Pores micropore filling

Adsorbents such as some silica gels and types of carbons and zeolites have pores of the order of molecular dimensions, that is, from several up to 10-15 A in diameter. Adsorption in such pores is not readily treated as a capillary condensation phenomenon—in fact, there is typically no hysteresis loop. What happens physically is that as multilayer adsorption develops, the pore becomes filled by a meeting of the adsorbed films from opposing walls. Pores showing this type of adsorption behavior have come to be called micropores—a conventional definition is that micropore diameters are of width not exceeding 20 A (larger pores are called mesopores), see Ref. 221a. 

The first stage in the interpretation of a physisorption isotherm is to identify the isotherm type and hence the nature of the adsorption process(es) monolayer-multilayer adsorption, capillary condensation or micropore filling. If the isotherm exhibits low-pressure hysteresis (i.e. at p/p° < 0 4, with nitrogen at 77 K) the technique should be checked to establish the degree of accuracy and reproducibility of the measurements. In certain cases it is possible to relate the hysteresis loop to the morphology of the adsorbent (e.g. a Type B loop can be associated with slit-shaped pores or platey particles). 

During the adsorption or occlusion of various molecules, the micropores fill and empty reversibly. Adsorption in zeoHtes is a matter of pore filling, and the usual surface area concepts are not appHcable. The pore volume of a dehydrated zeoHte and other microporous soHds which have type 1 isotherms may be related by the Gurvitch rule, ie, the quantity of material adsorbed is assumed to fill the micropores as a Hquid having its normal density. The total pore volume D is given by... 

Ghosh [548] used cellulose nitrate microporous filters (500 pm thick) as scaffold material to deposit octanol into the pores and then under controlled pressure conditions, displace some of the oil in the pores with water, creating a membrane with parallel oil and water pathways. This was thought to serve as a possible model for some of the properties of the outermost layer of skin, the stratum comeum. The relative proportions of the two types of channel could be controlled, and the properties of 5-10% water pore content were studied. Ibuprofen (lipophilic) and antipyr-ine (hydrophilic) were model drugs used. When the filter was filled entirely with water, the measured permeability of antipyrine was 69 (in 10 6 cm/s) when 90% of the pores were filled with octanol, the permeability decreased to 33 95% octanol content further decreased permeability to 23, and fully octanol-filled filters indicated 0.9 as the permeability. 

Type I isotherms are encountered when adsorption is limited to, at most, only a few molecular layers. This condition is encountered in chemisorption where the asymptotic approach to a limiting quantity indicates that all of the surface sites are occupied. In the case of physical adsorption, type I isotherms are encountered with microporous powders whose pore size does not exceed a few adsorbate molecular diameters. A gas molecule, when inside pores of these small dimensions, encounters the overlapping potential from the pore walls which enhances the quantity adsorbed at low relative pressures. At higher pressures the pores are filled by adsorbed or condensed adsorbate leading to the plateau, indicating little or no additional adsorption after the micropores have been filled. Physical adsorption that produces the type I isotherm indicates that the pores are microporous and that the exposed surface resides almost exclusively within the micropores, which once filled with adsorbate, leave little or no external surface for additional adsorption. 

Adsoiptive molecules transport through macropores to the mesopores and finally enter the micropores. The micropores usually constitute the largest portion of the internal surface and contribute the most to the total pore volume. The attractive forces are stronger and the pores are filled at low relative pressures in the microporosity, and therefore, most of the adsorption of gaseous adsoiptives occurs within that region. Thus, the total pore volume and the pore size distribution determine the adsorption capacity. 

Next we analyze the sorption kinetics of a sorbate with constant aqueous concentration, C°, sorbing into a porous spherical aggregate with radius rQ. More precisely, the macroparticle is a homogeneous aggregate of microparticles which are separated by micropores filled with water (Fig. 19.17). The sorbate diffuses in these pores and sorbs to the microparticles. It is not relevant whether sorption occurs at the surface or in the interior of the microparticles as long as we can assume that sorption equilibrium between the solute concentration and the microparticles at each position within the aggregate is attained instantaneously. 

Figure 19.17 Spherical macroparticle with radius ra consisting of an aggregate of microparticles separated by micropores filled with water. A chemical with constant concentration C° diffuses into the pore volume of the macroparticle. The local dissolved pore concentration Cw is at instantaneous equilibrium with the local sorbed phase C ( K d is microscopic equilibrium coefficient). Note that the macroscopic distribution coefficient Kd is time dependent (see Eq. 19-78.)...

The adsorption of vapors in complex porous systems takes place approximately as follows [1-3] at first, micropore filling occurs, where the adsorption behavior is dominated nearly completely by the interactions of the adsorbate and the pore wall after this, at higher pressures, external surface coverage occurs, consisting of monolayer and multilayer adsorption on the walls of mesopores and open macropores, and, at last, capillary condensation occurs in the mesopores. 

