Adsorption isotherm, water

There are complexities. The wetting of carbon blacks is very dependent on the degree of surface oxidation Healey et al. [19] found that q mm in water varied with the fraction of hydrophilic sites as determined by water adsorption isotherms. In the case of oxides such as Ti02 and Si02, can vary considerably with pretreatment and with the specific surface area [17, 20, 21]. Morimoto and co-workers report a considerable variation in q mm of ZnO with the degree of heat treatment (see Ref. 22). 

The problem of the theoretical description of biopolymer water adsorption isotherms has drawn the attention of researchers for a long time. In the works [19], [20] a rigorous statistical basis for equations describing the isotherms for the case of homogeneous adsorption surfaces and noninteracting adsorption sites of N different types has been suggested. The general equation is ... 

Fig. 5.12 (a) Water adsorption isotherms at 20°C on Graphon activated to 24-9 % burn-off, where its active surface was covered to varying extents by oxygen complex. (b) The results of (a) plotted as amount adsorbed per of active surface area (left-hand scale) and also as number of molecules of water per atom of chemisorbed oxygen (right-hand scale). (After Walker.)... 

Fig. 9. Water adsorption isotherms at 25°C (to convert kPa to mm Hg, multiply by 7.5). 4A and 5A designate molecular sieves.

It is also possible to measure surface area from the water adsorption isotherm, and this is arguably more relevant to aqueous pulp suspensions as it measures the surface area which is accessible to water. Values of up to 140 m2g-1 have been obtained from the... 

Hysteresis is observed not only in the sorption isotherms but also in calorimetric measurements of heat of wetting at different moisture contents, and it is thus a combined entropy and enthalpy phenomenon. A reliable explanation for this effect is not currently available, but there is speculation that it is due to the stresses which are induced as the cellulose swells. Since the swelling of cellulose is not completely reversible, mechanical recovery is incomplete and hysteresis will therefore be present both in the internal stress-strain curve of the sample, and also in the water adsorption isotherm. 

As water swells cellulose in an intercrystalline way (i.e. only within the non-crystalline amorphous regions), a relationship would be expected between accessibility and moisture uptake, and this is indeed found (Figure 5.5). Refining causes cellulosic fibres to swell and it would therefore be expected to cause a change in the water adsorption isotherm. This is indeed observed (Figure 5.6). 

Figure 5.6 Effect of refining (number of revolutions in a PFI mill) on the water adsorption isotherm of an unbleached pulp.

A similar roasting effect was shown for the water adsorption isotherms of hull flours except that the most severe roasting did not affect the EMC as dramatically as it did whole flours for the highest aw. The water asorption isotherms of high protein flours indicate that roasting did not affect EMC of these flours... 

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

The water adsorption isotherms were determined by the liquid-volumetric method [21] The rust samples were put in an atmosphere with certain amount of water up to obtaining an established equilibrium, the pressure is measured and the water adsorbed may be calculated. 

Fig. 5 (a) shows the nitrogen adsorption isotherms of aluminum hydroxy pillared clays after heat-treatment at 300-500°C. These are of the typical Langmuir type isotherm for microporous crystals. Fig, 5 (b) shows the water adsorption isotherms on the same Al-hydroxy pillared clays [27]. Unlike the water adsorption isotherms for hydrophilic zeolites, such as zeolites X and A, apparently these isotherms cannot be explained by Langmuir nor BET adsorption equations the water adsorption in the early stages is greatly suppressed, and shows hydrophobicity. Water adsorption isotherms for several microporous crystals [20] are compared with that of the alumina pillared clay in Fig. 6. Zeolites NaX and 4A have very steep Langmuir type adsorption isotherms, while new microporous crystals such as silicalite and AlPO -S having no cations in the...

Fig. 6. Comparison of the water adsorption isotherms of the alumina pillared clay and various micropor-ous crystals.

Figure 1.1 Water adsorption isotherm of a-chymotrypsin at 25 °C. Reprinted from Ref. [18].

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].

Sometimes it can be difficult to know if the system has come to a true equilibrium concerning water distribution. It has been noted that water adsorption isotherms sometimes show hysteresis effects, which means that the water content, for example, that bound to the enzyme, depends not only on the water activity, but also on the hydration history [6]. More water is thus bound if a specified water activity is approached from a higher value (dehydration direction) than if the enzyme is hydrated from a drier state. The hysteresis effects might be due to slow conformational changes in the enzyme. 

Already at this point, the MCM materials are extremely hydrophobic and are no longer miscible with water. This is further evidenced by measuring the water adsorption isotherms of a non-treated MCM-48 (Fig. 4 (a)) and the same sample, after reaction with DMDCS and... 

Figure 4 Water adsorption isotherms of (a) blank MCM-48 (b) after reaction with DMDCS (c) after VO grafting.

