Molecular Rotational Dynamics in Mixed CH₄–CO₂ Hydrates: Insights from Molecular Dynamics Simulations

Abstract

The impact of guest molecule composition on the rotational dynamics in CH 4 , CO 2 , and mixed CH 4 −CO 2 gas hydrates is investigated with classical molecular dynamics simulations. Rotational autocorrelation functions are calculated for the guest and host molecules in each hydrate composition from simulation trajectories at 10, 40, 190, and 270 K. Analysis of these functions for each molecule is further decomposed into cage type for each CH 4 and CO 2 guest and cage face for the H 2 O host. CH 4 becomes more constrained, and CO 2 gains freedom in the mixed guest systems. Mixing guest species in gas hydrates alters the intermolecular interaction environment, impacting the rotational motion of the guest molecules. This effect is also seen in the host lattice H 2 O molecules, as evidence of molecular rotations is seen in the CO 2 hydrate at 270 K during longer simulations.

Structural Stability of the CO2@sI Hydrate: a Bottom‐Up Quantum Chemistry Approach on the Guest‐Cage and Inter‐Cage Interactions

“…[7,11,12] CO 2 clathrate hydrates have been mainly found in the sI crystal structure, although some evidence of the CO 2 @sII type, and so far binary CO 2 @sH clathrates have also been reported. [2,[13][14][15][16][17][18] The stability of these clathrates relies on the nature of the guest molecule/s and its interactions with the hydrogen-bonded water network. Commonly, when considering properties of gas clathrate hydrates, semiempirical models are used, as are easier to handle, with a microscopic understanding of the underlying guest-host interactions as well as their effect on macroscopic properties being still far from complete.…”

“…CO 2 clathrate hydrates have been mainly found in the sI crystal structure, although some evidence of the CO 2 @sII type, and so far binary CO 2 @sH clathrates have also been reported [2,13–18] . The stability of these clathrates relies on the nature of the guest molecule/s and its interactions with the hydrogen‐bonded water network.…”

Local structure and distortions of mixed methane-carbon dioxide hydrates

“…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 .…”

A comprehensive review on molecular dynamics simulation studies of phenomena and characteristics associated with clathrate hydrates

“…With respect to the analysis of structural and energetic properties of the sI and sII CO hydrate it was revealed that increasing the content of CO molecules in the large cages can stabilize the sII but destabilize sI hydrate [399]. Using the rotational autocorrelation function (RACF) to study the host and guest rotational dynamics, it was highlighted that altering the rotational motion of both water and guest molecules affect the proportion of them in mixed CO2+CH4 hydrate [400]. Dynamical and energetic properties of H2+THF hydrate through EMD simulations determined that the van der Waals component with the surrounding water molecules in the constituent cavities is the largest contribution to the interaction energy of both guests [401].…”