohol mixtures in simple geometries are comparatively more scarce, although the topic has gained some attention during the last few years. [17][18][19][20][21][22][23][24][25][26] Experiments of adsorption of pure methanol were carried out mainly in carbon black, activated carbon, or in crystalline carbon pores with hexagonal shape. [27][28][29] The adsorption in a simple slit geometry was investigated in a few simulation studies, which showed that adsorption occurs below bulk saturation pressure for pores narrower than 2 nm and that methanol forms chains for narrow pores, which are perturbed as the pore size increases.…”

“…Diffusion rates are also affected by confinement to the point that they can be up to six orders of magnitude lower than in bulk. [17][18][19] Molecular dynamics (MD) simulations of water/alcohol mixtures confined between two graphene layers 21 show that, similar to water, molecules in these solutions are also structured in layers under parallel hydrophobic confinement. These simulations predict that as the pore size increases, a gradient of alcohol concentration is induced, with alcohol molecules arranged preferentially in the vicinity of the pore walls.…”

Effect of Surface Type on the Flow Characteristics in Shale Nanopores

“…It shows that the methane molecules could have a great slip velocity on the graphene surface, and such slip velocity could be up to 8.9 m/s, 16.3 m/s, and 28.6 m/s when the pore size is 5 nm, 10 nm, and 15 nm, respectively. Actually, in the slit-like graphene nanopore, such obvious surface slip velocity appears not only for CH 4 , but also in the flow of water, alkane, and some other geofluids [45,62,63]. The result also indicates that the pore size has nonnegligible effect on the slip velocity 5 Geofluids when CH 4 is confined in organic graphene nanopore.…”

Molecular dynamics simulation of methane transport in confined organic nanopores with high relative roughness

“…To improve computational efficiency, we used coarse-grained particles to represent gases moving like particles with zero charge. Therefore, the effect of Coulomb interactions between the gases and the wall on the fluid transportation is ignored, and the effect of dangling and/or broken bonds is weak and also ignored here (Leggesse et al, 2016;Gao et al, 2017). A buffer distance of 2.5 Å is set between each pair of graphite fragments to allow relaxation of the sheets while preventing direct overlap.…”