Adsorption localized

In order to have localized adsorption with only physical interaction it is clear that either the interaction must be strong, or the kinetic energy of the adsorbed molecules must be small. As an example of the latter condition we have the work of Keesom and Schweers (24, 25) for low temperature adsorption of hydrogen and neon on glass. They assumed that the actual area of the glass was equal to the apparent area, and the results in Table IX were worked out for = 3 on that basis. The 

As an example of strong interaction leading to localized adsorption we have the figures for the entropy of adsorption of water on different adsorbents, which are summarized in Table X. In all cases the heat of adsorption is quite large and the entropy values for the adsorbed material 

Adsorbent Ref. Temp. °K. AH kcals./ mole AS e.u. Svavor e.u. Sadsorbate e.u. 

0608 dynes/cm. (taking the area occupied by a water molecule to be 12 A.2). This is considerably higher than the experimental value of 

However, it was shown (9) that if all the water molecules were associated into dimers, assuming that the number of nearest neighbors 

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For localized adsorption, we need the contribution from the adsorption bond. Per degree of vibrational freedom, we obtain on applying Eq. XVII-20 to Eq. XVII-31,... 

We can now proceed with various estimates of the entropy of adsorption Two extreme positions are sometimes taken (see Ref. 14). First, one assumes that for localized adsorption the only contribution is the configurational entropy. Thus... 

Thus the entropy of localized adsorption can range widely, depending on whether the site is viewed as equivalent to a strong adsorption bond of negligible entropy or as a potential box plus a weak bond (see Ref. 12). In addition, estimates of AS ds should include possible surface vibrational contributions in the case of mobile adsorption, and all calculations are faced with possible contributions from a loss in rotational entropy on adsorption as well as from change in the adsorbent structure following adsorption (see Section XVI-4B). These uncertainties make it virtually impossible to affirm what the state of an adsorbed film is from entropy measurements alone for this, additional independent information about surface mobility and vibrational surface states is needed. (However, see Ref. 15 for a somewhat more optimistic conclusion.)... 

On the other hand, as applied to the submonolayer region, the same comment can be made as for the localized model. That is, the two-dimensional non-ideal-gas equation of state is a perfectly acceptable concept, but one that, in practice, is remarkably difficult to distinguish from the localized adsorption picture. If there can be even a small amount of surface heterogeneity the distinction becomes virtually impossible (see Section XVll-14). Even the cases of phase change are susceptible to explanation on either basis. 

Diffraction is not limited to periodic structures [1]. Non-periodic imperfections such as defects or vibrations, as well as sample-size or domain effects, are inevitable in practice but do not cause much difSculty or can be taken into account when studying the ordered part of a structure. Some other forms of disorder can also be handled quite well in their own right, such as lattice-gas disorder in which a given site in the unit cell is randomly occupied with less than 100% probability. At surfaces, lattice-gas disorder is very connnon when atoms or molecules are adsorbed on a substrate. The local adsorption structure in the given site can be studied in detail. 

The associative part of the adsorbing potential, Eq. (116), generates a highly localized adsorption which corresponds to the onefold, to the twofold bridging site, and to the fourfold hollow site adsorption dependence of the length L. Note that in the absence of the associative part, Eq. (119), and in the limit 0 the pore walls reduce to an array of hard spheres. 

J. P. Badiali, L. Blum, M. L. Rosinberg. Localized adsorption at solid-liquid interface the sticky site hard wall model. Chem Phys Lett 729 149-154, 1986. 

The thermodynamic properties of clathrates can be derived from a simple model which corresponds to the three-dimensional generalization of ideal localized adsorption. In ref. 52 the deriva-... 

Ideal localized adsorption, 5, 10 Infrared method, 373, 374 Initiators of polymerization, 148 Intramolecular London energies, 74, 78 Iodine molecule intramolecular London energy, 78... 

Contrary to the last two isotherms, which take into the account interactions between the neighboring molecities ortiy, the Kiselev model assumes the singlecomponent localized adsorption, with the specific lateral interactions among all the adsorbed molecules in the monolayer [4—6]. The equation of the Kiselev isotherm is given below ... 

Buckled surface models for the two other copper sulfur phases were proposed that rationalised the STM, LEED and SEXAFS data (Figure 10.5). Note that the local adsorption site of the sulfur adatoms in both the p(3 x 2) and p(5 x 2) structures is the four-fold hollow in agreement with the SEXAFS18 measurements but looking at the structure from above with the STM, the periodicity of the structure suggests a variety of different adsorption sites. 

Fig. 3.5.9 NMR measured local adsorption/desorption isotherms extracted from ROIs within the (a) Al203 and (b) ZnO ceramics shown in Figure 3.5.8. Adapted from Ref [21].

The adsorption/desorption isotherms measured by NMR (equivalent to conventionally measured isotherms), extracted from two different regions of the imaging field of view corresponding to the two ceramics, are shown in Figure 3.5.9. Once these local isotherms are extracted, they are simply the local adsorption for that point in space contained within the material, measured non-invasively and non-destructively. Conventional analysis techniques for adsorption isotherms (such as BET theory) can therefore be applied to the data, to determine the microstructural properties corresponding to that isotherm curve. 

The orientational contribution to the Hamiltonian for adsorbed nonpolar molecules can be represented in the most general form as a sum of local adsorption potentials... 

As a result, the energy preference of the local adsorption unit is switched to the opposite distortion/reconstruction and, thus, chiral lateral interactions are switched in the direction of the induction and propagation of the chiral assembly occurring in the mirror image construct, leading to a mirror chiral surface. Therefore, from this work, one may conclude that the overall global or local chirality is determined principally at the nucleation stage. 

If one develops this concept further it becomes evident that the general adsorption of the molecules at the surface proceeds continuously to a localized adsorption on distinct and specific surface sites. This appears entirely reasonable as an extreme case, i.e., for ion exchange interaction with basic or acidic surface sites. The consequence, of course, is that the effective area required by an adsorbed sample molecule increases. 

It is possible to calculate the entropies in cases where association or dissociation occurs at the same time as localized adsorption. Chang (17) gave an expression... 

The three cases of chemisorption examined all showed localized adsorption, but there may be other examples in which a considerable mobility is possessed by the adsorbed molecules or atoms. Again, this would be more likely to occur at high temperatures. Entropy values can give indications of dissociation or, more particularly, of association when localized adsorption takes place. This was clearly noticeable in the values for the entropy of water on mercury. Likewise a knowledge of the changes in entropy during the course of an isotherm may be of use... 

Other methods of surface area determination depend, in general, on adsorption under well defined conditions of various solute molecules of known dimensions (Sposito, 1984 Davis and Kent, 1990). Some of these are dipole molecules so that dipole interactions with the surface or H-bonding are involved. Water adsorbed at a fixed relative water vapour pressure (e. g. 0.2) to provide a monolayer is one example (Torrent et ah, 1990). An organic dipole frequently used for soils is ethylene glycol monoethylether (EGME) (Carter et al., 1965). The main problem with these dipole molecules lies in their mutual association which may lead to localized adsorption beyond a monolayer (capillary condensation), particularly on porous material. 

Quantitative determination of the local adsorption structure of carbonate on Ag(llO) has been done by Kittel etal. [110]. They have found that the carbonate species is essentially planar and adsorbs almost parallel to the surface at the off-atop site with respect to the outermost layer Ag atom. The C—Ag layer spacing was 0.264 0.009 nm, with a well-defined azimuthal orientation. This geometry is understood best in terms of the added-row model proposed by Guo and Madix. This model assumes that additional Ag atoms lie adjacent to the carbonate, such that the... 

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