Sorption multilayer

In very small pores the molecules never escape from the force field of the pore wall even at the center of the pore. In this situation the concepts of monolayer and multilayer sorption become blurred and it is more useful to consider adsorption simply as pore filling. The molecular volume in the adsorbed phase is similar to that of the saturated Hquid sorbate, so a rough estimate of the saturation capacity can be obtained simply from the quotient of the specific micropore volume and the molar volume of the saturated Hquid. 

Physical and ionic adsorption may be either monolayer or multilayer (12). Capillary stmctures in which the diameters of the capillaries are small, ie, one to two molecular diameters, exhibit a marked hysteresis effect on desorption. Sorbed surfactant solutes do not necessarily cover ah. of a sohd iaterface and their presence does not preclude adsorption of solvent molecules. The strength of surfactant sorption generally foUows the order cationic > anionic > nonionic. Surfaces to which this rule apphes include metals, glass, plastics, textiles (13), paper, and many minerals. The pH is an important modifying factor in the adsorption of all ionic surfactants but especially for amphoteric surfactants which are least soluble at their isoelectric point. The speed and degree of adsorption are increased by the presence of dissolved inorganic salts in surfactant solutions (14). 

Sorption mechanisms of Hg(II) by the nonliving biomass of Potamogeton natans was also elucidated using chemical and instrumental analyses including atomic absorption, electron microscopy, and x-ray energy dispersion analyses. The results showed a high maximum adsorption of Hg(II) (180 mg/g), which took place over the entire biomass surface. Nevertheless, there were spots on the surface where apparent multilayer sorption of Hg(II) occurred. The minimum concentration of Hg(II) in solution that can be removed appears to be about 4-5 mg/L.117... 

Many models have been developed that deal with the sorption properties of wood in the presence of moisture these have been discussed in a number of works (e.g. Skaar, 1972 Siau, 1984). They can be approximately divided into sorption models, such as the Brunauer-Emmett-Teller (BET) model, or solution models (such as the Hailwood-Horrobin, H-H, model). The sigmoidal shapes of sorption or desorption isotherms can be deconvoluted into two components. These are often taken to represent a monomolecular water layer (associated with the primary sorption sites, OH groups), and a multilayer component where the cell wall bound water molecules are less intimately associated with the fixed cell wall OH groups. 

Popper and Bariska (1972) studied the moisture sorption properties of wood chemically modified with acetic (or phthalic) anhydride and analysed the results using Brunauer-Emmett-Teller (BET) theory and the H-H model. Acetylation was found to reduce the number of sorption sites, whereas little effect was noted with phthaloylation. By dividing the sorption isotherm into a monolayer component and a multilayer component using the H-H model, it was shown that there was a large reduction in the... 

This means that for BET calculations from isotherms of highly microporous materials the obtained values have no physical meaning because the assumed sorption mechanism is wrong. In such cases a separate consideration of the mono and multilayer sorption and the micropore contribution becomes necessary. 

The Langmuir and Brumnauer, Emmett, and Teller (BET) models also have been used to describe nonlinear sorption behavior for environmental solids, particularly for mineral dominated sorption (Ruthven, 1984 Weber et al., 1992). The Langmuir model assumes that maximum adsorption corresponds to a saturated monolayer of solute molecule on the absorbent surface, that there is no migration of the solute on the surface phase, and that the energy of adsorption is constant. The BET model is an extension of the Langmuir model that postulates multilayer sorption. It assumes that the first layer is attracted most strongly to the surface, while the second and subsequent layers are more weakly held. 

Figure 7.42 Types of gas sorption isotherm - microporous solids are characterised by a type I isotherm. Type II corresponds to macroporous materials with point B being the point at which monolayer coverage is complete. Type III is similar to type II but with adsorbate-adsorbate interactions playing an important role. Type IV corresponds to mesoporous industrial materials with the hysteresis arising from capillary condensation. The limiting adsorption at high P/P0 is a characteristic feature. Type V is uncommon. It is related to type III with weak adsorbent-adsorbate interactions. Type VI represents multilayer adsorption onto a uniform, non-porous surface with each step size representing the layer capacity (reproduced by permission of IUPAC).

