Surface Area of Modified Adsorbent

The measurement of the silica surface area is well established however, the assessment of the surface area of chemically modified porous silica brings 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]. 

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 

Specific surface area of a hydrophobic adsorbent was determined using the value of con = 16.2 A, then the total adsorbent area would be underestimated. Correct comparison of the surface area of different modified adsorbents should also be done relative to 1 g of bare silica adsorbent. 

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As an example of the variation of the surface area of modified adsorbents, we discuss the series of silicas modified with alkylsUanes of different length [63]. In Figure 3-19 the dependence of the surface area on the number of carbon atoms of attached ligands are shown. 

The assessment of the surface area of modified adsorbent is extremely uncertain. Accounting for different types of corrections may lead to the significant differences in the calculated surface areas of the same material. Probably the dominating factor is the roughness of the surface of modified silica. Differences in the alkyl chain conformations may lead to the completely different values for the same material. Possible occlusion of nitrogen molecules between alkyl chains adds additional uncertainty [73]. 

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