Yu. A. Dyadin

Clathrate formation in water-noble gas (Hydrogen) systems at high pressures

Phase equilibria in helium-water, neon-water, and hxdrogen-water svstems were studied at pressures up to 15 kbar. The results are compared with the data for the previously investigated water systems with argon, crypton, and xenon. It is concluded that classical polyhedral clathrate hydrates are formed in all the systems, the stability of the hydrates diminishing from xenon to neon. In all the systems, except the xenon system, the hydrates are based on the crystalline framework of ice II. Their formation demands high pressures; the larger the guest molecule, the higher the pressure required. The xenon molecule seems to be too large to fit the cage of the ice II framework; therefore, the xenon hydrate CS-I remains stable up to at least 15 kbar.

Clathrate Hydrates of Tetrabutylammonium and Tetraisoamylammonium Halides

Clathrate formation was considered for two series of systems: (C4H9)4NG–H2O and i‐C5H11)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.

Clathrate polyhydrates of peralkylonium salts and their analogs

The structure, stoichiometry, and stability of the polyhydrates of tetraalkylammonium (tetraalkylphosphonium) salts and trialkylamine (trialkylphosphine, trialkylarsine) oxides are discussed in relation to the size and configuration of the hydrophobic part of the guest molecule and its ability to undergo hydrophilic interaction with the framework. A comparative analysis of their structures and characteristics with those of the ideal gas hydrates is given. The stability of the hydrates of the various structures under pressure is discussed. It is shown that the reactivity in the systems is determined largely by the size and shape of the guest molecules (ions) and by the ability of the water to construct frameworks with appropriate cavities and by their relative positions.

Clathrate Formation in Wateer-Peralkylonium Salts Systems

We discuss composition, stoichiometry and stability (phase diagrams) of peralkylonium salts and analogues (Alk3X0, where X=N,P,As) polyhydrates depending on the dimensions and the configuration of the hydrophobic part of a guest-molecule and its ability to interact in a hydrophilic way with the framework.

CONTACT STABILIZATION OF HOST COMPLEX MOLECULES DURING CLATHRATE FORMATION: THE PYRIDINE-ZINC NITRATE AND THE PYRIDINE-CADMIUM NITRATE SYSTEMS

Abstract Clathrate formation ranges of the phase diagrams of two binary systems Py-Zn(NO3)2 and Py-Cd(NO3)2 (Py = pyridine) were studied. A clathrate of composition [MPy4(NO3)2]·2Py (M = Zn, Cd) was observed in each of the systems. The space group Ccca (orthorhombic system) and the parameters of the unit cells of both clathrates were determined by X-ray analysis of their single crystals. The data obtained show them to be isostructural with the clathrate [NiPy4(NO3)2]·2Py whose structure is known and suggest the actual presence of the host molecules trans-[MPy4(NO3)2] (M = Zn,Cd) inside the clathrate phases. Host complexes do not form as separate compounds but can only arise in clathrate phases due to contact stabilization by the guest molecules. Both Zn- and Cd-clathrates are of constant composition and melt incongruently at 62.3(6) and 106.0(5)°C, respectively, yielding the complexes [ZnPy3(NO3)2] and [CdPy3(NO3)2], these melting congruently at 131.4(5) and 169.5(5)°C, respectively. During thermal decomp...

Effect of size and shape of cations and anions on clathrate formation in the system: Halogenides of quaterly ammonium bases and water

Abstract The effect of the hydrophobic and hydrophilic inclusion on the structure, stoichiometry and stability of polyhydrates of halogenides of quaternary ammonium bases is discussed. Phase diagrams of binary systems H2O-(i-C5H11)4−k(C4H9)kNA (k = 0, 1, 2, 3, 4; A = F−, Br−), H2O-(C4H9)4NA, −(i-C5H11)4NA (A = F−, Cl−, Br−, I−) and some structural and physico-chemical characteristics of polyhydrates forming in these systems are presented. A comparative analysis of presented data is given.

Theory of Clathrates

A review is presented of the theory of clathrate compounds, starting with the first quantitative theory suggested by van der Waals and Platteeuw and ending with the current ideas. A brief description is given of the theory of ideal and non-ideal solid clathrate solutions and of the theory allowing us to describe clathrates with an unstable, empty host lattice. The review is composed of results obtained by the different methods based on various approximations.In the concluding section predictions are made of the further development of the theory.

PHASE DIAGRAM OF THE XE-H2O SYSTEM UP TO 15 KBAR

The phase equilibria in the Xe–H2O system have been studied by the DTA technique under hydrostatic pressures up to 15 000 bar in a temperature range from -25 °C to 100 °C. We have shown that the cubic structure I xenon hydrate forming at ambient pressure does not undergo any phase transitions under the conditions studied. The temperature of its decomposition into water solution and gas (fluid) increases from 27 °C at 25 bar to 78.2 °C at 6150 bar. At higher pressures the hydrate decomposes into water solution and solid xenon. In the temperature range from 6800 to 9500 bar the decomposition temperature (79.0–79.5 °C) is practically independent of pressure, while further pressure increase results in a slow decrease to 67 °C at 15 000 bar.

Phase and X-ray study of clathrate formation in the tetraisoamylammonium fluoride-water system

The phase diagram of the binary (i-C5H11)4NF-water system has been studied in the clathrate formation region. Three polyhydrates have been discovered, two of which (1∶38.9 and 1∶32.7) are the known orthorhombic and tetragonal phases:Pbmn,a=11.88,b=21.53,c=12.70 Å,ρmeans=1.019 g cm-3 (0°C), m.p.=32.4°C andP42/m, a=23.729,c=12.466 Å,ρmeans=1.062 g cm-3, (0°C), m.p.=31.2°C, respectively. A single crystal X-ray analysis of the novel clathrate hydrate (i-C5H11)4NF·27 H2O is reported. This new clathrate hydrate is tetragonal,I4I/a, witha=16.894(5),c=17.111(2) Å,Z=4, (−50°C), and m.p.=34.6°C. Each (i-C5H11)4N+ cation occupies a four-chamber cavity built of 15-hedra 71635942 (idealized description), with small vacant 5444 cavities filling the intervening space.

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.