Single layer adsorption

Sorption and desorption are usually modeled as one fully reversible process, although hystersis is sometimes observed. Four types of equations are commonly used to describe sorption/desorption processes Langmuir, Freundlich, overall and ion or cation exchange. The Langmuir isotherm model was developed for single layer adsorption and is based on the assumption that maximum adsorption corresponds to a saturated monolayer of solute molecules on the adsorbent surface, that the energy of adsorption is constant, and that there is no transmigration of adsorbate on the surface phase. 

Typical adsorption and desorption isotherms for a porous soHd are indicated in Fig. 1.13. Following the adsorption branch, the steep initial rise (A) is caused by adsorption in the most energetic regions of the soHd (single layer adsorption). Thereafter, in the flat region (B), gas molecules adsorb on sites already occupied by other molecules (multilayer adsorption). 

The Langmuir isotherm is used to describe single-layer adsorption based on the concept that a solid surface possesses a finite number of sorption sites. When these active sites are filled, the site will no longer sorb solute from the solution ... 

The enhanced fluorescence detection capability of ZnO NR platforms is further assessed in relatively simple biological assays involving pure DNA and proteins. Unlike the previously described tests involving single layer adsorption of biomolecules, biological assays discussed from here on pertain to interactions between multilayered biomolecules. The following sections describe the results of fluorescence enhancement effect observed in these relatively simple but multilayered bioassays on ZnO NRs. 

Though the ideal gas assumption would cause some error in predicting result, the reasonableness of the above suggested models can be explained by HIO (Higashi, Ito, Oishi) model (Higashi et al., 1963) which is based on the random walk of molecules. The HIO model was same with the model 4 in this paper when the single layer adsorption was assumed. 

The type II isotherm is associated with solids with no apparent porosity or macropores (pore size > 50 nm). The adsorption phenomenon involved is interpreted in terms of single-layer adsorption up to an inversion point B, followed by a multi-layer type adsorption. The type IV isotherm is characteristic of solids with mesopores (2 nm < pore size < 50 nm). It has a hysteresis loop reflecting a capillary condensation type phenomenon. A phase transition occurs during which, under the eflcct of interactions with the surface of the solid, the gas phase abruptly condenses in the pore, accompanied by the formation of a meniscus at the liquid-gas interface. Modelling of this phenomenon, in the form of semi-empirical equations (BJH, Kelvin), can be used to ascertain the pore size distribution (cf. Paragr. 1.1.3.2). 

A rationale to explain the calculated results in Figures 4.18 and 4.19 is that at high concentrations of solvent in air, multilayer adsorption dominates because of the higher driving force in concentration. And at lower concentrations of solvent, the reduced concentration driving force only engenders single layer adsorption. 

The basic assumption is that the Langmuir equation applies to each layer, with the added postulate that for the first layer the heat of adsorption Q may have some special value, whereas for all succeeding layers, it is equal to Qu, the heat of condensation of the liquid adsorbate. A furfter assumption is that evaporation and condensation can occur only from or on exposed surfaces. As illustrated in Fig. XVII-9, the picture is one of portions of uncovered surface 5o, of surface covered by a single layer 5, by a double-layer 52. and so on.f The condition for equilibrium is taken to be that the amount of each type of surface reaches a steady-state value with respect to the next-deeper one. Thus for 5o... 

Molecular adsorbates usually cover a substrate with a single layer, after which the surface becomes passive with respect to fiirther adsorption. The actual saturation coverage varies from system to system, and is often detenumed by the strength of the repulsive interactions between neighbouring adsorbates. Some molecules will remain intact upon adsorption, while others will adsorb dissociatively. This is often a frinction of the surface temperature and composition. There are also often multiple adsorption states, in which the stronger, more tightly bound states fill first, and the more weakly bound states fill last. The factors that control adsorbate behaviour depend on the complex interactions between adsorbates and the substrate, and between the adsorbates themselves. 

