Ethylene oxide hydrate non-stoichiometry: measurements and implications

Abstract

Clathrate hydrates have long been known as non-stoichiometric compounds, but the relation between hydrate and overall system composition is often overlooked. Previous density measurements showed that the composition of ethylene oxide (EO) structure I (sI) hydrate changes when formed from different EO solutions. In our X-ray diffraction (XRD) experiments, it was found that at 263.15 K , the unit cell (11.987 A ̊ ) of EO hydrate formed from EO-lean (<11%) solutions was smaller than that formed from EO-rich (>14%) solutions (12.020 A ̊ ) . Since all conditions except overall composition were constant, the hydrate lattice parameter could only be changed via hydrate composition. Raman spectroscopy measurements also showed that the hydrate composition changes when formed from different solutions. The hydrate composition change was predicted using the statistical mechanics model developed by van der Waals and Platteeuw. The predictions agreed well with the measured data. An EO–H 2O phase diagram with a hydrate solid solution range is proposed based upon these measurements and calculations. It is probable that a solid solution range also exists in CH 4 and CO 2 hydrates. This may impact energy recovery from sea-floor methane hydrates and CO 2 sequestration in the deep ocean. Schematic phase diagrams are presented for CH 4+H 2O and CO 2+H 2O which account for the hydrate solid solution range.