Clathrate Formation in Tetraisopentylammonium Bromide-Water System

Three polyhydrates of tetraisopentylammonium bromide with 38, 32, and 26 water molecules and also the dihydrate were found in the i-Pent₄NBr-H₂O system.     

Clathrate Hydrates in the System Tetraisopentylammonium Iodide-Water

Two clathrate hydrates i-Pent₄NI·36H₂O and i-Pent₄NI·32H₂O were revealed in the system (i-Pent)₄NI-H₂O. The hydrates melt incongruently at 14.2 and 14.8°C, respectively. Along with the polyhydrates, tetraisopentylammonium dihydrate was found.     

Diisoamyldibutylammonium Bromide Clathrate Hydrates

In the system i-Am₂Bu₂NBr-H₂O, along with the known compound i-Am₂Bu₂NBr·38H₂O, three new clathrate hydrates were revealed: i-Am₂Bu₂NBr·32H₂O, i-Am₂Bu₂NBr·26H₂O, and i-Am₂Bu₂NBr·24H₂O. Crystals of all the hydrates were isolated, and their compositions and melting points were determined.     

Clathrate Hydrates of Tetrabutylammonium Dicarboxylates. System Tetrabutylammonium Adipate-Water

In the system tetrabutylammonium adipate-water, two stable and two metastable polyhydrates were identified. The melting points of the polyhydrates were correlated with the anion hydrocarbon chain length in the series H₂O-[(C₄H₉)₄N]₂(CH₂) _n C₂O₄ (n = 0-8).     

Physicochemical Properties of Ionic Clathrate Hydrates

Ionic clathrate hydrates are known to be formed by the enclathration of hydrophobic cations or anions into confined cages and the incorporation of counterions into the water framework. As the ionic clathrate hydrates are considered for their potential applicability in various fields, including those that involve solid electrolytes, gas separation, and gas storage, numerous studies of the ionic clathrate hydrates have been reported. This review concentrates on the physicochemical properties of the ionic clathrate hydrates and the notable characteristics of these materials regarding their potential application are addressed.     

氮气笼型水合物的激光拉曼光谱

在253K和16MPa的压力下,于实验室内合成了氮气水合物,用显微共焦拉曼光谱对其N-N和O-H键伸缩振动的光谱特征进行了研究．结果表明,氮气水合物中的N-N和O—H键的拉曼峰分别为2322．4和3092．1cm^-1,与天然的空气水合物中的数据十分接近．另外,还测定了液氮和溶解于水中的氮分子中N—N键的拉曼峰值,分别为2326．6和2325．0cm^-1．氮气笼型水合物分解的拉曼谱图表明,氮分子同时进入水合物的大笼和小笼中,但由于氮分子在大、小笼中的环境氛围十分接近,其拉曼位移相差不大,故拉曼谱图只能显示N—N键伸缩振动一个峰．     

Clathrate Hydrates of Tetrabutylammonium and Tetraisoamylammonium Halides

Clathrate formation was considered for two series of systems: (C₄H₉)₄NG–H₂O and iC₅H₁₁)₄NG–H₂O G = F^-, Cl^-, Br^-, I^-). Clathrate hydrates of tetraisoamylammonium halides were shown to melt at higher temperatures than those of the butyl series. In passing from fluoride to bromide, the stability of compounds of the butyl series falls significantly and tetrabutylammonium iodide does not produce polyhydrates. In the isoamyl series, the melting points of polyhydrates vary insignificantly for different halides. In addition, the highest melting hydrate of tetraisoamylammonium bromide melts at a slightly higher temperature than chloride hydrates, indicating not only a hydrophilic effect of the anion on clathrate formation.     

