Clathrate-like

The search for suitable borides of alkali metals recently led to a new lithium boride with hexagonal Bs-octahedra framework unknown until now. Strong evidence for the existence of planar LiaB molecules in the wide channels of the framework is given by the structural analysis ( ). This compound points out another aspect involving reactions of solid phases namely, the possible release of small molecules, which are clathrate-like, locked in the structure and are therefore stabilized. 

For clarity and because of space limitations, we will consider only examples of complexes in which the guest interacts with the host from above (or/and below) the cavity, and the structure has been confirmed by X-ray analysis. These complexes serve as models for the interaction of neutral molecules which are nearly concave/ convex complementary in their. spatial interaction. Although complementary in shape, some guest molecules simply fill spatial voids in the host lattice. These clathrate-like structures will not be considered. 

Mixture Models Broken-Down Ice Structures. Historically, the mixture models have received considerably more attention than the uniformist, average models. Somewhat arbitrarily, we divide these as follows (1) broken-down ice lattice models (i.e., ice-like structural units in equilibrium with monomers) (2) cluster models (clusters in equilibrium with monomers) (3) models based on clathrate-like cages (again in equilibrium with monomers). In each case, it is understood that at least two species of water exist—namely, a bulky species representing some... 

In the Frank and Evans iceberg model, ice-like structures form around hydrophobic entities, such as methane. In this model, the hydrophobic molecules enhance the local water structure (greater tetrahedral order) compared with pure water. Ordering of the water hydration shell around hydrophobic molecules has been attributed to clathrate-like behavior, in which the water hydration shell is dominated by pentagons compared to bulk liquid water (Franks and Reid, 1973). 

An alternative model of supercooled water comprises clathrate-like entities, which is consistent with model (2) for liquid water. Sorenson (1994) and Walrafen... 

The sharing of imperfect cluster faces of the clathrate-like clusters can be viewed as a thermodynamic tendency to minimize the negative entropies of solution. The tendency for face- or edge-sharing of individual solvation clusters, as Stillinger (1980) pointed out, is the same as the tendency for clustering of pure supercooled water. 

Despite the formation of clathrate-like clusters and complete 512 cages during these simulations, the increased ordering observed from the radial distribution functions and local phase assignments resulted in the authors concluding that their simulation results are consistent with a local order model of nucleation, and therefore do not support the labile cluster model. 

The process of guest release for the two types of inclusion compound of 9.4 can be followed by thermo-gravimetric analysis (TGA, Box 9.1). This reveals the clathrate-like behaviour of the y phase (trace 2) and contrasts significantly with the zeolite-like behaviour of the microporous /1-phase (trace 1), Figure 9.19b. The mass loss stages followed by both compounds after initial wetting in benzene are as follows ... 

Figure 5.11 Illustrations of possible water structure at hydrophilic and hydrophobic surfaces. Bulk water is shown by pentagonal and partial pentagonal circuits, which indicate structural entities being in equilibrium with monomeric water represented by arrows. Dipole-dipole interaction at a hydrophilic surface causes ordering of water molecules, leading to a notable disordered zone. Water molecules at a hydrophobic surface have extensive clathrate-like structure with a minimal disordered zone. From Nguyen and Schulze [53]. Copyright 2004, Dekker.

Fig. 2 A, B. Three layers model of water at the interface with mineral according to Dorst-Hansen164) O = clathrate-like ordering ] = water-dipole. A Vicinal water near non-polar surface. Extensive clathrate-like ordering near interface but minimal disordered region. B Vicinal water near polar surface, disordered region...

Hydrates with related clathrate-like structures are formed by tetramethyl and tetraethyl ammonium salts. The tetramethyl and tetraethyl ammonium salts form hydrates in which the cations occupy voids in hydrogen-bonded structures formed by the anions and the water molecules. Unlike the gas hydrates, the alkylamine hydrates, and the butyl and isoamyl ammonium salt hydrates discussed in the next section, these cage-like structures do not feature the pentagonal dodecahedron as a common structural component. 

The existence of clathrate-like water structure adjacent to the hydrophobic surfaces of macromolecules is an attractive hypothesis. Models have been proposed which have received some support from thermodynamical arguments [808]. However, this concept has proved ineffective as a basis for the interpretation of the structure associated with the many electron density solvent peaks, separated by 2.8 A to 3.0 A, which are frequently observed on the Fourier X-ray maps close to the surface of a protein [809, 810], Recently, however, some local clathrate-like water has been observed in special cases in the high-resolution studies of the small plant protein, crambin [811], in a hydrated deoxydinucleoside-phosphate drug complex [812], in (Phe4Val6) antamanide hydrate [8131 and in an oligodeoxy-nucleotide duplex [814],... 

The present evidence favors the view that clathrate-like water structure is observed in only a minority of biological structures. It is not clear, as yet, what particular features of these molecules induce this water structure. If it were simply the... 

Five fused water pentagons forming a clathrate-like structure are the predominant feature in the hydration of crambin, a small hydrophobic protein of molecular weight 4720 [811, 844]. 

Nakayama H, Watanabe K (1978) Hydrates of organic compounds. III. The formation of clathrate-like hydrates of tetrabutylammonium dicarboxylates. Bull Chem Soc Jpn 51 2518-2522... 

Starch-guest molecule compounds in inclusion complexes are usually nonstoichiometric species. On the other hand, in the case of amylose such compounds are stoichiometric, but their composition is not repeatable. Starch complexes may consist of partly physical mixtures, adsorbates, and true inclusion complexes formed by direct involvement of dipolar interactions, host-guest hydrogen bonds, and/or clathration-like interactions within the starch matrix. 

A final specific conclusion, and a provocative one, concerns the relevance of tetrahedrality of solvent structure or the clathrate likeness of the solvent in... 

Table VII presents a summary of calorimetric measurements of the differential heat of adsorption of ammonia, water, and carbon dioxide on the sodium form of ZSM-5 zeolite. Ammonia adsorption at 416 K (97.147) shows that NaZSM-5 zeolite is weakly acidic, whereas CO adsorption (147) indicates that in addition there are some weak basic sites. It should be noted that of the two samples studied with ammonia adsorption one was 70% H exchanged and the sodium content of the other was not given. Water adsorption on NaZSM-5 displayed unusual behavior, with a steep increase in the differential heat of adsorption at high surface coverages (166). An adsorption mechanism was proposed to explain these findings in which adsorption occurs first on the hydrophilic sites, consisting of sodium cations and framework anions where water molecules are bound by dipole-field interactions. Further adsorption takes place near these sites through weak interaction with zeolite surfaces, and when the number of water molecules close to these sites exceeds a certain value, they tend to reorient by forming clathrate-like struc-...

Because the cluster composition is not constant, a clathrate-Uke structure cannot be used for its representation. Nevertheless, the clathrate-like structure [(PrOH)8(H20)4o] suggested in ref 20 is compatible with the water-rich clusters for Xi > 0.35. Indeed, the alcohol molar fraction of about 0.15, calculated for the water-rich clusters when jci > 0.35, corresponds to the clathrate (PrOH)8(H2O)40. 

There are important differences between the literature models and our results. In our case, (i) the number of monomers is smaller than that in the Pauling model (where they are present in clathrate-like cages), and (ii) they coexist with a disturbed but still infinite, not disintegrated network of water molecules. In contrast, the models in refs 11 and 32 do not involve a network but only a distribution of clusters. 

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