Influence of Adsorption

Normal pulse voltammetry is very sensitive to adsorption of both the substrate and the product at the electrode surface. If the substrate is adsorbed, a peakshaped voltammogram is obtained. The stronger the adsorption or the lower the concentration of the substrate or the shorter the pulse time, the better defined is the peak. This is illustrated in Fig. II.2.6 by the normal pulse voltammograms of imidazoacridinone. This compound, at -0.4 V, is strongly adsorbed at the mercury surface, therefore the two consecutive reduction steps result in the formation of two peaks instead of two waves. 

The adsorption of the product leads in NPV to the formation of a pre-wave. A good example of such an electrode process is the electrooxidation of a mercury electrode in the presence of EDTA, a strong ligand [9]. The product, HgEDTA, strongly adsorbs on mercury. The lower the concentration of EDTA or the shorter the pulse time, the higher (relative to the main wave) is the pre-wave. 

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In this mode, the solid is no longer moving. The shifting of the inlet and outlet lines only simulates solid flow, and the solid flowrate downward is directly linked to the shift period. Proper selection of flowrates is required to stabilize the different fronts of species A and B in the proper zones. The adequate choice of the flowrates requires a minimum knowledge of the physico-chemical properties of the system. The influence of adsorption isotherms and plate numbers is simulated by the software. 

Synge, RLM, Experiments on Electrical Migration of Peptides and Proteins Inside Porous Membranes Influences of Adsorption, Diffusion, and Pore Dimensions, Biochemical Journal 65, 266,1957. 

The comparison of experimental data on adsorption of various particles on different adsorbents indicate that absorbate reaction capacity plays a substantial role in effects of influence of adsorption on electric conductivity of oxide semiconductors. For instance, the activation energy of adsorption of molecular oxygen on ZnO is about 8 kcal/mole [83] and molecular hydrogen - 30 kcal/mole [185]. Due to such high activation energy of adsorption of molecular hydrogen at temperatures of adsorbent lower than 100 C (in contrast to O2) practically does not influence the electric conductivity of oxides. The molecular nitrogen and... 

Fig. 4.27. Variation of electric conductivity of ZnO film under the influence of adsorption of CH3 radicals at room temperature for various pressures of acetone vapours 1,2 - 200 Torr 5-1 Torr 1, 2 - before and after immersion of the film in liquid acetone 4 - the film covered with a liquid layer.

Zoungrana et al. (1997) and Norde and Zoungrana (1998) investigated the influence of adsorption on the structure, structure stability and biological activity of a proteolytic enzyme, a -chymotrypsin. The enzyme was adsorbed from 0.01 M phosphate buffer at pH 7.0 and at 22°C onto solid surfaces of different hydrophobicities and morphologies. 

The influence of adsorption on the structure of a -chymotrypsin is shown in Fig. 10, where the circular dichroism (CD) spectrum of the protein in solution is compared with that of the protein adsorbed on Teflon and silica. Because of absorbance in the far UV by the aromatic styrene, it is impossible to obtain reliable CD spectra of proteins adsorbed on PS and PS- (EO)8. The CD spectrum of a protein reflects its composition of secondary structural elements (a -helices, / -sheets). The spectrum of dissolved a-chymotrypsin is indicative of a low content of or-helices and a high content of //-sheets. After adsorption at the silica surface, the CD spectrum is shifted, but the shift is much more pronounced when the protein was adsorbed at the Teflon surface. The shifts are in opposite directions for the hydrophobic and hydrophilic surfaces, respectively. The spectrum of the protein on the hydrophilic surface of silica indicates a decrease in ordered secondary structure, i.e., the polypeptide chain in the protein has an increased random structure and, hence, a larger conformational entropy. Adsorption on the hydrophobic Teflon surface induces the formation of ordered structural elements, notably an increase in the content of O -helices (cfi, the discussion in Sect. 3.1.4). 

Isotherm Subtraction. A second method (7) of determining the net proton coefficient from adsorption data is an adaptation of the thermodynamics of linked functions as applied to the binding of gases to hemoglobin (19). The net proton coefficient determined by this method is designated, Xp- The computational procedure makes a clear distinction between the influence of adsorption density and pH on the magnitude of the net proton coefficient. The fundamental equation used in the calculation of Xp is... 

Collection of in situ XAS data using a single cell fuel cell avoids problems associated with bubble formation found in liquid electrolytes as well as questions regarding the influence of adsorption of ions from the supporting electrolyte. However, the in situ study of membrane electrode assemblies (MEAs) in a fuel cell environment using transmission... 

Burkhard, N. and Guth, J. A. Chemical hydrolysis of 2-chloro-4,6-bis(alkylamino) -1,3,5 -triazine herbicides and their breakdown in soil under the influence of adsorption, Pestic. Sci, 21(l) 45-52, 1981. 

We observe that for the bonds depicted in Figs, lb and le, an atom M or an atom R, to which the chemisorbed particle C is attached, are more weakly bound to the lattice than the normal ions M+ or, respectively, Rr. As a result, in some cases we can expect that the molecule CM or CR may evaporate that is, the particle C upon desorption may carry off with it an atom of the lattice, thereby violating the stoichiometric composition of the crystal. In all cases such adsorption should facilitate surface creep which plays such an important role in the sintering, recrystallization, and disintegration of solids in reaction. This may also explain the well-known influence of adsorption on the surface mobility of the adsorbent atoms. 

