Mixtures of Hydrophilic and Hydrophobic Silicas

Specifically treated mixtures (mixed at 5000-18000 rpm) with hydrophilic and hydrophobic nanosilicas, water, and air can form two-shell microspheres. These microparticles can include both air and water, for example, water on the outside and air inside microspheres (a hydrophobic part is inside and a hydrophilic shell is outside) or vice versa depending on the preparation technique. 

The water-rich powders containing up to 98% (by weight) of water and a small amount of hydrophobic nanosilica and characterized by the same flow properties as dry silica powder ( dry water ) can be prepared by a mixing process at a high rpm value (Berthod et al. 1988, Hamer et al. 2001, Lahanas et al. 2001, Yoichiro et al. 2002, Koga et al. 2004, Dampeirou 2005, Hasenzahl et al. 2005, Binks and Murakami 2006, Forny et al. 2007, 2009, Saleh et al. 2011). Dry water materials with water droplets covered by the hydrophobic shells or related dried materials without water are of interest for applications in medicine, cosmetics, biotechnology, etc. 

If in a hydrophilic—hydrophobic silicas—water mixture (prepared at 10,000 rpm), the inner layer is composed of hydrophobic silica and the outer layer is of hydrophilic silica that air bubbles can be entrapped in the microspheres. The bulk density of this fresh gel-like mixture can be relatively low, 0.5-0.6 g/cm due to entrapped air (Mironyuk et al. 1994,1999). In the case of the opposite structure of the composition, water is located inside microparticles with the hydrophobic outer shell that give the dry water material mentioned earlier. In the dry water materials, instead of hydrophilic silica, starch or other superhydrophilic polymers or other hydrophilic compounds can be used to provide stronger and longer retention of water inside microparticles. 

Several techniques can be used to produce composite or hybrid materials based on the hydrophobic-hydrophilic mixtures with water or other liquids (de Gennes et al. 2004, Eshtiaghi et al. 2009, Boissiere et al. 2011). For instance, a mixture of hydrophilic nanosilica A-300 and hydrophobic nanosilica R812 was dispersed in a Schiff-base-type liquid crystal (LC), / -ethoxy(ben zylidene)-p-n-butylaniline, and analyzed using deuterium NMR spectroscopy (Milette et al. 2008) 

Nuclear Magnetic Resonance Studies of Interfacial Phenomena 

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Stable mixtures of hydrophilic and hydrophobic silicas with water at different ratio between the concentrations of these oxides and a constant total amount of oxides were also studied using electrophysical methods (Mironyuk et al. 1999). The conductivity (a) of the oxide blend at different frequencies of the applied electromagnetic field increases relative to that for hydrophilic silica. However, the a value at the constant electrical current decreases as the concentration of hydrophobic silica increases. An increase in the frequency from 0.1 to 10 kHz gives the exponential growth of the ratio between dielectric loss (e") and dielectric permittivity (e ) due to a strong decrease in the e value. These effects are caused by additional ordering in the system to reduce the contact area between hydrophobic silica particles and water that influences formation of micelles with a high polarizability. 

FIG. 13 Stabilization and inversion of toluene + water emulsions caused by mixtures of hydrophilic and hydrophobic silica particles, diameters between 15 and 30 nm. Water-continuous emulsions have high conductivity, whereas the conductivity of toluene-continuous emulsions is very low. It is clear that there is inversion, from water-in-oil (w/o) to oil-in-water (o/w) emulsion at a weight percent of hydrophilic silica of around 0.7 (see text). 

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