Silica critical pore diameter

Fig. 14.6. Filtration flux as a function of time of filtration for the filtration of O.Oi g/L silica particles in 0.001 M NaCI solution at pH 6 at a membrane of mean pore diameter 84 nm. The particle size was very close to the pore size. The critical transmembrane pressure for these conditions was calculated as 130 kPa. Operation below this pressure gives only a gradual decline in filtration flux with time. Operation above this pressure gives an initially higher filtration flux which declines rapidly with time. In the latter case the intial hydrodynamic force exceeds the electrical double layer repulsion between the membrane and the particles, causing the particles to block the membrane pores.

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... 

Phase transitions of the pore fluid were also observed for krypton in MCM-48 silica materials at 87 K, which is 26.5 K below the bulk triple point [13]. Remarkably, the width of the hysteresis loop decreases with decreasing pore size, which may indicate that at 87 K the pore fluid of C is much closer to pore criticality as the confined fluid in A (in case the observed phase transitions for krypton in MCM-48 silica reflect still a gas-liquid phase transition). For krypton/A an even broader hysteresis loop is observed at 77 K compared to 87 K [13]. Recent work on MCM-41 of mean pore diameter 4 nm indicated a solidification of the krypton capillary phase at 77 K [21]. The observed phase transitions of krypton in the MCM-48 silicas will be investigated in more detail in further studies [13]. 

The larger the average pore diameter, the lower the specific surface area and hence the chromatographic retention, A 50-nm pore size silica exhibits a specific surface area of about 50 m /g, about five times lower than that of a 15-nm pore size material. However, due to the multisite interactions of a biopolymer with a bonded silica in HPLC modes such as reversed phase or ion exchanger, the value of the specific surface area is not a critical parameter for the resolution of biopolymers. It has been demonstrated that even the low, entirely external specific surface area of about 1 to 2 m /g of 1- to 2-pm nonporous silica particles is sufficient to retain and to resolve biopolymeric analytes [12]. 

When 650 Torr N2F4 was introduced at room temperature above molecular sieves (Linde 13X, 10X, 5A, 4A, 3A with effective pore diameters of 9, 8, 5, 4, and 3 A, respectively), NF2 ESR spectra were obtained from 13X, 10X, and 5A sieves, but only N2F4 NMR spectra could be observed for 13X and 10X sieves. Neither adsorbed species was found for 4A or 3A sieves (the NF2 critical diameter is estimated to be 3.7 A). Only adsorbed N2F4 could be detected on silica gel. It was concluded that N2F4 and NF2 followed relatively independent adsorption isotherms because the concentration of adsorbed NF2 on 13X, 10X, and 5A sieves remained practically constant while the bulk adsorption of N2F4 changed by more than a factor of 20 [24]. 

Fig. 4.2. Effect of ratio of solute critical diameter to pore diameter on effective diffusivity in restricted diffusion solid circles correspond to non adsorbable molecules open circles show adsorbable molecules in silica-alumina beads (median pore diameter 3.2 nm).

Only by disrupting the silica or by overcoming the critical (rupture) shear stress for the polymer can the polymer be able to flow through the silica network. We estimate that this critical shear stress is about 1-5 bar at these conditions. This means that if the flow paths are long compared to pore diameter, the total pressure drop caused by the flow will be unreallistically high. 

Figure 6.20 Filtration fluxes as a function of time for filtration of particles very close In dimensions to the pore dimensions. Silica particles, 0.001 M NaCI, pH 6.0, mean particle diameter 86 nm, mean pore diameter 85 nm, calculated critical pressure = 130 kPa.

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