Solid silica mean pore diameter

This is liquid-solid chromatography in which the surface of microparticulate silica or other adsorbent constitutes the polar stationary phase. The silica particles are characterized by their shape (irregular or spherical), size and size distribution, and pore structure (mean pore diameter,... 

Figure 1 shows the pore size distribution of primary supports and the immobilized derivatives in silica matrix on an incremental (derivative) basis, dV/d Log (D), to highlight the differences among the immobilized derivatives in silica gels. The computation of pore size distribution, from gas adsorption, was based on the BJH method using the desorption branch. From these plots it was observed that all solid samples (PS, SPS, ADS, CB1, CB2, EN1, and EN2) exhibit a unimodal distribution of pores, and the smaller mean pore diameters were observed with the samples PS, ADS, CB1, and CB2 (Table 2). 

The parameters of the pore structure, such as surface area, pore volume, and mean pore diameter, can generally be used for a formal description of the porous systems, irrespective of their chemical composition and their origin, and for a more detailed study of the pore formation mechanism, the geometric aspects of pore structure are important. This picture, however, oversimplifies the situation because it provides a pore uniformity that is far from reality. Thorough attempts have been made to achieve the mathematical description of porous matter. Researchers discussed the cause of porosity in various materials and concluded that there are two main types of material based on pore structure that can be classified as corpuscular and spongy systems. In the case of the silica matrices obtained with TEOS and other precursors, the porous structure seems to be of the corpuscular type, in which the pores consist of the interstices between discrete particles of the solid material. In such a system, the pore structure depends on the pores mutual arrangements, and the dimensions of the pores are controlled by the size of the interparticle volumes (1). 

In Fig. 1 the diagrams of log Vs vs 1/T relationships for the column packings composed of n-octadecanol and silica gel (a specific surface area 27.3 and a mean pore diameter 125 nm) are presented. The retention volumes were determined with n-octane. According to Serpinet [26] Vs is the net retention volume related to 1 g of solid support in the column. Vs was chosen because the investigated column packings often contained a very small amount of stationary phase and because Vs is closely related to Vg, the retention volume generated by Ig of liquid stationary phase, due only to the partition of solute between the gas and liquid stationary phases... 

The morphologic characterization of the immobilized enzyme is important to correlate the biocatalyst performance with porous structure parameters. BET analysis, which is usually based on N2 isothermal adsorption at 77 K, allows determining the solid-specific surface area, total pore volume, pore size distribution, and mean pore diameter. It is not recommended for solids with a low specific surface area (<5 m g ). Table 2 shows the specific smface area, mean pore diameter, and total pore volume determined by BET for the pure sol-gel silica matrix having TEOS as the precursor and the same matrix with the encapsulated CGTase. 

As temperature is increased from 20 °C to 60 °C, pore volume and mean radius of silica and silica-alumina gels are increased, while pore surface remains quite constant. This can be explained from the increase of the diameter of elementary particles of silica (silica sol) induced by the temperature increase [1]. As a consequence, the mean pore radius increases (pores are cavities in the gel network and between the particles), the number of elementary particles decreases (solid content is constant) resulting to the increasing of the pore volume. The weak variations of pore surface could be due to compensating effects. 

Silica supports in HPLC have mesopores and macropores. The presence of micropores leads to slow desorption kinetics and a lower loading capacity than found in supports with mesopores. The pore system can be built up by a unidimensional, two-dimensional, and network system of interconnected and intersecting pores, respectively MCM-41, as a member of the family of M41S materials, for instance, exhibits a unidimensional pore system of hexagonally arranged channels with a pore diameter ranging from 2 to 6 nm (Fig. 1) [10]. As the pore walls of MCM-41 are made of amorphous silica, the material is not considered to be a crystalline solid in its strict physical meaning but is rather termed an ordered mesoporous silica [11]. 

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