Nonstoichiometric hydrates

Because it is impossible for all cavities to be occupied (an analog would be a perfect crystal) simple hydrates always have more water molecules than the ideal composition. Usually the ratios range from G -5(3/4) H20 to G 19H20, with typical fractional occupancies of the smaller cavities of 0.3-0.9, based on size restrictions. This variation causes clathrate hydrates to be called nonstoichiometric hydrates, to distinguish them from stoichiometric salt hydrates. 

Oxidation of sensitive divinyl alcohol 20 to dienone 4 is achieved by treatment with activated manganese dioxide. Commercially available active Mn02 21 is a synthetic nonstoichiometric hydrated material. This reagent provides mild conditions for oxidation of allylic, propargylic, and benzylic alcohols.10... 

The sodium bicarbonate can further decompose by reacting with another NaOH to give sodium carbonate and water. The sodium carbonate is nonvolatile, and water is trapped on the sodium hydroxide as a nonstoichiometric hydrate. 

Form Supplied in dark brown powder, widely available. The commercial active Mn02 used as oxidizing reagent for organic synthesis is a synthetic nonstoichiometric hydrated material. The main natural source of Mn02 is the mineral pyrolusite, a poor oxidizing reagent. 

DVS instrument, the mass of the sample is measured as a function of relative humidity (r.h.). Humidity is either changed continuously or stepwise and the mass of the sample is plotted as a function of humidity (or time). Figure 8.11 shows a typical DVS diagram of a substance that forms one hydrate. When humidity is increased from 0% to 95%, a sharp mass increase is observed at 80% th. From the magnitude of the mass increase, it can be calculated if a hemi, mono, sesqui, and so on hydrate is formed. When the humidity is decreased from 95% to 0%, a loss in mass is observed at w50% r.h. Such a hysteresis is typical for a substance that forms a stoichiometric hydrate and it is due to kinetic hindrance of hydration and dehydration. Some hydrates show extreme hystereses, such that hydration or dehydration may not be observed at all in a standard DVS experiment. Substances that form nonstoichiometric hydrates or that just adsorb surface water normally show a more continuous mass increase and decrease and almost no hysteresis. Substances that can form several hydrates may lead to complicated DVS traces (Figure 8.12). 

The accurate composition of the gas hydrates for a long time remained a controversial subject, since direct analysis leads to ambiguous results owing to decomposition of the hydrate and/or inclusion of mother liquor in the crystals. Thus it was firmly believed that the nonstoichiometric compositions of gas hydrates found experimentally were all due to errors in the analysis. But more recent determinations of the composition by the indirect... 

For instance, one of the various crystalline forms of polyoxacyclobutane is a hydrate [62], Syndiotactic polymethylmethacrylate also forms nonstoichiometric inclusion compounds with a variety of solvents [63,64]. 

PIO Gas hydrates are nonstoichiometric compounds consisting of hydrogen-bonded water molecules in a cagelike structure, which traps small-diameter gas molecules. (From Tuckerman, 1999)... 

Various preparative methods are adopted at nonstoichiometric formulations, incomplete dehydration or using oxide additives to obtain boron phosphate of varying purity for its catalytic applications. The compound also forms hydrates (tri- tetra-, penta-, and hexahydrates) which readily decompose in water to phosphoric acid and boric acid. 

Clathrate Formation. Ethylene oxide forms a stable clathrate with water (20). It is nonstoichiometric, with 6.38 to 6.80 molecules of ethylene oxide to 46 molecules of water in the unit cell (37). The maximum observed melting point is 11.1°C. An x-ray structure of the clathrate revealed that it is a type I gas hydrate, with six equivalent tetrakaidecahedral (14-sided) cavities fully occupied by ethylene oxide, and two dodecahedral cavities 20—34% occupied (38). 

It gradually became clear that the clathrate hydrates distinguished themselves by being both nonstoichiometric and crystalline at the same time, they differed from normal hexagonal ice because they had no effect on polarized light. 

A second classification of hydrates is obtained through consideration of the guest molecules. Such a classification is a function of two factors (1) the chemical nature of the guest molecule and (2) the size and shape (particularly in sH) of the guest. The size of the guest molecule is directly related to the hydration number and, in most cases, to its nonstoichiometric value. 

Hydrate Nonstoichiometry. The cause of the nonstoichiometric properties of hydrates has been considered. Evidence for the view that only a fraction of the cavities need to be occupied is obtained from both the experimental observations of variation in composition, and the theoretical success of the statistical thermodynamic approach of van der Waals and Platteeuw (1959) in Chapter 5. Typical occupancies of large cavities are greater than 95%, while occupancy of small cavities vary widely depending on the guest composition, temperature, and pressure. 

Glew (1959) suggested that the most nonstoichiometric guest molecules are those for which the size of the guest approaches the upper limit of the free volume of a cavity. For two molecules that approach the size limit of cavities, Glew and Rath (1966) presented experimental evidence that hydrate nonstoichiometry for both chlorine and ethylene oxide was due to the composition of the phase in equilibrium with the hydrates. 

A systematic determination of both hydration number (Cady, 1983) and relative cage occupancies (Davidson and Ripmeester, 1984) shows that molecules such as CH3CI and SO2 are the most nonstoichiometric. Although theoretical calculations using the van der Waals and Platteeuw model provides some rationale for the nonstoichiometry, experimental quantification of nonstoichiometry as a function of guest/cavity size ratio has yet to be determined. 

If all the cavities of structure I or structure II were occupied as a simple hydrate, for example, with xenon or argon, the minimum number of water molecules (5.75 and 5.67, respectively) would be obtained per guest molecule. Both these values yield a structure that is 85 mol.% water. If all three cavity types were completely filled in structure H with two guests, the water mole fraction would be 0.85 as well. As discussed in Section 2.1, due to the nonstoichiometric nature of hydrates, the mole fraction of water is invariably higher than 0.85. 

