Analysis of adsorption isotherms on hypercrosslinked polystyrenes

On the other hand, the isotherms for N2 and CO2 are linear only until p/ps = 0.018 and 0.01, respectively this very narrow range of the validity of the BET equation is strange in itself Furthermore, if one specifies the monolayer capacity on the isotherm calculated by Eq. [3.1], then must correspond to the position of the so-called B-point. This point on the isotherm indicates that the formation of a monomolecular adsorption 

It is of no surprise that the calculations of specific surface areas from the sorption isotherms for water and carbon dioxide also give totally different results. Thus, when calculated from the adsorption of water vapors, the specific surface area of the sample with 100% crosslinking degree amounts barely to 15m g, while the adsorption of carbon dioxide on the same polymer gives a 6 times greater value, lOOm g. 

Sorption of inert gases at low temperatures appears to produce more habitual data. Now, the values of the C constants do not significandy differ from those for conventional adsorbents also, they are sufficiendy high to allow a refiable estimation of the surface area (Table 10.1). The calculated monolayer capacities, a, are consistent with the position of the B-points. Also, the surface areas calculated from the adsorption isotherms for inert gases at —196°C prove to be many times greater than those calculated from the isotherms for water or carbon dioxide at room temperatures. 

The sorption isotherms for nitrogen at 20°C are worthy of note, because the hysteresis loops are located in the relative pressure range corresponding to the capillary condensation of N2 vapors in mesopores at low temperatures. However, it is obvious even to the naked eye that the capillary condensation of nitrogen at room temperature is impossible therefore, fundamentally different reasons should be responsible for both the S-shaped form of these isotherms and the presence of hysteresis loops. 

X (%) Adsorbate Temperature (°C) S (m /g) B-point capacity (mmol/g) Constants of BET equation dm C  

---