# Atomistic modeling of structure II gas hydrate mechanics: Compressibility and equations of state

## Abstract

This work uses density functional theory (DFT) to investigate the poorly characterized structure II gas hydrates, for various guests (empty, propane, butane, ethane-methane, propane-methane), at the atomistic scale to determine key structure and mechanical properties such as equilibrium lattice volume and bulk modulus. Several equations of state (EOS) for solids (Murnaghan, Birch-Murnaghan, Vinet, Liu) were fitted to energy-volume curves resulting from structure optimization simulations. These EOS, which can be used to characterize the compressional behaviour of gas hydrates, were evaluated in terms of their robustness. The three-parameter Vinet EOS was found to perform just as well if not better than the four-parameter Liu EOS, over the pressure range in this study. As expected, the Murnaghan EOS proved to be the least robust. Furthermore, the equilibrium lattice volumes were found to increase with guest size, with double-guest hydrates showing a larger increase than single-guest hydrates, which has significant implications for the widely used van der Waals and Platteeuw thermodynamic model for gas hydrates. Also, hydrogen bonds prove to be the most likely factor contributing to the resistance of gas hydrates to compression; bulk modulus was found to increase linearly with hydrogen bond density, resulting in a relationship that could be usedmore »

- Authors:

- Department of Chemical Engineering, McGill University, Montreal H3A 0C5 (Canada)

- Publication Date:

- OSTI Identifier:
- 22611395

- Resource Type:
- Journal Article

- Resource Relation:
- Journal Name: AIP Advances; Journal Volume: 6; Journal Issue: 8; Other Information: (c) 2016 Author(s); Country of input: International Atomic Energy Agency (IAEA)

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; BUTANE; CHEMICAL PHYSICS; COMPRESSIBILITY; COMPRESSION; DENSITY FUNCTIONAL METHOD; EQUATIONS OF STATE; EQUILIBRIUM; ETHANE; GAS HYDRATES; HYDROGEN; METHANE; OPTIMIZATION; PRESSURE RANGE; SIMULATION; SOLIDS; THERMODYNAMIC MODEL; VAN DER WAALS FORCES

### Citation Formats

```
Vlasic, Thomas M., Servio, Phillip, and Rey, Alejandro D., E-mail: alejandro.rey@mcgill.ca.
```*Atomistic modeling of structure II gas hydrate mechanics: Compressibility and equations of state*. United States: N. p., 2016.
Web. doi:10.1063/1.4961728.

```
Vlasic, Thomas M., Servio, Phillip, & Rey, Alejandro D., E-mail: alejandro.rey@mcgill.ca.
```*Atomistic modeling of structure II gas hydrate mechanics: Compressibility and equations of state*. United States. doi:10.1063/1.4961728.

```
Vlasic, Thomas M., Servio, Phillip, and Rey, Alejandro D., E-mail: alejandro.rey@mcgill.ca. Mon .
"Atomistic modeling of structure II gas hydrate mechanics: Compressibility and equations of state". United States.
doi:10.1063/1.4961728.
```

```
@article{osti_22611395,
```

title = {Atomistic modeling of structure II gas hydrate mechanics: Compressibility and equations of state},

author = {Vlasic, Thomas M. and Servio, Phillip and Rey, Alejandro D., E-mail: alejandro.rey@mcgill.ca},

abstractNote = {This work uses density functional theory (DFT) to investigate the poorly characterized structure II gas hydrates, for various guests (empty, propane, butane, ethane-methane, propane-methane), at the atomistic scale to determine key structure and mechanical properties such as equilibrium lattice volume and bulk modulus. Several equations of state (EOS) for solids (Murnaghan, Birch-Murnaghan, Vinet, Liu) were fitted to energy-volume curves resulting from structure optimization simulations. These EOS, which can be used to characterize the compressional behaviour of gas hydrates, were evaluated in terms of their robustness. The three-parameter Vinet EOS was found to perform just as well if not better than the four-parameter Liu EOS, over the pressure range in this study. As expected, the Murnaghan EOS proved to be the least robust. Furthermore, the equilibrium lattice volumes were found to increase with guest size, with double-guest hydrates showing a larger increase than single-guest hydrates, which has significant implications for the widely used van der Waals and Platteeuw thermodynamic model for gas hydrates. Also, hydrogen bonds prove to be the most likely factor contributing to the resistance of gas hydrates to compression; bulk modulus was found to increase linearly with hydrogen bond density, resulting in a relationship that could be used predictively to determine the bulk modulus of various structure II gas hydrates. Taken together, these results fill a long existing gap in the material chemical physics of these important clathrates.},

doi = {10.1063/1.4961728},

journal = {AIP Advances},

number = 8,

volume = 6,

place = {United States},

year = {Mon Aug 15 00:00:00 EDT 2016},

month = {Mon Aug 15 00:00:00 EDT 2016}

}