Nonmonotonous Lattice Distortion Model for Gas Hydrates

Area of Science:

• Physical Chemistry
• Materials Science
• Thermodynamics

Background:

• Gas hydrates form complex structures trapping gas molecules under specific temperature and pressure conditions.
• Existing theories on hydrate lattice distortion primarily explain cavity expansion, not contraction, based on guest size.
• This limitation leads to a simplified, monotonous relationship between guest size and lattice distortion.

Purpose of the Study:

• To develop a novel model for gas hydrate lattice distortion that accounts for both contraction and expansion.
• To investigate the non-monotonous relationship between guest size and lattice distortion.
• To establish a baseline for undistorted sII-type hydrate lattice distortion.

Main Methods:

• Statistical thermodynamics-based model combined with the modified Patel-Teja equation of state.
• Ab initio quantum mechanical calculations using spin-component-scaled second-order Møller-Plesset (SCS-MP2) perturbation theory.
• Calculation of reference chemical potential difference (RCPD) from experimental hydrate phase equilibrium data.

Main Results:

• A non-monotonous lattice distortion model was identified, where RCPD initially decreases then increases with guest size.
• Small guests were shown to contract the hydrate cavity, while larger guests cause expansion.
• The minimum lattice distortion (794.0913 J mol⁻¹) for an undistorted sII-type hydrate lattice was reported.

Conclusions:

• The proposed model accurately captures the complex, non-monotonous lattice distortion in gas hydrates.
• This research provides a more comprehensive understanding of guest-host interactions within hydrate structures.
• The findings are validated across a range of guest molecules, including methane, nitrogen, and propane hydrates.