Nitrogen molecular area

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 ). 

It has been shown that nitrogen occupies a larger area on hydrophobic surfaces than on polar surfaces. The nitrogen molecular area on hydrophobic surfaces is estimated to be between 19 and 22 A [71]. 

Adsorbent surface area is calculated as the product of the monolayer capacity estimated from BET equation and nitrogen molecular area, con,- If the... 

Molecular area, a (Ar) of argon at 77 K on graphitized carbon blacks (Argon BET plots constructed with p (liquid) surface areas determined by BET-nitrogen, with a (Na)... 

The survey in the present section shows quite clearly that it is not possible to assign a fixed value of a to a given adsorptive, which will remain valid for its adsorption on ail adsorbents. As demonstrated in Section 2.7, nitrogen and argon would seem to provide the best approximation to a constant effective molecular area, with = 16-2 A and a, (Ar) = 16-6 A. ... 

Harkins, W. D. and Jura, G. J. Chem. Phys. 11 (1943) 431. An adsorption method for the determination of the area of a solid without the assumption of a molecular area and the area occupied by nitrogen molecules on the surface of solids. J. Amer. Chem. Soc. 66 (1944) 1366. Surface of solids. Part XIII. 

The adsorption isotherm of N, on FSM-16 at 77 K had an explicit hysteresis. As to the adsorption hysteresis of N-, on regular mesoporous silica, the dependencies of adsorption hysteresis on the pore width and adsorbate were observed the adsorption hysteresis can be observed for pores of w 4.0nm. The reason has been studied by several approaches [5-8]. The adsorption isotherm of acetonitrile on FSM-16 at 303K is shown in Fig. 1. The adsorption isotherm has a clear hysteresis the adsorption and desorption branches close at PIP, = 0.38. The presence of the adsorption hysteresis coincides with the anticipation of the classical capillary condensation theory for the cylindrical pores whose both ends are open. The value of the BET monolayer capacity, nm, for acetonitrile was 3.9 mmol g. By assuming the surface area from the nitrogen isotherm to be available for the adsorption of acetonitrile, the apparent molecular area, am, of adsorbed acetonitrile can be obtained from nm. The value of am for adsorbed acetonitrile (0.35 nnr) was quite different from the value (0.22 nm2) from the liquid density under the assumption of the close packing. Acetonitrile molecules on the mesopore surface are packed more loosely than the close packing. The later IR data will show that acetonitrile molecules are adsorbed on the surface hydroxyls in... 

With most other adsorbates, adjusted molecular areas (determined, for exampie, by calibration with nitrogen adsorption data) which are usually in excess of those calculated from liquid densities must be used, and, moreover, for a given adsorbate, this adjusted value usually varies from solid to solid. This is due mainly to a certain amount of localisation of the adsorbed gas in the first monomolecular layer with respect to the variously distributed potential energy minima of the solid lattices. To avoid excessive localisation, a low value of —A//j is desirable however, a high value of c (i.e. —AHi > —AHl) is also required to give a well-defined point B. The main... 

Plot the adsorption isotherm and use the BET equation to calculate a specific surface area for the silica gel sample, taking the molecular area of nitrogen as 16.2 x 10-20 m2. 

Thus for nitrogen at its normal boiling point of 77 K, K0 is 9.2 10"4, with pressure in atmospheres, c0 taken to be 16.2 A2 pressure (actually, this is the estimated molecular area of Nitrogen) and x0 as 10 12 sec. 

An early normalizing procedure, proposed by Kiselev (1957) to compare adsorption isotherms of hydrocarbons, water vapour, etc. on a series of different adsorbents, was simply to plot the surface excess concentration F (=n/A), obtained from a knowledge of the BET-nitrogen surface area, A (BET), versus p/p°. It is also possible to plot, instead of f, the reduced adsorption , n/nm, which still relies on the BET method to determine the monolayer capacity nm but does not require knowledge of the molecular cross-sectional area a. 

The values of a(BET) in Table 9.1 are the BET-nitrogen areas, which were derived from the linear regions of the BET plots, with the molecular area assumed to be 0.162 nm2. The values of a(ext) in Table 9.1 are obviously much smaller than the corresponding values of a(BET). The question naturally arises does the BET method provide a reliable assessment of the total area (i.e. internal plus the external area) The non-linear character of the low-pressure region of each as-pk>t is a clear indication that the isotherm is distorted in the monolayer region and we may therefore conclude that a(BET) does not represent a real surface area. Additional support for this interpretation comes from the microcalorimetric data, which are discussed later in this section. More detailed discussion of some of the results in Table 9.1 is given in Chapter 12. 

For. a number of reasons, nitrogen (at 77 K) is generally considered to be the most suitable adsorptive for standard surface area determination and for this purpose it is usually assumed that the BET monolayer is close-packed (with the molecular area taken as 0.162 nm2). One advantage of nitrogen is that the path of its multilayer isotherm is not very sensitive to differences in adsorbent structure. A useful check on the validity of nm is that the value of C(BET) should be neither too low nor too high if C(BET) < 50, Point B is not sufficiently sharp if C(BET) > 200, there is either a significant micropore filling contribution or localized adsorption on specific sites. 

Amount of grafted modifier a (mg/m ), specific surface area with respect to nitrogen Sn (m /g), BET adsorption monolayer values for various substances am (mmol/g), water molecular area whjO (nm ) and heat of wetting q (mj/m ) for the initial and modified silicas [38]... 

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