Physisorption mechanisms

In recent years much attention has been given to the application of fractal analysis to surface science. The early work of Mandelbrot (1975) explored the replication of structure on an increasingly finer scale, i.e. the quality of self-similarity. As applied to physisorption, fractal analysis appears to provide a generalized link between the monolayer capacity and the molecular area without the requirement of an absolute surface area. In principle, this approach is attractive, although in practice it is dependent on the validity of the derived value of monolayer capacity and the tacit assumption that the physisorption mechanism remains the same over the molecular range studied. In the context of physisorption, the future success of fractal analysis will depend on its application to well-defined non-porous adsorbents and to porous solids with pores of uniform size and shape. 

In view of the complexity of physisorption mechanisms and the heterogeneity of most solid surfaces and pore structures, it is not surprising to find that all the theoretical models summarized in this chapter have practical limitations of one kind or another. Furthermore it must be re-emphasized that the range of fit of a particular equation is not enough by itself to establish the validity of the underlying theory. 

The six major types of isotherms in the IUPAC classification (see Figure 1.7) still provide a useful basis for the discussion of the various physisorption mechanisms,... 

The effect of corona discharge treatment on the surface physico-chemistry of PP and PET films has been investigated using surface energy measurements from contact angles, XPS and AFM. The information gathered from these techniques, in addition to practical adhesion measurements, has enabled identification of the dominant mechanisms of adhesion of silicones to these plastic films. The physisorption mechanism and the mechanism of mechanical interlocking are not the cause of... 

The hydrophobic character exhibited by dehydroxylated silica is not shared by the metal oxides on which detailed adsorption studies have been made, in particular the oxides of Al, Cr, Fe, Mg, Ti and Zn. With these oxides, the progressive removal of chemisorbed water leads to an increase, rather than a decrease, in the affinity for water. In recent years much attention has been devoted, notably by use of spectroscopic and adsorption techniques, to the elucidation of the mechanism of the physisorption and chemisorption of water by those oxides the following brief account brings out some of the salient features. 

Theoretically, these intermolecular interactions could provide adhesion energy in the order of mJ/m. This should be sufficient to provide adhesion between the adhesive and the substrate. However, the energy of adhesion required in many applications is in the order of kJ/m. Therefore, the intermolecular forces across the interface are not enough to sustain a high stress under severe environmental conditions. It is generally accepted that chemisorption plays a significant role and thus, physisorption and chemisorption mechanisms of adhesion both account for bond strength. 

J. Mai, W. von Niessen. The influence of physisorption and the Eley-Rideal mechanism on the surface reaction C0-(-02. Chem Phys 156 63-69, 1991. 

As noted before, thin film lubrication (TFL) is a transition lubrication state between the elastohydrodynamic lubrication (EHL) and the boundary lubrication (BL). It is widely accepted that in addition to piezo-viscous effect and solid elastic deformation, EHL is featured with viscous fluid films and it is based upon a continuum mechanism. Boundary lubrication, however, featured with adsorption films, is either due to physisorption or chemisorption, and it is based on surface physical/chemical properties [14]. It will be of great importance to bridge the gap between EHL and BL regarding the work mechanism and study methods, by considering TFL as a specihc lubrication state. In TFL modeling, the microstructure of the fluids and the surface effects are two major factors to be taken into consideration. 

The adsorption action of activated carbon may be explained in terms of the surface tension (or energy per unit surface area) exhibited by the activated particles whose specific surface area is very large. The molecules on the surface of the particles are subjected to unbalanced forces due to unsatisfied bonds and this is responsible for the attachment of other molecules to the surface. The attractive forces are, however, relatively weak and short range, and are called Van der Waals forces, and the adsorption process under these conditions is termed as a physical adsorption (physisorption) process. In this case, the adsorbed molecules are readily desorbed from the surface. Adsorption resulting from chemical interaction with surface molecules is termed as chemisorption. In contrast to the physical process described for the adsorption on carbon, the chemisorption process is characterized by stronger forces and irreversibility. It may, however, be mentioned that many adsorption phenomena involve both physical and chemical processes. They are, therefore, not easily classified, and the general term, sorption, is used to designate the mechanism of the process. 

