Surface area silica

FIGURE 33.17 Effect of silica surface area on wet-skid resistance of vulcanizates filled with different fillers. 

Silica-based stationary phases with a chemically bonded ligand on the surface can be characterized by the carbon content (grams of carbon per 100 g of packing) and by the bonding density (micromols of ligand bonded/square meter of initial silica surface area). 

The silica surface area-to-solution volume ratio was 2 x 10-3 m2 liter-1 for the streaming potential experiment and 1.0 m2 liter-1 for the electrophoresis experiment. Calculate HS at each pH from the electrophoresis data (rj = 0.00894 P, er = 78.5). Propose an explanation for the charge reversal behavior of the silica. Discuss the origin of the difference between HS and tsiPor in terms of this model. 

Taking into account the solvent volume, V, and the total silica surface area, F, the concentration, c, of silicic acid can be expressed as... 

Figure 15. Daily release of silicic acid from 1 sq. meter of silica surface area into 50 ml. of fresh solvent (0.9% NaCl plus 0.1% NaHCOt, pH = 8.4), repeated over 20 days for different silica modifications...

Naviroj et al. [28] reported that VS and MPS were adsorbed on silica (surface area of 130 m2/g) at 0.35 and 0.24 monolayer equivalents, respectively. Space projections of MPS molecules in the perpendicular and parallel orientations of the extended form were reported to be about 24 and 55 A2, respectively [48], Therefore, it is expected that a single molecular layer coverage of MPS would differ by a factor of about 2 for the two different orientations. The difference by a factor of about 2 from our experiments may also be due to similar effects. 

A silica gel with a higher surface area gives a higher capacity ratio, as demonstrated by the determination of famotidine [3] in the presence of its potential degradants and preservatives. The capacity factor increased by more than a factor of 3 when the silica surface area increased from 200 to 350 m7g. 

Fig. 2, Fumed silica (surface area 200 g ) - Transmission electron microscopy (TEM)...

The roughness of the silica surface could introduce the steric hindrance of the surface accessibility similar to the effect of the micropores. In the discussion above, we assume the ideal tubular geometry of the silica surface. The use of different probe molecules for the BET measurement of silica surface area (such as N2, Ar, Kr, benzene, etc.) leads to significant difference in the surface area values for the same silica sample. It was suggested that silica surfaces possess the property of fractals [16] this essentially means that molecules of different size will see a different surface area. 

The measurement of the silica surface area is well established however, the assessment of the surface area of chemically modified porous silica brings a... 

Figure 6.39 Hysteresis loops of viscosity versus shear rate of a 3% by weight suspension of fumed silica (surface area = 325 m /g) in poly(dimethyl-siloxane), (PDMS molecular weight = 67,000, ris 125 P) at 30°C. In each run, the shear rate was first increased up to a maximum shear rate /max located at the arrow, and then decreased. After a rest of 23 hours, another run was made, with a different /max, thus producing the series of curves shown. (Reprinted from J Non-Newt Fluid Mech 17 45, Ziegelbaur and Caruthers (1985), with kind permission from Elsevier Science NL, Sara Burgerhartstraat 25,1055 KV Amsterdam, The Netherlands.)...

Silica (surface area = 600 vcfilg) and 3-aminopropyltriethoxysilyl-silica were purchased from Fluka. 

Figure 4. Effects of organic carbon content (foc) on the ratio of K0r to KDpfor two soils having different silica surface areas (SA).

Figure 8.53. Effect of fumed silica surface area on peel adhesion of silicone sealant. [Data from Cochrane H, Lin C-S, Rubber World, 1985.]...

The acylation of anisole wifh AAN was examined in the presence of silica-supporfed PW.i The supporfed catalysfs are prepared by impregnating silica (surface area 300 m x g i) or mesoporous silica MCM-41 (surface area 1250 m x g i) wifh a mefhanol solution of PW. The acylations are carried out in liquid phase in a glass reactor charged with aromatic substrate and AAN, the substrate taken in excess over the acylat-ing agent no solvent is used. 

Measurement of the surface area and pore diameter of a silica by conventional techniques can be used to classify its surface structure, approximately as in Table 7-1. The ratio of reactive to total hydroxyls SJSt) is a more precise index of silica surface structure, and this quantity—along with silica surface area—can be determined easily by a simple procedure described in Appendix II. Most chromatographic silicas in use today, and especially those silicas used for thin-layer chromatography, have surface areas of 300-600 m /g and pore diameters of 100-250 A. For the most part they can be classed as large pore silicas. However, small pore silicas are also commercially available. For a further discussion of the surface structure of various commercial silicas and the variation of chromatographic properties with the quantity see Refs. (14,19). 

The ratio SJSf is then determined from Fig. II-3. S, may also be calculated from the silica surface area, and vice versa surface area (square meters per gram) = 3585f. Determination of Sf plus (S,/Sj) by silanization requires about 4 hr/sample. 

In general, a preferred range of operation of the invention is to add active silica at a rate of 1-4 g/h/1000 m at the beginning of the heating-up (transition) interval. Nevertheless, it is better to make actual determinations of the silica surface area and from there determine whether a given rate is suitable or not. 

Two clusters modeled the silylated substrates Si2Me, containing 2 (isolated), and Si6Me, containing 6 (associated) TMS groups, respectively. The equilibrated structures of these two silylated clusters, which represent a silica surface area with different degrees of silylation constructed from the same basic Si480H cluster, are shown in Fig. 3. 

In an excellent series of papers Burattin, Che and Louis [5-8] have proposed the molecular details of DP with urea studying the important system of nickel on silica. By variation of the silica specific surface area, nickel concentration and DP time, the authors concluded that turbostratic nickel hydroxide is the main phase deposited when short reaction times and low silica surface area are applied. Longer reaction times and higher silica surface area led to 1 1 nickel phyllosilicate of increasing crystallinity. The overall reaction mechanism is depicted in Figure 6.5 and is now discussed in some detail. Following the papers mentioned, the authors describe the key steps of the mechanism as follows. 

Silica Surface area (m g- ) pH Au loading (wt%) Thermal treatment dAu (nm) Reference... 

It can be noted that the brucitic layer of Ni(II) bonded to silica acts as nuclei for the growth of supported 1 1 nickel phyllosilicate or supported nickel hydroxide. The heterocondensation reaction is faster than the olation one, but it is limited by the concentration and diffusion in solution of silicic acid arising from silica dissolution, which itself depends on the silica surface area, i.e., on the extent of support-solution interface. 

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