Silica surface area from

The increasing of hydrolysis ratio, HjO/TEOS, up to 10 increases strongly the silica surface area from 100 to 600 m g, for values higher than 10 the surface area remains almost constant A similar behaviour is shown by the "basic" samples, however die effect is lower and the values of the surface area are lowo too. 

Experiments with silica powder chosen from five depths in a two-pound container gave surface areas from top to bottom of 9.8,10.2,10.4,10.5 and 10.7 m g . When the same sample was poured into a conical pile, five random samples produced surface areas of 10.3, 11.0, 10.4, 10.0 and 10.6 m g . However, when the sample was riffled in a spinning riffler with three size reductions, the subsequent analysis of the contents of five collectors gave 10.2,10.1,10.2,10.2 and 10.1 m g as the specific surface... 

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. 

The adsorption isotherm of N, on FSM-16 at 77 K had an explicit hysteresis. As to the adsorption hysteresis of N-, on regular mesoporous silica, the dependencies of adsorption hysteresis on the pore width and adsorbate were observed the adsorption hysteresis can be observed for pores of w 4.0nm. The reason has been studied by several approaches [5-8]. The adsorption isotherm of acetonitrile on FSM-16 at 303K is shown in Fig. 1. The adsorption isotherm has a clear hysteresis the adsorption and desorption branches close at PIP, = 0.38. The presence of the adsorption hysteresis coincides with the anticipation of the classical capillary condensation theory for the cylindrical pores whose both ends are open. The value of the BET monolayer capacity, nm, for acetonitrile was 3.9 mmol g. By assuming the surface area from the nitrogen isotherm to be available for the adsorption of acetonitrile, the apparent molecular area, am, of adsorbed acetonitrile can be obtained from nm. The value of am for adsorbed acetonitrile (0.35 nnr) was quite different from the value (0.22 nm2) from the liquid density under the assumption of the close packing. Acetonitrile molecules on the mesopore surface are packed more loosely than the close packing. The later IR data will show that acetonitrile molecules are adsorbed on the surface hydroxyls in... 

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. 

Fluorosilicones can be compounded by the addition of mineral fillers and pigments. Fillers for such compounds are most commonly silicas (silicon dioxide), because they are compatible with the elastomeric silicon-oxygen backbone and thermally very stable. They range in surface areas from 0.54 to 400 m2/g and average particle size from 100 to 6 nm. Because of these properties, they offer a great deal of flexibility in reinforcement. Thus, cured compounds can have Durometer A hardness from 40 to 80. Other fillers commonly used in fluorosilicones are calcium carbonate, titanium dioxide, and zinc oxide. 

Fig. 5. Surface area of silylated silica N2-adsorption BET-surface area ( ) Ar-adsorption BET-surface area ( ) surface area from TEM analysis (0) decrease of surface area according to mean thickness of silylation layer (V) [15]...

A catalyst produced from silica spheres III with the sequence WI, D, C, (350 C) DI, D reduced the surface area from 60 m /g to 5 m /g. The phosphoric acid had filled most part of the pores. Crushing strenght was maintained at 1.6 kg before and after preparation and after testing. 

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. 

Silica is the most widely used commercially available normal-phase packing in various forms, having standard particle sizes ranging from 3 to 10 /im and surface areas from 200 to 800 m /g [1]. It is classified as an acidic adsorbent because its surface consists of acidic hydroxyl groups that are covalently bound to the Si atoms (i.e., silanol groups). Silica contains weak Brpn-sted acidic sites and does not contain any Lewis-add sites. 

The range of specific surface areas from Recent radiolarians to sponge spicules may vary by nearly three orders of magnitude. Since Alexander (25) has shown that for a series of silica gel sols, solubility varies as a function of both specific surface area and internal structure, only general trends in solubility can be discussed, and those, conservatively. 

