Silica spherical microparticles

Spherical microparticles are more difficult to manufacture and can be prepared by several methods. One method prepares silica hydrogel beads by emulsification of a silica sol in an immiscible organic liquid [20,21,24,25]. To promote gelling a silica hydrosol, prepared as before, is dispersed into small droplets in a iater immiscible liquid and the temperature, pH, and/or electrolyte concentration adjusted to promote solidification. Over time the liquid droplets become increasingly viscous and solidify as a coherent assembly of particles in bead form. The hydrogel beads are then dehydrated to porous, spherical, silica beads. An alternative approach is based on the agglutination of a silica sol by coacervation [25-27], Urea and formaldehyde are polymerized at low pH in the presence of colloidal silica. Coacervatec liquid... 

In 1998, Avnir and coworkers patented the interfacial polymerization process in which the mild sol-gel production of silica glass is combined with emulsion chemistry to form sol-gel spherical microparticles in place of irregular granules. In brief, the emulsion droplets provide a microreactor environment for the hydrolysis and condensation reactions of Si alkoxides. All sorts of molecules can be entrapped and stabilized in similar ceramic microparticles with broad control over the release rate for a range of applications (such as drug delivery, release of specialty chemicals and cosmeceutical/ nutraceutical, and beyond, Figure 18.1), with no need for reformulation for different molecules. 

Samples of control (non-functionalized) pHlPE and pHIPE-GMA were added to vials containing 15% w/w modified sihca microparticles in methanol and THE. 10 drops of cone. HCl were added to each vial and the reactions were allowed to mn for 16 hours. Two types of silica particles were used (1) 3 pm spherical Si-Amine (Si(C3H6)NH2), and (2) 3 pm spherical Si-Thiol (Si(C3H6)SH). After the reaction time the samples were placed in a sonicator for 1 h to remove any unbound Si particles. 

The increase in surface area of the pHIPE is important to open up new application areas. One approach is to attach spherical particles to the surface of the monolithic material. This has been, for example achieved by electrostatic interaction between charged latex nanoparticles and a monolithic surface (62). In preliminary experiments we investigated if fimctional silica microparticles could be covalently adsorbed to the p(HIPE-g-GMA) surface so as to produce stable, functional pHIPE with an increased surface area. 

Again, summarizing what has been extensively described elsewhere, the two main techniques to prepare sol-gel microparticles start either from W/0 or from oil-in-water (0/W) emulsions to entrap, respectively, hydrophilic or lipophilic ingredients inside the spherical shell of amorphous silica or organosilica. 

Thus, the MCA treatment leads to significant diminution of AC microparticle sizes however, the microparticles were not completely destroyed. Textural pores in AC microparticles are non-slit-shaped (see Chapter 3). Therefore, the observed decomposition of AC microparticles (Figure 4.35) results in an increase in contribution of slit-shaped pores (Table 4.13, Qjt) and a decrease in contribution of textural pores (cylindrical pores [CcyJ and voids between spherical particles [Cvoidl) in silica/AC composites in comparison with initial AC. 

Two basic types of packings have been used in LC, pellicular and porous particle. The original pellicular particles were spherical, nonporous. glass or polymer beads with typical diameters of 30 to 40 pm. A thin, porous layer of silica, alumina, a polystyrene-divinyl-benzene synthetic resin, or an ion-exchange resin was deposited on the surface of these beads. Small porous microparticles have completely replaced these large pellicular particles. In recent years, small (- 5 pm) pellicular packings have been reintroduced for separation of proteins and large biomolecules. 

Scanning Electron Microscopy - Silica microcapsules are studied by ESEM. The spherical particles have average diameters ranging from 1-30 pm, shown in - Figure 15. The shells of microparticles present a smooth surface. 

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