Effective pore width

Both a uniform bulk fluid and an inhomogeneous fluid were simulated. The latter was in the form of a slit pore, terminated in the -direction by uniform Lennard-Jones walls. The distance between the walls for a given number of atoms was chosen so that the uniform density in the center of the cell was equal to the nominal bulk density. The effective width of the slit pore used to calculate the volume of the subsystem was taken as the region where the density was nonzero. For the bulk fluid in all directions, and for the slit pore in the lateral directions, periodic boundary conditions and the minimum image convention were used. 

Let us consider a slit-like pore of width D along whose walls the ip(x) potential is localized (Fig. 4). We shall regard the interaction of monomers with the walls as a short-range interaction and the characteristic radius of interaction as being of the order of the segment size a. The exact assignment of the form of the potential is immaterial for our purposes, since it describes the effective interaction of units with the pore walls, renormalized by the solvent molecules. Conditions are to be as follows ... 

In the past, much attention was given to the study of dye and iodine adsorption by active carbons (Bmnauer, 1945 Orr and Dalla Valle, 1959). Many studies have been made with dye molecules of well-known size, shape and chemical properties, but the results have not been easy to interpret (Giles et al., 1970 McKay, 1982, 1984). In a systematic study of iodine adsorption (from aqueous solution) on a carbon black and four activated carbons (Femandez-Colinas etal., 1989b), it was found that the iodine isotherms could be analysed by the as-method. In this way it was possible to assess values of the available volume in pores of effective width of 0.5-1.5 nm. The adsorption of iodine was also featured in a recent study by Ziolkowska and Garbacz (1997), who applied the Langmuir, Freundlich and other isotherm equations. 

Two kinds of pitch-based ACFs (P5 and P20 Osaka Gas Co.) were used. The microporous structure was determined by high-resolution N, adsorption isotherms at 77 K using a gravimetric method. The micropore structual parameters were obtained from high-resolution a, -plot analysis with subtracting pore effect (SPE) method. The average slit pore width w was determined from the micropore volume and the surface area. The adsorption isotherms of methanol and ethanol on carbon samples were gravimetrically measured at 303 K. The sample was preevacuated at 10 mPa and 383 K for 2h. 

A significant effret of very small micropores on SO2 adsorption was also noticed by Bagreev and coworkers [34]. Fig. 9 shows the dependence of Sft adsorption capacity on the volume of pores with widths between 0.679 and 0.858 nm. Two slopes distinguished in this figure suggest two different steps/mechanisms of adsorption. It was further concluded that the adsorption capacity is governed by two surfece features porosity and sur ce chemistry. Their contributions are impossible to separate and the combined effect is addressed in section 3.3. 

Here the parameter c is the minimum pore half width below which adsorbate molecules can not penetrate due to the steric effect, that is only pores having spacing greater than or equal to the diameter of the adsorbate molecule will allow the adsorption to proceed. 

Figure 6.11-2 shows the log-log plot of the effective width versus the relative pressure. Note the increase in the effective pore width with the pressure. The limit... 

In the microporous region the pore width is approaching molecular dimensions, for instance the effective width of the nitrogen molecule is given as 0.364nm. It is clear that the pore wall adsorption fields are almost overlapping and adsorbate molecules are in close proximity to them. This results in enhanced uptake of adsorbate at very low relative pressures. Horvath and Kawazoe (2) calculated that, for pores in carbons, pores filled at the low relative pressure shown in Table 1. 

General hydrodynamic theory for liquid penetrant testing (PT) has been worked out in [1], Basic principles of the theory were described in details in [2,3], This theory enables, for example, to calculate the minimum crack s width that can be detected by prescribed product family (penetrant, excess penetrant remover and developer), when dry powder is used as the developer. One needs for that such characteristics as surface tension of penetrant a and some characteristics of developer s layer, thickness h, effective radius of pores and porosity TI. One more characteristic is the residual depth of defect s filling with penetrant before the application of a developer. The methods for experimental determination of these characteristics were worked out in [4]. 

Let us consider the calculation of sensitivity threshold in the case when the cracks are revealing by PT method. Constant distance H between crack s walls along the whole defect s depth is assumed for the simplicity. The calculation procedure depends on the dispersity of dry developer s powder [1]. Simple formula has to be used in the case when developer s effective radius of pores IC, which depends mainly on average particle s size, is smaller than crack s width H. One can use formula (1) when Re is small enough being less than the value corresponding maximum sensitivity (0,25 - 1 pm). For example. Re = 0,25 pm in the case when fine-dispersed magnesia oxide powder is used as the developer. In this case minimum crack s width H that can be detected at prescribed depth lo is calculated as... 

These calculations lend theoretical support to the view arrived at earlier on phenomenological grounds, that adsorption in pores of molecular dimensions is sufficiently different from that in coarser pores to justify their assignment to a separate category as micropores. The calculations further indicate that the upper limit of size at which a pore begins to function as a micropore depends on the diameter a of the adsorbate molecule for slit-like pores this limit will lie at a width around I-So, but for pores which approximate to the cylindrical model it lies at a pore diameter around 2 5(t. The exact value of the limit will of course depend on the actual shape of the pore, and may well be raised by cooperative effects. 

The limits of pore size corresponding to each process will, of course, depend both on the pore geometry and the size of the adsorbate molecule. For slit-shaped pores the primary process will be expected to be limited to widths below la, and the secondary to widths between 2a and 5ff. For more complicated shapes such as interstices between small spheres, the equivalent diameter will be somewhat higher, because of the more effective overlap of adsorption fields from neighbouring parts of the pore walls. The tertiary process—the reversible capillary condensation—will not be able to occur at all in slits if the walls are exactly parallel in other pores, this condensation will take place in the region between 5hysteresis loop and in a pore system containing a variety of pore shapes, reversible capillary condensation occurs in such pores as have a suitable shape alongside the irreversible condensation in the main body of pores. 

We now proceed with the study of the phase behavior of associating fluids in pores. To elucidate the effects of changes of the association energy, again we have considered = 0, 7, and 10. The pore width was fixed and set to H = 6a. Figs. 14(a) and 14(b) show some examples of the... 

Figure 2. Density profiles Illustrating effect of pore width on layering structure. Theory with 6 - oo LJ fluid.

Figure 6. Pore dlfifuslvlty versus pore width. Theory Is for 6-oo LJ fluid with an effective hard sphere diameter cTgff = 0.972. Units of dlfifuslvlty are (3a/8)...

Effectiveness of selective adsorption of phenanthrene in Triton X-100 solution depends on surface area, pore size distribution, and surface chemical properties of adsorbents. Since the micellar structure is not rigid, the monomer enters the pores and is adsorbed on the internal surfaces. The size of a monomer of Triton X-100 (27 A) is larger than phenanthrene (11.8 A) [4]. Therefore, only phenanthrene enters micropores with width between 11.8 A and 27 A. Table 1 shows that the area only for phenanthrene adsorption is the highest for 20 40 mesh. From XPS results, the carbon content on the surfaces was increased with decreasing particle size. Thus, 20 40 mesh activated carbon is more beneficial for selective adsorption of phenanthrene compared to Triton X-100. 

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