Figure 3 shows the water-vapor adsorption isotherms measured on the silane-treated fibers along with those obtained on the untreated fibers. A complete discussion of the water adsorption isotherms of the untreated fibers has already been reported [8]. Here, two new features are immediately evident. First, the presence of the silane overlayer has greatly enhanced the water adsorption capacity of the fibers, and, second, the silane-treated fibers that contain 4% and 6% B,0, adsorb significantly more water than the 0% B20, fibers. It is important to note that these data have been normalized to the specific surface areas of the... 

Framework Dynamics Including Computer Simulations of the Water Adsorption Isotherm of Zeolite Na-MAP. See also J.-R. Hill, C. M. Freeman, and L. Subramanian, in Reviews in Computational Chemistry, K. B. Lipkowitz and D. B. Boyd, Eds., Wiley-VCH, New York, 2000, Vol. 16, pp. 141-216. Use of Force Fields in Materials Modeling. The shell model is also discussed by B. van de Graaf, S. L. Njo, and K. S. Smirnov, in Reviews in... 

In fig. 1.26 the effect of sample pretreatment is illustrated. The original sample is "Cab-0-Sir, a pyrogenic silica. It has a fairly low affinity for water. The isotherm type is between II and III (fig. 1.13). No hysteresis is observed. Stronger outgasslng (fig. (b)), further reduces the affinity for water the curve is now definitely of type II but also shows considerable hysteresis which was attributed to incomplete hydroxylation. In case (c) the surface is made hydro-phobic by methylatlon. The water adsorption isotherm (not shown) remains of type II but as Nj adsorption is not determined by hydrophilic groups, the corresponding Isotherm is of type III. Again, it is hysteresis-free. By application of the theories outlined before, information can be extracted from these isotherms in terms of available areas and enthalpies of adsorption. The authors extended this work with infrared studies. 

Figure 2 contains water adsorption isotherms corresponding to some of the samples. Figure 3 presents the water adsorption isotherms for the series of heat treated CF. In Figure 2 it can be seen that water isotherms are type V, characteristic of water adsorption in microporous AC [8]. It must be indicated that, contrary to CO2 adsorption and to the adsorption of many other gases and vapours, where adsorption in the microporosity takes place at low relative pressures (see Figure 1), in the case of water, adsorption takes place in... 

Comparing CO2 adsorption isotherms (at 273 K and 298 K) and water adsorption isotherms (Figure 1 and 2, respectively), it can be observed that, considering the total adsorption capacity, CO2 and water adsorption capacity follow a similar trend for all the samples. 

Figure 3 contains water adsorption isotherms corresponding to three heat-treated Kureha carbon fibres. For comparison purpose, this figure also includes the water isotherm... 

Figure 3. Water adsorption isotherm at 298 K on heat-treated carbon fibers.

The water and n-hexane adsorption isotherms of the zeolitic mesoporous materials obtained are compared to that of a 4S0 nm colloidal silicalite-1 in Figure 5. The water adsorption isotherms are distinctively type HI, whereas the n-hexane isotherms are type 1. The lowest water isotherm was for the colloidal silicalite-1, where the first point measured for the n-hexane isotherm was already at 80 mg g. The amount of n-hexane adsorbed reached 2S0 mg g at high pressure, which roughly corresponds to the filling of silicalite-1 micropores. 

Water affects the reaction rate through its effect on reaction kinetics and protein hydration, which is required for optimal enzyme conformation and activity. Enzymes need a small amount of water to maintain their activity however, increasing the water content can decrease the reaction rate as a result of hydrophilic hin-drance/barrier to the hydrophobic substrate, or because of denaturation of the enzyme (189). These opposite effects result in an optimum water content for each enzyme. In SCFs, both the water content of the enzyme support and water solubilized in the supercritical phase determine the enzyme activity. Water content of the enzyme support is, in turn, determined by the distribution/partition of water between the enzyme and solvent, which can be estimated from water adsorption isotherms (141, 152). The solubility of water in the supercritical phase, operating conditions, and composition of the system (i.e., ethanol content) can affect the water distribution and, hence, determine the total amount of water that needs to be introduced into the system to attain the optimum water content of the support. The optimum water content of the enzyme is not affected by the reaction media, as demonstrated by Marty et al. (152), for esterification reaction using immobilized lipase in n-hexane and SCC02- Enzyme activity in different solvents should, thus, be compared at similar water content of the enzyme support. 

NMR and used it to determine the pore size distribution of mesoporous silicates. They also derived the self-diffusion coefficient of water in MCM-41 and MCM-48 [94,107]. Llewellyn et aL [109] found that MCM-41 exhibits a type V water adsorption isotherm indicating an initial repulsive character followed by capillary condensation at higher pressures. 

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