Estudo teórico - computacional da adsorção de etanol, água e do dímero etanol - água em nanocapilares de grafeno de extremidade aberta

Abstract

Graphene has received much attention due to its properties and applicability in various areas. Among them, graphene nanocapillaries (GC) have been studied for molecular filters in separating water-ethanol mixtures. These nanofilters have great socio-economic and environmental potential, mainly because they can contribute to or even replace fractional distillation, which is used on a large scale to separate ethanol from water during the production of ethanol biofuel, requiring high energy expenditure. Thus, using a nanofilter to separate the mixture could significantly reduce this cost and the emission of gases such as carbon dioxide, making it a more sustainable alternative. In this work, the adsorption of ethanol, water, and ethanol-water dimers on open-edge graphene capillaries was evaluated employing first-principles calculations using Density Functional Theory (DFT), with the aim of providing a detailed understanding of the molecule-substrate interaction. Further, in order to better understand the general behavior of the forces acting on the systems, such as van der Waals forces, analytical calculations were carried out using the Lennard-Jones potential for a carbon atom, oxygen, hydrogen, and the carbon monoxide (CO) molecule in different arrangements interacting with the GC. The results obtained using DFT showed that the stability of the ethanol molecule adsorbed on the GC depends on its orientation within the structure, its position along the cross-section, and the distance between layers of the GC (d). The ethanol molecule was observed to be more stable when vertically aligned on the edge of the CG for 8 Å ≲ d ≲ 10 Å. On the other hand, the ethanol molecule in the center of the GC, oriented horizontally concerning the surface, represents the most stable configuration when d ≲ 8 Å and d ≳ 10 Å. The water molecule was found to be more stable at the edge of the GC, regardless of its orientation in relation to the substrate and the distance d. Ethanol is more stable than water in GCs with d ≳ 6 Å, and water is more stable at lower d values. The ethanol-water dimer finds its most stable position when adsorbed on the edge of the CG, with water being the donor and ethanol the acceptor in the hydrogen bridge bond. The water-CG bond is weakened in the dimer compared to the adsorbed molecule. In addition, the ethanol-CG interaction is not significantly affected after the formation of the dimer but is slightly intensified. It was found that after adsorption on the GC, ethanol, and water receive electrons in the chemical bond formed, but in the case of the ethanol-water dimer, ethanol donates. In contrast, water receives electrons compared to the molecule in the gas phase. Furthermore, the intensity of the chemical bond between the adsorbed molecules and the GC is not significantly altered depending on its width. The analytical results showed that there is a suction force that pulls the atoms and the CO molecule into the GC and that there are regions of stability within the GC, which depend on the atomic type and molecular orientation. The central region between the layers of the GC is a region of stable equilibrium for the CO atoms and molecule until d reaches a critical value, after which there are two regions of stable equilibrium equidistant from the center of the layers. Finally, the results provide relevant details on the accumulation of water and ethanol molecules at the edges of the GCs and the suction forces on atoms, providing information that can be useful for future studies and the development of nanofilters for ethanol-water mixtures based on graphitic systems.