Adsorption different orientations

Surface reconstruction or adsorption can often cause a vicinal surface with a single macroscopic orientation to facet into surfaces with different orientations. Generally the reconstruction occurs on a particular low-index flat face, and lowers its free energy relative to that of an unreconstructed surface with the same orientation. However the same reconstruction that produces the lower free energy for the flat face generally increases the energy of surface distortions such as steps that disturb the reconstmction. Thus reconstmction is often observed only on terraces wider than some critical terrace width Wc. When steps are uniformly distributed initially and if Wc is much greater... 

In Faujasites. Bezus et al. (49) reported in 1978 statistical calculations on the low-coverage adsorption thermodynamics of methane in NaX zeolite (Si/Al = 1.48). As for single-atom adsorbates described earlier, the agreement between their calculated values and a range of experimental values was excellent. Allowing for different orientations of the molecule, they calculated a value of 17.9 kJ/mol for the isosteric heat of adsorption at 323 K. Experimental values available for comparison at that time (134-136) ranged from 17.6 to 18.8 kJ/mol. Treating the methane molecule as a hard-sphere particle, with a radius of 2 A, resulted in a far lower heat of adsorption (12.6 kJ/mol). Further calculations (99) yielded heats of adsorption of 19.8 and 18.1 kJ/mol for methane in NaX and NaY zeolites, respectively. 

The adsorption isotherm was calculated from the measured concentration change. The number of points and their precision suggests that the adsorption values are good to 5%, except at the very lowest concentrations. The absolute accuracy depends on the cleanliness of the carbon surface, which could contain chemisorbed oxygen, and on the completeness of the dispersion process. These possible errors would lead to low values for the experimental surface excess. Comparison of the area per adsorbed ion at apparent surface saturation with the calculated area in different orientations suggests that the entire B.E.T. area is available for adsorption in the dispersions. 

L-cysteine is oriented roughly perpendicular to the step edge. These orientations agree with those predicted using density functional theory and indicate significantly different orientations of the two enantiomers of cysteine on the chiral Au(17, 11, 9) surfaces and significant differences in their adsorption energetics. 

We should not expect, of course, that the variation of any single attribute of a clean surface, be it the /-band center, the f-LDOS, the workfunction, etc., will in all circumstances correctly predict the variation in its chemisorption properties, but we may find that some of these numbers are more useful for the purpose than others. It is instructive to point out that for transition-metal surface whose Fermi energy cuts the rising tail of the J-band the Ff-LDOS is expected to correlate with the /-band center the lower the J-band center, the smaller the corresponding Ff-LDOS. Indeed, based on the results of calculations in [113], a correlation can be found between the surface Ff-LDOS (before CO adsorption) of Pt surfaces with different openness (i.e., different orientation) and the CO adsorption energy on these surfaces, of which the latter is conrelated to the /-band center see Figure 25. It would be very beneficial to the field of catalysis if a more general relationship between these two quantities could be established. 

Special examples of mixture adsorption are competitive adsorption of the different forms of the same substance, such as pH-dependent ionic and undissociated molecular forms, monomers, and associates of the same substance, as well as potential-dependent adsorption of the same compound in two different orientations in the adsorbed layer. Different orientations on the electrode surface—for example, flat and vertical—are characterized with different adsorption constants, lateral interactions, and surface concentrations at saturation. If there are strong attractive interactions between the adsorbed molecules, associates and micellar forms can be formed in the adsorbed layer even when bulk concentrations are below the critical micellar concentration (CMC). These phenomena were observed also at mineral oxide surfaces for isomerically pure anionic surfactants and their mixtures and for mixtures of nonionic and anionic surfactants (Scamehorn et al., 1982a-c). 

In experiments on nonionic surfactants, namely Triton X-405 Geeraerts at al. (1993) performed simultaneously dynamic surface tension and potential measurements in order to discuss peculiarities of nonionic surfactants containing oxethylene chains of different lengths as hydrophilic part. Deviations from a diffusion controlled adsorption were explained by dipole relaxations. In recent papers by Fainerman et al. (1994b, c, d) and Fainerman Miller (1994a, b) developed a new model to explain the adsorption kinetics of a series of Triton X molecules with 4 to 40 oxethylene groups. This model assumes two different orientations of the nonionic molecule and explains the observed deviations of the experimental data from a pure diffusion controlled adsorption very well. Measurements in a wide temperature interval and in presence of salts known as structure breaker were performed which supported the new idea of different molecular interfacial orientations. At small concentration and short adsorption times the kinetics can be described by a usual diffusion model. Experiments of Liggieri et al. (1994) on Triton X-100 at the hexane/water interface show the same results. 

Apparently, under these conditions the specificity of the different orientations manifests itself in a variation of the 2 -values, which is perfectly compensated by the change in the pre-exponential factor B, so that the actual rates are not very different. The values of B support the idea that in the adsorption step of the reaction a mobile transition state is involved. 

The chemical differences arising from the differences in the primary structure are also very important because the balance of polar, nonpolar and charged amino acid side chains determines the surface activity of proteins in a particular system, i.e., the possibility and mode of their location at interfaces of different types. This amphi-pathic nature of the protein molecule allows it to bind with surfaces of different chemical nature. A very important property is the protein hydrophobicity [17]. It influences adsorption and orientation of proteins at interfaces and in many cases correlates with surface activity [2,21]. 

It is a consequence of the additivity law that the adsorption potential will be greatest if a maximum number of atoms in a molecule are in close contact with the surface. If other forces constrain the molecule to be in a different orientation the dispersion potential will be considerably reduced. For example, electrostatic forces tend to localise atoms at positions and orientations favourable for maximising eleetrostatic energies which may not be those which maximise the dispersion potential. 

The chemisorption of benzoic acid on polycrystalline gold electrode (from 0.1 mol dm HCIO4) was reported by 2felenay et al. using the radiotracer technique. According to their conclusion drawn from adsorption data and model calculations, the adsorbed molecules could be present in two different orientations. Horizontal (parallel to the surface) orientation dominates at low potential values while the vertical orientation will be dominating at high potentials. 

Several conclusions can be drawn from the systematic calculations performed for the adsorption of the water molecule at different sites of the (100) surface of the three noble metals, and for different orientations of the water monomer at those sites. 

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