Molecular Study on Carbon Dioxide Hydrate Formation in Salty Water

Clathrate hydrate-based seawater desalination has the potential to be a low-energy, environmentally friendly technique. However, the underlying hydrate growth and ion fractionation mechanism remains unclear. Hence, systematic molecular dynamics simulations were conducted to investigate the carbon dioxide hydrate growth process in the presence of salt. We find that hydrate growth slows with the increasing solution salinity, and the number of ions bound in the hydrate structure is related to the growth rate and the salinity. Ions can replace the water molecules at the vertex of the cages, which has been proven to be energetically unfavorable. The temperature dependence of the hydrate growth rate shows a maximum, while the pressure has a negligible impact on the growth rate of carbon dioxide hydrate under our conditions. The ion concentration in the hydrate phase decreases with an increase in temperature and is hardly affected by the pressure. Further examination indicates that the trapping of ions in the hydrate phase is kinetically controlled by the diffusion properties of the ions. Our study not only expands our understanding of the process of hydrate formation in seawater but also has great benefits for hydrate-based seawater desalination and seabed carbon sequestration.

Automated summary

This study investigates the growth process of carbon dioxide hydrate in the presence of salt through systematic molecular dynamics simulations. The results show that the growth rate of hydrate decreases with increasing solution salinity, and the number of ions bound in the hydrate structure is related to the growth rate and salinity. The replacement of water molecules by ions at the vertex of the cages is energetically unfavorable. The temperature has a maximum effect on the growth rate, while pressure has a negligible impact. The ion concentration in the hydrate phase decreases with temperature and is hardly affected by pressure. The trapping of ions in the hydrate phase is kinetically controlled by ion diffusion properties.