Pore size distribution measurement

For a monodisperse system this result is in good agreement with the values obtained from pore size distribution measurements, but it can be significantly in error if one is dealing with a bimodal pore size distribution (see Section 6.4.2). 

The pore structure of a solid can contribute to the disintegration, dissolution, adsorption, and diffusion of a drug material [26,27]. Because of this, porosity and pore size distribution measurements have been used extensively to study tablets [28-30], granules [31,32], and excipients [33]. The following classification system of pore sizes has been developed based on the average pore radii [6] ... 

The surface area of PSs can be measured by means of quantitative electron microscopy (EM) [49-51], knowing the sizes of particles and pores [51,52], wetting heat [7,53] measurements, etc. But, the most universal methods are based on adsorption measurements [51,53-55], corresponding to the traditional methods of pore size distribution measurements in the range 0.3 to 100 nm. 

Laboratory data collected over honeycomb catalyst samples of various lengths and under a variety of experimental conditions were described satisfactorily by the model on a purely predictive basis. Indeed, the effective diffusivities of NO and NH3 were estimated from the pore size distribution measurements and the intrinsic rate parameters were obtained from independent kinetic data collected over the same catalyst ground to very fine particles [27], so that the model did not include any adaptive parameters. 

Figure 1. Core processing (scale in cm). Samples taken for area and pore-size distribution measurements (A), petrographic thin sections (Bj, B2, and B3), sorption experiments (Ci and C2), and permeability measurements (D).

The sintered compacts had a porosity of 32%. No difference in porosity was found between the AKP-30 and AKP-15 supports. Their pore-size distributions, measured by mercury poro-simetry, are given in Figure 1. The mean pore-radii were found to be 40 and 80 nm respectively. 

A Fisons Sorptomatic 1900 instrument was used to carry out pore size distribution measurements. The characterization techniques included the determination of nitrogen... 

The mesoporosity of these materials has been established by BET measurements and gas adsorption experiments. As the chain length of the surfactant was increased from Cs to G 5, the amount of adsorbed benzene was increased, indicating that there was a relationship between the size of the surfactant and the amount of gas adsorbent taken up by the MCM-41 material. In terms of a comparison to zeolite materials, experiments were done at 60 torr pressure and at 25< C. llie US-Y zeolite sample had an uptake that was about 4 times less than that of MCM-41. The above mentioned MCM-41 materials all show pore size distributions with broad bands centered around 40 A. The pore size distribution measurements are a true indication of the size of the pores and can be used to verify the existence of mesopores. Further evidence of mesoporosity comes from X-ray powder difraction experiments which were done to determine the crystallinity of these materials. The position of the (100) reflection was found to correlate with the amount of uptake by the different materials, or in therwords, with the mesoporosity of these systems. Pores of the MCM-41 materials were shown to form in a hexagonal shape by using high resolution transmission electron microscopy data. 10... 

As a result of the drastic chemical and structural changes of the bulk material, the BET surface area of the amorphous precursor increases from 0.025 to about 60 m2/g depending on the oxidation conditions. Pore-size distribution measurements using nitrogen capillary condensation indicated that the material contained mainly pores of 2-4 nm size besides some larger pores. 

Size can refer to volume, area, or length, and therefore pore-size distribution may be defined in terms of any one of these properties. In practice, the definition of size adopted is highly dependent upon the method of measurement. For example, the area size distribution of pores is often measured by image analysis of soil thin sections, while water retention data are usually interpreted in terms of the distribution of pore diameters (Bullock Thomasson, 1979). For consistency with the definition of the Peclet number, we have chosen to define size in terms of length, L. Dullien (1991) has proposed the following interrelationships between the different definitions of size L = VIS in three-dimensions or L=AJP in two-dimensions, where V is volume, S is surface area, A is cross-sectional area and P is perimeter. These relations can be used to compare pore-size distributions measured using different methods. 

Surface area and pore size distribution measurements were carried out by nitrogen adsorption at 77 K on Carlo Erba Sorptometer(1800). 

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. 

These curves show that the pore size distributions measured by the two techniques are different, although the total pore volumes are similar. This observation suggests that although the two techniques are measuring the same pore structure, either the models used in the interpretation of the data are inappropriate and do not adequately describe the pore structure or the structure is modified during analysis. Earlier workers (I, 2)... 

N2 adsorption/desorption isotherms for BET surface area and pore size distribution measurements were collected at liquid nitrogen temperature using a Micromeretics ASAP 2010 apparatus. Before the measurement, approx. 50 mg of the sample was dehydrated under vacuum at 573 K overnight. Pore diameters were calculated using the BJH method on the adsorption branch of the isotherm. X-ray diffraction (XRD) powder patterns were recorded between 1 and 10° (or 3 and 80°) (20) on a Bruker (Siemens) D 5005 diffractometer using CuKa2 radiation with steps of 0.02° and 10 s (or 1 s) per step. 

Fig. 2. Pore size distribution measurements of Davisil 654 before and after steam treatments. (Average pore diameter of 250 A for untreated and 290 A for steam-treated sample.)...

To interpret the results from pore size distribution measurements, it is usually assumed that the pores are tubular. However, this is far from the truth. The nature of the pores depends to some degree on the method used to synthesize the particles, but in all cases they form an interconnected network of irregularshaped channels. For most preparations the nature of the pore network is much like the interstitial space formed in a packed bed. Electron microscopy is used to examine the internal structure of HPLC packings. Except for some special cases, the internal structure is homogeneous throughout the particle. 

The pore size distribution in cement paste can be also determined from the Defay s relation. Unking the radins of meniscus and the freezing point of water in the capiUary [61]. The information dealing with the continuity of pores system can be derived from the hysteresis loop between the heating and cooling. However, because of many assirmptions, the results are only approximate the most accurate relate to the larger pores. In Fig. 5.29 the pore size distribution measured with this method, and with merciuy porosimetry is shown [61]. 

Fig. 5.4. Log-normal pore size distribution measured from AFM images. Reprinted from [20]. Copyright 1998, with kind permission from Elsevier...

The experimental materials and procedures are described in detail by Kaiser et al. (7), including the characterization of the activated caibon fabrics used such as BET aurface area and pore size distribution measurements by well established methods (2, 5), the composition of the carrier of the adsorption solution, and of the adsorption test procedure.. The key difference is the use of of a 70 ppm solution of chemical agent Bis (2-chloroethyl) sulfide (HD) in HFE-7100 as a challenge solution instead of a 70 ppm solution of CEES in HFE-7100. The HD was obtained from the US Army Edgewood Chemical and Biological Center s... 

Figure 4.7 Pore-size distribution measured by nitrogen adsorption of electrodes after various operating times at a current density of lOOmAcm". Source DLR.

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