Porosimetry, mercury intrusion

Mercury intrusion porosimetry is used extensively for the characterization of various aspects of porous media, including porous membranes and powders, and is applicable to pores from 30 A to 900 A in diameter. It is well commercialized. 

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]  

Here a = a (p, T) is the surface tension of mercury which on principle is a pressure and temperature dependent quantity , 0 = 140° is the contact angle between the mercury meniscus and the pore wall. Though the exact value of this parameter normally is unknown, a practical value of 14(T turned out to lead to physically reasonable results in many cases and hence is recommended for practical use [1.44], 

The total volume of mercury VHg(p) penetrating the pores of the material at pressure p leads via equation (1.2) to the integral volume Vp(r) of all pores with radii (p) larger than r p oo, i. e. Vp(r) = VHg(p). By differentiation to the pore radius r this yields the differential pore size distribution of the material. This method is valuable to investigate macro- and mesopores (lUPAC, cp. Sect. 3), but not for micropores, i. e. it is limited to pore radii r 1 nm. 

Mercury intrusion data also may be misleading for porous materials having many inkbottle type pores, cp. middle portion of Fig. 1.1. In such situations high pressures are needed to overcome resistance of mercury to pass the narrow neck of the pore, i. e. the wider portion of the inkbottle pores will not be adequately reflected in the experimentally taken Vhj = Vng (p) curve. However, despite these disadvantages, mercury intrusion experiments often gives valuable information concerning the macro- and mesopores of a sorbent and hence very well may be used for comparative measurements and quality tests of sorbent samples. 

MIP is a method for the direct determination of pore diameters (or a distribution of pore diameters) based on the volume of penetrating mercury as a not-wetting liquid at a certain pressure being applied. 

The principle of measurement is based on the fact that mercury does not wet most substances and thus, it will not penetrate pores by capillary action. Surface tension opposes the entrance of any liquid into pores, provided that the hquid exhibits a contact angle greater than 90° [115,116]. Therefore, external pressure is required to force the liquid (mercury in this case) into the pores of the material. The pressure that has to be applied to force a liquid into a given pore size is given by the Washburn equation. 

It has to be noted that this relation is only valid for pores, possessing cylindrical shape. From Equation 1.1, it gets apparent that under zero pressure, none of the nonwetting liquid will enter the pores of the immersed material. If now the pressure is raised to a certain level, the liquid will penetrate pores possessing radii greater than that calculated from Equation 1.1. Consequently, the higher the pressure that is applied, the smaller the pores that are penetrated by the liquid. 

These raw data, provided by an MIP measurement, enable the calculation of a number of parameters that are necessary and helpful for the interpretation of a porous structure  

Pore dimensions vary with the volume density of nanofibers in the mat and therefore also with the yield and collection time (Hong, Y., et al. 2006), but 

TABLE 5.2 Porosimetric and modulus data illustrating the effect of changing solvent composition on electrospun segmented polyurethane nanofiber mat characteristics 

Percent DMF Fiber Radius (nm) Porosity (%) (s/s,r Young s Modulus (MPa) 

---

In addition, mercury intrusion porosimetry results are shown together with the pore size distribution in Figure 3.7.3(B). The overlay of the two sets of data provides a direct comparison of the two aspects of the pore geometry that are vital to fluid flow in porous media. In short, conventional mercury porosimetry measures the distribution of pore throat sizes. On the other hand, DDIF measures both the pore body and pore throat. The overlay of the two data sets immediately identify which part of the pore space is the pore body and which is the throat, thus obtaining a model of the pore space. In the case of Berea sandstone, it is clear from Figure 3.7.3(B) that the pore space consists of a large cavity of about 85 pm and they are connected via 15-pm channels or throats. 

Pore shape is a characteristic of pore geometry, which is important for fluid flow and especially multi-phase flow. It can be studied by analyzing three-dimensional images of the pore space [2, 3]. Also, long time diffusion coefficient measurements on rocks have been used to argue that the shapes of pores in many rocks are sheetlike and tube-like [16]. It has been shown in a recent study [57] that a combination of DDIF, mercury intrusion porosimetry and a simple analysis of two-dimensional thin-section images provides a characterization of pore shape (described below) from just the geometric properties. 

Although a number of methods are available to characterize the interstitial voids of a solid, the most useful of these is mercury intrusion porosimetry [52], This method is widely used to determine the pore-size distribution of a porous material, and the void size of tablets and compacts. The method is based on the capillary rise phenomenon, in which excess pressure is required to force a nonwetting liquid into a narrow volume. 

