Competitive Adsorption Measurements

The amount of solute molecules preferentially adsorbed from dilute solution onto a given solid can be measured in a separate adsorption experiment, independently of the calorimetry measurement. In the case of the titration calorimetry procedure, this is even the only possibility to determine the amount adsorbed after each injection step and subsequently calculate the differential molar enthalpy of adsorption. The main difficulty here, contributing to a significant uncertainty of the experimental result, is related to the necessity of reproducing strictly the same experimental conditions in both types of experiment (i.e., the same solid surface-to-solution volume ratio, evolution of the pH and ionic strength in the equilibrium bulk solution, charging behaviour of the solid surface, etc.). 

The quantity of adsorption is usually measured by means of the solution depletion method [6] in glass stoppered tubes or fiasks (Fig. 6.14). A known mass of the solid 

The amount of solute adsorbed at the Solid-Liquid interface is calculated by means of the following formulas  

For a given adsorption system, the amount of solute adsorbed at equilibrium depends on the temperature T, the pressure P, and the composition of the equilibrium solution phase p. The experimental results of adsorption measurements are usually reported in the form of individual adsorption isotherms showing the quantity or 

The appearance of an adsorption plateau region at equilibrium concentrations (molalities) approaching the solubility limit indicates that the phenomenon involves only single solute species that are individually dissolved in the solvent. When the adsorption plateau is observed at lower concentrations (molalities), it is usually argued that surface sites of a given type have been saturated by the adsorbing solute species. 

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We would like to express thanks to our co-workers, especially Mr. K. Birkner for the sputtering work. Dr. G. Hellwig for protein adsorption studies. Dr. B. Schindler for developing plasma deposition, and Mr. N. Stroh and Mr. M. Timmermann for filtration experiments. We are grateful to Dr. Lemm, Ber-for competitive adsorption measurements. 

The competitive adsorption isotherms were determined experimentally for the separation of chiral epoxide enantiomers at 25 °C by the adsorption-desorption method [37]. A mass balance allows the knowledge of the concentration of each component retained in the particle, q, in equilibrium with the feed concentration, < In fact includes both the adsorbed phase concentration and the concentration in the fluid inside pores. This overall retained concentration is used to be consistent with the models presented for the SMB simulations based on homogeneous particles. The bed porosity was taken as = 0.4 since the total porosity was measured as Ej = 0.67 and the particle porosity of microcrystalline cellulose triacetate is p = 0.45 [38]. This procedure provides one point of the adsorption isotherm for each component (Cp q. The determination of the complete isotherm will require a set of experiments using different feed concentrations. To support the measured isotherms, a dynamic method of frontal chromatography is implemented based on the analysis of the response curves to a step change in feed concentration (adsorption) followed by the desorption of the column with pure eluent. It is well known that often the selectivity factor decreases with the increase of the concentration of chiral species and therefore the linear -i- Langmuir competitive isotherm was used ... 

The precise measurement of competitive adsorption isotherms not only of theoretical importance but may help the optimization of chromatographic processes in both analytical and preparative separation modes. The methods applied for the experimental determination of such isotherms have been recently reviewed [90], Frontal analysis using various flow rates can be successfully applied for the determination of competitive adsorption isotherms [91]. 

Tables VI and VIII contain in parentheses several sets of adsorption coefficients of aromatic hydrocarbons that have been estimated from competitive experiments or adsorption measurements. The problems with the interpretation have been mentioned in Section V,A,2. Other series that have been correlated with Type A and Type B expressions are summarized in Table IX 48, 52, 74, 82, 96,100,103,156-159). The series showing parallel...

Amidine derivatives are effective dehalogenation inhibitors for the chemoselective hydrogenation of aromatic halonitro compounds with Raney nickel catalysts. The best modifiers are unsubstituted or N-alkyl substituted formamidine acetates and dicyandiamide which are able to prevent dehalogenation even of very sensitive substrates. Our results indicate that the dehalogenation occurs after the nitro group has been completely reduced i.e. as a consecutive reaction from the halogenated aniline. A possible explanation for these observations is the competitive adsorption between haloaniline, nitro compound, reaction intermediates and/or modifier. The measurement of the catalyst potential can be used to determine the endpoint of the desired nitro reduction very accurately. 

