Adsorption cooperativity

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. 

Evans J W 1993 Random and cooperative sequential adsorption 1993 REv. Mod. Phys.65 1281-1329... 

These calculations lend theoretical support to the view arrived at earlier on phenomenological grounds, that adsorption in pores of molecular dimensions is sufficiently different from that in coarser pores to justify their assignment to a separate category as micropores. The calculations further indicate that the upper limit of size at which a pore begins to function as a micropore depends on the diameter a of the adsorbate molecule for slit-like pores this limit will lie at a width around I-So, but for pores which approximate to the cylindrical model it lies at a pore diameter around 2 5(t. The exact value of the limit will of course depend on the actual shape of the pore, and may well be raised by cooperative effects. 

In the higher pressure sub-region, which may be extended to relative pressure up to 01 to 0-2, the enhancement of the interaction energy and of the enthalpy of adsorption is relatively small, and the increased adsorption is now the result of a cooperative effect. The nature of this secondary process may be appreciated from the simplified model of a slit in Fig. 4.33. Once a monolayer has been formed on the walls, then if molecules (1) and (2) happen to condense opposite one another, the probability that (3) will condense is increased. The increased residence time of (1), (2) and (3) will promote the condensation of (4) and of still further molecules. Because of the cooperative nature of the mechanism, the separate stages occur in such rapid succession that in effect they constitute a single process. The model is necessarily very crude and the details for any particular pore will depend on the pore geometry. 

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

The weakness of the adsorbent-adsorbate forces will cause the uptake at low relative pressures to be small but once a molecule has become adsorbed, the adsorbate-adsorbate forces will promote the adsorption of further molecules—a cooperative process—so that the isotherms will become convex to the pressure axis. 

With the availabihty of computers, the transfer matrix method [14] emerged as an alternative and powerful technique for the study of cooperative phenomena of adsorbates resulting from interactions [15-17]. Quantities are calculated exactly on a semi-infinite lattice. Coupled with finite-size scaling towards the infinite lattice, the technique has proved popular for the determination of phase diagrams and critical-point properties of adsorbates [18-23] and magnetic spin systems [24—26], and further references therein. Application to other aspects of adsorbates, e.g., the calculation of desorption rates and heats of adsorption, has been more recent [27-30]. Sufficient accuracy can usually be obtained for the latter without scaling and essentially exact results are possible. In the following, we summarize the elementary but important aspects of the method to emphasize the ease of application. Further details can be found in the above references. 

Similar results were found in a study of aromatic carboxylates with one to six carboxyl groups (Scott, Jackson Wilson, 1990). Adsorption increased with the number of carboxyl groups and was also dependent on the spacing between the carboxyl groups. With the benzene dicarboxylates, maximum permanent adsorption was obtained with the 1,3-dicarboxylate, while the 1,4-dicarboxylates was not adsorbed at all. This is again evidence of the cooperative effect between carboxyl groups. 

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. 

Tegoulia VA, Cooper SL (2000) Leukocyte adhesion on model surfaces under flow effects of surface chemistry, protein adsorption, and shear rate. J Biomed Mater Res 50 291-301... 

Scotchford CA, Gilmore CP, Cooper E, Leggett GJ, Downes S (2002) Protein adsorption and human osteoblast-like cell attachment and growth on alkylthiol on gold self-assembled monolayers. J Biomed Mater Res 59 84-99... 

Tegoulia VA, Rao W, Kalambur AT, Rabolt JF, Cooper SL (2001) Surface properties, fibrinogen adsorption, and cellular interactions of a novel phosphorylcholine-containing self-assembled monolayer on gold. Langmuir 17 4396-4404... 

Frumkin isotherms for a few different values of the interaction parameter g. Positive values of g broaden the isotherm because the adsorbed particles repel each other for negative values of g the isotherms are narrow because adsorption is then a cooperative effect. The case g = 0 corresponds to the Langmuir isotherm. 

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

The rate enhancement for cyclohexane dehydrogenation observed for submonolayer copper deposits may result from changes in the geometric (6) and the electronic (8) properties of the copper overlayer relative to bulk copper. Alternatively, the two metals may catalyze different steps of the reaction cooperatively. For example, dissociative adsorption on bulk copper is unfavorable because of an activation barrier of approximately 5 kcal/mol (33). 

A different approach was taken by Hao and Cooper (1994), who used a combination of the him linear muffin-tin orbital (LMTO) method and an ab initio molecular quantum cluster method, to investigate S02 adsorption on a Cu monolayer supported by 7—AI2O3. Emphasis here was on the geometry of adsorption sites, with the conclusion that the preferred adsorption site is the Al—Al bridging one. 

Before our theory was fully developed, extensive work by J. Koral in cooperation with R. Ullman (15) confirmed in detail and with considerable accuracy all previously known features. They ascertained, in addition, the particulars of the adsorption isotherms for a number of polymers and dispersed adsorbates and established the remarkable degree to which most isotherms could be approximated by 2-parameter equations, like Langmuir s isotherm for monolayers of small molecules. They found the dependence of the adsorption on MW to be weak and determined the area per adsorbed molecule. 

The rate enhancement observed for submonolayer Cu deposits may relate to an enhanced activity of the strained Cu film for this reaction due to its altered geometric and electronic properties. Alternatively, amechansim whereby the two metals cooperatively catalyze different steps of the reaction may account for the activity promotion. For example, dissociative Hj adsorption on bulk Cu is unfavorable due to an activation barrier of approximately 5 kcal/mol . In the combined Cu/Ru system, Ru may function as an atomic hydrogen source/sink via spillover to/from neighboring Cu. A kinetically controlled spillover of Hj from Ru to Cu, discuss above, is consistent with an observed optimum reaction rate at an intermediate Cu coverage. 

EARN distributions the yield along the azimuth 4>= —30° was preferentially reduced with respect to = + 30°. In agreement with intuition, the calculations confirm that the oxygen atom resides in the C-site. Thus from the cooperation of EARN experiments and computer simulations the coverage and nature of the adsorption site of 0/Rh(l 11) has been determined. It will be of interest to see if other surface structure techniques can be used to confirm these s Kcific surface structures. 

Cooper, R.S. and Liberman, D.A. (1970) Fixed-bed adsorption kinetics with pore diffusion control. Ind. Chem. Eng. Fund., 9, 620. 

Amy, G.L., Narbaitz, R.M., and Cooper, W.J. Removing VOCs from groundwater containing humic substances by means of coupled air stripping and adsorption, J. Am. Water Works Assoc., 79(l) 49-54, 1987. 

Bobe, A., Coste, C.M., and Cooper. J.-F. Factors influencing the adsorption of fipronil on soils, J. Agric. Food Chem., 45(12) 4861-4865, 1997. 

As an analogous example, the behavior of sulfonium salts can be mentioned. At mercury electrodes, sulfonium salts bearing trialkyl (Colichman and Love 1953) or triaryl (Matsuo 1958) fragments can be reduced, with the formation of sulfur-centered radicals. These radicals are adsorbed on the mercury surface. After this, carboradicals are eliminated. The carboradicals capture one more electron and transform into carbanions. This is the final stage of reduction. The mercury surface cooperates with both the successive one-electron steps (Scheme 2.23 Luettringhaus and Machatzke 1964). This scheme is important for the problem of hidden adsorption, but it cannot be generalized in terms of stepwise versus concerted mechanism of dissociative electron transfer. As shown, the reduction of some sulfonium salts does follow the stepwise mechanism, but others are reduced according to the concerted mechanism (Andrieux et al. 1994). 

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