Adsorption-Inhibition of Clathrate Hydrates by Self-Assembled Nanostructures

The mechanism by which safranine O (SFO), an ice growth inhibitor, halts the growth of single crystal tetrahydrofuran (THF) clathrate hydrates was explored using microfluidics coupled with cold stages and fluorescence microscopy. THF hydrates grown in SFO solutions exhibited morphology changes and were shaped as truncated octahedrons or hexagons. Fluorescence microscopy and microfluidics demonstrated that SFO binds to the surface of THF hydrates on specific crystal planes. Cryo-TEM experiments of aqueous solutions containing millimolar concentrations of SFO exhibited the formation of bilayered lamellae with an average thickness of 4.2±0.2 nm covering several μm². Altogether, these results indicate that SFO forms supramolecular lamellae in solution, which might bind to the surface of the hydrate and inhibit further growth. As an ice and hydrate inhibitor, SFO may bind to the surface of these crystals via ordered water molecules near its amine and methyl groups, similar to some antifreeze proteins.     

Clathrate Hydrates of Sulfur Hexafluoride at High Pressures

The pressure dependence (0.4 Mpa–1.3 GPa) of the hydrate decomposition temperatures in the sulfur hexafluoride-water system has been studied. In addition to the known low-pressure hydrate SF₆17H₂O of Cubic Structure II, two new high-pressure hydrates have been found. X-ray analysis in situ showed the gas hydrate forming in the sulfur hexafluoride-water system above 50 MPa at room temperature to be of Cubic Structure I. The ability of water to form hydrates whose structures depend on the guest molecule size under normal conditions and at high pressures is discussed.     

Theoretical Study of Clathrate Hydrates with Multiple Occupation

In this work, the electronic, structural, dynamic andthermodynamic properties of structure II, H and tetragonalAr clathrate hydrates have been calculated and the effectof multiple occupancy on their stability has been examinedusing first-principles and lattice dynamics calculations.The dynamic properties of these clathrates have beeninvestigated depending on the number of guest moleculesin a clathrate cage. It has been found that selectedhydrate structures are dynamically stable. The calculatedcell parameters are in agreement with experimental data.We also report the results of a systematic investigationof cage-like water structures using first-principles calculations. Ithas been observed that Ar clusters can be stabilized indifferent water cages and the stability is strongly dependenton the number of argon atoms inside the cages.     

Guest-Host Interaction Study in Clathrate Hydrates Using Lattice Dynamics Simulation

Lattice dynamics simulation of several gas hydrates （helium, argon, and methane） with different occupancy rates has been performed using TIP3P potential model. Results show that the coupling between the guest and host is not simple as depicted by the conventional viewpoints. For clathrate hydrate enclosing small guest, the small cages are dominantly responsible for the thermodynamic stability of clathrate hydrates. And the spectrum of methane hydrate is studied compared with argon hydrate, then as a result, shrink effect from positive hydrogen shell is proposed.     

Guest–Host Hydrogen Bonding in Structure H Clathrate Hydrates

Molecular dynamics simulations of the structure H (sH) clathrate of tert‐butylmethylether show the prevalence of ether–water hydrogen bonding (see picture), absent in the neo‐hexane sH clathrate. This affects guest–cage dynamics and host–water dielectric relaxation dynamics. The ¹³C and ¹H NMR relaxation times for both guests are measured, and the differences are explained in terms of guest–host interactions in the two clathrates.

     

Abnormal Thermal Expansion of Clathrate Hydrates Induced by Asymmetric Guest Molecules

We investigated for the first time the abnormal thermal expansion induced by an asymmetric guest structure using high‐resolution neutron powder diffraction. Three dihydrogen molecules (H₂, D₂, and HD) were tested to explore the guest dynamics and thermal behavior of hydrogen‐doped clathrate hydrates. We confirmed the restricted spatial distribution and doughnut‐like motion of the HD guest in the center of anisotropic sII‐S (sII‐S=small cages of structure II hydrates). However, we failed to observe a mass‐dependent relationship when comparing D₂ with HD. The use of asymmetric guest molecules can significantly contribute to tuning the cage dimension and thus can improve the stable inclusion of small gaseous molecules in confined cages.     

