Water sorption isotherms

Fig. 3. Water sorption isotherms of poIy(trans-3,4-dihydroxytetrahydiopyran-6,2-diyloxy-methylene) 24 and its related polymers at 20 °C.

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... 

Hageman, M. J., Possert, P., Bauer, J. M. Prediction and characterization of the water sorption isotherm for bovine somatropin recombinant. Journal of Agricultural Food Chemistry, 40(2), p. 342-347, 1992. 

Lewicki, P.P. and Pomaranska-Lazuka, W. 2003. Errors in static desiccator method of water sorption isotherms estimation. Int. J. Food Prop. 6, 557-563. 

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. 

Water sorption isotherms may be determined experimentally by gravimetric determination of the moisture content of a food product after it has reached equilibrium in sealed, evacuated desiccators containing saturated solutions of different salts. Data obtained in this manner may be compared with a number of theoretical models (including the Braunauer-Emmett-Teller model, the Kuhn model and the Gruggenheim-Andersson-De Boer model see Roos, 1997) to predict the sorption behaviour of foods. Examples of sorption isotherms predicted for skim milk by three such models are shown in Figure 7.12. 

Figure A2.3.4 Effect of temperature on water sorption isotherm. (A) Shift without intersection. (B) Shift showing the point of intersection. Reprinted from Rahman and Labuza (1999), courtesy of Marcel Dekker.

Water sorption isotherms on low rank coals have a standard type II sigmoid shape typical of porous adsorbents (4. ... 

Figure 10.35. (a) Nitrogen adsorption isotherms at 77 K on zircoma gel ARZr3 outgassed at 25°C and 200°C (closed circles for desorption) (b) Water sorption isotherms on same gel outgassed at 25°C Three successive runs Runs 1, 2 and 3 start at points A, B and C, respectively D is the final desorption point of run 3 (solid symbols for desorption) (Gimblett et al, 1981)... 

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. 

The conversion of disulfide groups to thiol groups does not greatly affect the water sorption isotherm (Mellon et al., 1949 Watt, 1963), the maximum regain increasing by only 1 %. Extensive reduction in n-propanol, however, followed by methylation in the same solvent (Maclaren, 1962) causes... 

In spite of the composite nature of the stratum corneum, its water sorption isotherm is qualitatively identical to those of the more simple protein systems shown, suggesting that water interacts predominately with the protein components of the corneum. This conclusion is supported further by the results of chloroform-methanol (3/1 by volume) extraction which removed as much as 25% of the original dry weight (lipids and low molecular weight water-soluble components) but did not quantitatively alter the isotherm in the low relative humidities (18). The application of the Zimm-Lundberg cluster theory (56, 57) to the isotherm yields additional information as to the state of the sorbed water in the corneum. The tendency of water to cluster is expressed in this theory by the cluster function CiGn ... 

Paik, S.W. Gilbert, S.G. Water sorption isotherms on sucrose and starch by modified inverse frontal gas chromatography. J. Chromatogr. 1986, 351, 417-423. 

Fig. 2.2. Typical water sorption isotherms (T, > 7", > Tf. (A) Based on water pressure. (B) Based on water activity. A water activity, p water pressure exerted by the sorbed water molecules in the solid at a given temperature, / water pressure over pure water at the same temperature.

Modified Frontal Chromatographic Method for Water Sorption Isotherms of Biological Macromolecules... 

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