Mercury porosimetry limitations

Since in practice the lower limit of mercury porosimetry is around 35 A, and the upper limit of the gas adsorption method is in the region 100-200 A (cf. p. 133) the two methods need to be used in conjunction if the complete curve of total pore volume against pore radius is to be obtained. 

Mercury porosimetry has somewhat the same constraints at the narrow pore end of its range, in that the same questions arise regarding the constancy of surface tension and wetting angle for mercury as exist for an adsorbate. Consequently, both methods have nearly the same lower limit which is about 18 A pore radius for mercury intrusion (e.g. bOOOOpsia). However, at the wide-pore end porosimetry does not have the limitation of the Kelvin equation and for example, at 1.0 psia pore volumes can be measured in pores of 107 micrometer radius or 1.07 x 10 A. 

The distribution of pore sizes can be obtained from both mercury porosimetry and capillary flow porometry. These distributions are only representations of the actual scaffold structure reflecting the limitations of the underlying physics behind each technique. For this reason it is very difficult to compare pore size distributions for complex structures, such as particulate-leached tissue scaffolds. 

For description of textural properties of carbonaceous adsorbents, adsorption/desorption isotherms of vapours and gases in static conditions as well as mercury porosimetry are used. The latter method often leads to destruction of porous structure of investigated materials while the usage of the former one is affected by the specific properties of molecular sieves described above. Taking into account these limitations, in this work the authors have made an attempt of determination of porous structure of carbon molecular sieves with the used of the pycnometric technique. 

The critically important pore size and its distribution can be determined by a host of measurement techniques. The bubble point test is used primarily for delecting any defects or hairline cracks and for estimating the average pore size. Traditionally mercury porosimetry and nitrogen adsorption/desorpiion have been the workhorse for determining the pore size distribution of a porous membrane. They arc, however, usually limited to a pore diameter range of >3 nm and 1.5-100 nm, respectively. Determination... 

The porous structure of active carbons can be characterized by various techniques adsorption of gases (Ni, Ar, Kr, CO ) [5.39] or vapors (benzene, water) [5,39] by static (volumetric or gravimetric) or dynamic methods [39] adsorption from liquid solutions of solutes with a limited solubility and of solutes that are completely miscible with the solvent in all proportions [39] gas chromatography [40] immersion calorimetry [3,41J flow microcalorimetry [42] temperature-programmed desorption [43] mercury porosimetry [36,41] transmission electron microscopy (TEM) [44] and scanning electron microscopy (SEM) [44] small-angle x-ray scattering (SAXS) [44] x-ray diffraction (XRD) [44]. 

Mercury porosimetry is performed nearly exclusively on automatic commercial instruments that differ mainly in the highest operative pressure, which determines the size of smallest attainable pores. The highest pressure is limited by the uncertainty about the validity of the Washburn equation, which forms the basis of data evaluation. In pores with sizes similar to the mercury atom the assumption that physical properties of liquid mercury (surface tension, contact angle) are equal to bulk properties is, probably, not fully substantiated. For this reason the up-to-date instruments work with pressures up to 2000 - 4000 atm, only. 

In parallel with mercury porosimetry in which a non wetting liquid is used, we can mention the suction porosimetry in which a wetting liquid like water (0 <0 < Jc/2) is held within the porous solid [5]. In this case the Laplace equation predicts that it will experience a reduced hydrostatic pressure, inversely proportional to the radius of pores in which menisci are formed. The lower limit of pore size accessible to this technique is around a few tens of microns. 

Froment discusses pore network influences in deactivation. He concludes that, "Evidently, the parameters associated with the pore and network structure should be determined from independent physical measurements adsorption, mercury porosimetry, electron microscopy..." Unfortunately, no experimentally based studies have been published that have employed one or a combination of techniques to determine the pore network structure and its changes during deactivation. The few experimental determinations of pore structure are limited to determination of pore dimensions usually from the intrusion data in Hg porosimetry or the desorption data from nitrogen physical sorption (often incorrectly referred to as BET analyses). 

Several of the more popular models for deactivation involve "pore mouth plugging" wherein the transport limiting constrictions within the pore network are selectively reduced in dimension. If one realizes that intrusion mercury porosimetry and desorption measurements specifically characterize the constriction ("throat") dimensions then decreases in these dimensions would be greater than the changes found in the retraction porosimetry or in the desorption (which measure the opening dimensions). To understand the changes in network structure on the deactivation process it seems necessary to measure and analyze each aspect of the porous structure. 

The average pore size of modem analytical HPLC packings is 100 A range 60-120 A. Figure 5 shows the internal surface area versus pore diameter for four commercial 5 im silicas with pore sizes ranging from 60 to 120 A as determined by mercury porosimetry (33). This technique can measure pore diameters down to 30 A, which is the upper limit of the size range for micropores. Note that the data in Figure 5 are biased toward the smallest pore sizes, which... 

In pores of size comparable to that of the nitrogen molecule, the low kinetic energy of the gas (77 K) may limit their accessibility. In such cases, adsorption of CO2 at 273 K can be used to reveal supermicroporosity. Narrow porosity also can be detected by using probe molecules smaller than nitrogen (often combined with immersion calorimetry) or, for example, by small angle x-ray scattering (SAXS), while several methods, very often mercury porosimetry can be chosen for materials with wider porosity (Fig. 6). 

Mercury porosimetry can distort the pore size owing to the elastic nature of the carbon-PTFE composite also, for thin electrodes and for electrodes consisting of two or three layers of different porosity, this method is of limited application (Abell et al., 1999)... 

The dimensions of pores may differ from millimeters to nanometers. The most universal method for the determination of pore dimensions is a mercury porosimetry ASTM C-493-98 [34]. It gives the pore size distribution. Sample one-mode, bi-mode, and nonmode pore sizes appear in Fig. 1.4. Forty to fifty years ago, limited attention was paid to the sizes of pores. The main criterion for the optimization of refractory properties was low porosity. Now people from research and industry have come to the conclusion that in many applications, the pore size should be taken into account. Of course, the Andreasen equation [35] for the optimization of grain size composition, with the aim to diminish the porosity of the material, as well as variants of the Andreasen equation [36] are valid in R D practice, yet the pore size distribution (Fig. 1.5) is being taken into account more and more frequently. 

---