Yun-Ho Ahn

Experimental verifications of Mpemba-like behaviors of clathrate hydrates

The exact mechanism of the Mpemba effect of water was recently investigated microscopically and explained on the basis of the cooperative relationship between intramolecular polar-covalent bonds (O-H) and intermolecular hydrogen bonds (O:H). We posited that this relationship might exist in clathrate hydrates since they consist of hydrogen-bonded host water frameworks and enclathrated guest molecules. The formation of tetrahydrofuran (THF) hydrate, which is the simplest clathrate hydrate, was investigated by using differential scanning calorimetry and Raman spectroscopy. THF hydrates show the Mpemba effect at lower initial temperatures, but formation times were delayed at higher initial temperatures because the evaporated THF molecules should be liquefied to correspond to the stoichiometric concentration of guest molecules to form sII clathrate hydrates. However, even though the formation time was delayed at higher initial temperatures, the rates of heat emission during THF hydrate formation, measured in a bulk state, roughly increased as the initial temperature increased. Moreover, we observed that the O: H stretching phonons of water in the THF hydrate showed a blue shift, and the O-H stretching mode showed a redshift as temperature decreased. Both the rate of heat emission and the Raman shift of these two bonds imply that a cooperative relationship between the covalent bond and the hydrogen bond exists in THF hydrate as pure water. The formation kinetics of THF hydrate therefore might depend on its initial temperature, thus showing Mpemba-like behavior.

Thermodynamic and spectroscopic identification of aldehyde hydrates

Abstract-It has been reported that some aldehyde compounds have formed simple sII clathrate hydrates without help-gas molecules, showing a self-forming effect. However, the structure of aldehyde hydrates is quite unstable due to the “gem-diol reaction”. According to the previous studies, the aldehyde hydrate slowly decomposes at atmospheric condition with the conversion of aldehyde to gem-diol. We investigated binary aldehyde (acetaldehyde, propionaldehyde, and isobutyraldehyde)+methane clathrate hydrate with spectroscopic and thermodynamic analyses. Similar to the simple aldehyde hydrate, the binary hydrates also formed a sII hydrate. During the hydrate formation process, we found that most of the aldehydes converted to gem-diols and were then incorporated into the large cages of the sII hydrate. Depending on the equilibrium constant of the gem-diol reaction caused by the molecular structures of the three aldehydes, different phase equilibrium curves of aldehyde+methane hydrates were obtained.

Thermodynamic Stability of Structure II Methyl Vinyl Ketone Binary Clathrate Hydrates and Effects of Secondary Guest Molecules on Large Guest Conformation

Clathrate hydrates have received massive attention because of their potential application as energy storage materials. Host water frameworks of clathrate hydrates provide empty cavities that can capture not only small molecular guests but also radical species induced by γ-irradiation. In this work, we investigated structure II methyl vinyl ketone (MVK) binary clathrate hydrates with CH4, O2, and N2 and the effects of secondary guest species on MVK conformation in the cavity of hydrate and on the thermodynamic stability of unirradiated and γ-irradiated hydrate phases. The present findings provide meaningful information to understand the nature of guest–host interactions in γ-irradiated clathrate hydrates and to open up practical applications for hydrate-based nanoreactors.