Multilayer adsorption mechanism

These sizes can be determined from the aspect of N, adsorption at 77 K, and hence N2 molecules are adsorbed by different mechanisms -multilayer adsorption, capillary condensation, and micropore filling for macropores, mesopores, and micropores, respectively. The critical widths of 50 and 2 nm are chosen from empirical and physical reasons. The pore width of 50 nm corresponds to the relative pressure of 0.96 for the adsorption isotherm. Adsorption experiments above that are considerably difficult and applicability of the capillary condensation theory is not sufficiently examined. The smaller critical width of 2 nm corresponds to the relative pressure of 0.39 through the Kelvin equation, where an unstable... 

Langmuir referred to the possibility that the evaporation-condensation mechanism could also apply to second and higher molecular layers, but the equation he derived for the isotherm was complex and has been little used. By adopting the Langmuir mechanism but introducing a number of simplifying assumptions Brunauer, Emmett and Teller in 1938 were able to arrive at their well known equation for multilayer adsorption, which has enjoyed widespread use ever since. 

Adsorption of dispersants at the soHd—Hquid interface from solution is normally measured by changes in the concentration of the dispersant after adsorption has occurred, and plotted as an adsorption isotherm. A classification system of adsorption isotherms has been developed to identify the mechanisms that may be operating, such as monolayer vs multilayer adsorption, and chemisorption vs physical adsorption (8). For moderate to high mol wt polymeric dispersants, the low energy (equiUbrium) configurations of the adsorbed layer are typically about 3—30 nm thick. Normally, the adsorption is monolayer, since the thickness of the first layer significantly reduces attraction for a second layer, unless the polymer is very low mol wt or adsorbs by being nearly immiscible with the solvent. 

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. 

Since this model was far too complex to serve any practical purpose, Brunauer, Emmet and Teller made some simplifying assumptions (the main one being that in all layers the evaporation-condensation mechanisms are identical) to derive their famous BET equation, to be used in the multilayer-adsorption region of the adsorption isotherm ... 

These limits are to some extent arbitrary since the pore filling mechanisms are dependent on the pore shape and are influenced by the properties of the adsorptive and by the adsorbent-adsorbate interactions. The whole of the accessible volume present in micropores may be regarded as adsorption space and the process which then occurs is micropore filling, as distinct from surface coverage which takes place on the walls of open macropores or mesopores. Micropore filling may be regarded as a primary physisorption process (see Section 8) on the other hand, physisorption in mesopores takes place in two more or less distinct stages (monolayer-multilayer adsorption and capillary condensation). 

By introducing a number of simplifying assumptions, Brunauer, Emmett and Teller (1938) were able to extend the Langmuir mechanism to multilayer adsorption and obtain an isotherm equation (the BET equation), which has Type II character. The original BET treatment involved an extension of the Langmuir kinetic theory of monomolecular adsorption to the formation of an infinite number of adsorbed layers. 

In principle, the as-method is not restricted to nitrogen adsorption and can be applied to any gas-solid physisorption system irrespective of the shape of its isotherm it can be used to check the validity of the BET area and also to identify the individual mechanisms (monolayer-multilayer adsorption, micropore filling or capillary condensation). Numerous examples of different as-plots are to be found in subsequent chapters. Here, we are concerned with the general principles of the as-method of isotherm analysis with particular reference to the evaluation of surface area. The distinctive features of various hypothetical as-plots are revealed in Figure... 

In a critical appraisal of the different methods for determining surface fractal dimensions, Neimark (1990) has stressed the importance of taking account of the different mechanisms of physisorption (e.g. at high p/p° the combination of multilayer adsorption and capillary condensation). Conner and Bennett (1993) have also warned of the risk of an oversimplistic interpretation of a linear log-log fractal plot. 

There are a number of possible explanations to account for the lack of agreement between various values vp in Table 11.5. First, it must be kept in mind that the total uptake at high p/p° is controlled by three mechanisms (1) the intracrystalline filling at low p/p°, (2) the multilayer adsorption on the external surface, and (3) capillary condensation within a secondary pore structure. Process (2) and process (3) are manifested in the form of a finite multilayer slope and by a hysteresis loop in the capillary condensation range (Kenny and Sing, 1990). 

According to present day Insight type I isotherms (fig. 1.13) are not representative of a Langmulr-llke mechanism, but of micropore filling (in the initial steep part) followed by multilayer adsorption on the small external surface (plateau with a low slope). Therefore, the Langmuir. BET and other equations may not be used to determine the surface area at best, something can be said about the external area. On the other hand, if the relatively small amount adsorbed on the external surface is subtracted from the total amount (and the absence of mesopores has been ascertained) the remaining amount may be identified as that present in micropores. 

To unravel the detailed mechanism, substrate adsorption, quenching, inhibition and kinetic studies were conducted for the ZnS-catalyzed photodehydrodimeriza-tion of 2,5-DHF [107, 148]. A plot of the amount of 2,5-DHF adsorbed ( eq) against the residual concentration in solution (cgq) exhibits saturation plateaus at eq(max) of 2.8 X 10 and 65 X 10 mol g . The first plateau is due to the formation of a mixed solvent-solute surface monolayer and the second corresponds to multilayer adsorption. Assuming that the formation of the monolayer can be described by competitive adsorption between water and 2,5-DHF, the data can be analyzed according to Hiemenz (Eq. 30) [149] ... 

