Successive adsorption

Some research groups also tried to build up bi- and multilayers of latex particles by using self-assembly techniques [92-94,97]. Either the alternate adsorption technique outlined in Figure 14 was used, in which cationic and anionic particles are successively adsorbed, or a slightly modified version of successive adsorption of anionic particles and a cationic poly-... 

The adsorption of at least one reactant is the first step of the mechanism of any catalytic reaction. This step is followed by surface interactions between adsorbed species or between a gaseous reactant and adsorbed species. In many cases, these interactions may be detected by the successive adsorptions of the reactants in different sequences. Heat-flow microcalorimetry can be used with profit for such studies (19). 

Fig. 27. Differential heats versus coverage for the successive adsorptions, at 30°C, of carbon monoxide (A), oxygen(B), and, again, carbon monoxide (C) on the surface of lithium-doped nickel oxide. Reprinted from (54) with permission J. Chim. Phys.

Figure 1.4 gives an example of the adsorption of a compound to suspended sediment, modeled as two resistances in series. At first, the compound is dissolved in water. For successful adsorption, the compound must be transported to the sorption sites on the surface of the sediment. The inverse of this transport rate can also be considered as a resistance to transport, Ri. Then, the compound, upon reaching the surface of the suspended sediment, must find a sorption site. This second rate parameter is more related to surface chemistry than to diffusive transport and is considered a second resistance, R2, that acts in series to the first resistance. The second resistance cannot... 

In extreme cases, such as boiling HC1 in contact with stainless steel, inhibitors can still be found that work very well indeed. Ellipsometric measurements show that they can form polymer layers five and six molecular layers thick—a kind of in situ formation of a paint film, built up by successive adsorption, layer upon layer. 

A series of tests was made by measuring the helium flow rate after successive adsorption and desorption of ethane to 10% of the weight of the carbon. With a disk which originally exhibited only Knudsen flow, Poiseuille flow appeared and... 

Let us now consider the continuous, quasi-equilibrium procedures. Since the safest and simplest check is to look for the superimposition of two successive adsorption isotherms, this requires one to be able to operate at two different flow rates and, if necessary, to reduce the rate until the test is satisfied. For this reason, it is inadvisable to employ any technique which would not allow the possibility of reducing the flow rate beyond the value finally selected. Also, equipment designed to maintain the pressure over the adsorbent at a predetermined level does not necessarily guarantee equilibrium conditions again, this should be checked by two successive experiments using different flow rates. 

Previous reports on starch-alcohol interactions assumed that only physical sorption is involved, despite observations of the irreproducibility of successive adsorption-desorption isotherms for starch-methanol and starch-ethanol systems. This irreproducibility was assumed to be the result of swelling.354... 

The external versus internal surface has also been investigated in the case of Co-H-zeolites active in the selective catalytic reduction of NO by methane [20]. The highly hindered nitrile, ortho-toluonitrile (oTN) has been used, co-adsorbed with CO and NO. The pre-adsorption of oTN poisons the external cationic sites of Co-H-ZSM5. Successive adsorption of CO shows that some of the Co ions are on the inner surface and some on the outer surface. The coadsorption of oTN and NO shows that trivalent Co ions are actually located in the internal cavity surface of Co-H-ZSM-5 while divalent Co ions distribute almost equally in the internal and external surface. 

Selective depositions of copper on pre-patterned TiSi2, W, Cr, A1 and Zr substrate layers were also carried out successfully". Adsorption studies with copper(II) S-diketonates, such as Cu(acac)2 (7a) and Cu(hfac)2 (7c)" proved that the appropriate precursors adsorb dissociatively under typical CVD conditions under reducing hydrogen atmosphere, as depicted in Scheme 2. 

Calorimetry can help elucidate the mechanisms of heterogeneously catalyzed reactions (1, 3). Possible reaction steps can be studied in the calorimeter by means of successive adsorption sequences, whereas reaction mechanisms, including different series of steps, can be tested in thermochemical cycles. However, the following limitations of the approach should be noted (i) (1) adsorption sequences only involve irreversibly adsorbed species, since... 

It has been suggested" that, for MgO heated at temperatures up to 523 K, the so called free OH groups are able to transfer one electron (OH groups type-A according to Horlock and Anderson ). For species heated at temperatures above 973 K the 0 anions have the same ability." Through successive adsorption of Hammett indicators and indicators having one-electron donor properties it was possible to establish that 0 " anions cannot simultaneously serve as one-electron and electron-pair sources. This phenomenon is completely different from that of alumina," and has so far not been explained, although it is very important from the catalytic point of view. 

Fio. 17. Differential heats for the successive adsorptions at 30° of oxygen and carbon monoxide on hydrated nickel oxide. 

From the successive adsorption of polyelectrolyte layers on a porous support one can, first, expect that the pores of the supporting membrane become coated and, second, that the thickness of the separating membrane gradually increases. Consequently, the total flux should decrease and eventually-when all the pores are coated-the separation should improve. Experiments indicated that this is indeed the case [66, 67]. With increasing number of deposited PVA/PVS bilayers, the water content of the permeate increases from 10 wt% for 10 bilayers to 92 wt% for 50, and 98 wt% for 60 bilayers, if the ethanol/water feed solution contains 6.3 wt% water. Simultaneously the total flux decreases from about 1 kg m 2 h 1 to about 150 g m 2 h 1 for the 50 bilayer sample, and to 110 g m 2 h 1 for 60 bilayers. The decrease mainly reflects the usual inverse relationship between flux and thickness of a membrane. From the measurements it can be derived that the adsorption of at least 50 polyelectrolyte bilayers on the highly porous PAN/PET support is necessary to get an optimum separation. Most of our experiments were therefore carried out on membranes containing 60 bilayers. 

Mathauer, K. Frank, C. W., Binary self-assembled monolayers as prepared by successive adsorption of alkyltrichlorosilanes, Langmuir 1993, 9, 3446-3451... 

Fig. 3. FT-IR spectra of a 10 % CuO/Ti02 catalyst disk (a) after activation by outgassing at 350 °C (b) after adsorption of water 5 Torr and outgassing at r.t (c) after successive adsorption of ammonia (5 Torr) and outgassing at r.t.

The first step is necessarily adsorption of the virion onto the cell surface. Physical contact, collision frequency, contact area size, and the ionic conditions of the environment are all factors, but they are not usually sufficient to account for successful adsorption. For many viruses, specific cellular receptor sites have been demonstrated on the plasma membranes. For example, the adsorption of the influenza virus to red blood cells has been shown to be through binding of virion hemagglutinin, in the form of surface spikes on the virion envelope, to cell receptors. 

For NH4-X and NH4-A OH bands are not clearly detectable especially after solid state ion-exchange. The introduction of cations can be checked, therefore, by the successive adsorption of NHa and pyridine (vide infra). 

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