Porosity, Porosimetry, Structure

Porosity, Porosimetry, Structure. - The increase in experimental sensitivity by hyperpolarized xenon, is remarkable. A number of recent developments in Xe NMR spectroscopy were reviewed with direct applications to the study of mesopore space in solids (37 references). Experiments illustrated include the rapid characterization of the void space in porous solids, including the in situ study of processes such as diffusion and hydration, and imaging with chemical shift resolution. 

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. 

From the mercury porosimetry data, porosity can be calculated. A higher porosity means a more open pore structure, thus generally providing a higher permeability of the membrane. Porous inorganic membranes typically show a porosity of 20 to 60% in the separative layer. The porous support layers may have higher porosities. 

Since the porosity of carbons is responsible for their adsorption properties, the analysis of the different types of pores (size and shape), as well as the PSD, is very important to foresee the behavior of these porous solids in final applications. We can state that the complete characterization of the porous carbons is complex and needs a combination of techniques, due to the heterogeneity in the chemistry and structure of these materials. There exist several techniques for the analysis of the porous texture, from which we can underline the physical adsorption of gases, mercury porosimetry, small angle scattering (SAS) (either neutrons—SANS or x-rays—SAXS), transmission and scanning electron microscopy (TEM and SEM), scanning tunnel microscopy, immersion calorimetry, etc. 

Scaffold porosity and information on the pore size distribution can be obtained from intrusion techniques. The most commonly used methods are mercury porosimetry and capillary flow porometry. In mercury porosimetry the pressure required to fill a tissue scaffold with non-wetting mercury is monitored over a set period of time. Higher pressures are required to fill small pores than large pores a fact that can be exploited using the Washburn equation13 to extract structural information where D is the diameter of the pore at a particular differential... 

Porosity and pore structure are properties that control diffitsive transport, selective reaction, and sorption-based separations of gases in adsorbents and catalysts [1,2]. Sorption porosimetry may be used to characterize the porosity of both mesoporous and microporous solids. The pore size distribution F H) is obtained from the experimental isotherm /[/ )... 

The case of mEl is somewhat more surprising, whilst mercury porosimetry and thermoporometry detect a peak just above 10 nm, the gas adsorption results show no indication of any porosity in this range. This may be due to a non-rigid structure that can not be detected using the former techniques. 

The openness (e.g., volume fraction) and the nature of the pores affect the permeability and permselectivity of porous inorganic membranes. Porosity data can be derived from mercury porosimetry information. Membranes with higher porosities possess more open porous structure, thus generally leading to higher permeation rates for the same pore size. Porous inorganic membranes, particularly ceramic membranes, have a porosity... 

The porous supports, in disc or tubular shaped form, were produced commercially (Velterop Company, Netherlands). The discs (25 mm in diameter and 2 mm in thickness) were available with different macropore sizes (0.08, 0.15, 2 and 9 pm). These macropores, which were formed between the sintered alumina grains are shown typically for a disc in Figure 1. The tubes with an outer diameter of 14 mm and a wall thickness of 3 mm were manufactured with pores of 2.5 pm and 9 pm. The macropore structure of these different types of supports was analysed by mercury porosimetry. Changes in the porosity which occurred after the hydrothermal treatment were also monitored. Figure 2 shows the highly uniform pore structure of a series of supports with different nominal pore sizes. 

For determination of the overall porosity, pycnometric methods are recommended that use imbibition of the material in a light inert gas and mercury. Mercury porosimetry is the method of choice for assessing macro- and meso-pores. The use of intrusion and extrusion measurements is necessary to understand more complex pore structures. 

As new membranes are developed, methods for characterization of these new materials are needed. Sarada et al. (34) describe techniques for measuring the thickness of and characterizing the structure of thin microporous polypropylene films commonly used as liquid membrane supports. Methods for measuring pore size distribution, porosity, and pore shape were reviewed. The authors employed transmission and scanning electron microscopy to map the three-dimensional pore structure of polypropylene films produced by stretching extended polypropylene. Although Sarada et al. discuss only the application of these characterization techniques to polypropylene membranes, the methods could be extended to other microporous polymer films. Chaiko and Osseo-Asare (25) describe the measurement of pore size distributions for microporous polypropylene liquid membrane supports using mercury intrusion porosimetry. 

Mercury porosimetry is the most widely used technique for characterizing macroporosity in solids this technique covers a wide range of pore sizes, which also includes the majority of mesopores. Mercury porosimetry is based on the penetration of mercury, under pressure, into porosity. As mercury does not wet the carhon surface, pressui e is required to force the mercury into the structure. The relationship between pore radius, r, and mercury intrusion follows the so-called Washburn equation, already suggested in 1921 ... 

The support is a microporous PVC-silica sheet having a porosity in the 70-80% range. The pore size as determined by mercury intrusion porosimetry is in the 0.2 u to 2.0 urn range. The support is extremely hydrophilic, has a negative charge, and a surface area of 80 m /g. The material is non-compressible under normal conditions, is steam sterilizable, and has a low dry density of 0.45 g/cm. The microporous support has received FDA approval for direct food contact. The tortuosity of the pore structure requires that the substrate make intimate contact with the active enzyme as it passes through the support material. The active sites are attributed to the silica contained within the porous matrix which allows the addition of organic functionality. 

Pore structure of freeze dried PHEMA seafFolds was characterized on a mereuiy poro-simeter Pascal 140 and 440 (Thermo Finigan, Rodano, Italy). It woiks in two pressure intervals, 0-400 kPa and 1-400 MPa, allowing determination of meso- (2-50 nm), macro- (50-1000 nm) and small snperpores (1-116 pm). The pore volnme and most freqnent pore diameter were calculated under the assumption of a cyUndrical pore model by the PASCAL program. It employed Washburn s equation describing capillary flow in porous materials [33]. The volumes of bottle and spherical pores were evaluated as the difference between the end values on the volume/pressure curve. Porosity was calculated according to Equation 2, where cumulative pore volume (meso-, macro- and small supeipores) from mercury porosimetry was used for R. 

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