Hydrophobic silica

All of the eommereial alkyl eyanoaerylate monomers are low-viseosity liquids, and for some applications this can be an advantage. However, there are instances where a viseous liquid or a gel adhesive would be preferred, sueh as for application to a vertical surface or on porous substrates. A variety of viscosity control agents, depending upon the desired properties, have been added to increase the viscosity of instant adhesives [21]. The materials, which have been utilized, include polymethyl methacrylate, hydrophobic silica, hydrophobic alumina, treated quartz, polyethyl cyanoacrylate, cellulose esters, polycarbonates, and carbon black. For example, the addition of 5-10% of amorphous, non-crystalline, fumed silica to ethyl cyanoacrylate changes the monomer viscosity from a 2-cps liquid to a gelled material [22]. Because of the sensitivity of cyanoacrylate esters to basic materials, some additives require treatment with an acid to prevent premature gelation of the product. 

Fig. 4. Comparison of activity-coupling yield (r %) and yield of grafting for samples of CRL immobilized on silica hydrophobic matrices by different methodologies.

Mariscal R., Lopez-Granados M., Fierro J. L. G., Sotelo J. L., Martos C. and Van Grieken R., Morphology and surface properties of titania-silica hydrophobic xerogels, Langmuir 16 (2()00) 9460-9467. 

FIGURE 5.62 (a) Critical pressure for globule entry, (full circles), and efficiency (empty circles) of a silicone oil-silica compound in 10 mM AOT solution, as functions of the time of silica hydrophobization. (b) The dependence of the compound efficiency on for solutions of three different surfactants 10 mM anionic AOT, 0.6 mM nonionic APG, and 1 mM nonionic Triton X-100. (Adapted from Marinova et ah, Langmuir, 18, 3399, 2002.)... 

VOC removal during automotive cold starts High-silica, hydrophobic medium-and large-pore zeolites Achieved 35-70% reduction... 

The results for the immobilization of CGTase from Thermoanaerobacter sp. in glyoxyl-silica, hydrophobic adsorption in octadecyl-sepabeads, and encapsulation into the sol-gel matrix are shown at Table 1. 

Figure 19.8 Influence of the nature of the silica surface on the release of [P4444][lbu] at 20% loading in PBS (pH 7.4) non-pre-calcined silica (hydrophilic silica red) and precalcined silica (hydrophobic silica blue).

Silica (hydrophobic) Prolonged drug release Delayed drug forms... 

Preparation of C-18 silica for reversed-phase extractions using SPE tubes. The process changes polar silica (hydrophilic material) to nonpolar silica (hydrophobic material). 

Lime and Fatty Acids. At high pH (over 11) silica adsorbs calcium iohs which in turn adsorb stearate ions, making the silica hydrophobic enough to be removed from iron ore by flotation. Starch at this pH is apparently adsorbed on the iron oxide preventing interaction with stearate so that iron remains in suspension, according to Sorensen and Frommer (332). 

Mehanna et al. (2011) showed 93.45 % of tadalafil could be released from Syloid 244FP silica-based SD in 10 min, while its physical mixture with silica could release only 21.78 %, and no release was observed from pure native drug. From the observations of dissolution studies, one can predict the efficiency of silica to improve dissolution of poorly soluble drugs. Takeuchi et al. (2004) used dissolution studies to compare SD of tolbutamide prepared with hydrophilic silica and hydrophobic silica. Hydrophobic silica showed much slower dissolution of tolbutamide, confirming the role of hydrophilic silica in increasing the rate of drug dissolution (Takeuchi et al. 2004). 

Silicone oils Pyrogenic or precipitated silicas, hydrophobized with chemically bound methyl groups ". Special reference is made to the relative ineffectiveness of the oil alone As claimed 2-8 (preferably 5)... 

The possibility that the spreading coefficient of PDMS oils is modified by the addition of hydrophobic particles had been explored by Povich [209]. Here the initial spreading coefficient for PDMS was shown to slightly decrease upon the addition of hydrophobed silica (despite the effectiveness of this silica in promoting the antifoam behavior of the oil). Povich [209] attributed this to adsorption of oil-soluble surface-active impurities on the silica. Hydrophobed silica has also been shown to be without significant effect on the spreading behavior of a liquid paraffin [43,71]. 

Pyrogenic or predpitated silicas hydrophobed with chemically bound methyl groups (probably sHanized)... 

Aerogel, fume, or precipitated silicas hydrophobed by any suitable method... 

Silicas hydrophobed with silicone oil or with alkyl chlorosHanes + most of the oils claimed... 

Precipitated silica hydrophobed with polymethylsiloxane and mixed with various mineral oils... 

Polypropylene glycd with silica (hydrophobed with trimethylsilaDe or isobutanol)... 

Silica hydrophobed with dimethylpolysiloxane and dispersed in naphthenic, mineral, or mineral seal oil... 

Silica hydrophobed with dimethyl or trimethyl silyl groups... 

These products (fumed sihca) are highly dispersed, amorphous, white in color and spherically shaped. The average diameters of the primary particles are in the range of 7-40 nm, according to the AEROSIL grade [13]. The company offers many products made from silica, e.g. hydrophihc fumed silica, hydrophobic fumed silica or fumed mixed oxides (mixtures of Si02 and AI2O3) [27]. 

Elution chromatography based on liquid-solid adsorption for small molecules typically employs adsorbents such as alumina, charcoal, silica, hydrophobic silica, etc. Surface area, water content and the chemical nature of the adsorbent (e.g. polar or nonpolar) distinguisb one adsorbent from another. For larger molecules/macromolecules/charged species, other adsorbents needed are briefly touched upon in Sections 7.1.5.1.6-7.1.5.1.8. 

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