Porous solids having a regular pore structure have gathered much attention in the fields of chemistry and physics[l-7]. Those solids are expected to elucidate the interaction of gas with pores from the microscopic level. lUPAC classified pores into micropores, mesopores, and macropores using pore width w ( micropores w< 2nm, mesopores 2 nm < w< 50 nm, and macropores w> 50 nm)[8]. Physical adsorption occurs by the mechanism inherent to the pore width. Vapor is adsorbed on the mesopore wall by multilayer adsorption in the low pressure range and then vapor is condensed in the mesopore space below the saturated vapor pressure P . This is so called capillary condensation. Capillary condensation has been explained by the Kelvin equation given by eq. (1). 

Pores, and espjecially mesopores and micropores, play an essential role in physical and chemical properties of industrially important materials like adsorbents, membranes, catalysts etc. The description of transport phenomena in porous materials has received attention due to its importance in many applications such as drying, moisture transport in building materials, filtration etc. Although widely different, these applications present many similarities since they all depend on the same type of transport phenomena occurring in a porous media environment. In particular, transport in mesoporous media and the associated phenomena of multilayer adsorption and capillary condensation have been investigated as a separation mechanism for gas mixtures. 

The mechanism of the photodehydrodimerisation of 2,5-dihydrofuran on suspended ZnS powders has been investigated using a variety of techniques. Both mono- and multilayer adsorption participate, and the substrate appears to be adsorbed perpendicularly to the surface at all of the available zinc sites. Dissociative electron transfer occurs from the adsorbed substrate to a reactive hole affording a proton and a dihydrofuryl radical. 

The statistical mechanics of phase transitions is briefly reviewed, with an emphasis on surfaces. Flat surfaces of crystals may act as a substrate for adsorption of two-dimensional (d = 2) monolayers and multilayers, offering thus the possibility to study phase transitions in restricted dimensionality. Critical phenomena for special universality classes can thus be investigated which have no counterpart in d = 3. Also phase transitions can occur that are in a sense in between different dimensionalities (e.g., multilayer adsorption and wetting phenomena are transitions in between two and three dimensions, while adsorption of monolayers on stepped surfaces allows phenomena in between one and two dimensions to be observed). 

Sorption equilibria and kinetics are influenced by the nature of the adsorbent and the adsorbate, by the mechanism of adsorption, and by environmental parameters such as temperature, relative humidity, concentration of the adsorbate, and air velocity and turbulence past the adsorbent surface. Air velocity and turbulence only affect sorption kinetics the other parameters also affect equilibria. In general, low adsorbate saturation vapor pressure, low temperature, and high adsorbate concentration in the air increase adsorption. Relative humidity does not always affect adsorption. Colombo et al. (1993) found a 35 % decrease in adsorbed mass when relative humidity was changed from <10 % to 35 %, but only an 8 % decrease when the humidity was increased from 35 % to 70 %. Building materials, which are exposed to indoor air in the normal humidity range of 35-70 %, will typically already be covered by at least one monolayer of adsorbed water, and the formation of multilayers will only have a limited influence on sorption properties for other airborne substances. Kirchner et al. (1997) found that an increase in air velocity increased the rate of desorption of a VOC mixture from painted gypsum, but not from carpet. The air velocity of air above the tuft may be insignificant for the desorption processes of carpet fibers deeper in the tuft. 

For this reason, we consider it hardly possible to cite all of the publications. Let us focus only on the following examples. Hydroxamic acids have already been for a long time subject of the classical analytical chemistry. In [71], the possibility of using these compounds in flotation of rare-earth minerals is shown. It has been concluded that on a mineral surface cerium chelates are formed. Besides, chemisorption is accompanied by a physical multilayer adsorption of hydroxamic acid derivatives formed by reaction with cations in the water phase. A number of chelate-forming compounds including hydroxamic acids has been tested in flotation of niobium ores [72]. The best results are obtained when using alkyl phosphonic acids. Chemisorption mechanism and the structure of the surface compounds are established by spectroscopic methods. 

Under certain circumstances it is possible to obtain adsorption isotherms that correspond to a layer-by-layer mechanisms. This is a very well known fact in gas adsorption on homogeneous flat surfaces [151] and is known as layering. At the solid-liquid interface stepwise isotherms are also observed, nevertheless one step and sometimes two are found [114]. Sellami et al. [152] studied the adsorption of 2,5-dimethylpyridine (DMP) on silica from aqueous solutions. The experimental results [153] clearly show the stepwise character of the isotherms, which could either mean multilayer adsorption or layering. In their paper [152] they addressed the question of using the adsorption isotherm to discriminate between... 

In Table 3.9-1, the range of pore radii is chosen from the lower limit to the upper limit of the mesopore (according to the lUPAC classification, 2 < d < 50 nm). We note that the capillary condensation starts to occur in the smallest mesopore (d = 2 nm = 20 A) at the relative pressure of 0.39. It is reminded here that the upper limit of the relative pressure for the validity of the multilayer theory is about 0.35. This reduced pressure is usually regarded as the demarcation point between the multilayer adsorption and the capillary condensation mechanism, and it is satisfied by many adsorption systems. 

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