From the minimum in the susceptibility through the maximum as the amounts of palladium increase, it is clear, from the sorption data, that multilayers of palladium are building up. It is suggested that at least two factors may be involved in the susceptibility changes given in this part of the curves. 

With the two aromatic hydrocarbons, Barrer and Kelsey (1961) found that the dm spacing increased steadily with the increase in p p°, but with the alkanes there was very little change. In the former case, it appeared that most of the uptake was in the interlamellar region. As indicated by the shape of the Type Hb isotherms, the sorption of the other organic vapours probably included an appreciable amount of multilayer adsorption on, and between, the clay platelets (cf. Figure 11.5). 

In the sub-monolayer range, the amount adsorbed on the external area of a 1 pm cubic zeolite crystal is very small in comparison with the adsorption within the micropore structure (the intracrystalline sorption). Also, apart from a small multilayer adsorption on the external surface, there should be no additional uptake at higher pip0. However, there are three ways in which the non-zeolitic contribution may be increased (a) the binder may have a relatively large specific surface (b) the zeolite crystallite size may be much smaller than 1 pm and (c) the zeolite may contain some amorphous aluminosilicate or silica. In practice, one or more of these effects can result in a significant distortion from the classical form of the Type I isotherm (see Sayari etal., 1991). 

Water vapor adsorption isotherms have been obtained on cotton from room temperature up to 150°C [303,304]. Theoretical models for explaining the water vapor sorption isotherms of cellulose have been reviewed [303]. Only adsorption theories will be discussed here at ambient temperatures. The shape of the isotherm indicates that multilayer adsorption occurs and thus the Brunauer, Emmett and Teller (BET) or the Guggenheim, Anderson and deBoer (GAB) theory can be applied. In fact, the BET equation can only be applied at relative vapor pressures (RVPs) below 0.5 and after modification up to a RVP of 0.8 [305]. The GAB equation, which was not discussed in the chapter in the book Cellulose Chemistry and Its Applications [303], can be applied up to RVPs above 0.9 [306]. Initially as the RVP... 

Lioutas et al. (1986) measured the 0 and resonances of lysozyme powders and solutions, in experiments like those carried out for H by Fullerton et al. (1986). They similarly interpreted discontinuities in the NMR response in terms of three populations of water 20 mol of water per mol of protein (corresponding to 0.025 h) with a correlation time of 41 psec, 140 mol of water (0.17 h) with a correlation time 27 psec, and 1400 mol of water (1.7 h) with a correlation time 17 psec. The differences between these results and those of Fullerton et al. (1986) indicate the difficulty of estimating water correlation times. Lioutas et al. (1987) extended these results by analyzing H resonance data through comparison with the sorption isotherm. D Arcy-Watt analysis of the sorption isotherm gave 19 mol of tightly bound water per mol of lysozyme, 148 mol of weakly bound water, and 2000 mol of multilayer water. These classes plus two more types, corresponding to water in solutions... 

On the basis of a molecular model for sorption kinetics Jdntti introduced a method to calculate equilibria shortly after a change of the pressure of the sorptive gas. In the present paper we apply that method for the description of multilayer adsorption. 

Waksmundzki et al. extensively examined the surface areas and microporosities of imprinted silica surfaces [44]. It was found that although the template itself had little effect on the total surface area, the sizes of the micropores were positively correlated to the size of the template. Subsequent studies on the sorption of template to silicas imprinted with pyridine [45-50], quinoline and acridine [45-47], and 2-picoline, 2,4-lutidine and 2,4,6-collidine [50], combined with thermodynamic studies on the heat of wetting of template or methanol/water sorption [47,51-53], led to the conclusion that these templates were adsorbed as multilayers to the silica. This observation supported the association mechanism hypothesis. The possibility of a footprint mechanism and an association mechanism coexisting in a concentration dependent fashion does not appear to have been considered. 

