Heats of sorption

Fig. 5. Differential heats of sorption in nature chaba2ite (4). A = N2 B = Ar Q = O2 <) = CO = CO2. See Table 2 for polarizabiUty, dipole moment, and quadmpole moment values for the gases. Volume adsorbed is expressed as cm of adsorbate as Hquid. To convert kj to kcal, divide by 4.184.

Fig. 5 Net differential heat of sorption for water vapor by native potato starch at 20°C as a function of amount of water sorbed per mass of dry starch. (From Ref. 14.)...

The parameter determined from simulations that can be most readily compared to an experimentally observable quantity is the heat of sorption. The mean total internal energy determined by the simulations, (V), can be equated to the isosteric heat of adsorption, Qst, in the limit of low occupancy, as follows ... 

June et al. investigated the sorption and spatial distribution of butane and three hexane isomers within the pores of silicalite, using a Metropolis MC method (87) and MD simulations (85). Perturbations of conformation as a result of confinement within the pore were also reported. Heats of adsorption and Henry s law coefficients were found to be in good agreement with experimental values for butane (48-51 kJ/mol) (142,148,150,163-165) and n-hexane (70-71 kJ/mol) (163, 166, 167). The heats of sorption of the other two hexane isomers, 2- and 3-methylpentane, were predicted to be 5 kJ/mol lower than that of n-hexane. 

The standard heat of sorption, Afl°, can then be interpreted in terms of appropriate energy contributions and the corresponding entropy, AS°, in terms of various degrees of freedom of the guest molecules relative to the host lattice (Table II) (10). An example of the use of the above method is given elsewhere in this volume (11). In comparing observed standard entropies with those based on the models of Table II, only empirical methods (12) are available for a priori estimates of the frequencies v. Nevertheless, comparisons have been of considerable interest (10). 

As a consequence of the assumed temperature dependence of the molecular volume of the sorbate, the isosteric heats of sorption, calculated... 

Figure 6. Differential heats of sorption, A i (— AHy = Qst) as functions of amount sorbed, ri, per cm3 of intracrystalline free volume, tj is given in cm3 at stp per cm3 of intracrystalline free volume, (a) Xe in BarL (curve 1), in Na-L (curve 2), and in K-L (curve 8).

The difference between IGC and conventional analytical gas-solid chromatography is the adsorption of a known adsorptive mobile phase (vapour) on an unknown adsorbent stationary phase (solid state sample). Depending on experiment setup, IGC can be used at finite or infinite dilution concentrations of the adsorptive mobile phase. The latter method is excellent for the determination of surface energetics and heat of sorption of particulate materials [3]. With IGC at finite dilution, it is possible to measure sorption isotherms for the determination of surface area and porosity [4], The benefits of using dynamic techniques are faster equilibrium times at ambient temperatures. 

Temperature Effects. Equilibrium studies have been performed on the HC1, 10 2M NaCl system at temperatures of 2°, 25°, and 50°C. No significant differences were noted in the resulting isotherms, so that heats of sorption have not been calculated. The results of rate studies on the same systems at the three different temperatures are shown in Figures 13... 

An intriguing aspect of these measurements is that the values of D determined from NMR and from sorption kinetics differ by several orders of magnitude. For example, for methane on (Ca,Na)-A the value of the diffusion coefficient determined by NMR is 2 x 10 5 cm2 sec-, and the value determined for sorption rates only 5 x 10"10 cm2 sec-1. The values from NMR are always larger and are similar to those measured in bulk liquids. The discrepancy, which is, of course, far greater than the uncertainty of either method, remained unexplained for several years, until careful studies (267,295,296) showed that the actual sorption rates are not determined by intracrystalline diffusion, but by diffusion outside the zeolite particles, by surface barriers, and/or by the rate of dissipation of the heat of sorption. NMR-derived results are therefore vindicated. Large diffusion coefficients (of the order of 10-6 cm2 sec-1) can be reliably measured by sorption kinetics... 

Since the columns used in the various breakthrough experiments varied from run to run (see experimental section), certain questions needed to be resolved with the help of the above-mentioned computer programs, along with the LRC coefficients for prediction of the multicomponent loadings and heats of sorption. These questions included the following ... 

The isosteric heats of sorption reported in Table VI are also in acceptable agreement with values based on sorption measurements made near ambient temperature by other investigators, as the comparisons in Table VIII show. 

Table VIII. Reported Isosteric Heats of Sorption (kcal mol...

The curves of Figure 2 may be thought of as "titration" curves for the sites involved, those in zeolite NaA equalling the number of 8-rings. That sites of similar nature are involved is also shown by the observed constancy of isosteric heats of sorption with coverage. It is then clear that the initial, steep fall of the CaA zeolite curves corresponds to sites of greater number in the A structure. These curves have the appearance of being the sum of two lines of different slope. 

The rate of uptake slows down considerably as p decreases, p affects the slopes of both the initial and the later portions of the uptake curve. This shows that uptake rate is very sensitive to the adsorbate loading and the heat of sorption. For a Langmurian system, p may be written as ... 

A variety of studies have shown that as temperature increases, sorption may increase, decrease, or remain the same, with isosteric heats of sorption being very low (Table 21.7). These studies have used the batch slurry technique, so the impact of temperature on water-triazine interactions may mask surface-triazine interactions. In contrast, at 10% soil moisture isosteric heats of atrazine sorption ranged from -10 to -12kcal/mol determined with the SF technique (Koskinen and Rochette, 1996). Sorption coefficients in field-moist soils were much greater than are typically obtained with the batch slurry system, while heats of sorption were much more negative, indicating greater sorption at low moisture contents. 

It is, therefore, understandable that Bagg and Tompkins explain the constancy of the heat of sorption that they found for oxygen on iron films at room temperature in terms of immobility. In view of what we learned in Sec. VII,6, however, a solution of oxygen in the surface layers is more probable. 

Maxted and Hassid (858), and later Kwan (859), found the heat of sorption of hydrogen on platinum to be independent of the amount that was taken up. It is quite probable that, at the temperatures they used in their investigations, hydrogen penetrates the surface layers of the metal. 

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