Effect of anion size on clathrate formation in butyltriisopentylammonium halide-water systems

Three clathrate hydrates i-Pent₃BuNCl · 38H₂O, i-Pent₃BuNCl·32H₂O, and i-Pent₃BuNCl·27H₂O, as well as triisopentylammonium dihydrate were found in the system i-Pent₃BuNCl-H₂O. Crystals of all the hydrates were isolated, and their compositions were determined. The effect of anion size on the formation of clathrates involving the triisopentylammonium cation was studied.     

Hydrate formation in the system n-propanol–water

The phase diagram of the binary system n-propanol alcohol–water was investigated with use of differential thermal analysis and powder X-ray diffraction. The phase diagram has three groups of thermal effects, which can be considered as peritectic melting of three different hydrates (?60.0, ?53.5, and ?41.5 °C). At the same time, powder X-ray diffraction data indicate the existence of only one compound in this system (cubic unit cell, a = 12.09 ± 0.01 Å and 12.15 ± 0.01 Å at ?109 to ?66 °C, respectively). The most probable explanation of this contradiction seems to be the existence of several hydrates belonging to the same structural type but different in composition.     

Stoichiometry of clathrates

Two aspects of clathrate stoichiometry are discussed: structural stoichiometry and variation in composition due to variable occupation of host cavities in the framework by guest molecules. The solid solutions that are due to the variable occupation of cavities (iskhoric solutions) are classified into two types according to the stability of the hollow clathrate framework of the host. The first type involves compounds with stable hollow frameworks (the occupancies change from zero), and the second type are compounds with metastable hollow frameworks (the occupancies change from certain positive values). Special attention is paid to a wide class of clathrate compounds of constant composition (currently all clathrates are regarded as nonstoichiometric compounds). Clathrates of constant composition are formed when the hollow framework of the host is absolutely unstable. Reasons for instability of the frameworks are discussed, and theoretical models designed on the basis of the available data are considered. Examples of alloxenic (with one guest replaced by another) and allokiric (with replacements in the host subsystem) solid clathrate solutions are given. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 36, No. 6, pp. 1088–1141, November–December, 1995. Translated by L. Smolina     

Phase diagram of a tetrapropylammonium bromide-water system according to DTA data

The phase diagram of a tetrapropylammonium bromide-water binary system is studied by means of differential thermal analysis. Three tetrapropylammonium bromide hydrates are identified that include one stable compound of 1: 4 composition (bromide: water; mp, ?32.6°C) and two metastable hydrates of 1: 6 (mp, ?70.4°C) and 1: 24 compositions (mp, ?57.0°C).     

Hydrate formation in the system tetraethylammonium bromide-water

Hydrate formation in the binary system tetraethylammonium bromide-water was studied by differential thermal analysis. Three stable solid phases were detected, as well as two hydrates melting in metastable regions.     

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.     

New Clathrate Hydrates in the System Triisopentylammonium Bromide-Water

In the system i-Pent₃BuNBr-H₂O, along with the known compound i-Pent₃BuNBr-38H₂O, three new clathrate hydrates were identified: i-Pent₃BuNBr-32H₂O, i-Pent₃BuNBr-26H₂O, and i-Pent₃BuNBr-24H₂O. Crystals of all the hydrates were isolated, and their compositions were determined.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 2, 2005, pp. 192–195.Original Russian Text Copyright © 2005 by Aladko, Dyadin.     

Hydrate formation in the system tetraethylammonim chloride-water

The phase diagram of the binary system tetraethylammonim chloride-water was studied by the differential-thermal analysis method. In addition to two known compounds (tetraethylammonim chloride monohydrate and tetrahydrate) five crystal phases were detected: two solid-phase modifications of tetraethylammonim chloride pentahydrate (mp 33.2 and 10.0°C) and three phases melting at −83.5, −110.0, and −135.0°C.     

Double clathrate hydrates of tetrabutylammonium fluoride + helium, neon, hydrogen and argon at high pressures

Decomposition curves of double ionic clathrate hydrates of tetrabutylammonium fluoride with helium, neon, hydrogen and argon were studied at pressures up to 800 MPa. Formation of double hydrates with helium, neon and hydrogen does not lead to any significant increase of the temperatures of decomposition of these hydrates; at high temperatures the hydrates may decompose even at lower temperatures than the hydrate of pure tetraalkylammonium salt does. Decomposition temperatures of double hydrates with argon in all cases were 4–8 °C higher in comparison with the decomposition temperature of ionic clathrate hydrates of tetrabutylammonium fluoride. We suppose that this behavior is caused by simultaneous effect of three factors on hydrate decomposition temperature: (1) partial filling of the small cavities in the framework of the hydrate with water molecules, (2) weakness of the van der Waals interactions between the gas molecules and the host water molecules, and (3) dissolution of helium, hydrogen and neon in the solution of tetrabutylammonium salt causing a decrease of melting temperatures of the hydrates formed from these solutions.     

Phase diagram and high-pressure boundary of hydrate formation in the ethane-water system

Dissociation temperatures of gas hydrate formed in the ethane-water system were studied at pressures up to 1500 MPa. In situ neutron diffraction analysis and X-ray diffraction analysis in a diamond anvil cell showed that the gas hydrate formed in the ethane-water system at 340, 700, and 1840 MPa and room temperature belongs to the cubic structure I (CS-I). Raman spectra of C-C vibrations of ethane molecules in the hydrate phase, as well as the spectra of solid and liquid ethane under high-pressure conditions were studied at pressures up to 6900 MPa. Within 170-3600 MPa Raman shift of the C-C vibration mode of ethane in the hydrate phase did not show any discontinuities, which could be evidence of possible phase transformations. The upper pressure boundary of high-pressure hydrate existence was discovered at the pressure of 3600 MPa. This boundary corresponds to decomposition of the hydrate to solid ethane and ice VII. The type of phase diagram of ethane-water system was proposed in the pressure range of hydrate formation (0-3600 MPa).