Distribution of Butane in the Host Water Cage of Structure II Clathrate Hydrates

To understand host–guest interactions of hydrocarbon clathrate hydrates, we investigated the crystal structure of simple and binary clathrate hydrates including butane (n‐C₄H₁₀ or iso‐C₄H₁₀) as the guest. Powder X‐ray diffraction (PXRD) analysis using the information on the conformation of C₄H₁₀ molecules obtained by molecular dynamics (MD) simulations was performed. It was shown that the guest n‐C₄H₁₀ molecule tends to change to the gauche conformation within host water cages. Any distortion of the large 5¹²6⁴ cage and empty 5¹² cage for the simple iso‐C₄H₁₀ hydrate was not detected, and it was revealed that dynamic disorder of iso‐C₄H₁₀ and gauche‐nC₄H₁₀ were spherically extended within the large 5¹²6⁴ cages. It was indicated that structural isomers of hydrocarbon molecules with different van der Waals diameters are enclathrated within water cages in the same way owing to conformational change and dynamic disorder of the molecules. Furthermore, these results show that the method reported herein is applicable to structure analysis of other host–guest materials including guest molecules that could change molecular conformations.     

Dynamics of Some He and Ar Clathrate Hydrates. Computer Simulation Study

Molecular dynamics simulation of solid solutions of He and Ar inice II at T 200 K has shown that amplitudes of water moleculeoscillations diminish when noble gas atoms fill the cavities ofthe hydrogen-bonded framework. The effect of Ar atoms ismore pronounced. Slow diffusion of He along trigonal axis isobserved when not all the cavities are filled. He and Ar atomsexert little effect on frequencies of translational and librationalvibrations of the water molecules. Type II empty gas hydrateframework (CSII) is quite stable at T 200 K. Amplitudes ofoscillations of water molecules which occupy differentcrystallographic positions are different. Filling of the cavitiesin the CSII framework with Ar atoms causes diminutionof the amplitudes of water molecule vibrations, and differencebetween amplitudes of vibration of molecules occupyingdifferent positions becomes less pronounced. Large cavities inthe CSII framework can accommodate two Ar atoms withoutdistortion. No diffusion of guest Ar atoms was observed at 200 Kin CSII framework.     

Effect of Guest–Host Hydrogen Bonding on the Structures and Properties of Clathrate Hydrates

To provide improved understanding of guest–host interactions in clathrate hydrates, we present some correlations between guest chemical structures and observations on the corresponding hydrate properties. From these correlations it is clear that directional interactions such as hydrogen bonding between guest and host are likely, although these have been ignored to greater or lesser degrees because there has been no direct structural evidence for such interactions. For the first time, single‐crystal X‐ray crystallography has been used to detect guest–host hydrogen bonding in structure II (sII) and structure H (sH) clathrate hydrates. The clathrates studied are the tert‐butylamine (tBA) sII clathrate with H₂S/Xe help gases and the pinacolone + H₂S binary sH clathrate. X‐ray structural analysis shows that the tBA nitrogen atom lies at a distance of 2.64 Å from the closest clathrate hydrate water oxygen atom, whereas the pinacolone oxygen atom is determined to lie at a distance of 2.96 Å from the closest water oxygen atom. These distances are compatible with guest–water hydrogen bonding. Results of molecular dynamics simulations on these systems are consistent with the X‐ray crystallographic observations. The tBA guest shows long‐lived guest–host hydrogen bonding with the nitrogen atom tethered to a water HO group that rotates towards the cage center to face the guest nitrogen atom. Pinacolone forms thermally activated guest–host hydrogen bonds with the lattice water molecules; these have been studied for temperatures in the range of 100–250 K. Guest–host hydrogen bonding leads to the formation of Bjerrum L‐defects in the clathrate water lattice between two adjacent water molecules, and these are implicated in the stabilities of the hydrate lattices, the water dynamics, and the dielectric properties. The reported stable hydrogen‐bonded guest–host structures also tend to blur the longstanding distinction between true clathrates and semiclathrates.