L. Zhou, Y. P. Zhou, and Y. Sun, Studies on the mechanism and capacity of hydrogen uptake by physisorption-based materials, Int. J. Hydrogen Energy, 31, 259-264 (2006). 

Scheme III shows Liberman s associative ring closure mechanism 19). The participation of surface hydrogen atoms (26) in the cyclization-ring-opening complex is noteworthy. The other atoms of the C5 ring are claimed as lying on the metal linked to it by physisorption forces.

The majority of physisorption isotherms (Fig. 1.14 Type I-VI) and hysteresis loops (Fig. 1.14 H1-H4) are classified by lUPAC [21]. Reversible Type 1 isotherms are given by microporous (see below) solids having relatively small external surface areas (e.g. activated carbon or zeolites). The sharp and steep initial rise is associated with capillary condensation in micropores which follow a different mechanism compared with mesopores. Reversible Type II isotherms are typical for non-porous or macroporous (see below) materials and represent unrestricted monolayer-multilayer adsorption. Point B indicates the stage at which multilayer adsorption starts and lies at the beginning of the almost linear middle section. Reversible Type III isotherms are not very common. They have an indistinct point B, since the adsorbent-adsorbate interactions are weak. An example for such a system is nitrogen on polyethylene. Type IV isotherms are very common and show characteristic hysteresis loops which arise from different adsorption and desorption mechanisms in mesopores (see below). Type V and Type VI isotherms are uncommon, and their interpretation is difficult. A Type VI isotherm can arise with stepwise multilayer adsorption on a uniform nonporous surface. 

In order to develop a more realistic model, additional aspects have been taken into account. The CO desorption can be modeled by equation (9.1.43). The additional aspect of CO desorption leads to the disappearance of the CO-poisoned state [15] because at every value of Yco adsorbed CO molecules are able to leave the surface. Many other investigations under various conditions have been performed, like energetic interactions [16], the aspect of physisorption and reaction via the Eley-Rideal mechanism [17]. 

Weakly bonded hydrogen, which is released in an amount of less than 0.5 wt% upon heating to room temperature, can reasonably be assigned to physisorption, which is considered to be the dominant mechanism of hydrogen absorption by carbon materials at pressures below 12 MPa and not too high temperatures. 

Morrall2 used a HPLC system with two columns. The first column was loaded with the controlled pore glass (CPG) to be modified. The second column was used for separation of the reaction effluents. This column was coupled to a refractive index detector, allowing for quantitative detection of the effluents. The reaction was initiated by injecting an APTS/toluene mixture and stopped by injection of pure toluene. With this so-called stop-flow mechanism reaction times down to 18 seconds could be used. From these analyses it became evident that upon mixing of the aminosilane with the silica, a very rapid physisorption occurs. The initial adsorption of the APTS (from toluene solution on dried CPG) occurred before the 18 second minimum time delay of the stop-flow apparatus. For non-aminated silanes the adsorption proved to be much slower. This study also revealed the pivotal role of surface water in the modification of siliceous surfaces with alkoxysilanes, as discussed in the previous chapter. 

As the additional physisorption has built up additional layers on top of the first monolayer, as for modification at higher concentration, the additional layers will analogously be desorbed. This mechanism is pointed out by thickness measurements, performed on non-porous modified silica layers.15 Upon curing at 473 K on air the thickness of the APTS layer appears to be reduced from 1.8 to 0.7 nm, indicating that after curing only one layer of APTS is left. 

Figure 9.30 Flip mechanism for APTS reaction in dry conditions, (a) physisorption, (b) condensation, (c) main structure after curing.

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