Drying adsorption isotherms

The water removal mechanism is adsorption, which is the mechanism for ad Class 4 drying agents. The capacity of such materials is often shown in the form of adsorption isotherms as depicted in Figures 9a and 9b. The initial adsorption mechanism at low concentrations of water is beheved to occur by monolayer coverage of water on the adsorption sites. As more water is adsorbed, successive layers are added until condensation or capidary action takes place at water saturation levels greater than about 70% relative humidity. At saturation, ad the pores are fided and the total amount of water adsorbed, expressed as a Hquid, represents the pore volume of the adsorbent. 

The storability of the dried product depends to a large extend on the selected type, e. g. strawberries, carrots and green beans [4.7]. For meat, the fat content can be important. Karel [4.8] studied the influence of the water content in stored dried food, and found that not only was the amount of water of influence, but also the kind of bond to the solids. This link can be described by adsorption isotherms, as shown in Fig. 4.1. In food technology, the bond of water is often given by the term water activity, aw ... 

Figure 6 Water adsorption isotherms by dry-roasted navy bean flour fractions at room temperature (20°C).

Fig. 5.1 Examples of adsorption isotherms. S-type aldrin on oven dry kaolinite from aqueous solution. L-type parathion on oven-dry attapulgite from hexane solution. H-type methylene blue at pH = 6 on montmorillonite from aqueous solution. C-type parathion on clay soil from hexane solution (Yaron et al. 1996)...

The adsorption isotherms of xenon were measured at 34°C using a classical volumetric apparatus. The 29xe-NMR measurements were performed at the same temperature on a Bruker CXP-200 spectrometer operating at 55.3 MHz. The n-hexane adsorptions were conducted at 90°C on a Stanton Redcroft STA-780 thermoanalyzer. The samples were submitted to a preliminary calcination under dry air up to 650°C with a heating rate of 10°C/min. 

Figure 1.2 Water adsorption isotherms at 25°C for Celite and preparations obtained by mixing Celite with different solutions (l.OmLg 1 Celite) and drying. Pure Celite ( ) Celite and a-chymotrypsin in water (4.0mgmD )(D) Celite and a-chymotrysin (4.0mgmL l) in 50mM sodium phosphate buffer, pH 7.8 (O). Reprinted from Ref. [18].

All zeolites have a highly hydrophilic surface and are very efficient desiccants. Contrary to other nonzeolitic desiccants such as silica gel and activated alumina, zeolite adsorbents have type I adsorption isotherms for water—i.e., a high water adsorption capacity at a low concentration of water. To obtain extremely dry gases and liquids, therefore, zeolite adsorbents are strongly preferred over amorphous desiccants. The 3A mo-... 

Hageman et al. [3.13] calculated the absorption isotherms for recombinant bovine somatotropin (rbSt) and found 5-8 g of water in 100 g of protein, which was not only on the surface but also inside the protein molecule. Costantino et al. [3.72] estimated the water monolayer M0 (g/100 g dry protein) for various pharmaceutical proteins and for their combination with 50 wt% trehalose or mannitol as excipient. They compared three methods of calculating MQ (1) theoretical (th) from the strongly water binding residues, (2) from conventional adsorption isotherm measurements (ai) and (3) from gravimetric sorption analysis (gsa) performed with a microbalance in a humidity-controlled atmosphere. Table 3.5 summarizes the results for three proteins. The methods described can be helpful for evaluating RM data in protein formulations. 

Fig. 7. N2 adsorption isotherms, measured at 25° C, for Ag/LSX (a) after drying at room temperature followed by vacuum dehydration at 450° C, (b) after drying at room temperature followed by vacuum dehydration at 350° C, (c) after drying in air at 100° C followed by vacuum dehydration at 350°C and (d) after drying in air at 100°C in air, followed by heat treatment in air at 450° C and, finally, vacuum dehydration at 450° C (Hutson and Yang, 2000).

The adsorption isotherm up to 200 mm. Hg was completed and afterwards the sample was evacuated. Evacuation was started at —183° and continued for 40 minutes at that temperature. Thereafter the temperature was increased to —78° and evacuation continued at that temperature for thirty minutes. The sample was cooled down in He and evacuated. A second CO isotherm was taken at —183°. The difference between the two isotherms was determined to be 2.7 % of a monolayer. To insure that this sample had not, in some way or other, been oxidized, the temperature was raised to —78° with the sample still kept in the CO atmosphere. The sample was kept at — 78° for a few minutes only and the dry-ice bath again substituted by the liquid oxygen bath. During the temperature cycle more CO had been adsorbed so that the amount of chemisorption was now equal to 10.6% of a monolayer, 7.9% having been taken up during the temperature cycle. 

A classical volumetric adsorption apparatus equipped with absolute capacitance pressures transducers can be used for the estimation of adsorption isotherms in the pressure range 10-3 mbaradsorption measurements the carbon black samples are extracted with toluene and water/methanol (1 1) and after drying degassed overnight at 200 °C at a pressure below 10 4 mbar. The time allowed for equilibrium of each point of the isotherm is 5-90 min depending on the sample and the adsorbed amount. 

Figure 1-33 Adsorption Isotherm of Freeze-Dried Meat...

Figure 1.35. Schematic monolayer adsorption isotherms (surface coverage a versus P/Psat) for the two cases in Table 1.2. At P/ Psat = 1, the solid surface is wet in case (1) and nearly dry in case (2).

The presence of surface OH groups or H2 O molecules can play a primary role in adsorption. For example, a microcalorimetric study of the adsorption of stearic acid, from heptane solution, on ferric oxide (Husbands et al., 1971) revealed that preadsorbed water enhanced adsorption of stearic acid. When adsorption takes place from a dry organic liquid, residual surface water may act as special agent. This was shown for the adsorption of a silane coupling agent (y-amino-propyl-triethoxysilane) on silica covered with water molecules for 6 < 1 (Trens and Denoyel, 1996). By the simultaneous determination of adsorption isotherms and the enthalpies of displacement (of heptane by various silanes) it was demonstrated that the amine function was able to displace some of the surface water and make it available for the hydrolysis of the silane into trisilanol, whereas the residual water was able to promote the formation of siloxane bonds between the trisilanol molecules and the surface. 

Fig. 33.—The adsorption isotherm of dry potato starch layers in the range of 20-90°C. (Reprinted from reference 409, pp. 611-622 by courtesy of Marcel Dekker, Inc.)...

The synthesis of phenolic-formaldehyde and melamine-formaldehyde resins in the presence of fumed silica allows obtaining porous organic materials with a differentiated porous structure and surface properties. The pore characteristics of the studied resins in dry state were determined from nitrogen adsorption isotherms. The differences in surface character of the synthesized polymers were estimated satisfactorily by XPS spectra showing the presence of various functional groups. The adsorption/desorption mechanism of water and benzene on the investigated porous polymers was different due to differentiated hydrophobicity of the bulk material. 

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