“Liquid-like” Water in Clathrates Induced by Host–Guest Hydrogen Bonding

Abstract

Clathrate structures are sustained by a host lattice formed by hydrogen-bonding water molecules, which encapsulates guest molecules. Up to now, all water molecules in the host lattice are considered ice-like crystallized. Here, we discovered the occurrence of “liquid-like” water molecules and the resulting defects in (polar) tetrahydrofuran clathrates by liquid-state 1H NMR experiments. The liquid-like water molecules start occurring at 271 K, well below the apparent dissociation point (277 K) of the clathrate matrix, via extracting water molecules from the host lattice by host–guest H-bonding. We found an intriguing two-stage dissociation of the clathrate: Partial dissociation at 271 K converting one-third of water molecules into liquid-like followed by complete dissociation at 277 K. The clathrate structure is molecularly heterogeneous in the region between 271 K and 277 K. No liquid-like water exists in (nonpolar) cyclopentane clathrates. This work uncovers the essentiality of host–guest interaction for clathrate structures and the ability to tune their stability using polar molecules.

Clathrate Adhesion Induced by Quasi-Liquid Layer

“…Complete dissociation observed at 4.0 °C results in a THF- d 8 solution interacting with the sphere (inset in Figure a). The present observation of gradual dissociation of THF- d 8 clathrate agrees with our recent nuclear magnetic resonance experiments . The THF- d 8 clathrate does not have a fixed dissociation point; instead, it dissociates gradually in the temperature range from 2 to 4 °C …”

“…The present observation of gradual dissociation of THF-d 8 clathrate agrees with our recent nuclear magnetic resonance experiments. 34 The THFd 8 clathrate does not have a fixed dissociation point; instead, it dissociates gradually in the temperature range from 2 to 4 °C. 34 Our experiments reveal that the capillary force between the clathrate surface and the hydrophilic sphere is largest in the premelting region.…”

“…34 The THFd 8 clathrate does not have a fixed dissociation point; instead, it dissociates gradually in the temperature range from 2 to 4 °C. 34 Our experiments reveal that the capillary force between the clathrate surface and the hydrophilic sphere is largest in the premelting region. For the original silica sphere, the capillary force is on the order of 1000 mN/m.…”

“…We used the clathrate of deuterated tetrahydrofuran (THF-d 8 , C 4 D 8 O 99.5 atom % D, Carl Roth GmbH) because we used the same stock solution for nuclear magnetic resonance (NMR) experiments. 34 The independent NMR experiments were performed in parallel addressing the origin of phase transitions in clathrates. 34 Hereafter, the THF-d 8 clathrate may be simply referred to as "clathrate".…”

“…34 The independent NMR experiments were performed in parallel addressing the origin of phase transitions in clathrates. 34 Hereafter, the THF-d 8 clathrate may be simply referred to as "clathrate". Clathrates were prepared in a freezer at atmospheric pressure through the following steps.…”