Porous solids mercury penetration

Mercury porosimetry (or intrusion) Measurement of the specific porous volume and of the pore size distribution function by applying a continuous increasing pressure oti liquid mercury such that an immersed or submerged porous solid is penetrated by mercury. If the porous body can withstand the pressure without fracture the Washburn equation, relating capillary pressure to capiUaiy diameter allows converting the pressure penetration curves into a size distribution curve. If a sample is contracted without mercury intrusion, a specific mechanical model based on the buckling theory must be used... 

For porous materials pp < Pabs and cannot be measured with such methods. A mercury porosimeter can be used to measure the density of coarse porous solids but is not reliable for fine materials, since the mercury cannot penetrate the voids between small particles. In this case, helium is used to obtain a more accurate value of the particle density. Methods to measure the particle density of porous solids can be found in Refs. 2 and 5. 

The porosity of a catalyst or support can be determined simply by measuring the particle density and solid (skeletal) density or the particle and pore volumes. Particle density pp is defined as the mass of catalyst per unit volume of particle, whereas the solid density p, as the mass per unit volume of solid catalyst. The particle volume Vp is determined by the use of a liquid that does not penetrate in the interior pores of the particle. The measurement involves the determination by picnometry of the volume of liquid displaced by the porous sample. Mercury is usually used as the liquid it does not penetrate in pores smaller than 1.2/m at atmospheric pressure. The particle weight and volume give its density pp. The solid density can usually be found from tables in handbooks only in rare cases is an experimental determination required. The same devices as for the determination of the particle density can be used to measure the pore volume V, but instead of mercury a different liquid that more readily penetrates the pores is used, such as benzene. More accurate results are obtained if helium is used as a filling medium [10]. The porosity of the particle can be calculated as ... 

During the past decade, percolation theory has been successfully used to analyze condensate desorption from porous solids (14-34), mercury penetration into porous solids (35 -43), and the kinetics of catalytic deactivation... 

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. 

If the contact angle between liquid and solid is greater than 90 , then at equilibrium the pressure on the convex side of the meniscus must be greater than on the concave side. Thus if a porous solid is immersed in a nonwetting liquid such as mercury there will be no penetration of the pores until the... 

Mercury porosimetry is featured in many of the contributions to this volume. Indeed, it is now one of the most popular methods available for the characterization of a wide range of porous materials and the derived pore sizes are often quoted in the patent and technical literature. The method is based on the non-wetting nature of mercury and the application of the Washburn equation. The volume of mercury penetrating into a porous solid is determined as a function of the applied pressure, which is assumed to be directly related to the pore width. 

Because the shape of the pores is not exactly cylindrical, as assumed in the derivation of Equation 3.12, the calculated pore size and pore size distribution can deviate appreciably from the actual values shown by electron microscopy. If a pore has a triangular, rectangular or more complicated cross-section, as found in porous materials composed of small particles, not all the cross-sectional area is filled with mercury due to surface tension mercury does not fill the comers or narrow parts of the cross-section. A cross-section of the solid, composed of a collection of nonporous uniform spheres during filling the void space with mercury is shown in Figure 3.3. Mayer and Stowe [15] have developed the mathematical relationships that describe the penetration of mercury (or other fluids) into the void spaces of such spheres. 

The porous material is immersed in a non-wetting liquid, preferably mercury (Hg). Increasing the pressure in the liquid will cause it to penetrate into the pores of the solid until equilibrium against the surface tension (o) in the smaller and smaller pores is attained. The respective mechanical equilibrium condition leads to the so-called Washburn equation for the limiting pore radius (r) into which mercury at pressure (p) can penetrate [1.1, 1.2, 1.43, 1.44] ... 

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