Multilayers formation

The immediate site of the adsorbent-adsorbate interaction is presumably that between adjacent atoms of the respective species. This is certainly true in chemisorption, where actual chemical bond formation is the rule, and is largely true in the case of physical adsorption, with the possible exception of multilayer formation, which can be viewed as a consequence of weak, long-range force helds. Another possible exception would be the case of molecules where some electron delocalization is present, as with aromatic ring systems. 

The very considerable success of the BET equation stimulated various investigators to consider modifications of it that would correct certain approximations and give a better fit to type II isotherms. Thus if it is assumed that multilayer formation is limited to n layers, perhaps because of the opposing walls of a capillary being involved, one... 

As a general rule, adsorbates above their critical temperatures do not give multilayer type isotherms. In such a situation, a porous absorbent behaves like any other, unless the pores are of molecular size, and at this point the distinction between adsorption and absorption dims. Below the critical temperature, multilayer formation is possible and capillary condensation can occur. These two aspects of the behavior of porous solids are discussed briefly in this section. Some lUPAC (International Union of Pure and Applied Chemistry) recommendations for the characterization of porous solids are given in Ref. 178. 

The adsorption isotherms are often Langmuirian in type (under conditions such that multilayer formation is not likely), and in the case of zeolites, both n and b vary with the cation present. At higher pressures, capillary condensation typically occurs, as discussed in the next section. Some N2 isotherms for M41S materials are shown in Fig. XVII-27 they are Langmuirian below P/P of about 0.2. In the case of a microporous carbon (prepared by carbonizing olive pits), the isotherms for He at 4.2 K and for N2 at 77 K were similar and Langmuirlike up to P/P near unity, but were fit to a modified Dubninin-Radushkevich (DR) equation (see Eq. XVII-75) to estimate micropore sizes around 40 A [186]. 

I (curve D). Thus the micropores had been able to enhance the adsorbent-adsorbate interaction sufficiently to replace monolayer-multilayer formation by micropore filling and thereby change the isotherm from being convex to being concave to the pressure axis. 

Figure 5.19 shows an idealized form of the adsorption isotherm for physisorption on a nonporous or macroporous solid. At low pressures the surface is only partially occupied by the gas, until at higher pressures (point B on the curve) the monolayer is filled and the isotherm reaches a plateau. This part of the isotherm, from zero pressures to the point B, is equivalent to the Langmuir isotherm. At higher pressures a second layer starts to form, followed by unrestricted multilayer formation, which is in fact equivalent to condensation, i.e. formation of a liquid layer. In the jargon of physisorption (approved by lUPAC) this is a Type II adsorption isotherm. If a system contains predominantly micropores, i.e. a zeolite or an ultrahigh surface area carbon (>1000 m g ), multilayer formation is limited by the size of the pores. 

On the basis of this argument, the mechanism for the current oscillation and the multilayer formation can be explained as follows. First note that U is kept constant externally with a potentiostat in the present case. In the high-current stage of the current oscillation, the tme electrode potential (or Helmholtz double layer potential), E, is much more positive than U because E is given hy E=U —JAR, where A is the electrode area, R is the resistance of the solution between the electrode surface and the reference electrode, andj is taken as negative for the reduction current. This implies that, even if U is kept constant in the region of the NDR, is much more... 

The alternate-multilayer formation in the alloy deposit can also be explained on the basis of the above mechanism. First, we have to note that thej value in the low-current... 

There is no influence of pH on these results over a wide range (pH 3-8). The process by which such multilayer formation might be caused is discussed. 

If we consider multilayer formation to be the result only of sequential desorption (adsorption) of polymers one on top of the other at each surface site, we can write the following partition function for one such "chain"... 

The intrinsic viscosity of the collagen preparation supports the notion that a significant fraction of the adsorbed collagen is present in the form of the dimer, i.e. of two triple helical, 3000 A long cylinders, flexibly joined together. Whereas at 7.7°C these dimers may mainly be folded and have two of the non-helical ends on the surface. It is conceivable that one end is permitted to lift off at the higher temperature and that the doubling in thickness is thus produced by a mechanism not related to a BET-like multilayer formation. 

Miyashita, T. Mizuta, Y. Matsuda, M., Studies on Langmuir Blodgett multilayer formation from preformed polyW alkylacrylamides), Br. Polym. J. 1990, 22, 327 331... 

The determination of the specific surface area of a zeolite is not trivial. Providers of zeolites typically give surface areas for their products, which were calculated from gas adsorption measurements applying the Brunauer-Emmet-Teller (BET) method. The BET method is based on a model assuming the successive formation of several layers of gas molecules on a given surface (multilayer adsorption). The specific surface area is then calculated from the amount of adsorbed molecules in the first layer. The space occupied by one adsorbed molecule is multiplied by the number of molecules, thus resulting in an area, which is assumed to be the best estimate for the surface area of the solid. The BET method provides a tool to calculate the number of molecules in the first layer. Unfortunately, it is based on a model assuming multilayer formation. Yet, the formation of multilayers is impossible in the narrow pores of zeolites. Specific surface areas of zeolites calculated by the BET method (often termed BET surface area) are therefore erroneous and should not be mistaken as the real surface areas of a material. Such numbers are more related to the pore volume of a zeolite rather than to their surface areas. 

As the concentration of the macromolecules in equilibrium with the interface increases, so that the coil population becomes crowded, a sudden reaching of a very gently upward sloping plateau is observed, but hardly ever any multilayer formation, since no proper outer boundary of the adsorbed layer of coils develops (see below). Instead, as more coils compete for the same interfacial area, adsorption of further molecules occurs as interpenetration is still resisted, at the expense of the site areas held per molecule, i.e., the adsorbed trains become shorter, the loops longer, and the area under the coils smaller the coils become sideways and upwards compressed, (15) (16), Fig. 3. Experimentally,... 

Auer et al. [134] presented an example for multilayer formation and controlled deposition of functionalized nanoparticles on SAM of mercaptohexadecanoic acid (MHA) using electrostatic interactions. As a pH-sensitive switchable linker between the SAM of MHA and negatively charged gold nanoparticles, bis-benzami-dine bolaamphiphiles having different alkyl spacers were used [135]. This strategy resulted in a potentially tunable and switchable property of the entire assembly. For example, the kinetics of adsorption as well as the final particle layer thickness can be controlled by the kind of bis-benzamidine used as the linker (Fig. 9.16). 

Adsorption isotherms for n-decylamine on Ni, Fe, Cu, Pb, and Pt at the potential of maximum adsorption are shown in Figure 10.4. It is seen that a limiting coverage is approached in each case except on Pt, where multilayer formation occurs. The coverage 0 in this case is defined as... 

It is obvious that such processes involving monomolecular film transfers will easily be disturbed by defects arising from various sources. As will be shown in the following text, these defects are in most cases easily detected. The structural analysis of the molecular ordering within a single LB monolayer is important both to understand how the environment in the immediate vicinity of the surface (i.e., solid) affects the structure of the molecular monolayer and to ascertain how the structure of one layer forms a template for subsequent layers in a multilayer formation. 

Type III This is a somewhat special type with usually only a multilayer formation, such as nitrogen adsorption on ice. 

Jordan, C. E. and Corn, R. M. (1997) Surface plasmon resonance imaging measurements of DNA hybridization adsorption and strepta-vidin/DNA multilayer formation at chemically modified gold surfaces. Anal. Chem. 69, 1449-1456. 

Fig. 20. Multilayer formation by adsorption, using bifunctional silane surfactants as monolayer building units [193]...

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