Adsorbed molecules, area nitrogen

Surface Area. The most important features influencing the performance of carbon blacks are aggregate size and surface area. Surface area is measured by gas- and Hquid-phase adsorption techniques, and depends on the amount of adsorbate required to form a surface monolayer. If the area occupied by a single-adsorbate molecule is known, a simple calculation will yield the surface area. A low temperature nitrogen absorption method, based on the original method of Bmnauer, Emmett, and Teller (BET) (30), has been adopted by ASTM as standard method D3037-86 (2). 

Surface areas are deterrnined routinely and exactiy from measurements of the amount of physically adsorbed, physisorbed, nitrogen. Physical adsorption is a process akin to condensation the adsorbed molecules interact weakly with the surface and multilayers form. The standard interpretation of nitrogen adsorption data is based on the BET model (45), which accounts for multilayer adsorption. From a measured adsorption isotherm and the known area of an adsorbed N2 molecule, taken to be 0.162 nm, the surface area of the soHd is calculated (see Adsorption). 

Data taken from the adsorption leg of the isotherm of Figure 17.11 are listed in the first two columns of the following table. Test the applicability of the following equilibrium theories (a) Langmuir (b) infinite BET and (c) Harkins and Jura. From (a) and (b) obtain estimates of the surface area of the adsorbent and compare the values with that obtained by the point B method. One molecule of nitrogen adsorbed on alumina occupies 0.162 nm2. 

Since (C — 1)/ (C — 1), (0q )m > calculated from a BET plot, there exists a potential means of predicting the cross-sectional area variation relative to nitrogen. On surfaces that contain extensive porosity, which exclude larger adsorbate molecules from some pores while admitting smaller ones, it becomes even more difficult to predict any variation in the adsorbate cross-sectional area by comparison to a standard. 

In summary, it may be concluded that the uncertainty in calculating absolute cross-sectional areas, the variation in cross-sectional areas with the BET C value and the fact that on porous surfaces less area is available for larger adsorbate molecules all point to the need for a universal, although possibly arbitrary, standard adsorbate. The unique properties of nitrogen have led to its acceptance in this role with an assigned cross-sectional area of 16.2 usually at its boiling point of —195.6 °C. 

The next basic assumption of the Emmett-Brunauer technique is that the average cross-sectional area of the adsorbed molecules is the same as that corresponding to the plane of closest packing in the solidified gas. It may be shown that when nitrogen is used as an adsorbate at —195.8 deg C, each cu cm adsorbed represents 4.38 sq m of surface. 

This ratio was determined from the number of metal atoms per unit area, from crystallographic data, to the number of adsorbate molecules per unit area, from the adsorption data. It is the number of nitrogen molecules and of argon atoms per unit area. 

The isotherms and corresponding as-plots in Figures 10.8 and 10.9 are for the adsorption of nitrogen on representative mesoporous and microporous silica gels (Bhambhani et al., 1972). The derived values of the specific surface area are given in Table 10.8. The values of the BET nitrogen area, a(BET), in Table 10.8 are based on the usual assumption that the adsorbed molecules were close-packed in the completed monolayer (i.e. a(N2) 0.162 nm2). The corresponding values of a(S, N2) were calculated from the initial slope of the as-plots by die relation... 

From measuring adsorption isotherms one obtains n(p), and the integral can be solved. The specific surface area is determined by measuring the adsorption isotherm n(p) with a reference gas (e.g.. nitrogen), where the area occupied by an adsorbed molecule is known. Therefore the adsorption isotherms are fitted... 

The estimate of the surface area of chromatographic silica support is a complicated issue. It is usually performed via the BET method using low temperature nitrogen adsorption (N2.- sorptometry). The total surface area of the adsorbent is the product of the number of adsorbed molecules and the surface area per molecule. However, if the pore size distribution is not very narrow, an estimate of bonding density on the basis of carbon load and surface area may yield a large error because the smallest pores are not available for derivatization and the calculated bonding density is lower than the actual one. 

It is generally assumed that a nitrogen molecule occupies 16.4 on the polar silica surface. The adsorbent surface area is then calculated as a product of the total amount of nitrogen in the monolayer (n ) and the nitrogen molecular area (16.4 A ). 

The calculation of the adsorbent surface area using BET theory involves the estimation of the molecular cross-sectional area of nitrogen molecule [15]. In general it is assumed to be equal to 16.2 per nitrogen molecule on the surface, but this value is the subject of intense criticism during the past 30 years [72]. 

As evidenced by curves on figures 4 and 5, changes in texture characteristics are quasi-linearly related to increasing carbon content. It also appears that calculated surface areas strongly depend on the physisorption conditions, while measured adsorbate volumes are less affected. This observation is in favor of erroneous assumptions on nitrogen molecule area for the calculations of specific surface area [10]. But, as mentioned above, nitrogen molecule could penetrate in smaller pores, increasing measured micropore volume and surface areas. 

An answer to the first question would be to suppose a wrong choice of the cross-sectional area of the methylene chloride molecule. In the case of nitrogen, a standard molecule for the determination of the specific surface area of solids, various values were also postulated (16.2, 12.9 or 17.7A ) depending on the nature of the adsorbent and the orientation of the adsorbed molecule. 

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