Crystals, however, are not always so perfectly ordered. Atomic mobility exists within the crystal lattice however, it is greatly reduced relative to the amorphous state. Partial loss of solvent from the lattice can result in static disorder within the lattice where the atomic positions of a given atom can deviate slightly within one asymmetric unit of the unit cell relative to another. Lattice strain and defects occur for many reasons. Solvents can be present within channels of the lattice in sites not described by the symmetry of the crystal structure itself, resulting in disordered solvent molecules or incommensurate structures and potentially nonstoichiometric solvates or hydrates. 

Mixed oxides have a widespread application as magnets, catalysts, and ceramics. Often, nonstoichiometric mixtures with unusual properties can be prepared for example, Fe203 and ZnO have been milled for the production of zinc ferrite [40], while mixed oxides of Ca(OH)2 and Si02 were described by Kosova et al. [77]. Piezoceramic material such as BaTi03 from BaO and anatase Ti02 has been prepared [78], while ZnO and Cr203 have been treated by Marinkovic et al. [79] and calcium silicate hydrates from calcium hydroxide and silica gel by Saito et al. [80]. The thermal dehy-droxylation of Ni(OH)2 to NiO or NiO-Ni(OH)2 nanocomposites has also been investigated [81]. 

Abstract. Nanopowders of nonstoichiometric tungsten oxides were synthesized by method of electric explosion of conductors (EEC). Their electronic and atomic structures were explored by XPS and TEM methods. It was determined that mean size of nanoparticles is d=10-35 nm, their composition corresponds to protonated nonstoichiometric hydrous tungsten oxide W02.9i (OH)o.o9, there is crystalline hydrate phase on the nanoparticles surface. After anneal a content of OH-groups on the surface of nonstoichiometric samples is higher than on the stoichiometric ones. High sensitivity of the hydrogen sensor based on WO2.9r(OH)0.09 at 293 K can be connected with forming of proton conductivity mechanism. 

The surface of the nanocrystalline nonstoichiometric tungsten oxide conditioned on air (sample NN1) is almost completely formed of crystalline hydrate W03 (H20). Thus on the W4f-spectra (Fig. 2-3) the main component is comp, e with Ep=36.1 eV, on the Ols-spectra (Fig. 3-3) along with the 02 -states, the contributions from OH-groups (comp, g) and from H20 (comp, m) are present. 

After the anneal on air of the nonstoichiometric tungsten oxide nanopowder the signal from crystalline hydrate WO3 (H20) (comp, e) in the spectrum of W4f-level (sample NN2, Fig. 2-4) disappeared. The component d from W6+-states of the oxide dominated in the spectrum of W4f-level after anneal on air and the component c from W5+-states (EpW4f7/2=34.8 eV) appeared in the low energy region. 

As will be discussed below, hydrates and solvates represent a relevant portion of the known polymorphic forms for many substances. While representing a complication in the study of polymorphism, their existence can be extremely advantageous. It is not uncommon to have several forms of the same molecule or salt that differ in the degree of solvation and the number and type of solvent molecules. The problem is complicated by the often nonstoichiometric, fractional or variable presence of solvent molecules. 

Manganese dioxide, a grey to black solid, occurs in ores such as pyrolusite, where it is usually nonstoichiometric. When made by the action of 02 on Mn at a high temperature, it has the rutile structure found for many other oxides M02 (e.g., those of Ru, Mo, W, Re, Os, Ir, and Rh). However, as normally made by heating Mn(N03)2-6H20 in air ( 530°C), it is nonstoichiometric. A hydrated form is obtained by reduction of aqueous KMn04 in basic solution. Manganese(IV) occurs in a number of mixed oxides. 

These differences may result, in part, from the higher coordination number of aluminum in AIF3. In it, each aluminum atom is surrounded by a distorted octahedron of six fluorine atoms. Each fluorine is two-coordinate, being comer-shared by pairs of octahedra. Thus, the stmcture is similar to ReOs and has a relatively open lattice. As a result, numerous sites exist for water molecules, leading to the occurrence of a wide range of nonstoichiometric and stoichiometric hydrates (AIF3 (H20) n = 1, 3, 9). Quite curiously, no hexahydrate corresponding to [Al(H20)6]Cl3 is known. 

Clathrate hydrates are crystalline but nonstoichiometric compounds and all the cages are not always occupied. Clathrate hydrates are stable only when the interaction between guest and water molecules dominates over sum of the unfavorable two terms (1) entropy decrease arising from confinement of guest molecules in small void cages, and (2) free energy for formation of empty clathrate hydrate structure from ice or liquid water. 

However, the decomposition temperature ranges differ widely in accordance with the nature of the metal involved and the wide variation in thermal stability of the intermediate metal carbonates. Aluminium oxalate is anomalous in that its nonstoichiometric water of hydration is retained in the final residue, which corresponds to hydrated alumina. ... 

A further important interaction, namely, that between an active substance and water, can be studied by DSC and TSC (28). To illustrate this case, we will present results obtained with a drug manufactured in three states labeled hydrated hemihydrate (HEMI), monohydrate (MONO), and freeze-dried (FD) forms. The last is mostly amorphous and absorbs water in nonstoichiometric quantities. The other two forms are highly crystalline and contain specific ratios of water per active molecule. Thermogravimetric experiments have shown that water is linked between two active molecules for the hemihydrate, whereas the distribution is homogeneous and the ratio unimolecular for the monohydrate. Moreover, two different hemihydrate batches obtained by two different industrial processes have been analyzed. Complementary studies have shown that the batch labeled PR, is more crystalline and more stable than the PR2 batch in terms of the transition to the monohydrate form. 

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