Unraveling hydrate stoichiometry in water/alkyl alcohol systems by thermodynamic modeling

01.Oct.2024

The solid–liquid equilibrium (SLE) phase diagram of water/alkyl alcohol systems indicated the formation of several hydrates. However, due to kinetic limitations and significantly low melting temperatures of these hydrates, assessing their stoichiometry remains an unresolved challenge. To this end, we utilized thermodynamic modeling of SLE in four binary water/alkyl alcohol systems: water/methanol, water/ethanol, water/iso-propyl alcohol (IPA), and water/tert-butyl alcohol (TBA), to check the consistency between the measured SLE phase diagram and the postulated hydrate stoichiometry in each system. We found that two possible hydrates could be formed in the water/methanol system: a stable 1:1 hydrate and a metastable 2:1 hydrate. On the other hand, 4:1 and 2:1 hydrates were formed in the water/ethanol system, and the latter was found to be more difficult to crystallize. The SLE phase diagram of the water/IPA system was more complex compared to the other studied systems, and the determination of the exact stoichiometry of the hydrates was challenging. We found that at least three hydrates should form to describe the measured SLE phase diagram. The stoichiometry of the three hydrates formed in the water/IPA system was 5:1, 2:1, and 3:2. In contrast, the stoichiometry of the hydrates formed in the water/TBA was previously confirmed by single-crystal X-ray diffraction, and the SLE phase diagram of the water/TBA was accurately modeled when the formation of 7:1 and 2:1 hydrates was considered. This work contributes to the fundamental knowledge of hydrate science and offers valuable guidance for the development of more efficient methods for hydrate characterization.

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