Porosity interstitial

For diffusion between soil and sediment particles, n represents the interstitial porosity. For diffusion within soil and sediment particles, n represents the in-traparticle porosity. Values of m close to 1 are common for diffusion in porous media [29-31]. However, in low porosity materials this value can increase [32]. A more thorough treatment of this topic can be found in Grathwohl [33]. 

When monolithic sihca columns are prepared in a fused sihca capillary, the silica network structure can be bonded to the tube wall. They can be used as a column directly after preparation, or as a reversed-phase adsorbent after alkyl or some other type of modihcation. The porosity of monolithic silica columns is much greater than that of a particle-packed column. A major difference is seen in interstitial porosities 65-70% for monolithic sihca prepared in a mold, and higher than 80% for those prepared in a capillary, compared to 40% for a particle-packed column. A comparison of the separations of cytochrome triptic digest on packed and monolithic colums is shown in Figure 3-23. The separations are nearly identical except that on monolithic column it is ten times faster. Figure 3-24 shows the dependence of the backpressure generated on the system as a function of the flow rate for packed column and a set of different monolithic columns. The slope on all monohthic columns is the same, and it is approximately live times lower than that on a packed column. Additional information on fast FIPLC on monolithic columns is given in Chapter 17. 

The constant Bo characterises the permeability of the column, which depends on the interstitial porosity of the column, c, (with regularly packed columns, e, is usually close to 0.40) and increases with the second power of the mean particle diameter, dp. From the Koz.eny-Carman equation [16.171 it follows ... 

The strength of pellets depends, to a great extent, on the physical forces that bond the primary particles together and their resulting interstitial porosity. The size of the gap at the perimeter of the disc is adjusted based on how much evaporation is desired. In some types of equipment and for certain operations in others, this air... 

When considering a packed bed the interparticle or interstitial porosity amounts to approximately 40%. The average diameter of these interstitial voids calculates for a fairly regular packing to 0.4 times of the particle diameter, e.g. a regularly packed bed with 15 p,m particle creates interstitial pores of 6 pm. A bimodal particle size distribution of silicas employed for preparative columns can often be found, e.g. a major distribution around 15 pm and a minor peak at 4 pm. These small particles are thought to occupy the interstices between the 15 pm diameter particles and thus to stabilize the bed. 

Interparticle or interstitial porosity, b - Fraction of the column volume outside the pores, occupied by the bulk mobile phase and available for the mobile phase flow through the column. Typically around 40-41% in analytical columns, this fraction is somewhat less in compressed beds. 

As we know from experience, the backpressure of a column packed with small particles is larger than the backpressure of a column packed with larger particles. The specific permeability depends on the particle size d, and the interstitial porosity e, of the packed M. The relationship is known as the Kozeny-Carman equation (16,17) ... 

Normally, the interstitial porosity of a packed bed is around 0.40. With this value, the constant coefficient is approximately and the specific permeability simply becomes... 

The external surface of a packing increases in inverse proportion to the particle size. For a normal packed bed with an interstitial porosity of 40%, the formula for the surface area per volume is 3.6/dp. Similarly, the internal surface of a particle increases with decreasing pore size. Typical surface areas are lOOmVg for a pore size of 30nm, 300mVg for a pore size of lOnm and SOOmVg for a 6-nm pore size. 

Engelhardt reported 0.42 for the interstitial porosity of solid glass beads and 0.80-0.88 for the mobile-phase porosity of totally porous supports (57). 

Figure 6.5 Increase in packing density achieved by (a) filling the interstices between large spheres with small spheres and (b) replacing small spheres and their interstitial porosity by large spheres.

Asymmetric hollow fiber membranes of polysulfone, polyethersulfone, and polyphenylsulfone can be prepared by phase inversion spinning solvent/nonsolvent dopes, i.e., N-methylpyrrolidone/formamide. These asymmetric hollow fiber membranes possess a microscopically observable skin supported by a porous open cellular network. The walls of the open cells of the matrix are composed of arrays of interconnected spherical micelles. With increasing proximity to the outer surface, the packing density of the spherical micelles increase with a concomitant decline in interstitial porosity. At the outer surface layers, the packing of the micelles becomes... 

In the following sections, selected characterization tests based on several common anticipated uses for nanofiber materials are presented. The assessment of interstitial porosity properties, analysis of fiber dimension by microscopy, and mechanical integrity of fiber mats are the more important metrics in several application areas. Some of these will eventually be adopted as standard test methods in the field — both the American Sodely for Testing Materials (ASTM) and the International Standards Organization (ISO) have initiated efforts to standardize the nomenclature and test methods relating to nanomaterials, including nanofibers. Here, the reader wiU be afforded a flavor of the rather limited research literature on single-fiber measurements. 

The terms pore and porosity are used in this chapter to mean the interstitial porosity associated with a fiber mat, where the pores are defined by a set of nanofibers lying at the periphery of an interstitial volume. This is contrasted with fiber porosity, which refers to porous features that occur on the surface or within individual nanofibers (dealt with in detail in Chapter 9). 

Fractional void volume, or total interstitial porosity H, is defined in terms of the apparent density of the fiber mat, pmat (g/cm ), and bulk density of the polymer, Pp (g/cm ), of which it is made ... 

The terms porosity and pore are used in this chapter to mean the features on the nanofiher itself as opposed to the interstitial porosity of mats dealt with in Chapter 5. Note that porous nanofibers (with surface pores on the fiber) are electrospun into porous mats with interstitial pores as well. 

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