The characteristic inflection in Type II behavior occurs when multilayer sorption starts. The Brunauer, Emmett, Teller (BET) equation, shown as Eq. (4), can describe isotherms where multilayer sorption is evident ... 

Although the sorption isotherm is fundamental to the characterization of moisture interaction with water, it is generally not possible to make any judgments about the effect of water on the substrate from the isotherm alone. For example, it is not possible to determine if an increase in moisture content is due to multilayer adsorption, swelling of the substrate, or some combination of the two. 

The isotherm for aspirin would be classified as Type III, indicating a low affinity for water followed by multilayer sorption. These four isotherms cover a broad range of moisture interaction with solids of pharmaceutical interest. 

Throughout the preceding section we have tabulated the measure of sorption expressed in terms of that obtained by the application of the BET theory. In general, the theory and data are compatible over the classical range limited to about 0.05 Pq to 0.35 Pq. Such a correlation should not be construed to serve as proof of the BET mechanism or even to measure a true specific surface area. The BET theory is, by its very nature and derivation, a theory for multilayer formation. Such a process is probably in play for nitrogen sorption but is questionable for water and... 

A three dimensional capillary network model has been developed, aiming to the simulation of sorption by several model mesoporous adsorbents, such as the one mentioned in the previous section. The model offers realistic simulation conditions and is able to provide satisfactory prediction of adsorption-desorption isotherms of CCI4 and C5H1 for different porosities, temperatures and adsorbates. The expected desorption branch hysteresis is estimated as a two component summation of the thermodynamic (single pore) and the network hysteresis. Similarly, the overall sorbed volume is the two component summation of the volume due to multilayer adsorption and to the volume due to capillary condensation. 

These assumptions are justifiable as the heat of adsorption of the small inert sorbate (e.g., N2 or Ar) is rather low and, hence, differences between sorption sites at the surface will be very small. Similarly, the interaction between the first and the following layers will be close to the heat of condensation, as the effect of polarization by the surface will be small beyond the first layer (screening of the long-range van der Waals forces). From its conception, the BET theory extends the Langmuir model to multilayer adsorption. It postulates that under dynamic equilibrium conditions the rate of adsorption in each layer is equal to the rate of desorption from that layer. Molecules in the first layer are located on sites of constant interaction strength and the molecules in that layer serve as sorption sites for the second layer and so forth. The surface is, therefore, composed of stacks of sorbed molecules. Lateral interactions are assumed to be absent. With these simplifications one arrives at the BET equation... 

Since the product of flow rate, time and concentration equal the input mass, a constant input concentration permits the calculation of mass from either time or retention volume. Empty columns provide an essentially constant ratio of input mass to time at constant flow rate with the concentration of water vapor fixed by the temperature of the carrier gas saturated with water vapor (100Z RH or Aw of 1 see Figure 6). This state can be achieved with substrates that do not dissolve in water when saturated (for example, starches and many proteins), or when the relative humidity is constant but insufficient to allow uptake to produce a highly multilayered or clustered water state in the substrate equivalent to a continuous water phase or solution. This condition requires a source of humidified gas as in (7.). The sorption isotherm equation is then given by... 

According to the association-induction theory proposed by Ling (1962), fixed charges on macromolecules and their associated counterions constrain water molecules to form a matrix of polarized multilayers having restricted motion, compared with pure water. The monolayer of water molecules absorbed on the polar sorption site of the molecule is almost immobilized and thus behaves, in many respects, like part of the solid or like water in ice. It has different properties than additional water layers defined as multilayers have. The association-induction theory has been shared by many researchers for many years. Unfortunately, elucidation of the nature of individual layers of water molecules has been less successful, due to the complexity of the system and lack of appropriate techniques. 

Figure 3.3. Sorption equations fitted to experimental data for the sorption of water by wood at 40°C (Simpson, 1980). The Langmuir isotherm is parabolic, corresponding to the formation of a BET monolayer. Multilayer adsorption describes sorption behaviour better but sorption at the highest moisture contents, where capillary condensation occurs, is underestimated.

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