Orientations for adsorption

Some of the discrepancies in the reported recoveries of different solutes from various waters by different investigators who have used the same functional polymers from different manufacturers can be rationalized by considering the discussions of pore size and surface area. Even when the pore sizes and surface areas are specified, awareness of the uncertainty in their determination is needed. Two polymers having the same listed pore size and surface area can behave quite differently as accumulators of organic solutes surface area does not specify surface orientation for adsorption pore size is not uniform, so the quoted value is an average and experimentally uncertain number. 

Figure 3. Possible orientations for adsorption of pyrazine on a metallic surface.

There is now available a substantial amount of information on the principles and techniques involved in preparing evaporated alloy films suitable for adsorption or catalytic work, although some preparative methods, e.g., vapor quenching, used in other research fields have not yet been adopted. Alloy films have been characterized with respect to bulk properties, e.g., uniformity of composition, phase separation, crystallite orientation, and surface areas have been measured. Direct quantitative measurements of surface composition have not been made on alloy films prepared for catalytic studies, but techniques, e.g., Auger electron spectroscopy, are available. 

The second problem of interest is to find normal vibrational frequencies and integral intensities for spectral lines that are active in infrared absorption spectra. In this instance, we can consider the molecular orientations, to be already specified. Further, it is of no significance whether the orientational structure eRj results from energy minimization for static dipole-dipole interactions or from the competition of any other interactions (e.g. adsorption potentials). For non-polar molecules (iij = 0), the vectors eRy describe dipole moment orientations for dipole transitions. 

Figure 2. Likely orientations for the adsorption of isoquinoline on mercury.

A sharp decrease in adsorption enthalpy between 10 and 30% surface coverage of SAL can also be seen in Figure 2. This decrease may indicate that only a small number of surface sites are favorably oriented for SAL-goethite bond formation, although possible SAL-SAL interactions on the surface may also have an effect. Separate measurements of SAL adsorption on goethite, gave relatively small adsorption maxima (when compared to the phosphate and fluoride adsorption maxima discussed above) of 22 and 11 pmol/g at pH 4.8 and 6.3, respectively, in either 0.001 M NaN0 or 0.001 M KC1 06). J... 

The influence of the organocation structure on the exchange adsorption becomes evident from the data in table V. 4,4 Bipyridinium cations adsorb two times more energetically (AH s 2j2 kJ Eq ) than do 2,2 bipyridinium cations (AH° = 11 f5 Eq ). The former adapt a planar orientation (dnm = f 26 nm) in contrast to the inclined position of the latter ( qq = 1.4 nm), despite the fact that sufficient surface is available for adsorption in a flat configuration. Smaller enthalpy terms are consistent with smaller electrostatic interaction energies. The reason for the tilting is unknown however. 

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. 

A simple linear plot of the data allows AA to be obtained. The results of a set of experiments (Table 5) are surprising. AA is independent of the size or molecular weight of the protein. Although the cross-sections of the proteins studied range from 1000 to 10,000 A2, AA is nearly constant at 100 to 200 A2. Conclusion . . only a small portion of the protein molecule needs to enter the interface in order for adsorption to then proceed spontaneously (Ref.3), p. 290). It is as if only a small foothold or handhold is required to stabilize the molecule against desorption. Now firmly planted at the interface, the molecule can optimize its interfacial interactions by time-dependent orientation and perhaps conformational changes. The size of the foot is obviously relevant to the exchange discussion in Sect. 4.5. 

Elaboration of a new mathematical software for the kinetic steady- and non-steady-state experiments in particular, the reliable provision for the primary interpretation of kinetic data, new methods (program-adaptive and completely adaptive) of performing informative steady-state kinetic experiments and radically new methods of carrying non-steady-state experiments oriented for the establishment of reaction mechanisms. Finally, it is the development of complex methods involving a combination of kinetic and physical (adsorptive, isotopic, spectroscopic) studies. 

Either approach can be modified to account for orientational effects if the molecule must be in a specific configuration for adsorption. 

---