Hydrophobic colloidal silica

Winslow (1949) reported that silica gel in a low-viscosity oil showed this effect under an electric field of 3 kV/mm. The fluid can be sheared with a force proportional to the square of the electrical field. For example, a 25% by volume of hydrophobic colloidal silica spheres of 0.75 pm diameter in 4-methylcyclohexanol showed ER responses at 40-4,000 Hz, although dc fields are also viable. Dispersants are often added to the suspension in order to prevent the settling of the solids. 

III.l Experimental Procedure The samples were made of variable amounts of hydrophobic colloidal silica (Aerosol R972 Degussa) mixed with an index matching solvent (ethanol + ethanolamine) containing the tracer molecule (fluorescein isothiocyanate. Molecular Probes). We have checked that tracer adsorption on the silica was negligible. Further details on the sample preparation can be found in reference [16]. 

Modifications of monodisperse colloidal silica, of 10 or 500 nm in diameter, were carried out using trialkoxysilane-terminated polymer in a low polar solvent, such as acetone or 1,2-dimethoxyethane. without coagulation during the coupling reaction (35,37-42). In this modification, the hydrophobic polymer can be efficiently bound to hydrophilic colloidal silica surface. The reaction mechanism of the binding... 

FIGURE 7.13 Change in hydrophobicity of silica upon adsorption on nonionic surfactants. (Erom Soma-sundaran, R, Snell, E. and Xu, Q., J. Colloid Interface ScL, 144(1), 159, 1991. With permission.)... 

Two possibilities are open amend the DLVO theory or design a new theory based on typical properties of colloidal silica. Lyklema (34) believes the first approach is promising if features such as solvation layers, hydrophobic bonding, and surface charge fluctuations are incorporated into the DLVO theory. 

To create particles with a hydrophobic core and a hydrophilic shell, preformed hydrophobic polymer particles can be used as substrate for the covalent grafting of hydrophilic polymer chains. Aqueous emulsion polymerization is thus the method of choice for synthesis of core particles with submicrometric diameter (due to the scope of this review article, discussion will be restricted to aqueous emulsion polymerization, although some examples of particle substrates created via suspension polymerization exist however, with much larger diameters). The grafting methods are very close to those used for the surface modification of inorganic materials, such as for instance silicon wafers or colloidal silica particles [46,48-52], Various strategies can be employed such as ... 

TEOS is the best silica precursor in the synthesis of MCM-48, which generates ethanol during the hydrolysis. Polar ethanol tends to enter the hydrophobic zone but cannot penetrate into the surfactant micelle core [39]. The enlarged surfactant hydrophobic volume decreases the interfacial curvature around the surfactant micelles, and thus results in the formation of MCM-48. Although Ryoo and coworker synthesized MCM-48 using colloidal silica as a source, adding ethanol was also necessary [38]. 

FIGURE 7.3 SEM images of dry yeast 5. cerevisiae cells (5-10 tm in diameter in native state) at different magnification (a)4,500x (bar lOjim) and(b) 15,000x (bar 1 (im). (Adapted from 7. Colloid Interface ScL, 283, Turov, V.V., Gun ko, V.M., Bogatyrev, V.M. et al.. Structured water in partially dehydrated yeast cells and at partially hydrophobized fumed silica surface, 329-343, 2005, Copyright 2005, with permission from Elsevier.)... 

Oleogels may be mixtures of liquid paraffin with polyethylene (Hydrophobic Base Gel DAC [43]) or fatty oils combined with anhydrous colloidal silica, aluminium or zinc soaps as viscosity enhancer. Hydrophobic gels are prepared by mixing the oil phase with the viscosity enhancer. If the... 

The interaction of hydrophobic groups adsorbed on the surface of silica involves hydrophobic bonding. Tanford (291) has described the effect in detail. Long chain amines, and particularly long chain alkyl-substituted quaternary ammonium salts, are powerful flocculating agents for colloidal silica. Thus Smith and Turnbull (292) add a quaternary ammonium salt to cause a degree of flocculation so as to make the v colloidal silica thixotropic when used as a binder in refractory powder slurries. 

Colloidal silica flocculated through hydrophobic bonding between the hydro-phobic spots on silica particles is characterized by the fact that the addition of a watermiscible alcohol such as propanol redisperses the flocculate by wetting the surface. The hydrophobic propyl groups are oriented toward the hydrophobic surface so that the outwardly disposed alcoholic hydroxyls render them hydrophilic. When such silica is recovered and dried the alcohol evaporates leaving a fully hydro-phobic surface. 

The theory of filling submicroscopic surface irregularitiu with colloidal silica, as on painted surfaces, has been described by Rusher and Yates (579). To reduce soiling of a hydrophilic surface such as upholstery fabric by making it both hydrophobic and soil resistant, a polydimethylsiloxane oil is incorporated with colloidal silica... 

Adsorption from solution has already been mentioned in Chapter 4 relating to the specific surface area of colloidal silica. These and other methods have also been applied to silica powders and gels. Adsorbates include organic compounds from organic and aqueous solutions as well as organic and inorganic cations and the OH ion from water. Polar compounds in nonpolar (hydrocarbon) solvents are adsorbed on the polar SiOH surface. Adsorption on the dehydrated siloxane surface is not as well understood. However, there is evidence that in water, the siloxane surface is hydrophobic. It adsorbs only hydrophobic materials or groups. 

Retention of flow control properties of fumed silica can be improved dramatically by applying a hydrophobic surface coating to the silica. Accelerated aging of epoxy adhesives containing hydrophobic fused silica (4 weeks at 60°C) showed no increase in adhesive sag, while untreated silicas, asbestos, and clay all showed some sag increase. Colloidal silica is also claimed to possess good sag resistance over time. ... 

---