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.