Hydrated molecules

According to these authors all gas hydrates crystallize in either of two cubic structures (I and II) in which the hydrated molecules are situated in cavities formed by a framework of water molecules linked together by hydrogen bonds. The numbers and sizes of the cavities differ for the two structures, but in both the water molecules are tetrahedrally coordinated as in ordinary ice. Apparently gas hydrates are clathrate compounds. 

Since the components of a compoimd are linked to each other in such a way that they have lost same of their free translational mobility, the volume of hydrated molecule is always smaller than the sum of the volumes of its components, the hydration is accompanied by a decrease of the total volume. 

Our inhibitor design strategy was based on the premise that structural modifications in the base of purine riboside that enhance purine base hydration without impairing the binding of the hydrated species to the ADA binding site would result in purine riboside (PR) analogues with high ADA inhibitory potency. Since the apparent inhibition constant (Kj (app)) is related to the hydration equilibrium constant (Keq) and the inhibitory constant for the hydrated molecule (Kj ) by... 

FIG. 3. Geometry of hydrated molecules cylinders associate to a lamellar liquid crystal, cones to a hexagonal and an inverse hexagonal. Adapted from The Physical Chemistry ofMembranes (Silver, B., ed.), Allen Unwin, Inc. Solomon Press, Winchester, MA, 1985. 

FIG. 5. Schematic cross-sections of vesicles with each line representing a bilayer of hydrated molecules. From Muller-Goymann, C.C., Fliissigkristalline Systeme in der Pharmazeutischen Technologic, PZ Prisma, 5, 129-MO (1998). 

A guiding principle for the solvent extraction chemist is to produce an uncharged species that has its maximum coordination number satisfied by lipophilic substances (reactants). Eor trivalent lanthanides and actinides (Ln and An, respectively), the thermodynamic data suggest a model in which addition of one molecule of TBP displaces more than one hydrate molecule ... 

In Cliapter 2 we learned how ions in solution are solvated. Some of the water molecules that form the solvation sheet are left behind when ions random walk and drift around, while others—the primary hydration molecules—show a stronger attrac -tion to the ion and follow it in its thermal, random movements. 

In Chapter 2 it was said that the number of hydration molecules as well as the hydration energy depends mainly on the charge and size of the ion. Thus, small cations have a large number of hydration molecules attached strongly to the ion. They cannot easily get rid of their hydration sheet. They will not contact adsorb on the electrode. On the contrary, large anions and large cations have only a few primary water molecules or none loosely attached to the ion. For them, the ion-solvent interactions are less important than other interactions. They will tend to contact adsorb on the electrode (Fig 6.91). 

The absorption spectra of the aqueous solutions indicate that the sulphur dioxide is mainly present as S02 molecules, some hydrate molecules also being present, but only small amounts of H2S03 and its ions.5... 

In this way, it was found that quinazoline (neutral species) has one hydrated molecule for every 5500 anhydrous ones. Altogether twenty-eight variously substituted quinazolines were examined,27 and the ratio was seen to vary from 10-2 to 10-5. It was concluded that (—1) substituents favor hydration, and, in general, the relative effects of various substituents on the ratio was very similar to that which they had earlier been found to exert on the cations, although much less intense. 

FIGURE 1.5 In water, ions are hydrated that is, they are surrounded by a cluster of water molecules bonded loosely to the ion. There is a constant interchange between the bonded water molecules and those in the bulk solvent. Note that a cation (a) is surrounded by water molecules with the O atoms closer to the ion, whereas an anion (b) has water molecules attached through their hydrogen atoms. The number of hydrating molecules depends on the size of the ion, but for most ions, it is approximately six. 

A bit of explanation is required here for those readers unfamiliar with the condensation concept, a key notion to describe polyelectrolytes. Consider as here a polyanion. If the charges are brought closer to one another, on the average, below a critical distance their mutual repulsion is such that — in order to continue to obey first principles electrostatics such as the Poisson equation — they screen themselves with an atmosphere of counterions. This atmospheric condensation, which can coexist with ionic binding at the individual sites, boosts the local concentration of counterions in the space surrounding the polyelectrolyte by as much as three orders of magnitude. The nmr measurements analyzed here focus on these water hydration molecules coordinated to condensed sodium counterions, next to the surface of the tactoids (see Fripiat s chapter). 

On adding water the intensity of the band 3400 cm 1 decreases while that of the other bands increases. It seems very likely that the bands at -3400 cm 1 belong to nitric acid molecules linked by hydrogen bond (pp. 7-8), while the lines at 2550, 2700 and 3150 cm 1 probably belong to nitric acid hydrate molecules. 

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