Measurements of particle porosity are a valuable supplement to studies of specific surface area, and such data are particularly useful in the evaluation of materials used in direct compression processes. For example, both micromeritic properties were measured for several different types of cellulosic-type excipients [53]. Surface areas by the B.E.T. method were used to evaluate all types of pore structures, while the method of mercury intrusion porosimetry used could not detect pores smaller than 10 nm. The data permitted a ready differentiation between the intraparticle pore structure of microcrystalline and agglomerated cellulose powders. 

One of the most popular methods to measure the pore size distribution in diffusion layers is mercury intrusion porosimetry (MIP) this technique is... 

Fig. 5.16 Mercury intrusion porosimetry curves of C3S pastes showing differences in capillary porosity distribution.

Figure 2. Cumulative pore volume vs. pore radius for AC-ref, SC-100 and SC-155 Mercury intrusion porosimetry.

Pore size distributions are often determined by the technique of mercury intrusion porosimetry. The volume of mercury (contact angle c. 140° with most solids) which can be forced into the pores of the solid is measured as a function of pressure. The pore size distribution is calculated in accordance with the equation for the pressure difference across a curved liquid interface,... 

WINSLOW, D.N., Advances in experimental techniques for mercury intrusion porosimetry , in reference 9 13, 259-286 (1984)... 

Similarly, we polymerized the same mixture used for the preparation of capillary columns in glass vials and used the product for mercury intrusion porosimetry. Since we found that a strong correlation exists between the "dry" porous properties of the monoliths and their chromatographic performance, even dry porosity measurements may be used to tailor column performance. 

C.M. Nielsen-Marsh, R.E.M. Hedges, Bone porosity and the use of mercury intrusion porosimetry in bone diagenesis studies, Archaeometry 41 (1999) 165-174. 

Table 4. Porosities and pore size distributions derived from mercury intrusion porosimetry.

V. Maquet, S. Blacher, R. Pirard, J.-P. Pirard, M. N. Vyakamum, and R. Jerome, Preparation of macroporous biodegradable poly(L-lactic-co-e-caprolactone) foams and characterization by mercury intrusion porosimetry, image analysis and impedance spectroscopy, J. Biomed. Mater. Res. 66A, 199-213 (2003). 

A. B. Abell, K. L. Willis, and D. A. Lange, Mercury intrusion porosimetry and image analysis of cement-based materials, J. Colloid Interf. Anal. 211, 39-44 (1999). 

The sample was dried completely and a second water penetration experiment was carried out. The maximum uptake of water occurred once again after 24 h of soaking however, the slope of the profile was less pronounced, as illustrated in Fig. 17. Furthermore, the peak intensity was lower in the second experiment, which is evidence of a smaller pore size distribution in the sample, suggesting that some degree of rehydration took place in this sample during the course of the first water penetration study. This result was further verified by mercury intrusion porosimetry. 

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... 

Incorporation of the measured contact angle in mercury intrusion porosimetry data is essential for an accurate determination of the pore size distribution. Both the advancing and static angle methods are suitable to carry out this measurement, leading to very similar results. For most oxidic materials and supported oxides, the contact angle is 140° and incorporation of the actual contact angle is less critical in the pore size determination. However, important deviations are observed in carbon and cement-like materials, with contact angles of > 150° and < 130°, respectively. This has been shown by comparison of the pore size distribution obtained from mercury porosimetry and N2 adsorption measurements. 

In this study mercury intrusion porosimetry (MIP) analyses were employed to determine the pore size distribution and pore volume over the range of approximately 100 pm down to 7.5 nm diameter, utilising CE Instruments Pascal 140/240 apparatus, on samples previously dried overnight at 150°C. The pressure/volume data were analysed by use of the Washburn Equation [14] assuming a cylindrical nonintersecting pore model and taking the mercury contact angle as 141° and surface tension as 484 mN m [10]. For the monolith... 

Mercury-intrusion porosimetry and a cantilever beam-proximity transducer balance were used to monitor... 

In addition to surface fractal dimension ( Mercury Intrusion Porosimetry, under Boundary and Surface Fractal Dimensions ), this method can also be employed to determine mass fractal dimension of porous particles. Once the relative density of the particle at different pore volume, p, is obtained, then Dm can be deduced according to Eq. (20) ... 

---