Molecular dynamics simulations of carbon dioxide hydrate growth in electrolyte solutions of NaCl and MgCl₂

Abstract

Molecular dynamics simulations are performed to study the growth of carbon dioxide (CO₂) hydrate in electrolyte solutions of NaCl and MgCl₂. The kinetic behaviour of the hydrate growth is examined in terms of cage content, density profile, and mobility of ions and water molecules, and how these properties are influenced by added NaCl and MgCl₂. Our simulation results show that both NaCl and MgCl₂ inhibit the CO₂ hydrate growth. With a same mole concentration or ion density, MgCl₂ exhibits stronger inhibition on the growth of CO₂ hydrate than NaCl does. The growth rate of the CO₂ hydrate in NaCl and MgCl₂ solutions decreases slightly with increasing pressure. During the simulations, the Na⁺, Mg²⁺, and Cl⁻ ions are mostly excluded by the growing interface front. We find that these ions decrease the mobility of their surrounding water molecules, and thus reduce the opportunity for these water molecules to form cage-like clusters toward hydrate formation. We also note that during the growth processes, several 5¹²6³ cages appear at the hydrate/solution interface, although they are finally transformed to tetrakaidecahedral (5¹²6²) cages. Structural defects consisting of one water molecule trapped in a cage with its hydrogen atoms being attracted by two Cl⁻ ions have also been observed.

Keywords:

• molecular dynamics simulation
• CO₂ hydrate
• crystal growth
• NaCl
• MgCl₂

Acknowledgements

We thank Prof. Amadeu K. Sum for kindly sharing the code for identifying cages in the hydrate with us and National Supercomputing Center of Shenzhen for allocations of computational time.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [grant number 51176192]; CAS Program [grant number KGZD-EW-301]; 863 Program [grant number 2012AA061403-03].