Isotherms pore filling degree

Figure 9.9 A scheme of distribution of liquid phase (black at the successive stages of slow drying (a), (b), (c), and (d). The insert shows the corresponding points at the desorption branch of isotherm, where U is the degree of pore filling and P/P the partical pressure of solvent vapor [3],...

Pore filling model comparisons have also been reported for other porous solids. The inside pore diameters of MCM-41 type adsorbents have been calculated to a high degree of consistency from nitrogen and argon porosimetry (Fig. 4) by using a DFT model of gas adsorption in cylindrical oxide pores to interpret the experimental isotherms... 

The first stage in the interpretation of a physisorption isotherm is to identify the isotherm type and hence the nature of the adsorption process(es) monolayer-multilayer adsorption, capillary condensation or micropore filling. If the isotherm exhibits low-pressure hysteresis (i.e. at p/p° < 0 4, with nitrogen at 77 K) the technique should be checked to establish the degree of accuracy and reproducibility of the measurements. In certain cases it is possible to relate the hysteresis loop to the morphology of the adsorbent (e.g. a Type B loop can be associated with slit-shaped pores or platey particles). 

The calculation of porosity from the water vapour adsorption isotherm leads to the higher value. Obviously, the porosity becomes higher with increasing w/c ratio because the hydration products cannot fill the rising volume of pores (Fig. 5.30). However, the porosity is substantially lower when nitrogen is applied as an adsorbate. The hydraulic radius of pores, determined from the water vapour isotherm, decreases also with the rise of hydration degree. The ratio of micropores is also increasing [54] with the time of hydration. When the samples are dried only at 11 % RH (over the LiCl H O saturated solution), the porosities from water vapour determination are close to these obtained with helium or methanol [62]. 

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