Synthesis and characterization of clathrate hydrates containing carbon dioxide and ethanol

This paper reports an experimental study on the formation of a clathrate hydrate containing ethanol as a guest substance together with carbon dioxide. Phase equilibrium measurements in the system of CO2 + ethanol + water have been performed in the temperature range from 254 K to 268 K. The measured equilibrium pressure at a given temperature below 264 K was lower by 20 kPa to 30 kPa than the corresponding equilibrium pressure of a simple CO2 hydrate in the system without ethanol. Powder X-ray diffraction measurements indicated that the hydrate formed is structure I. This crystallographic structure is the same as that of simple CO2 hydrate. However, the lattice constant of the hydrate formed in the system with ethanol was determined to be 11.8577(5) Å at 113 K, which is larger than the corresponding lattice constant (11.8434(8) Å) of simple CO2 hydrate at the same temperature. These experimental results strongly suggest the formation of a new hydrate containing both ethanol and CO2 as guest substances, thus contributing toward future applications considering CO2 hydrates. To further understand the inclusion of ethanol molecules in the structure I hydrate and the measured increase in the crystal lattice constant, molecular dynamics simulations were performed to support the experimental results.