Adsorption horizontal interaction

Halsey (55), however, taking into account horizontal interactions of the above type in an equivalent but somewhat more qualitative way, has concluded that multilayer adsorption in steps would indeed occur on an ideally uniform surface, and that the reason steps are practicallyjiever... 

As shown in the work of Hill (54) and Halsey (55) mentioned above, if one recognizes that liquid and adsorbed molecules actually have horizontal as well as vertical interactions, then adsorbed molecules, especially in second and higher layers, will have very different properties than in the BET theory. Halsey discusses these properties but unfortunately in addition makes the misleading remark that on the basis of the BET hypotheses the adsorption for any value of c is actually confined to the amount accommodated in the first layer. What is obviously meant is that this result would obtain if a liquid with both vertical and horizontal interactions is incorrectly allowed only vertical interactions on adsorption. Actually, the BET hypotheses are self-consistent and lead to multimolecular adsorption for any value of c the hypotheses include the assumption that both liquid and adsorbed molecules have only vertical interactions. Halsey s remark incorrectly implies that the BET theory uses the inconsistent hybrid assumptions mentioned above. 

Although the BET theory is used extensively, it still suffers from a number of criticisms. The first is that surfaces of real solids are heterogeneous while the model assumes that all the adsorption sites are energetically identical. The second reason is the assumption of the vertical force between adsorbent and adsorbate molecules. It neglects the horizontal interaction between adsorbed molecules. The third reason was put forward by Halsey (1948) and is detailed below. 

Fig. 10. The ESR signal produced at various points on the resonant line in a magnetic field modulated spectrometer. The vertical magnetic field modulation interacts with the bell-shaped adsorption curve [F(H)1 to produce the horizontal ESR signal. Here AH is the half amplitude line width and Hu is the center of resonance (S3).

As explained in Section 1.7, enhanced adsorbent-adsorbate interactions occur in micropores of molecular dimensions. A decrease in the micropore width results in both an increase in the adsorption energy and a decrease in the relative pressure at which the micropore filling occurs. The narrow range of relative pressure necessary to attain the plateau is an indication of a limited range of pore size and the appearance of a nearly horizontal plateau indicates a very small external surface area. The limiting adsorption is dependent on the available micropore volume. 

Up to now, infrared spectroscopy has been used mainly to determine the types of hydroxyl groups and the acidity of zeolites (39). The frequencies of the vertical and horizontal vibrations (with respect to the cavity wall) of H2O molecules adsorbed in zeolite A were determined by measurements in the far infrared ( 220 and —75 cm" ) (37). These values are in agreement with a simple theoretical model. A number of ultraviolet and ESR studies are reviewed (33). The difference has been established between the specific molecular interaction of aromatic molecules on zeolites cationized with alkali cations and the more complex interactions involving charge transfer in CaX and deca-tionized X and Y zeolites. These more complex interactions with CaX zeolites containing protonized vacancies and with decationized zeolites are similar. These phenomena are related to the interactions of molecules with acidic centers in zeolites which are stronger, as compared with the molecular adsorption. 

The adsorption isotherms of methanol and ethanol on ACF samples at 303 K were shown in Figure 2. The horizontal axes are expressed by the logarithm of P/P to show explieitly the adsorption in the low relative pressure range. The ad.sorption isotherms of ethanol on both P5 and P20 have a greater uptake in the lower relative pressure than those of methanol. Therefore, the interaction of an ethanol molecule with the micropore is much greater than that of a methanol molecule. However, the methanol molecule can interact more strongly than a H,0 molecule, because almost no water molecule is adsorbed below P/P = 0.3." ... 

Fig. 5 shows the low pressure adsorption isotherms of n-nonane by the micropore entrance modified ACF and the pristine ACF. These adsorption isotherms were determined under the almost equilibrium conditions. A remarkable enhancement of n-nonane adsorption with the micropore entrance modification is observed in the low P/Po region, although the adsorption amounts at high P/Pq region almost coincide with each other. The fractional filling of n-nonane at saturation is almost constant irrespective of the surface modification with TTS the ratios of the saturation n-nonane adsorption Wo(nonane) to the saturation Nj adsorption Wo(N2) for the modified ACF and ACF were 0.72 and 0.70, respectively. Thus, the low pressure uptake depends sensitively on the chemical state of the external surface, while the fractional filling at saturation does not change. Consequently, the slight uptake of the pristine ACF should be caused by the limitation of micropore diffusion. The diffusion limitation can be removed by application of n-nonane pressure of P/Pq >0.1 according to the result shown in Fig. 5. Accordingly, a marked enhancement of low pressure adsorption by the micropore-entrance modification is associated with enrichment of n-nonane molecules at the entrance of the micropore due to favourable interaction of n-nonane with hydrocarbon chains of TTS. The amount of the n-nonane enrichment can be estimated from the comparison of both adsorption isotherms in Fig. 5. With the adsorption amount indicated by the horizontal broken line, the equal amount of adsorption for both samples is obtained at different relative pressures of 0.065 (for ACF) and 0.02 (for TTS-modified ACF). That is, application of P/Pq = 0.065 is necessary for the prescribed adsorption in the case of ACF, whereas the TTS-modified ACF does not need such a high P/Pq. Application of P/Po = 0.02 is sufficient for the adsorption by the TTS-modified ACF. Thus, the TTS-modification increases the concentration...

Surfactant adsorption close to the cmc may appear Langmuirian, but this does not automatically imply a simple orientation. For example, rearrangement from a horizontal to a vertical orientation or electrostatic interaction and counterion binding may be masked by simple adsorption isotherms. It is essential, therefore, to combine the adsorption isotherms with other techniques such as microcalorimetry and various spectroscopic methods in order to obtain a full picture of surfactant adsorption. 

Such characteristic horizontal bands, as shown in Figure 5.43, might be termed adsorption multiplicity since they arise primarily from the interactions of external film heat and mass transfer resistances with the adsorption resistance. For smaller adsorption resistance, the horizontal bands disappear giving way to the more familiar multiplicity regions, arising from the interactions of physical transport and surface reaction resistances. 

The are two general mechanisms for the adsorption of a monomolecular film, as we have seen from earlier chapters physical adsorption and chemisorption. The simplest type of adsorption is the physical adsorption of materials such as hydrocarbons in which the only attractive factor is dispersion force interactions. If the solid surface is relatively hydrophobic and the hydrocarbon chain is relatively long, such adsorption can be quite strong. However, the adsorbed molecules will generally lie more or less horizontal along the surface (Fig. 18.13fl), which means that the thickness of the monolayer will be very small, about 0.25 nm. As a result, the effectiveness of such films will be limited and will decrease rapidly with repeated passing of the surfaces. 

The assembly of the colloidal particles on the patterned silane layers was achieved through adsorption from a droplet of latex suspension deposited onto the horizontal substrate (drop coating method). For the interaction of the particles with the silane surface several scenarios have to be considered. The PBA particles are composed of an intrinsically hydrophobic polymer, but they... 

The current understanding of the adsorption and aggregation process is as follows. At very low concentrations, hydrophobic attraction causes the tailgroup to adsorb horizontally on the surface, and an anisotropic interaction with the surface lattice causes the tailgroup to orient itself parallel to an underlying symmetry axis. Because the tail-surface interaction for this configuration is typically... 

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