An improvement in foam stability was observed as R was increased to >0.15 (Figure 17). This was accompanied by the onset of surface diffusion of a-la in the adsorbed protein layer. This is significantly different compared to our observations with /8-lg, where the onset and increase in surface diffusion was accompanied with a decrease in foam stability. Fluorescence and surface tension measurements confirmed that a-la was still present in the adsorbed layer of the film up to R = 2.5. Thus, the enhancement of foam stability to levels in excess of that observed with a-la alone supports the presence of a synergistic effect between the protein and surfactant in this mixed system (i.e., the combined effect of the two components exceeds the sum of their individual effects). It is important to note that Tween 20 alone does not form a stable foam at concentrations <40 jtM [22], It is possible that a-la, which is a small protein (Mr = 14,800), is capable of stabilizing thin films by a Marangoni type mechanism [2] once a-la/a-la interactions have been broken down by competitive adsorption of Tween 20. 

In studies of competitive adsorption, the usually measured quantity is the overall composition of the adsorbed phase for a given composition of the bulk phase in equilibrium with it. It has been found that chemical shifts can provide a more detailed description. In a mixture of Xe and Kr in N 4 zeolite it was possible to observe the individual signals from XenKr mixed clusters as well as the Xen clusters under magic angle spinning (28). The absolute 129Xe chemical shifts of the XenKr mixed clusters and the increments between XenKr and the Xen+1 in various Xe-Kr mixtures in Na4 zeolite, are shown in Table I. 

Transfer of solute with and relative to the moving water and competitive adsorption of solutes are central to amelioration of saline and alkali soils, agricultural chemical location in soils and management of wastes in soils. This paper illustrates how space-like coordinates based on the distribution of the solid and the water help analyse these problems. We focus on the macroscopic or Darcy scale of discourse [6], which permits unambiguous measurement of the key elements of the flow equations, and we restrict ourselves to 1-dimensional flow, because that seems to limit analysable experiments. 

Using SFS, Davies and co-workers [77-79] reported enhanced adsorption and competitive adsorption at the hydrophobic surface, reminiscent of that seen at the air-solution interface. For the SDS/PEO mixture [79], competitive adsorption was observed at low concentrations, whereas at higher SDS concentrations, PEO was depleted from the surface. Similar observations were made from IR-ATR measurements by Poirier et al. [80] on CieTAB/PSS mixtures at the silica-solution interface. However, the technique could not distinguish between depletion or surface complex formation. Similar trends were also reported by Fielden et al. [76] for SDS/AM-MAPTC mixtures on mica. For the PEI/SDS mixture at the hydrophobic interface [76], the SFS measurements indicated a higher degree of order and hence adsorption due to complexation at the interface. This was also shown to be strongly pH dependent [81],... 

The deactivation of catalysts concerns the decrease in concentration of active sites on the catalyst Nj. This should not be confused with the reversible inhibition of the active sites by competitive adsorption, which is treated above. The deactivation can have various causes, such as sintering, irreversible adsorption and fouling (for example coking or metal depositions in petrochemical conversions). It is generally attempted to express the deactivation in a time-dependent expression in order to be able to predict the catalyst s life time. An important reason for deactivation in industry is coking, which may arise from a side path of the main catalytic reaction or from a precursor that adsorbs strongly on the active sites, but which cannot be related to a measurable gas phase concentration. For example for the reaction A B the site balance contains also the concentration of blocked sites C. A deactivation function is now defined by cq 24, which is used in the rate expression. 

Adsorption processes may be particularly important in influencing species concentrations, since the arsenic present in the pore waters will probably be in equilibrium with arsenic adsorbed on solid surfaces. Arsenic in any species measured in pore waters may be only a fraction of the total amount of that species present in the sediments, the rest being adsorbed to or incorporated into particulate matter. Thus, it is important to study the sorptive characteristics of each of the arsenic species in the sediments. In the Menominee River sediments studied, the four oxygenated arsenic species (arsenate, arsenite, monomethyl arsonic acid and cacodylic acid) are often present together and competing among themselves and with phosphate for the same sorption sites. The competitive adsorptive characteristics of the species could greatly influence... 

Vesely [93] has expressed the effects of a number of additives in terms of competitive adsorption at the catalyst site by the insertion of a term Ki, [D] in the denominator of eqn, (6). Values of quoted for propene polymerization by a-TiCl3/AlEt3 are given in Table 2. However, if the additives are removed by chemical reaction the use of an equilibrium constant as a measure of their ability to coordinate with the catalytic centre would appear to be a considerable over-simplification. 

This UPD system is characterized by a significant positive Me-S lattice misfit (do.Tl = 0.3400 nm, doAg - 0.2890 nm) and by the formation of two T1 monolayers in the UPD range. Cyclic voltammograms and l E) isotherms of the system Ag(M/)/Pb, H, CIO4, measured with the FTTL technique [3.105], are presented in Figs. 3.3 and 3.10, respectively. The electrosorption valency was found to be = z = 1 in the entire UPD range (Fig. 3.12b). This means that cosorption or competitive adsorption processes of anions can be excluded in this system. 

