Hydraulic pore width

To obtain the values of mean pore width, Wp, in Table 10.15, we have assumed the validity of the BET area and all pores to be slit-shaped. Then by adopting the hydraulic pore width principle we have... 

The constancy of the hydraulic pore width (Figure 1 (b)), and the fact that it is the same for both samples, is consistent with a uniform particle structure which probably consists of parallel layers of magnesium oxide microcrystallites Intercalated with micropores of mean width 0.93 nm, which can be formally considered as 4 missing (111) MgO planes [9]. 

As already indicated, by applying the Kelvin equation (assuming hemispherical meniscus formation) and correcting for the adsorbed layer thickness, we are able to calculate the ranges of apparent pore width recorded in Table 12.5. The values of mean pore diameter, w, are obtained from the volume/surface ratio, i.e. by applying the principle of hydraulic radius (see Chapter 7) and assuming the pores to be non-intersecting cylindrical capillaries and that the BET area is confined to the pore walls. 

The glass frit in the Teorell oscillator can be replaced by nanoporous Alter membranes [33-35], or even capillary electrophoresis tubes [36]. By these means, important parameters such as salt and hydraulic permeability, salt concentration, pore width, and fixed charge relevant to electro-osmotic flow (which may in turn depend on pH) can be varied systematically. 

It has been proposed that the slit-shaped micropores may be regarded as spaces between (111) planes in the freshly formed MgO structure. Thus, if the width of each micropore was equivalent to four (111) planes, its width would be 0.96 nm, which is quite close to the hydraulic values. It is not surprising to find pore widening to have occurred as the decomposition neared completion. 

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