Water sorption, hydrates

Toteja RSD, Jangida BL, Sundaresan M, Venkataramani B (1997) Water sorption isotherms and cation hydration in dowex 50w and amberlyst-15 ion exchange resins. Langmuir 13 2980-2982... 

To illustrate this a model transesterification reaction catalyzed by subtilisin Carls-berg suspended in carbon dioxide, propane, and mixtures of these solvents under pressure has been studied (Decarvalho et al., 1996). To account for solvent effects due to differences in water partitioning between the enzyme and the bulk solvents. Water sorption isotherms were measured for the enzyme in each solvent. Catalytic activity as a function of enzyme hydration was measured, and bell-shaped curves with maxima at the same enzyme hydration (12%) in all the solvents were obtained. The activity maxima were different in all media, being much higher in propane than in either CO2 or the mixtures with 50 and 10% CO2. Considerations based on the solvation ability of the solvents did not offer an explanation for the differences in catalytic activity observed. The results suggest that CO2 has a direct adverse effect on the catalytic activity of subtilisin. 

There are many examples of second-order analyzers that are used in analytical chemistry including many hyphenated spectroscopic tools such as FTIR-TGA, IR-microscopy, as well as GC-MS, or even two-dimensional spectroscopic techniques. Another hyphenated technique that is being developed for the study of solid-state transitions in crystalline materials is dynamic vapor sorption coupled with NIR spectroscopy (DVS-NIR).26 DVS is a water sorption balance by which the weight of a sample is carefully monitored during exposure to defined temperature and humidity. It can be used to study the stability of materials, and in this case has been used to induce solid-state transitions in anhydrous theophylline. By interfacing an NIR spectrometer with a fiber-optic probe to the DVS, the transitions of the theophylline can be monitored spectroscopically. The DVS-NIR has proven to be a useful tool in the study of the solid-state transitions of theophylline. It has been used to identify a transition that exists in the conversion of the anhydrous form to the hydrate during the course of water sorption. 

Chapek et a/.349,350 pointed out that, among the two types of water of hydration, a significant major proportion of the water is weakly bound, weakly oriented, or osmotic. Its heat of adsorption is approximately 2 kcal/ mol. Later studies351352 with the NMR spin-echo technique and dielectric methods illustrated that barely 3% or less of the water enters discrete cavities (micropores that are comparable in size to the size of water molecules) in the first stage of sorption. Subsequent sorption proceeds in these micropores with the formation of clusters of water molecules. 

Water sorption isotherms for hep show marked hysteresis. Powers and Brownyard (P20) found that, while it was difficult to obtain reproducible desorption curves, the low-pressure part of the water vapour resorption curve varied little with w/c ratio, between different Portland cements, or, if allowance was made for the contents of unreacted cement, with the degree of hydration. This was their main direct evidence for the conclusion (Section 8.2.1) that the properties of the hydration product considered in their model were essentially independent of these variables. However, the water sorption iostherms obtained by different investigators have varied considerably (e.g. Refs P20 and S79), and it is not clear to what extent the above conclusion would stand had different desorption conditions been used. 

Zundel reports on the hydration of thin polyelectrolyte ion-exchange membranes subject to progressive increases in water sorption. Spectroscopic observations of these systems reflect the hydration of polyion charge centers in the membranes but in the presence of associated counterions, which in turn are progressively hydrated. Zundel worked with polysulfonates and found the spectra of unhydrated polymer salts the cation is attached unsymmetrically to the SOj groups. This mode of attachment leads to a loss of degeneracy in the antisymmetric vibrations. 

Plateau in sorption isotherm (60-220) Small differences in partial molar quantities for transfer of water into hydration layer, which, with increasing hydration, decrease to near 0 average values AGi = -0.2, AHj = —1.1 kcal/mol, AVj = 0 for A a 0.15... 

Adsorption is the adhesion of molecules of a gas, liquid, or dissolved substance or of particles to the surface of a solid substance. Absorption is the assimilation of molecule into a solid or liquid subsunce, with the formation of a solution or a compound. Sometimes the word sorption is used to indude both of these phenomena. We say that a heated glass vessel adsorbs water vapor from the air on cooling, and becomes coated with a very thin layer pf water a dehydrating agent such as concentrated sulfuric add absorbs water, forming hydrates. Activated alumina sorbs water vapor, probably both by adsorption (the adhesion of a layer of water vapor to the surface of rhe particles.) and by absorptimi (the formation ipf aluimnunt hydroxMe). 

Water of hydration Efflorescence Deliquescence Monolayer capacity Water present in regular positions within a crystal lattice. (There is a specific stoichiometry with the other molecule.) Spontaneous loss of water of hydration (not thermally induced). Sorption sufficient to produce dissolution of the substance. Volume of water (STP) adsorbed at STP when the surface... 

Hailwood-Horrobin Solution Sorption Theory. The Hailwood-Horrobin (57) model treats moisture sorption as hydration of the polymer, taken here to be dry wood, by some of the sorbed water called water of hydration, m. The hydrate forms a partial solution with the remaining sorbed water, called water of solution, m,. An equilibrium is assumed to exist between the dry wood and water and the hydrated wood with an equilibrium constant K. Equilibrium is also assumed to exist between the hydrated wood and water vapor at relative vapor pressure h, with equilibrium constant K2. A third constant is defined as the moisture content corresponding to com-... 

Although there is no bulk liquid with which adsorbed cations can be exchanged in the experiments made in atmospheres of controlled relative humidity, successive experiments can be made with different adsorbed cation populations, and it is found that the succession of dehydration steps varies with size and hydration energy of the cation (Posner and Quirk, 1964). There is also a dependence upon the type and extent of substitution in the aluminosilicate framework White (1958), for example, observed that two montmorillonites with the same cation-exchange capacity, loaded with the same cation, may have different water sorption and swelling properties. 

Jefferson, M.E., 1938. Structures of kaolin and talc—pyrophyllite hydrates and their bearing on water sorption of clays. Am. Mineral., 23 863—875. 

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. 

The results of ion and water sorption measurements for the two polymers under these solution conditions help to explain this difference. Table 5 " lists the concentrations of various sorbed species and the mole ratio of water to cation/anion in the polymer phase for NaCl and NaOH solution environments. This ratio decreases both in the polymers and in solution with increasing concentration. In solution, the ratio varies from 10.8 to 4.0 over the concentration range of 5-12.5 M NaOH, so that ions in the polymer phase exist in a significantly less aqueous environment compared to the solution phase. As noted by Mauritz and co-workers for perfluorosulfonate membranes, these water contents are insufficient to provide even primary hydration spheres for sodium ions, sorbed anions, and exchange sites, and the likelihood... 

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