Adsorption cooperative

Fig. 4.33 Model of cooperative adsorption in a slit-shaped pore.

Calculations were made at the desorbent concentrations used in Tests 3,6,7 and 8 in Table HI. Table IV below gives the respective adsorptions of sulfonate and desorbent as well as their equilibrium concentration. A comparison with the corresponding experimental values in Table HI shows good agreement with regard to sulfonate from the micellar slug. On the other hand, losses of desorbent are systematically underestimated. This shows that the assumption of the independent adsorption of both surfactants on the solid is incorrect and that presumably cooperative adsorption of desorbent and sulfonate takes place. Accordingly the model used needs to be improved. 

Figure 19. Schematic representation of the cooperative adsorption and desorption of DOPC molecules between an air/water interface and a sublayer.

Coadsorption, in which two different kinds of particles are chemisorbed on the solid surface, may be classified into cooperative adsorption and competitive adsorption. Cooperative adsorption takes place with two different adsorbate particles of opposite characteristics, such as electron-donating particles and electron-accepting particles (e.g. Na and S), and the two adsorbate particles are adsorbed uniformly on the solid surface. On the other hand, competitive adsorption involves two different particles of similar characteristics, i.e. both being electron-donating or electron-accepting particles (e.g. O and S), which are adsorbed separately on the solid surface. 

The S-shaped isotherm has an initial slope that increases with increasing equilibrium solute concentration and has two causes. Giles et al. (1974) attributed the S-shape to cooperative adsorption due to solute-solute interactions. These interactions stabilized the solute at the solid surface, and therefore the first adsorbed molecules enhance the adsorption of the next solute molecules. At high concentration, when the sites of the solid surface are saturated with solute the slope of adsorption isotherm start to decrease again. Sposito (1984) explained the S-shaped isotherm by a competing reaction within the solution. Solution ligands compete with surface... 

Using a cooperative adsorption isotherm developed by Ling (5) to derive a cooperative absorption isotherm that explicitly relates intracellular potassium content to external potassium concentration, Reisin and Gulati (4) have expressed the cooperative isotherm for the intracellular potassium as a function of Kex/Naex. This isotherm Ka(j is defined... 

Type III isotherms are not common and are characteristic of very weak adsorbate-adsorbent interaction and are typical of cooperative adsorption (i.e., adsorption of water vapor on graphitized carbon blacks). 

Coadsorption phenomena in heterogeneous catalysis and surface chemistry quite commonly consider competitive effects between two reactants on a metal surface [240,344]. Also cooperative mutual interaction in the adsorption behavior of two molecules has been reported [240]. Recently, this latter phenomenon was found to be very pronounced on small gas-phase metal cluster ions too [351-354]. This is mainly due to the fact that the metal cluster reactivity is often strongly charge state dependent and that an adsorbed molecule can effectively influence the metal cluster electronic structure by, e.g., charge transfer effects. This changed electronic complex structure in turn might foster (or also inhibit) adsorption and reaction of further reactant molecules that would otherwise not be possible. An example of cooperative adsorption effects on small free silver cluster ions identified in an ion trap experiment will be presented in the following. 

Cooperative Coadsorption Effects on Small Gold Clusters. Two examples of cooperative adsorption effects on small gold cluster anions identified in temperature dependent rf-ion trap experiments (see Chemical and Catal3dic Properties of Gas-Phase Clusters for experimental details) will be presented in the following. Au3 does not react with O2 in the ion trap experiment at any reaction temperature [34]. It, however, adsorbs a maximum of two CO molecules at reaction temperatures below 250 K [185]. If the gold trimer is exposed simultaneously to CO and O2 inside the octopole ion trap, still no reaction products are observed at reaction temperatures above 250 K as can be seen... 

Fig. 1.62. Temperature dependent mass spectra of the reaction of Aus with O2 and CO. The production distributions are analyzed after trapping Au3 for 500ms inside the octopole ion trap filled with 0.02Pa O2, 0.05Pa CO, and 1.23Pa He. Reaction temperatures (a) 300 K, (b) 200 K, (c) 100 K [34,371]. The proposed cooperative adsorption mechanism is indicated schematically. Large spheres indicate gold atoms, small, bright spheres carbon atoms, and small, dark spheres oxygen atoms...

When (S-lg adsorbs at the air-water interface in the presence of PS three phenomena can occur (i) the polysaccharide adsorbs at the interface on its own in competition with the protein for the interface (competitive adsorption) (ii) the polysaccharide complexates with the adsorbed protein mainly by electrostatic interactions or hydrogen bonding (Dickinson, 2003), and (iii) because of a limited thermodynamic compatibility between the protein and polysaccharide, the polysaccharide concentrates the adsorbed protein. In a previous work we have shown that the existence of competitive or cooperative adsorption between the (3-lg and the PS could be deduced from the comparison of rr-time curves for the single biopolymers and for the mixtures (Baeza et al., 2005b). 

The 77 values of (3-lg/PGA films (0.1 wt% PGA in the bulk phase) showed an antagonic behavior when compared to the tto( single (3-lg and PGA films, which should be attributed to their high degree of esterification (higher hydrophobicity) that allows them to rapidly adsorb at the interface. However, in the presence of KO at 0.5% (Figure 25.1a), the system showed a more cooperative adsorption. Similarly, KLVF at 0.5% increased tt of the mixed system. The increased cooperativity as PGA increased from 0.1 to 0.5% may be ascribed to an increase of segregation phenomena in the bulk solution. 

In the mixed systems, the behavior was similar to that observed for surface pressure. In the presence of surface-active PGA (Figure 25.3a and b) at low concentrations in the bulk phase (0.1 wt%), competition between the biopolymers at the interface results in a lower Ed than that expected from the observation of the single components. However, at higher concentrations of PS and long adsorption times, a cooperative adsorption can be deduced. This result could be explained by a concentration of (3-lg at the interface caused by the incompatibility with different biopolymers (that is more evident at higher concentrations). These phenomena would lead to an increase in the protein association in the film with the resultant increase in viscoelasticity. 

The S-type isotherm suggests cooperative adsorption, which operates if the adsorbate-adsorbate interaction is stronger than the adsorbate-adsorbent interaction. This condition favors the clustering of adsorbate molecules at the surface because they bond more strongly with one another than with the surface. 

The case of the S-shaped isotherm, representing cooperative adsorption, is a special situation with a similar derivation. Suppose that there is a tendency for the adsorbate, A, to adsorb in pairs on the surface. Then, in addition to reaction 10.5, the following adsorption reaction must be considered ... 

This is an S-shaped function for which the degree of cooperative adsorption is measured by the relative magnitude of the equilibrium constants, AT, and K2, for the monomer and dimer. 

Derive from the mass action law the general equation for cooperative adsorption ... 

In the case of coexistence of BSA with Na2Chs, the zeta potential changed stepwise in phosphate buffer solutions from 40 to -60 mV without reaching the iep. The results were explained as due to a complex formation on the kaolin surface via concurrent and/or cooperative adsorption of two polymers. 

Equation (22), derived for cooperative adsorption, concerns only the monolayer. As we have seen, if only the heat of liquefaction is available beyond the first layer, no further adsorption is to be expected. We must assume therefore that a van der Waals field transmits energy to the second and higher layers. If we assume a uniform surface, with the van der Waals energy falling off with distance according to a power law AE l/z", an isotherm equation can be derived. Hill (11) has... 

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