Hydration of salt

Protein separation by hydrophobic interaction chromatography is dependent upon interactions between the protein itself, the gel matrix and the surrounding aqueous solvent. Increasing the ionic strength of a solution by the addition of a neutral salt (e.g. ammonium sulfate or sodium chloride) increases the hydrophobicity of protein molecules. This may be explained (somewhat simplistically) on the basis that the hydration of salt ions in solution results in an ordered shell of water molecules forming around each ion. This attracts water molecules away from protein molecules, which in turn helps to unmask hydrophobic domains on the surface of the protein. 

In a number of other cases, however, notably in the exothermic decompositions of solids which can become explosive and in the endothermic transformation of hydrates of salts and of carbonates (to oxides), the slow processes seem to be nucleation. The rate laws for such nucleation-con-trolled processes can be very complex, and the rate studies are difficult to make and to reproduce. In many of these cases smj amounts of impurities play an important role in governing the de fed t nff and growth of nuclei, and there are a number of instances of sm bA ounts of water vapor having a significant catalytic effect. ... 

We shall be concerned in this chapter with the clathrate hydrates and the hydrates of salts, acids, and hydroxides. The structures of zeolites are described in Chapter 23, as also are the clay minerals, which can take up water between the layers. Examples of hydrated basic salts and of hydrates of heteropoly acids and their salts are also discussed in other chapters. 

In the preformulation study, the comprehension of physicochemical properties regarding water-solid surface interaction is beneficial to the handling, formulation, and manufacture of the finished products. Data on sorption/de-sorption isotherm, hydration of salts of drug product, water sorption of pharmaceutical excipients, and kinetics of water adsorption or desorption of a substance can be obtained effectively by the dynamic vapor sorption method. The knowledge may be utilized for dosage form design and supports the understanding of the mechanism of action. 

Of particular interest for the determination of water content is the use of a liquid reagent such as 2,2-dimethoxypropane, which yields liquid products (acetone and methanol) during hydrolysis [79—81]. To accelerate the hydrolysis it is conducted on a water-bath with heating in the presence of methylsulphuric acid (catalyst). The hydrolysis lasts 1 min. The method was applied to the determination of water in organic solvents and crystal hydrates of salts. 

On the other hand, the ion-solvent interactions manifest themselves directly in the magnitude of the free energy of solvation. Free energies of solvation (or hydration) of salts have been frequently discussed and corrdated. These quantities are also of interest because they are directly related to the chemical potentials of ions at infinite dilution, which, in turn, together with the chemical potential of the undissociated molecule, govern the dissociation equihbrium at infinite dilution. 

Since swelling is intrinsically a process of dissolution, be it one of partial dissolution, or even, in its initial phases bound up with the formation of addition compounds (or hydrates if water is concerned), the volume contractions do not represent any new or specific phenomenon In cellulose and gelatin gels, by far the main part of the total contraction coincides with the range of hydrate formation. (cf. p. 541) and hence, may be connected with a chemical reaction, like the contraction occurring in the system sulphuric acid-water, or upon formation of crystalline hydrates of salts. It is known from numerous other examples that compound formation gives rise to a volume contraction. 

This class conprises not only all the family of the reactions of oxidation of metals by gases (see section 15.2) but also other reactions such as the hydration of salts ... 

Hydration of Salts Another common characteristic of alkaline earth compounds is the formation of hydrates. Typical hydrates are MX2 6 H20,where M = Mg, Ca, or Sr, and X = Cl or Br. The Ba " " ion has a low charge density and shows little or no tendency to retain its hydration sphere in the solid state. The formulas for the hydrates of the alkaline earth nitrates illustrate that the degree of hydration typically decreases as the charge density of the metal ion decreases Mg(N03)2 6 H2O, Ca(N03)2 4 H2O, Sr(N03)2 4 H2O, Ba(N03)2-... 

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