Guest–Host Interactions in Mixed CH₄–CO₂ Hydrates: Insights from Molecular Dynamics Simulations

Abstract

Classical molecular dynamics simulations of mixed CH4–CO2 gas hydrates provide a thorough analysis of the energetics of both pure CH4 and CO2 hydrates and three intermediate compositions along the (CH4)1–x (CO2) x ·5.75­(H2O) solid solution. The energy is broken into guest–guest, guest–host, and host–host contributions. Radial distribution functions and three-dimensional density distributions provide insight into the changes in guest orientation and interactions with the host framework as a function of guest composition. Both energetic and structural analyses provide complementary information to previous experimental studies of the system. The experimentally observed isotropic orientation of CH4 molecules in both small and large cages is confirmed and the description of the anisotropic orientation of CO2 molecules in the large cage is confirmed and further enhanced. In mixed hydrates, the presence of CH4 alters the orientation of CO2, an indication that the interactions between guests, either direct (guest–guest) or mediated through interactions with the host (host–guest), are an important phenomenon in these systems.

Local structure and distortions of mixed methane-carbon dioxide hydrates

“…Consistent with former experimental and computational results, the CH 4 molecule in both the MD and experimental distributions in Fig. 4a-f, m-r, respectively, orients with a spherical distribution in both the large and small cages 7,18 . Previously, the CO 2 molecule was shown to orient in an oblate shape parallel to the hexagonal face of the large hydrate cage, along the longer axis 7,10,24,35 .…”

“…Local structure analysis of the guest-host RDFs in Fig. 3 lead to an understanding of how the guest molecules in different compositions interact with the host and affect cage filling, formation, stability, and decomposition behavior 3,7,18 . RDFs corresponding to the CO 2 molecule center (the central C) for pure CO 2 and mixed CH 4 -CO 2 hydrates are plotted in the top row of Fig.…”

“…Computational studies suggest that mixing guests in the hydrate structure impacts the guest-host (CH 4 or CO 2 -H 2 O), guest-guest, and host-host PDFs. CH 4 is found to have weaker interactions with the host, providing a more ordered lattice, whereas CO 2 interacts strongly with the host and other guests, leading to disorder 18,21 . Complementary local structure experiments investigating the impact of CH 4 , CO 2 , and their mixture on the structure and stability of gas hydrates have not been reported to the best of our knowledge.…”

“…This structure is made up of eight cages: two small dodecahedral and six large tetrakaidekahedral cages, which can occlude up to one guest molecule 3 . The average crystal structure of sI CH 4 and CO 2 hydrate has been well characterized with diffraction [6][7][8][9][10][11] , spectroscopy [12][13][14][15] , MD simulations [16][17][18][19][20][21][22][23] , and density functional theory calculations 11,[24][25][26] , which suggest a high degree of disorder within the crystal. This disorder is evident in the crystallographic model of sI CH 4 hydrate which requires four partially occupied proton positions for each H 2 O molecule and many positions to represent the unbonded, freely rotating CH 4 or partially constrained CO 2 molecules 27 .…”

“…This disorder is evident in the crystallographic model of sI CH 4 hydrate which requires four partially occupied proton positions for each H 2 O molecule and many positions to represent the unbonded, freely rotating CH 4 or partially constrained CO 2 molecules 27 . The intermolecular interactions that result in structural disorder are frequently studied with pair distribution functions (PDFs) calculated from simulated atomic models 18,22,23,[28][29][30] . Computational studies suggest that mixing guests in the hydrate structure impacts the guest-host (CH 4 or CO 2 -H 2 O), guest-guest, and host-host PDFs.…”