Porosity characteristics

The elution volume, F/, and therefore the partition coefficient, is a function of the size of solute molecule, ie, hydrodynamic radius, and the porosity characteristics of the size-exclusion media. A protein of higher molecular weight is not necessarily larger than one of lower molecular weight. The hydrodynamic radii can be similar, as shown in Table 4 for ovalbumin and a-lactalbumin. The molecular weights of these proteins differ by 317% their radii differ by only 121% (53). 

High porosity carbons ranging from typically microporous solids of narrow pore size distribution to materials with over 30% of mesopore contribution were produced by the treatment of various polymeric-type (coal) and carbonaceous (mesophase, semi-cokes, commercial active carbon) precursors with an excess of KOH. The effects related to parent material nature, KOH/precursor ratio and reaction temperature and time on the porosity characteristics and surface chemistry is described. The results are discussed in terms of suitability of produced carbons as an electrode material in electric double-layer capacitors. 

KOH-activated carbons, porosity characteristics, chemical structure, electrochemical capacitors. 

The treatment of semi-coke or mesophase (3 1 mixture) for 2 h at 600°C gives a material of pore volume VT about 0.7 cm3/g and surface area Sbet about 1700 m2/g which is typically microporous with rather narrow micropores (average size LD below 1.2 nm). Activated carbons produced within the temperature range of 700-800°C have fairly similar porosity characteristics, VT about 1 cm3/g and Sbet near 2500 m2/g. It is interesting to note that within the wide temperature range of 600-800°C the bum-off is at a reasonably low level of 20-23 wt%. 

Porosity characteristics were determined by N2 adsorption-desorption at 77 K (Dubinin method) with a Thermoquest Sorptomatic 1990. Powder samples were outgassed (10 4 Torr) and heated to 450°C before each test. 

It is also important to take into account that one of the main issues with the carbon fiber paper or cloth used as the DL is the uncontrolled variation in porosity (and other localized properties) of these manufactured conventional diffusion layers that is, the porosity characteristics between carbon papers are not repeatable [57]. These materials are difficult to improve because only average pore sizes and volume densities can be measured and much of the development has been based on empirical parameters. Thus, extensive work has focused on optimizing the MPL in order to reduce the differences within carbon paper fiber and cloth diffusion layers. 

Porosity characteristics also influence the degradation rate of blends containing intact starch grains. Amylase removal of starch from these films was not highly correlated with starch content, since films whose starch content was above possible "percolation thresholds" (6) were degraded at very different rates when starch content was not very different (Table I). 

Synthesis conditions and porosity characteristics specific surface area (Sbet), pore volume (V)... 

The results observed with a representative set of test-liquids are recorded in Fig. 11. The square of the correlation coefficient (r2), determined by linear regression analysis, was in every case >0.99 when the slope was >1, but r2 for those with slopes less than 1 were sometimes less than 0.98, owing to decrease in accuracy as a result of decrease in the difference (S — v) where v is the porosity characteristic of the respective composite film samples. In such cases the determination was repeated to obtain sets of averaged values, which typically exhibited r2 > 0.99. 

Many applications of AFM to pillared clays or zeolites have not specifically addressed the porosity characteristics, but rather the occurrence of adsorbed surface Al species in, e.g., pillared montmorillonite [41], or the crystal growth processes, adsorption on porous surfaces and the surface structure of natural zeolites [42]. Sugiyama et al. [43] succeeded to reveal the ordered pore structure of the (001) surface of mordenite after removal of impurities that clogged the pores. The authors indicated that resolution in AFM imaging of zeolites is significantly affected by the magnitude of the periodical corrugation on the crystal surface, so that if the surface contains deep pores only the pore structure, but not the atomic structure, can be resolved. 

TABLE 2 Surface Porosity Characteristics of the Active Carbons Investigated... 

Porosity is greatly determined by the particle shape. Ball shape allows for the minimum void volume in an irregular filling. However, the layer porosity characteristics obtained for the simplest case of ball-shaped evenly-sized particles cannot be applied for determining the type of the filling, or the shape of the channels formed by the particles and, consequently, for estimating the layer resistance... 

Mesoporous carbon aerogels [138-142] were produced by Pekala and co-workas by pyrotysis of organic aerogels of resorcinol and formaldehyde. Recently, their porosity characteristics [143] and applications in catalysis [144] have tsrai revkwed. 

Table 13.15. Measuring the porosity characteristics of oak samples (porosity estimated by image analysis) (Vivas, 1997)...

The choice of solvent is critical for achieving good imprints and for successful rebinding isotherms or chromatography results. One must use a solvent that dissolves all of the components of the prepolymerization mixture, allows for optimal template-monomer interaction, and contributes to good porosity characteristics in the final MIP. 

---