Support critical pore diameter

The microstructure of a sol, in terms of the nature and concentration of sol particles, significantly affect a membrane s properties such as average pore diameter and pore size distribution. Other critical parameters are the temperature and rate of evaporation used in the drying and calcination steps. Even though it is economical to obtain the final product form quickly, too rapid a drying rate forms cracks in the membrane. Crack formation is also dependent on thickness of the membrane. The work of Cini et al. [49], who prepared supported microporous y-A C membranes as catalyst supports, illustrates many of these effects. 

Similarly, Sano et al. [1994] added colloidal silica to a stirred solution of tetrapropylammonium bromide and sodium hydroxide to synthesize a hydrogel on a stainless steel or alumina support with a mean pore diameter of 0.5 to 2 pm. The composite membrane is then dried and heat treated at 500 C for 20 hours to remove the organic amine occluded in the zeolite framework. The silicalite membranes thus obtained are claimed to be free of cracks and pores between grains, thus making the membranes suitable for more demanding applications such as separation of ethanol/water mixtures where the compound molecules are both small. The step of calcination is critical for synthesizing membranes with a high permselectivity. 

Supports for SEC of proteins are designed to be neutral and very hydrophilic to avoid disruption of protein structure and interaction of the solutes with the support by ionic or hydrophobic mechanisms. The base matrix can be either silica or polymer efforts are made to totally mask its properties with a carbohydratelike stationary phase. The pore structure is critical to successful SEC. Not only must the total pore volume (F,) be adequate for separation, the pore diameter must be consistent and nearly homogeneous for attainment of maximum resolution between molecules with relatively small differences in molecular size (radius of gyration or molecular weight). A twofold difference in size is usually required for separation by SEC. Pore homogeneity can be assessed from the slope of the calibration curve of the logarithm of the molecular weight versus the retention time or the partition coefficient (Kd) = (F - Fq)/F , where F is... 

The critical parameter governing the initial layer formation was found to be the ratio of the particle diameter (in the colloidal suspension) to the pore diameter of the support. Given a certain colloidal suspension (characterised by its alumina concentration, kind and concentration of peptising acid used and ageing time) gel films could be formed on a support with pores below a critical diameter as shown in Table 8.1 [2,3]. 

Maximum allowable (critical) alumina and titania supported membrane thickness under given conditions on an a-alumina support with a pore diameter of about 0.2 pm... 

As discussed in Section 1.4.2.1, the critical condensation pressure in mesopores as a function of pore radius is described by the Kelvin equation. Capillary condensation always follows after multilayer adsorption, and is therefore responsible for the second upwards trend in the S-shaped Type II or IV isotherms (Fig. 1.14). If it can be completed, i.e. all pores are filled below a relative pressure of 1, the isotherm reaches a plateau as in Type IV (mesoporous polymer support). Incomplete filling occurs with macroporous materials containing even larger pores, resulting in a Type II isotherm (macroporous polymer support), usually accompanied by a H3 hysteresis loop. Thus, the upper limit of pore size where capillary condensation can occur is determined by the vapor pressure of the adsorptive. Above this pressure, complete bulk condensation would occur. Pores greater than about 50-100 nm in diameter (macropores) cannot be measured by nitrogen adsorption. 

The high sensitivity, resolving power, ease, and speed of the method make this an excellent means of critical assessment of purity of amino acid derivatives [88,89]. Typically, elution of derivatives is from a small-diameter (5-10 m, 300 A pore size) reversed phase (RP) support using a linear gradient of acetonitrile in aqueous buffer. Gradients are usually from 30% B to 100% B over 30 min. The following buffer systems are widely used, and it is recommended that at least one be employed for the determination of derivative purity. 

It is known that the pores of a catalytic support can be divided into three classes according to their diameter dp in nm (or lOr m). Micropores dp have a diameter less than 2 nm in mesopores, 2 < dp < 50, and in macropores dp > 50. The values of the previously calculated critical bubble radius Rk change from 8.6 x 10 to... 

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