Silica capsule

Hollow and porous polymer capsules of micrometer size have been fabricated by using emulsion polymerization or through interfacial polymerization strategies [79,83-84, 88-90], Micron-size, hollow cross-linked polymer capsules were prepared by suspension polymerization of emulsion droplets with polystyrene dissolved in an aqueous solution of poly(vinyl alcohol) [88], while latex capsules with a multihollow structure were processed by seeded emulsion polymerization [89], Ceramic hollow capsules have also been prepared by emulsion/phase-separation procedures [14,91-96] For example, hollow silica capsules with diameters of 1-100 micrometers were obtained by interfacial reactions conducted in oil/water emulsions [91],... 

FIG. 10 SEM micrographs of (a) sUica nanoparticle/polymer [Si02/PDADMAC)3]-coated PS lat-ices and (b) hollow silica capsules. The hollow sUica capsules were obtained by calcining coated particles as shown in (a). The calcination process removes the PS core and the polymer bridging the silica nanoparticles, while at the same time fusing the silica nanoparticles together. Some of the silica capsules were deliberately broken to demonstrate that they were hollow (b). (From Ref. 106.)... 

FIG. 11 TEM images of (a) a [(Si02/PDADMAC)2]-coated PS particle and hollow silica capsules produced from PS latices coated with (b) one, (c) two, or (d) three Si02 layers. The hollow silica capsules maintain the shape of the original PS particle template. Removal of the core by calcination is confirmed by the reduced electron density in the interior of the capsules (compare b-d with a). The images of the hollow silica capsules show the nanoscale control that can be exerted over the wall thickness and their outer diameter. (From Ref. 106.)... 

FIG. 12 TEM micrograph of a cross section of hoUow silica capsules. The hollow capsules were prepared by calcining PS latices coated with [Si02/PDADMAC)3], (From Ref. 110.)... 

Solubility in an Acid-Alcohol Medium Place 1 g of sample into a flask containing 500 mL of the following mixture 3 g of glacial acetic acid, 10 mL of ethanol, and sufficient water to make up the volume to 100 mL. Allow the contents of the flask to rest for 24 h. Filter on a filter screen with a porosity of 2.5 pun, then on a filter screen with a porosity of 0.8 pun. Concentrate the filtrate over a water bath. Finish evaporation over the water bath in a 70-mm diameter tared silica capsule. The dry residue remaining after evaporation must be less than 10 mg, taking into account any residue left by the evaporation of 500 mL of the acetic acid-ethanol mixture. 

Table 21. Representative compositions in the preparation of silica capsules...

Enthalpy data for high purity samples in quartz or vitreous silica capsules have been reported for the range 1698-1915 K by Kantor et al. ( ) and for the range 1686-1825 K by Olette (2). Due to the limited temperature range and the experimental uncertainty, the data do not appear to justify more than a constant heat capacity. A value of 6.5 cal K" mol" is selected, intermediate between the values of 6.75 and 6.15 obtained from separate experiments. 

For example, the transparent UV-Pearls that are the basic ingredients for advanced sunsaeens need to be totally leach-proof in order to retain the encapsulated UV absorber and ensure prolonged action of the lotion applied to the skin. Hence, the porosity of the silica capsules must be negligible such as in the case of capsules obtained from accelerated basic sol-gel polycondensation. On the other hand, the Cool Pearls doped with BPO to treat acne need to be porous to ensure controlled... 

Another issue posed by silica capsules has to do with the brittle nature of the glass shell that often requires (see the following text for encapsulated waxes) the use of organically modified silanes in the precursor mixture along or in place of TEOS. 

Recently, thus, the mechanical properties of the capsules doped with paraffin were considerably improved by another Chinese research team using MTMS in place of TEOS. The sol-gel encapsulation of paraffin in methylsilica takes place with both high encapsulation ratio (82%) and efficacy (no leaching of the melted paraffin) of the silica capsules, whereas the thermal stability and lowered flammability of the silica-microencapsulated paraffin is retained. 

For example, data in Table 18.4 from one patent application of a flavor and fragrance company illustrate how once plied through an antiperspirant (AP) roll-on base, silica capsules doped with... 

The phase equilibria in the ternary system Ce-Gd-Si have been established by Mokra (1979) by means of X-ray and metallographic analysis of 105 alloys prepared by arc melting and subsequent annealing in evacuated silica capsules for 800 h at 600 °C. The samples were finally quenched in water. Starting materials were Ce 99.56%, Gd 99.85% and Si 99.99%. 

Four ternary compounds have been reported for the Ho-Cu-Si system. The occurrence of the AlB2-type of structure was investigated by Rieger and Parthe (1969a) by means of X-ray powder diffraction of an arc-melted alloy with the composition HoCuo ySii 33 Cu and Si atoms were concluded to statistically occupy the 2d sites of P6/mmm a = 3.988, c = 3.976. The magnetic behavior of an alloy HoCuSi, prepared by heating a powder mixture in an evaluated silica capsule for 7 d at 800 to 900 °C, was studied in the temperature range of 77 300 K by Kido et al. (1983). X-ray powder data were reported to reveal an AlBj-type structure. HoCuSi... 

FIGURE 2.2.6 Stimuli-free on/off-lype drug release from LbL films of mesoporous silica capsule. 

The Mn diffusion normal to c appeared to be similar to diffusion parallel to c, and diffusivities in natural and synthetic (free of significant amounts of rare earth or other minor constituents) samples were the same, within experimental error. Experiments which were run in air, where Mn was in a more oxidized state, yielded diffusivities similar to those for experiments run in sealed silica capsules, given by above relationship. 

Chemical diffusion of the rare-earth element was measured in natural calcite under anhydrous conditions, using rare-earth carbonate powders as the sources. Experiments were run in sealed silica capsules together with finely ground calcite to ensure stability of the single-crystal samples during diffusion anneals. Rutherford back-scattering spectroscopy was used to measure diffusion profiles. The Arrhenius relationship at 600 to 850C was ... 

X. Deng, L. Mammen, Y. Zhao, R Lellig, K. Mullen, C. Li, H.J. Butt and D. Vollmer. Transparent, thermally stable and mechanically robust superhydrophobic surfaces made from porous silica capsules. Adv. Mater, 23,2962-2965 (2011). 

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