However, as follows from the results presented in Fig. 1(b), the behavior of the PMF for the case of adsorbed dispersion in the matrix at Pm< m — 0.386 contains interesting features in addition to those shown in Fig. 1(a). We observe that the PMF is modulated by the presence of solvent species and in addition is modulated by the presence of matrix particles. The structural repulsive barrier appears, due to matrix particles. An additional weak attractive minimum exists at separations corresponding to matrix-separated colloids. It is interesting that the effects of solvent modulation of the PMF in the adsorbed dispersion are seen for matrix separated colloids. The matrix particles are larger than colloids adsorption of solvent species on the surface of a matrix particle is stronger than on the surface of a colloid. Therefore, the solvent modulating effects of the PMF result from colloids separated by a matrix particle covered by a single layer of solvent species. 

Figure 9.10 STM images of a triangular single-layer M0S2 nanocluster showing the adsorption of thiophene at low temperatures, (a) Below 200 K there are two states, both molecular, one adsorbed on top of the bright rim associated with an edge (Type B) and the other adsorbed at the perimeter of the nanocrystal (Type A) in (b), only Type A exists between 200 and 240 K (c) above 240 K no thiophene is present. (Reproduced from Ref. 33).

The maximum adsorption capacity (,Sj11M) represents coverage of only a single layer of molecules. 

It is often convenient to think of adsorption as occurring in three stages as the adsorbate concentration increases. Firstly, a single layer of molecules builds up over the surface of the solid. This monolayer may be chemisorbed and associated with a change in free energy which is characteristic of the forces which hold it. As the fluid concentration is further increased, layers form by physical adsorption and the number of layers which form may be limited by the size of the pores. Finally, for adsorption from the gas phase, capillary condensation may occur in which capillaries become filled with condensed adsorbate, and its partial pressure reaches a critical value relative to the size of the pore. 

In some systems, three stages of adsorption may be discerned. In the activated alumina-air-water vapour system at normal temperature, the isotherm is found to be of Type IV. This consists of two regions which are concave to the gas concentration axis separated by a region which is convex. The concave region that occurs at low gas concentrations is usually associated with the formation of a single layer of adsorbate molecules over the... 

Figure 5.5 shows the variation of the pore size distribution as a function of cycles of surface-modification-based N2 adsorption isotherms. The pore size decreases with the modification cycle number. The reduction of the mesopore size for each cycle should be about twice the single-layer thickness. Accordingly, the effective singlelayer thickness is about 6 to 7 A based on the above BET measurements. This value is close to those estimated from the frequency changes of a quartz crystal balance for ultrathin fihns prepared by the surface sol-gel process on 2-D substrates." " ... 

Double layer paint provides additional protection since such coatings would be less porous than single layer paint. It Is also noted that In all specimens that are not rinsed there Is a tendency to show Inductive loops In the impedance plot. It is not clear If this Is due to the adsorption of Inhibitor on steel surface or due to the formation of oxides or due to Increased porosity (28). 

A clean, solid surface is actually an active center for adsorption from the surroundings (e.g., air or liquid). A perfectly cleaned metal surface, when exposed to air, will adsorb a single layer of oxygen or nitrogen (or water). Or, when a completely dry glass surface is exposed to air (with some moisture), the surface will adsorb a mono-layer of water. In other words, the solid surface is not as inert as it may seem to the naked eye. This has many consequences in industry, such as with corrosion control. Accordingly, solid surfaces should always be exposed to vacuum prior to any kind of adsorption studies. 

Because chemisorption involves a chemical bond between the adsorbate and adsorbent, only a single layer of chemisorption can occur. Physical adsorption on top of the chemisorbed layer and diffusion of the chemisorbed layer beneath the surface can obscure the fact that chemisorbed material can be only one layer in depth. 

Consider a concentrated electrolytic solution. For all intents and purposes, the entire Gouy-Chapman diffuse charge will be located on the OHP (Section 6.6.4). Further, let there be no contact adsorption, so that the IHP is unpopulated. What is being considered, therefore, is a single layer of charge on the solution side of the interface. 

If adsorption really takes place in this way, the surface must become saturated as soon as it is covered with a single layer of molecules of the adsorbed gas only in exceptional instances will the formation of a second layer be possible. 

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