Sorption isotherms specific surface areas

Sorption isotherms, using water, Nj or other sorbates, have been widely used to study the pore structure of hep. The results obtained by dififerent investigators show considerable variations, which arise in part from dififer-ences in the conditions of preliminary drying. 

Water sorption isotherms for hep show marked hysteresis. Powers and Brownyard (P20) found that, while it was difficult to obtain reproducible desorption curves, the low-pressure part of the water vapour resorption curve varied little with w/c ratio, between different Portland cements, or, if allowance was made for the contents of unreacted cement, with the degree of hydration. This was their main direct evidence for the conclusion (Section 8.2.1) that the properties of the hydration product considered in their model were essentially independent of these variables. However, the water sorption iostherms obtained by different investigators have varied considerably (e.g. Refs P20 and S79), and it is not clear to what extent the above conclusion would stand had different desorption conditions been used. 

In principle, isotherms at low partial pressures of the sorbate may be used to determine specific surface areas by the Brunauer-Emmett-Teller (BET) method (G64). In this method, it is assumed that molecules of the sorbate are adsorbed on surfaces that can include the walls of pores, provided that the distance between molecules on opposing walls is large compared with molecular dimensions. From a plot derived from the isotherm, and given the effective cross-sectional area of the sorbate molecule, the specific surface area of the sorbent and the net heat of adsorption are obtained. Using water as sorbate, specific surface areas of about 200 m per g of D-dried paste have typically been obtained for mature cement pastes of normal w/c ratios 

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Sorption Analysis. Specific surface areas and porosity can be calculated from the adsorption isotherm of nitrogen at — 196 °C. The method of Brunauer, Emmett, and Teller [4.29] is generally accepted for the evaluation of specific surface areas (BET surface area in square meters per gram). The two-parameter equation is applicable to carbon black. The BET surface area comprises the outer surface area as well as the surface area of the pores. 

Zierath et al. (1980) measured sorption isotherm data for benzidine on sediments and soils. Using Missouri River sediment withy,. = 0.023 kg oc kg-1 solid, CEC = 190 mmol kg-1, and a specific surface area Aswf = 131 m2 g 1, they obtained the following sorption data ... 

Brunauer and co-workers (B55,BI08) considered that the gel particles of the Powers-Brownyard model consisted of either two or three layers of C S-H, which could roll into fibres. D-drying caused irreversible loss of interlayer water, and the specific surface area could be calculated from water vapour sorption isotherms, which gave values in the region of 200m g for cement paste. Sorption isotherms using N2 give lower values of the specific surface area this was attributed to failure of this sorbate to enter all the pore spaces. 

These comments apply also to studies of pore size distribution or specific surface area, which have been widely studied using sorption isotherms or, in the former case, mercury intrusion porosimetry (MIP). Gregg and Sing... 

Upper part of Table 2 gives specific surface area (Sbet), surface areas of micropores (Smicro) and mesopores (Smeso), total pore volume (Vt) and micropore volume (Vmicro) of fresh, equilibrium, 2.17 %C and 4.2 %C catalyst samples. Lower part of table 2 gives the same characteristics of equilibrium, calcined equilibrium, calcined coked 2.17 %C catalysts (calcination is operated at 873 K, during 24 hours, in air flow) as determined from N2-77 K or Ar-87 K isotherms. All values depend on the probe molecule and sorption temperature, which confirm the observations deduced from the isotherm shape. 

The aim of the present paper was to investigate the structural and surface characteristics of two types of porous polymers. The sorption isotherms of nitrogen at 77 K, and benzene and water vapor at room temperature were measured by the static method. The specific surface areas, pore volumes and pore dimensions were derived for the investigated polymers from different experimental data. The structural characteristics of the investigated porous polymers differ considerably for various types of adsorbates and temperatures. The surface characterization of both resins was made by XPS method. 

Table 1. Characterization of the pristine MCM-48 silica B and the host/guest compound B-Fe203 with respect to specific surface area, pore size and pore volume using various methods (BET-specific surface area Sbet, BJH-mode pore diameter dp(BJH), average pore diameter Dh = 4 Vpore/SBET- In addition argon sorption isotherms at 87 K for both materials were analyzed with respect to pore size (dp(SF)) and pore volume (Vp(SF)) by applying the Saito-Foley method.

The studies have shown that irrespective of the composition of the initial mixture, as a result of uniform distribution of components in the structure of samples, the shape of their adsorption isotherms is transformed gradually from the typical isotherm for one individual component to the isotherm for the second component. In this case, as the content of the second component, having a higher sorption capacity, increases, transitional isotherms accumulate features inherent in its structure the micro- and mesopore volume, limiting sorption capacity Vs, and specific surface area Ssp increase. The latter increases because of the fact that along with development of porosity of samples, their more open structure accessible to adsorbate molecules is formed. 

Adsorbents, co-precipitated at 80°C, whose sorption capacity is 20-25% lower than that of the initial compounds and the specific surface area is 1.4 to 1.7 times lower, are an example of the systems of the first type (Table 10). Moreover, a substantial difference is found in the shape of the sorption isotherms, which have a smoother rise at low relative pressures and narrow hysteresis loops as compared to the initial materials. 

Nitrogen sorption isotherms were recorded at 77 K with a Micromeritics ASAP 2000 instrument after outgassing the samples at 423 K for 12 h under a pressure of 0.1 Pa. BET formalism was applied for determination of the specific surface area. Table 1 presents the surface area of the prepared catalysts. 

The nitrogen sorption isotherms of S41, C41 and Z41(X) are shown in Fig. 3 and the specific surface area Sbet), the external specific surface area (Sexd, the total specific pore volume (F,o<) and the specific micropore volume (Vmia-o) are reported in Table 1. 

The apparent specific surface area of hypercrosslinked polystyrenes is much higher than that of traditional macroporous styrene-DVB copolymers. However, when discussing this subject, we have to pay attention to the fact that the adsorption and desorption branches of the isotherm given in Fig. 7.26 do not coincide in the whole range of relative pressures, including the range of very small pressures. The sorption hysteresis at low p/p is... 

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