To better understand the effect of
size and flexibility of large molecule guest substances (LMGSs) on
crystal lattice structure and thermodynamic stability of structure
H (sH) clathrate hydrates, we performed powder X-ray diffraction (PXRD)
measurements and Parrinello–Rahman molecular dynamics (MD)
simulations on six alkane and cycloalkane LMGS in the presence of
methane help gas. We first quantitatively analyze the dependence of
the experimental lattice constants and formation pressure on the average
maximum length of the LMGS guests as determined by MD simulation.
The PXRD results show that within a family of LMGSs with similar molecular
structures molecules of optimum size give better stability of sH hydrate
phase. The most stable hydrate in each class has larger lattice constants
along the a-axis and smaller lattice constant along
the c-axis. The lattice expansion/shrinkage is well
reproduced in the MD simulations. From the MD simulations, we determine
the changes in the conformation as a result of encapsulation and the
tilt angle with respect to the long axis of the sH large cages of
the LMGSs. The results indicate that the molecular shapes inside the
sH large cages can significantly differ from those of the most stable
molecular structure in the gas phase. In the case of flexible molecules,
such as 2-methylbutane, the 1–4 dihedral angles and effective
molecular sizes change upon encapsulation. Molecules with shorter
length generally have larger tilt angles in the large cages; however,
the effective width dimension of the LMGS also affects the tilt angle.
Understanding the stability of sH hydrates of various LMGSs requires
a consideration of guest molecule size, structural flexibility, and
tilt angle in the cages. None of these quantities alone explain trends
in the stability. During simulation trajectories, we observe changes
in conformation in the LMGS molecules in the large cages. The effects
of these different factors make a priori structural
determinations of the large cages guests extremely complex.
Experiments were carried out to synthesize and characterize a structure H clathrate hydrate containing CO(2) and 3,3-dimethyl-2-butanone (pinacolone) by means of phase equilibrium and powder X-ray diffraction measurements. Molecular dynamics simulations of this structure H hydrate were performed to understand the nature of guest-host molecular interactions.
This paper report analyses of thermodynamic stability of structure-H clathrate hydrates formed with methane and large guest molecules in terms of their gas phase molecular sizes and molar masses for the selection of a large guest molecule providing better hydrate stability. We investigated the correlation among the gas phase molecular sizes, the molar masses of large molecule guest substances, and the equilibrium pressures. The results suggest that there exists a molecular-size value for the best stability. Also, at a given molecule size, better stability may be available when the large molecule guest substance has a larger molar mass.
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