The quasi-reversihle electroreduction processes of Zn(II) in the absence and in the presence of Ai,Ai -dimethylthiourea (DMTU) were quantitatively compared by Sanecld [91], It has been shown that in the presence of DMTU enhanced response of cyclic voltammetry and normal pulse polarography was complex and could be resolved into its regular reduction part and a part caused by the catalytic influence of adsorption of organic substance. 

Inukai et al. [512] have used STM to study Hg UPD on Au(lll) in sulfuric and perchloric acid solutions. For sulfuric acid, the influence of adsorption of bisulfate was indicated. It has been found that after the formation of the first UPD adlayer, two different structures were simultaneously formed on the same terrace. For perchloric acid, only a single structure was found. These results reflected a significant influence of the supporting electrolyte anions on the UPD structure. Recently, Abaci et al. [513] have presented the temperature-dependent studies on the influence of counteranions on Hg UPD on Au(lll). 

Scamehorn et al (1,2) have studied the influence of adsorption of isomerically pure surfactants and their mixtures on the occurrence of adsorption maxima. We have investigated, therefore, the flotability of barite in solutions of pure Na-alkylsulfates (C, nd... 

The influence of adsorption of polyelectrolytes on bimolecular phospholipid leaflets was studied. All polyelectrolytes studied were adsorbed on the surface of the film, as demonstrated by greatly increased drainage times. Only some of the polyelectrolytes investigated are able to decrease the d.c. resistance, notably a protein derived from ox erythrocyte ghosts and a Na-K polyphosphate. The combination of these latter substances proved particularly effective. It is concluded that the decrease of d.c. resistance is caused by adsorption and penetration of the polyelectrolytes into the membrane, resulting in the formation of pores or water channels, and not by the possibility of transport of charged macromolecules through the membrane. 

These two investigations, the study of the influence of adsorption on physical properties and the study of the adsorption equilibrium will form the subject of the next section. 

Fubini, B., Della Gata, G. and Venturello, G., 1978. Energetics of adsorption in alumina-water systems. Microcalorimetric study on the influence of adsorption temperature on surface processes. 3. of Colloid and Interface Sci., 64 470. 

Fig. 40. Influence of adsorption in zone and two-dimensional electrophoresis. (A) In zone electrophoresis adsorption causes contamination of the fractions. (B) In two-dimensional electrophoresis adsorption has no influence on the purity of the fractions.

Figure 14. Influence of adsorption on the Raman spectrum of ethanol. The penetration depth for the upper spectrum is on the order of 500 nm whereas in the lower spectrum it is only about 50 nm. Therefore, the Raman intensity is proportionally decreased. This is shown by the intensity ratio of the two Rayleigh lines, since the Raman spectrum is normalized to the 882 cm 1 band.

The factors which decrease evaporation rate with time are (1) retreat of an initially continuous deposit into discrete small areas as it gets too thin to be coherent, (2) influence of adsorption or solution in the porous, oily, or wet surface, (3) retreat of a deposit, initially lying on the outer surface, into deeper capillary spaces. 

Wadsworth and coworkers (13, 14) have found considerable evidence for surface polarization in double-beam infrared spectroscopy. Not only do new differential peaks due to adsorption appear in the spectrograms but also the bands due entirely to the adsorbent are frequently appreciably shifted by adsorption. This occurred, for example, in calcium fluorite treated with oleic acid, in samples of bentonites taken from aqueous solutions of different pH, and in various minerals treated by flotation collectors. In fact, it is more the rule than the exception that the spectrograms of finely divided solids dispersed in the KI or KBr window exhibit distortion due to adsorption, whether adsorption occurs at the solid-aqueous solution or at the solid-vapor interface. For example, Eyring and Wadsworth (13) found that two (differential) peaks were produced by adsorption on willemite either from the vapor or aqueous solution of hexanethiol. These peaks were due to the influence of adsorption of the hexanethiol on the Si-O bands of the willemite and occurred at about 9.2 and 12.3 microns. 

While there have been many SHG studies at the solid electrode/liquid interface as both an in situ probe of the electrode interface and the influence of adsorption at the surface of the electrode, there have been far fewer studies of electrochenucal processes occurring at the boundary between two immiscible electrolyte solutions. 

This plot represents the variation of an excessively adsorbed amount of acetonitrile with the variation of the equilibrium concentration of acetonitrile in the bulk solution. In the adsorption system the influence of adsorption forces exerted by the adsorbent surface are limited in their distance consequently, we should have limited volume where adsorbed analyte accumulates. It is also assumed that liquid is uncompressible and that molar volumes of both components do not change under the influence of adsorption forces. This leads to the displacement adsorption mechanism. 

Examples of the Influence of Adsorption on Kinetics. Three cases of arsenic species transformations influenced by adsorption are considered in this section. The three cases considered are 1) reactant not adsorbed, product adsorbed (e.g. demethylation of cacodylic acid to arsenate), 2) reactant adsorbed, product not adsorbed (e.g. methylation of arsenate to cacodylic acid),... 

Like most other organic arsenic containing compounds, the arsonic acids are more or less susceptible to adsorption, and because the influence of adsorption on the half-wave potential is dependent on substituents, concentration and the presence of surfactants, it is... 

However, additional factors must be considered in order to understand and fuially predict the rheological behavior of silica-resin mixtures. In particular, the type and strength of particle interactions and the influence of adsorption processes on these interactions are key factors which govern the rheology of liquid media containing pyrogenic silica for rheology control. 

The Arrhenius plots for Ya> l d smd (-rA)o are shown in figure 2. It can be observed that both (-rA)o ctnd Va increase when the temperature is increased, while d decreases. This could be interpreted as a strong influence of adsorption stages in the overall activation energy of the coking process. 

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