Mercury intrusion effects

Fig. 3J0 Plot of cumulative pore volume against logarithm of r the effective pore radius, (o) For charcoal AY4 A by mercury intrusion O by capillary condensation of benzene, (b) For zinc chloride carbon AYS A by mercury intrusion O by capillary condensation of benzene x by capillary condensation of benzene, after mercury intrusion followed by distillation of mercury under vacuum at temperature rising to 350°C. (Courtesy...

In Unger and Fischer s study of the effect of mercury intrusion on structure, three samples of porous silica were specially prepared from spherical particles 100-200 pm in diameter so as to provide a wide range of porosity (Table 3.16). The initial pore volume n (EtOH) was determined by ethanol titration (see next paragraph). The pore volume u (Hg, i) obtained from the first penetration of mercury agreed moderately well with u fEtOH),... 

Sieve analysis using standard mesh screens is commonly used to determine particle size and size distribution of pellets and the reader is referred to standard texts for further information (61). Several types of densities have been defined for pellets based on interparticulate (void fraction) and intraparticulate pore volumes and include true, apparent, effective, bulk and tapped. The bulk and tapped densities may be obtained using simple devices, such as that used to evaluate granulations in tableting, while the true and apparent densities need more complex techniques based on mercury intrusion, gas flow, powder displacement, imaging or minimum fluidization velocity (62). 

Table II provides another illustration of this effect. In these experiments analysis was done by mercury intrusion, which is sensitive to a broader range of pores. In this series of experiments, samples of hydrogel were washed in various alcohol-water mixtures to yield pore volumes ranging from high, for the sample washed in pure alcohol, to low, for the sample washed in pure water. This procedure does not affect the surface area, which remained constant at 375 m2/g The activity of the finished catalyst increases with the pore volume. Notice that all samples have about the same volume inside...

A diameter of 20 A represents approximately the limiting pore size that can be measured by mercury intrusion. In pores smaller than this, transport becomes increasingly affected by molecule-pore wall interactions, and conventional theories based on molecular and Knudsen diffusion break down. The classification is somewhat arbitrary, however, since the point at which such effects become important also depends on the size of the diffusing molecule. Adsorption equilibrium in microporous adsorbents also depends to some extent on the pore size as well as on the nature of the surface, so control of the pore size distribution is important in the manufacture of an adsorbent for a particular separation. 

We now consider application of percolation theory to describing mercury intrusion into porous solids. First we briefly recall the main physical principles of mercury porosimetry (in particular, the Washburn equation). These principles are treated in detail in many textbooks [e.g., Lowell and Shields 49)]. The following discussions (Sections IV,B and IV,C) introduce general equations describing mercury penetration and demonstrate the effect of various factors characterizing the pore structure on this process. Mercury extrusion from porous solids is briefly discussed in Section IV,D. 

The results of simulations demonstrating the effect of various factors on mercury intrusion into porous solids are shown in Figs. 24-26. All the intrusion curves presented have been calculated for the same void radius distribution [Eq. (41) with v = 3000 A and = 0.5]. The mercury intrusion process is seen to start at higher pressures with decreasing Zo (Fig. 24), r (Fig. 25), and o- (Fig. 26). 

Equations (44) and (46) take into account the overlapping of the neck and void radius distribution. However, these equations have been derived employing the assumptions similar to those used in the mean-field approximation in statistical physics. In particular, Eqs. (44) and (46) ignore the effect of pore-size correlations on mercury intrusion. The latter effect has been recently studied in detail by Tsakiroglou and Payatakes (4J) employing the Monte Carlo method. Simulations were made on a square lattice... 

From Eq. (47) we can conclude that the effect of interconnection of various pores on mercury intrusion is very strong for laige necks at relatively low pressures (when Zoq < 2.5). Mercury penetration into large necks is blocked by small necks. For small necks at relatively high pressures (when Zoq > 2.5), the percolation probability 9 b2(zo ) is close to q (see, e.g., Fig. 7b), and Eq. (47) yields the same result as the model of independent cylinders. 

Therefore, it is anticipated that an increase in the granulometric size of the SDDP particles during scale up will probably not effect the SDDP particles dissolution rate of the powder as long as the texture of the SDDP is not modified Hence the critical parameters to be followed during scale up are the internal porosity and the intraparticular pore diameter as determined by Mercury Intrusion Porosimetry or by Scanning Electron Microscopy... 

Static methods Mercury intrusion Laplace (Washburn) Cylindrical 5 nm-15 pm Pore size distribution (including dead-end pores) Porosity Outgassed (dry) samples. Measurement of pore entrance. Destructive method. For small pore sizes damage of the porous structure may occur. Network percolation effects derived. 

Measuring the nitrogen sorption isotherms of a number of silicas both before and after analysis by mercury intrusion demonstrates that mercury intrusion can lead to compression of silica structures and that this compression can account for differences in pore size distributions measured by the nitrogen sorption and mercury intrusion techniques. These techniques are widely employed in the structural characterization of porous solids, often independently, despite the fact that very often the pore size distributions obtained by the two techniques fail to agree. Compression effects must be recognized because use of incorrect information can lead to misconceptions regarding the structure of a material. 

The effect of mercury intrusion analysis on structure was examined for a series of silica xerogels with different pore size distributions. This analysis was achieved by applying nitrogen sorption analysis to the silicas both before and after mercury intrusion analysis. The study required the development of a method for the removal of mercury from a sample after the initial intrusion measurement that does not damage the structure. The results show the potential for an elastic deformation of the structure during compression as well as irreversible compression during mercury intrusion. 

Effects of Mercury Intrusion. The facts that there is very little residual mercury in the solids after extraction and that the extraction method used does not cause any detectable structural modification have been established now the earlier suggestion (I) that mercury intrusion causes compression of the pore structures can be examined. 

Mercury Intrusion Experiments with Silica Spheres. These silica spheres (S980 G1.7 from Shell) were not examined in the same detail as were the other silica samples, but the photographs are included because they illustrate the effect of mercury intrusion on the integrity of the solid. These particular spheres have a typical pore volume of 1 cm3/g and a pore diameter of 60 nm. The particles are also much larger than the Sorbsil materials (1.7 mm in diameter, compared to 40 to 60 pm for the Sorbsil materials). 

Figure 6. Effect of mercury intrusion on Sorbsil C500. The pore size distribution of the original material measured by nitrogen desorption is shown as curve a and that measured by mercury intrusion is shown as curve b. Curve c shows the nitrogen desorption pore size distribution after mercury intrusion and the removal of mercury.

Figure 7. Effect of mercury intrusion on silica spheres (Shell). Top, starting material and bottom, material after mercury intrusion (no attempt was made to remove the mercury).

These results confirm the suggestions of earlier workers (I, 2) that the mercury intrusion method can lead to structural deformation of solids during analysis. For silicas, there appears to be both an elastic deformation and an irreversible compression effect that contribute to the differences in... 

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