Single-component isotherm parameters cannot always predict elution profiles with satisfied accuracy [122, 123], Therefore, to be able to predict accurate overloaded multi-component elution profiles where competition occurs competitive adsorption isotherm parameters are often necessary. Measurement of isotherms from a mixture is also often necessary because the pure enantiomers are not always accessible in large quantities. However, there exist only a small number of reports on the determination of multi-component adsorption isotherm parameters. FA can be used to determine binary isotherm data but it is time-consuming. The PP method is an alternative method to determine isotherm parameters from binary mixtures. It has been reported that the PP method works well up to weakly non-linear conditions [118, 119],... 

Figure 6.28 compares measured and simulated profiles for the batch separation of EMD53986. Very good agreement between theory (solid lines) and experiment (symbols) is achieved using the multi-component modified-Langmuir isotherm (Fig. 6.21). Also shown are the simulation results neglecting component interaction by using only the single-component isotherms (dashed line), which deviate strongly from the observed mixture behavior. Typical for competitive adsorption is the displacement of the weaker retained R-enantiomer and the peak expansion of the stronger adsorbed S-enantiomer.

Figure 6.30 shows the very close agreement between these methods for this enantiomer system. Especially when considering the effort necessary to measure the multi-component isotherms, IAS theory or its extensions may provide a good estimate for the component interaction in the case of competitive adsorption. Therefore, it is advisable to simulate elution profiles using the IAS theory after singlecomponent isotherms have been measured. These calculations should then be compared with a few separation experiments to decide if measurements of the multi-component isotherm are still necessary. 

This isotherm model has been used successfully to accoimt for the adsorption behavior of numerous compounds, particularly (but not only) pairs of enantiomers on different chiral stationary phases. For example, Zhou et ah [28] foimd that the competitive isotherms of two homologous peptides, kallidin and bradyki-nine are well described by the bi-Langmuir model (see Figure 4.3). However, most examples of applications of the bi-Langmuir isotherm are found with enantiomers. lire N-benzoyl derivatives of several amino acids were separated on bovine serum albumin immobilized on silica [26]. Figure 4.25c compares the competitive isotherms measured by frontal analysis with the racemic (1 1) mixture of N-benzoyl-D and L-alanine, and with the single-component isotherms of these compounds determined by ECP [29]. Charton et al. foimd that the competitive adsorption isotherms of the enantiomers of ketoprofen on cellulose tris-(4-methyl benzoate) are well accounted for by a bi-Langmuir isotherm [30]. Fornstedt et al. obtained the same results for several jS-blockers (amino-alcohols) on immobilized Cel-7A, a protein [31,32]. 

If we apply one of these equations to single-component isotherm data, we see that Eqs. 4.54 and 4.55 can be applied to the competitive adsorption data for a binary mixtiue only if Eq. 3.31 applies to the single-component data for each component. Then the six parameters can be derived from the single-component isotherms and only the coefficient b has to be measured with the mixture. Using more complicated models, Lin et al. [70] and Moreau et al. [71] have derived similar isotherms. Attempts at reducing the number of independent parameters as well as at determining these parameters from sets of experimental data have had limited success so far. 0onsiderable attention is required to clarify this issue. 

There is a dearth of competitive adsorption data, in a large part because they are difficult to measme, but also because little interest has been devoted to them, as, until recently, there were few problems of importance whose solution depended on their understanding. Besides the static methods, which are extremely long and tedious and require a large amoimt of material, the main methods of measurement of competitive isotherms use column chromatography. Frontal analysis can be extended to competitive binary isotherms [14,73,93-99], as well as pulse techniques [100-104]. The hodograph transform is a powerful method that permits an approach similar to FACP for competitive binary isotherms [105,106]. 

Figure 4.26 Comparison of the competitive adsorption isotherm measured by FA and calculated by two different methods. p-Cresol (Left) and phenol (Right), Top Data from the mass balance method (MMB, binary frontal analysis) at molar ratios of 3 1 (Q)/ Id ( ) and 1 3 (A). Solid hnes calculated by the method of composition velocity (MMC). Bottom Comparison of the competitive isotherms obtained by MMB (Q) and HBBM (square s)nnbol) (n) for p-ciesol and phenol in three concentration regimes. Reproduced with permission from J. Jacobson and J. Frenz, ]. Chromatogr., 499 (1990) 5 (Figs. 2 and 5).

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