Change upon adsorption

The standard molar free energy change upon adsorption of the probe gas is thus given by... 

It can be expected that the electronic structure changes would be reflected by the heats of adsorption of suitable chosen molecules. Indeed, Shek et al (17) report that one maximum in the thermal desorption profile of CO shifts to lower temperatures when the Cu content of alloys increases. If the variations in the entropy changes upon adsorption can be neglected (probably - they can) this would indicate a lower heat of adsorption of CO on alloys than on Pt from abt. 33 Kcal/mol on pure Pt,to 26 Kcal/mol for an alloy with abt. 20% Cu. 

The site-selectively derived thermodynamic parameters obtained by adaptation of Equation 1.17 (Table 1.8) clearly revealed that the heat of adsorptions are exothermic on both enantioselective and nonenantioselective sites, and the difference in the adsorption enthalpies on enantioselective and nonenantioselective sites is about 10 and 15 kJ mol for/ - and 5-enantiomers, respectively. The differential enthalpy change upon adsorption of R- and 5-enantiomers at the enantioselective site AAEIg... 

Adsorption of ions or molecules on metal clusters markedly affects their optical properties. It was shown that the intensity and the shape of the surface plasmon absorption band of silver nanometric particles, which is close to 380 nm, change upon adsorption of various substances [125]. The important damping of the band generally observed is assigned to the change of the electron density of the thin surface layer of the... 

Cyclohexene adsorbed on the Pt(l 11) surface produces a (4 j) surface structure at 300 K. The work function change upon adsorption is —1.7 eV. As the temperature is increased to 450 K a new (q °) surface structure appears. 

The attenuated total reflection (ATR) Fourier transform infrared spectroscopic (FT-fR) studies of Gendreu, Jakobsen, and others79 have the potential for direct determination of conformational changes during the adsorption process due to shifts in the infrared absorption bands. Sakurai et al. 80,81), have used ATR-FT1R, as well as CD, to probe conformational changes upon adsorption. 

Although a number of proteins of interest (human serum albumin, for example) contain a single Trp, most contain two or more. Thus the spectrum observed is the sum of all active Trp fluors, making it difficult to deduce the local environment of each fluor. Nevertheless, the UV fluorescence emission spectrum is useful in deducing orientation and/or conformation changes upon adsorption. 

Clearly specific antibodies, and particularly monoclonal antibodies, may be very useful in probing the properties of adsorbed proteins. Specific antibodies have been used to probe the structure of antigens in solution 88). Consider the adsorption of a simple protein with a small number of reasonably well-defined epitopes (surface sites with antibody binding activity), as in Fig. 19. Clearly epitopes E and A are not accessible for binding, while B, C, and D would be sterically accessible. One could also envision a conformational change upon adsorption which produces an epitope... 

In conclusion, TIRF promises to be exceedingly useful in the study of protein-substrate interactions. It gives in situ, possibly remote, real-time information about protein adsorption-desorption parameters, conformational changes upon adsorption and hopefully, nanosecond time-resolved fluorescence lifetime information about adsorbed proteins 156). 

It should be realized that selective chemisorption titrates only atoms on the surface, but that the surface composition can be changed upon adsorption (75). A difficulty in the interpretation of chemisorptive titration is the possible presence of electronic effects, which can alter the chemical nature of the titrated atoms. Adsorbates can also show strong interactions with each other on pure metals, and decreases in this interaction can cause an increase in the number of molecules adsorbed per active metal atom exposed. The surface composition of active metal atoms deduced from these measurements will then be too high. 

Besides, the free energy change upon adsorption can be calculated as follows [2,13] ... 

Similar problems arise with the surface excess Gibbs energy G°, which is defined in table 1.2 in sec. 1.3. However, a number of enthalpy changes (upon adsorption, immersion, etc.) can be obtained and from them useful thermodynamic information can be deduced, see sec. 1.3. Some of these measurements contribute to the understanding of surface heterogeneity (in the energetic sense). In principle such information can also be obtained by isotherm analysis, see sec. 1.7. 

Thermodynamic analysis can of course also start from directly measured adsorption enthalpies. Detailed and accurate data are required because in the elaboration step often interpolations are needed for Instance, on oxides is required at constant pH or but in calorimetry these parameters usually change upon adsorption. 

Silica aerogels are highly porous solids with specific surfaces up to 1000 m g [1]. The doping of aerogels with transition metal oxides like vanadia to form efficient catalysts has been a subject of great interest [2-5]. Vanadia doped silica gels show in addition colour changes upon adsorption of small molecules such as water, ammonia or formaldehyde [6] and may therefore used as optical sensors. 

We are not aware of theoretical predictions for the technologically most important high-salt regime. Still these results confirm charge reversal upon polyelectrolyte binding which has been observed in the fabrication of polyelectrolyte multilayers. It is also obvious that the complexity of electrostatic conditions at the interface and their change upon adsorption will cause complicated adsorption kinetics, as we have discussed above. 

In the case of the simplest mechanism of repeated adsorption-desorption events of the unaltered molecules, the retention time and the peak shape are insensitive to the composition of the carrier gas — now we would add provided that it constantly modifies the column surface. Some of the more complex mechanisms of migration are indistinguishable from the outside because they are analogously insensitive. It is, for instance, simple chemisorption, when the initial electronic structure of the adsorptive substantially changes upon adsorption, but is restored at the desorption stage. Such is also the microscopic history of metallic adatoms in metal columns. Another case is the chromatography of molecular halides in columns loaded with alkali halides the adsorbed state is a surface complex between the two halides see Sect. 1.5.1. In both examples the structure of the original adsorption sites is not necessarily restored. It is, of course, unimportant in the experiments with tracers. 

More recently, it was demonstrated that the thermistor approach could be used to monitor specific interactions of fluoride ions with silica-packed columns in the flow injection mode. A thermometric method for detection of fluoride [56] was developed that relies on the specific interaction of fluoride with hydroxyapatite. The detection principle is based on the measurement of the enthalpy change upon adsorption of fluoride onto ceramic hydroxyapatite, by temperature monitoring with a thermistor-based flow injection calorimeter. The detection limit for fluoride was 0.1 ppm, which is in the same range as that of a commercial ion-selective electrode. The method could be applied to fluoride in aqueous solution as well as in cosmetic preparations. The system yielded highly reproducible results over at least 6 months, without the need of replacing or regenerating the ceramic hydroxyapatite column. The ease of operation of thermal sensing and the ability to couple the system to flow injection analysis provided a versatile, low-cost, and rapid detection method for fluoride. 

In RPLC, the influence of pressure on the chromatographic behavior is related to the hydrophobic interactions involved in the retention mechanism and to the change upon adsorption in the numbers of acetonitrile and water molecules in the solvent shells of the protein molecule and of the bonded layer. The importance of the changes in the retention factor and the saturation capacity with a change in the average column pressure will thus depend on the retention mode used and will vary with the hydrophobicity of the molecule [128]. In RPLC, it is larger with polymeric than with monomeric bonded phases [126]. 

It is evident that the positive entropy change upon adsorption is by far the major contributor to the negative values of the free energy change and thus the main driving force for adsorption at the interface in these compounds. The —AG°d per —CH2— group at 25°C is 3.0-3.5 kJ increase in the length of the alkyl chain therefore increases the tendency of the compound to adsorb. 

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