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). 

In monolayer adsorption all the adsorbed molecules are in contact with the surface layer of the adsorbent. In multilayer adsorption the adsorption space accommodates more than one layer of molecules so that not all adsorbed molecules are in direct contact with the surface layer of the adsorbent. In capillary condensation the residual pore space which remains after multilayer adsorption has occurred is filled with condensate separated from the gas phase by menisci. Capillary condensation is often accompanied by hysteresis. The term capillary condensation should not be used to describe micropore filling because this process does not involve the formation of liquid menisci. 

In the case of micropore filling, the interpretation of the adsorption isotherm in terms of surface coverage may lose its physical significance. It may then be convenient to define a monolayer equivalent area as the area, or specific area, respectively, which would result if the amount of adsorbate required to fill the micro-pores were spread in a close-packed monolayer of molecules (see Section 11.2.1.8). 

It is generally recognized that the mechanism of physisorption is modified in very fine pores (i.e. pores of molecular dimensions) since the close proximity of the pore walls gives rise to an increase in the strength of the adsorbent-adsorbate interactions. As a result of the enhanced adsorption energy, the pores are filled with physisorbed molecules at low p/p°. Adsorbents with such fine pores are usually referred to as microporous. 

The limiting dimensions of micropores are difficult to specify exactly, but the concept of micropore filling is especially useful when it is applied to the primary filling of pore space as distinct from the secondary process of capillary condensation in mesopores. 

The O2 isotherms for these AlPO molecular sieves are essentially Type I, typical of micropore filling (12,13). In Figure 16 the O2 isotherms of AlP0i.-5, -11, -17 are compared with those of a typical zeolite, NaX, and a silica molecular sieve, silica-lite. A1P04-5 and -17 have saturation pore volumes of approximately 0.2 cm /g similar to that of silicalite, while A1P0 -11 has a relatively small O2 pore volume (ca. 0.1 cm3/g). 

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. 

That micropores are filled reversibly in the p/p° range below the normal onset of capillary condensation is now indisputable, but the mechanisms involved in micropore filling are still under discussion - as they have been for over 40 years. However, it is now apparent that the micropore filling process is dependent on both the ratio of the pore width to the molecular diameter (w/d) and the pore shape. 

The filling of these molecular-sized pores, which is associated with the isotherm distortion at very low pip0, has been called primary micropore filling . Wider micro-pores are filled by a secondary , or co-operative, process over a range of higher p/p0 (Sing, 1979). These processes are discussed in some detail in subsequent chapters. 

In principle, the processes of capillary condensation and evaporation should occur reversibly in a closed tapering pore (Everett 1967). At low relative pressures there is an enhanced concentration of adsorbed molecules in the narrow end of the pore (i.e. a micropore filling effect) as in Figure 7.4. At a certain p/p°, a meniscus begins to form which, with the increase of p/p°, then moves steadily up towards the pore entrance. Evaporation proceeds in the reverse direction but involves the same elemental steps (i.e. the meniscus configurations) and therefore the entire isotherm is reversible. 

In addition, we should keep in mind that the micropore filling capacity is dependent on both the available pore volume and the packing of the adsorbed molecules. 

Because of its diatomic nature and permanent quadrupole moment, the physisorp-tion of nitrogen at 77 K presents special problems. The application of DFT is facilitated if the molecules are assumed to be spherical, which was the approach originally adopted by Seaton et al. (1989) and also by Lastoskie et al. (1993). The analytical procedures already outlined in Chapter 7 (Section 7.6) do not depend on the meniscus curvature and are in principle applicable to both capillary condensation and micropore filling. The non-local version of the mean field theory (NLDFT), which was used by Lastoskie, gave excellent agreement with computer simulation when applied to the carbon slit pore model. However, as pointed out earlier, these computational procedures are not entirely independent since they involve the same model parameters. 

Backward extrapolation of the linear multilayer section of the as-plot allows us to assess the total micropore capacity (as indicated in Chapter 8) and hence to evaluate the effective micropore volume, v(mic, S). The values of vp(mic, S) in Table 9.1 were obtained by making the usual assumption that the pores are filled with liquid nitrogen (density 0.808 g cm-3). 

As indicated earlier, an extensive range of linearity of a DR plot is usually associated with primary micropore filling. However, it must be kept in mind that the micro-pore filling mechanism is dependent on the nature of adsorption system and the temperature as well as on the pore size. Since it contains an additional adjustable parameter, the DA equation is obviously more adaptable than the simple DR... 

From the limits of the two stages of pore filling in Table 9.2 we are able to arrive at the tentative ranges of pore size summarized in Table 9.3. However, caution must be exercised in the acceptance of these limits in view of the the complexity and nonrigidity of the pores in activated carbons. Since there appears to be no sharp boundary between the completion of co-operative micropore filling and the beginning of reversible capillary condensation, we cannot at present define the upper limit of secondary micropore filling. 

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