Transport Mechanism of Guest Methane in Water-Filled Nanopores

Abstract

We computed the transport of methane through 1 nm wide slit-shaped pores carved out of selected solid substrates using classical molecular dynamics simulations. The transport mechanism was elucidated via the implementation of the well-tempered metadynamics algorithm, which allowed for the quantification and visualization of the free energy landscape sampled by the guest molecule. Models for silica, magnesium oxide, alumina, muscovite, and calcite were used as solid substrates. Slit-shaped pores of width 1 nm were carved out of these materials and filled with liquid water. Methane was then inserted at low concentration. The results show that the diffusion of methane through the hydrated pores is strongly dependent on the solid substrate. While methane molecules diffuse isotropically along the directions parallel to the pore surfaces in most of the pores considered, anisotropic diffusion was observed in the hydrated calcite pore. The differences observed in the various pores are due to local molecular properties of confined water, including molecular structure and solvation free energy. The transport mechanism and the diffusion coefficients are dependent on the free energy barriers encountered by one methane molecule as it migrates from one preferential adsorption site to a neighboring one. It was found that the heterogeneous water distribution in different hydration layers and the low free energy pathways in the plane parallel to the pore surfaces yield the anisotropic diffusion of methane molecules in the hydrated calcite pore. Our observations contribute to an ongoing debate on the relation between local free energy profiles and diffusion coefficients and could have important practical consequences in various applications, ranging from the design of selective membranes for gas separations to the sustainable deployment of shale gas.

Exploration of Capturing CO2 from Flue Gas by Calcite Slit‐Nanopores: A Computational Investigation

“…While in contrast to CO 2 , the N 2 molecules have weaker adsorption density onto the calcite surface due to the weaker adsorption interactions between the N 2 molecules and the surface. From Figure 4b, it can be found that the adsorption sites occupied by the N 2 molecules on the calcite surface is consistent with the areas of the free energy distribution of CH 4 molecules adsorbed on calcite surface, [28] which demonstrates that the adsorption intensity of N 2 molecules on calcite surface might be similar to the CH 4 , but which is much weaker than CO 2 .…”

Supramolecular Organization in Confined Nanospaces

“…observed a higher tendency of ethanol to selectively adsorb on the pore walls of boron nitride, and showed that the ethanol/water partial demixing was relatively unaffected by the pore width (Figure b) . Similar microphase‐separated tubular structures seem to be a recurrent feature of binary liquid mixtures in mesopores, and are also supported by experimental evidences . For example, neutron diffraction experiments on MCM‐41 filled with a tert‐butanol‐toluene mixture evidenced inhomogeneous distribution of the components, suggesting that the pore surface is covered by tert‐butanol molecules, while toluene preferentially locates in the central region of the pore …”

“…[394] Similar microphase-separated tubular structures seem to be a recurrent feature of binary liquid mixtures in mesopores, and are also supported by experimental evidences. [398][399][400][401] For example, neutron diffraction experiments on MCM-41 filled with a tert-butanol-toluene mixture evidenced inhomogeneous distribution of the components, suggesting that the pore surface is covered by tert-butanol molecules, while toluene preferentially locates in the central region of the pore. [400] Theoretical models well illustrate the role of surface silanols in the arrangement of guest species.…”

The Effect of Hydration on the Structure and Transport Properties of Confined Carbon Dioxide and Methane in Calcite Nanopores

“…An evaluation of the diffusivity of methane, nitrogen and CO 2 confined in calcite slit-shaped nanopore showed that the diffusivity of the simulated fluids close to the surface differs from the molecules away from the solid interface and toward the center of the pore (Franco et al, 2016). This anisotropic behavior was evident in the diffusivity of methane through hydrated calcite pores (Bui et al, 2017). In the absence of any dissolved gas solutes, the translational and rotational dynamics of confined water were found to be dependent on the local density variation and the local hydrogen bonds connectivity, respectively (Mutisya et al, 2017).…”

“…Calcite is present in almost all geological systems and represents the main constituent of limestone which forms most of the world's hydrocarbons reservoirs (Meldrum and Cölfen, 2008;Geysermans and Noguera, 2009;Addari and Satta, 2015;Bovet et al, 2015;Côté et al, 2015). For this reason, several studies focused on understanding gas interactions in calcite nano-pores (Franco et al, 2016;Sun et al, 2016bSun et al, , 2017Wang S. et al, 2016a,b;Bui et al, 2017;Simoes Santos et al, 2018). Further, shale formations such as the Middle Bakken region in North America are characterized by a significant fraction of calcite which contribute to pore sizes that are 2 nm or greater (Liu et al, 2017).…”