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 surface of ACF of w = 1.45 nm was modified with molecular adsorption-decomposition method using SiCU. SiCU was adsorbed on the ACF and then hydrolyzed by introduction of H2O vapour at 298 K. Afterwards, residual SiCU and produced HCl vapours were removed, and then the treated ACF was heated at 573 K. The amount of the produced hydrated silica was determined by the measurement of the weight change. The micropore structure of the silica-coated ACF was examined by N2 adsorption the t-plot analysis of the N2 adsorption isotherm showed that the micropore width decreases with the silica coating by 0.2 nm the silica coating decreased the micropore volume and surface area from 1.49 ml/g and 2280 m /g to 0.68 ml/g and 1100 m /g, respectively. No spherical silica particles were observed on the external surface of the silica-coated ACF by scanning electron microscopy with a resolution of 10 nm. Therefore, hydrated silica should be deposited entirely on the micropore walls of the ACF. 

The coalescence of the two small droplets in contact is driven by the tendency for the doublet to approach a spherical shape corresponding to minimum surface free energy for the doublet volume. For liquid ptulicles including glassy materials like. silica, coalescence takes place by viscous flow. For a Newtonian liquid, during the initial stages of coale.scence of two liquid spheres of equal diameter, a neck forms at the contact point with a radius that grows as r(Frenkel, 1945). After a short initial period in which coalescence is very rapid, the rate of decrease in the surface area becomes linear in the deviation of the doublet surface area from the sphere of the same volume as the doublet (Koch and Friedlander, 1990) ... 

As the surface tension of the pore liquid was lowered in these experiments, the total pore volume increased. Nitrogen adsorption indicated that the pores also became larger. This is an inevitable outcome of maintaining constant surface area (from the definition of a cylinder, or any other shape of pore, pore volume must be proportional to pore area multiplied by pore diameter). The least active silica in the series, which had the lowest pore volume, also contained most of its volume within small pores (less than 60 A in diameter). In contrast, the most active samples, which were dried by organic solvent extraction, had the highest total pore volumes and most of that volume was inside much larger (meso) pores. 

Figure 16. Concentration of the surface hydroxyl groups (the silanol number) ocoh for silicas having different specific surface areas S when the surface has been hydroxylated to a maximum degree. The letters a through i represent different types of amorphous silica (see text). The shaded area is a range of experimental data (100 samples of Si02 with different specific surface area, from 9.5 to 950 m2/g). The average value of aoH is 4.6 OH groups per square nanometer (least-squares method). (Reproduced with permission from reference 53. Copyright 1993.)...

Besides the principal silicon oxide, common silica fillers include other oxides as minor inclusions or traces, such as AI2O3, Fc203, Ti02, CaO, MgO, Na2S04, and trace elements. Specific gravity (density) of silica is 1.9-2.2 g/cm, moisture content is typically from 0.1 to 7%, and can go up as high as 20%. Particle sizes of commercial silica grades, which are typically white powder, are from less than 1 to 40 pm, oil absorption between 20 and 330 g/100 g, specific surface area from 1 to 800 mVg. 

Silica-gel ndsotbents are composed of a rigid three-dimensional network of spherical particles of amorphous colloidal silica (Si02). They were reparted as early as 1640 and commercial production was begun around 1919. The surface area is generated by the very flue size of the colloidal panicles. They exhibit surface areas from as little as 100 m2/g for the "aerogels" to over 800 m2/g, The product is provided both in granular and spherical forms. 

Most solids are able to adsorb species from gases and liquids. However, only a few have a sufficient selectivity and capacity to make them serious candidates for commercial sorbents. A good sorbent must have a large specific area (area per unit mass). Typical commercial sorbents, such as activated carbon or silica gel, have specific surface areas from 300 to 1200 m2/g. Thus, a few grams of activated carbon have a surface area equal to that of a football field Such a large area is possible by a particle porosity from 30 to 85% with average pore diameters from 10 to 200 A. They... 

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. 

The properties of amorphous silicas of high specific surface area, from the smallest colloidal particles to macroscopic gels, depend largely on the chemistry of the surface of the solid phase [1]. 

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