Encapsulation of silica particles

Also, here, the effect of the adsorption layer of HPC on encapsulation of silica particles in polymerization of styrene in the presence of silica particles has been investigated. Encapsulation is promoted greatly by the existence of the adsorption layer on the silica particles, and the dense adsorption layer formed at the LCST makes composite polystyrene latices with silica particles in the core (7.). This type of examination is entirely new in polymer adsorption studies and we believe that this work will contribute not only to new colloid and interface science, but also to industrial technology. 

It was apparent that the dense adsorption layer of HPC which was formed on the silica particles at the LCST plays a part in the preparation of new composite polymer latices, i.e. polystyrene latices with silica particles in the core. Figures 10 and 11 show the electron micrographs of the final silica-polystyrene composite which resulted from seeded emulsion polymerization using as seed bare silica particles, and HPC-coated silica particles,respectively. As may be seen from Fig.10, when the bare particles of silica were used in the seeded emulsion polymerization, there was no tendency for encapsulation of silica particles, and indeed new polymer particles were formed in the aqueous phase. On the other hand, encapsulation of the seed particles proceeded preferentially when the HPC-coated silica particles were used as the seed and fairly monodisperse composite latices including silica particles were generated. This indicated that the dense adsorption layer of HPC formed at the LCST plays a role as a binder between the silica surface and the styrene molecules. 

According to Hergeth and coworkers [55], a minimum surface of the inorganic particles is needed to prevent secondary nucleation. To estimate this amount, a formula was derived for seeded emulsion polymerization with spherical particles and a water-soluble initiator [55]. This formula was based on the observation that primary particles are produced by a coUapse and micellization process of oligomeric chains. An upper limit for the particle size was estimated to be 100 nm for the encapsulation of silica with polyvinyl acetate. A relatively water-soluble monomer is applied here for more hydrophobic monomers this upper limit will be higher. Because the surface area needed to prevent secondary nucleation is proportional to the monomer conversion per unit of time, the encapsulation efficiency can be improved by using monomer-starved conditions. So far, mainly submicrometer particles have been encapsulated with this method. The encapsulation of the larger filler particles... 

Alternative approaches involving molecules that combine the properties of a monomer with those of a surfactant (so-called polymerizable surfactants) have also been reported. For example, quaternary alkyl salts of dimethyl aminoethyl methacrylate (CnBr) surfactants were used to promote polymer encapsulation of silica gels in aqueous suspension [43, 44]. The polymerizable surfactant formed a bilayer on the silica surface, the configuration of which enabled the formation of core-shell particles. The CnBr amphiphilic molecule was either homopolymer-ized or copolymerized with styrene adsolubilized in the reactive surfactant bilayer. This concept of admicellar polymerization is detailed in Sect. 3.1. In the recent... 

Another approach of chemical modification of silica for rubber nanocomposite for the same aim, i.e. to reduce the filler-filler interaction, is differential microemulsion polymerization. In general, the process involves two main continuous steps (i) pre-treatment/chemical bonding of silica particles with coupling agent, and (ii) polymerization of the monomer in a reaction medium containing the pre-treated silica. The resultant product is a core-shell structure where nano-silica is the core encapsulated by a nanopolymer shell. 

N. Ruramoto, Polymerization of surface-active monomers. III. Polymer encapsulation of silica gel particles by aqueous polymerization of quaternary salt of di-methylaminoethyl methacrylate with lau-ryl bromide,/.Appi. Polym. Sci. 1989, 38, 2183 2189 (b) K. Nagai, Polymerization of surface-active monomers and applications, Macromd. Symp. 1994, 84, 29-36. 

Bourgeat-Lami, E. and Lang, J. (1999) Encapsulation of inorganic particles by dispersion polymerization in polar media 2. Effect of silica size and concentration on the morphology of silica-polystyrene composite particles. Journal of Colloid and Interface Science, 210,281-289. 

SWCNT forest electrodes have been employed in electrochemical and electrochemilumines-cent (ECL) immunosensors. Antibody-modified SWCNT forest electrodes and HRP-labeled detection antibodies were first employed to measure the small molecule biotini" and model protein human serum albumini" and later for cancer biomarker proteins in serum. These nanostructured immunosensors offer 3-10-fold gains in sensitivity compared to similar sensors based on flat bulk electrodes.i i Analysis of the SWCNT forest electrode-based sensor revealed that dense packing of the earboxylated ends of SWCNTs enables attachment of a large surface concentration of capture antibodies, whieh, along with other favorable properties of the nanotubes, contributes to the observed sensitivity improvements of SWCNT-modified sensors over sensors based on bulk electrode materials. " ECL-based SWCNT forest immunosensors for eancer biomarker proteins relied on Ru(bpy)3 + encapsulated in silica particles (Section 13.3.6) that were decorated with antibodies.i" " Eor the prostate cancer biomarker prostate-specific antigen, SWCNT forest ECL immunosensors exhibited 34 times higher sensitivities and 10 times lower DLs than analogous... 

The sol-gel reaction during the formation of silica particles in the multiple emulsion system started in the external oil phase containing the precursor alkoxide type (tetraethyl orthosilicate, TEOS), as shown in Figure 7.25. Under stirring, the TEOS molecules can penetrate the surfactant layer surrounding the aqueous phase, and then hydrolysis can start. As hydrolysis proceeds, the Si-OH based molecules diffuse and dissolve in the aqueous phase. A gel network is formed by condensation, yielding the insoluble hydrated silica encapsulating the retinol molecules. The water content in the multiple emulsion was demonstrated to impart the final shape and size distribution of the particles. 

Lee MH, Oh SG, Moon SK, Bae SY. 2001. Preparation of silica particles encapsulating retinol using O/W/O multiple emulsions. J Colloid Interface Sci 240 83-89. 

Lim et al. [24] observed that hydrophilic polyacrylamide (PAM) nanofibers effectively wrapped the hydrophilic silica colloids, while hydrophobic polyacrylonitrile (PAN) nanofibers embedded large amounts of hydrophobic silica colloids. Moreover, the addition of a small amount of hexanol to the PEO solution successfully confined all silica colloids inside PEO fibers. Hexanol acted as a surfactant, decreasing the interfacial tension between the silica colloids and PEO (Fig. 16.4). Based on the observations, it was postulated that encapsulation of silica colloids was mainly governed by the wettability of the polymers on the particles. 

Scheme 2. Encapsulation of size- and shape-controlled Pt nanoparticles under neutral hydrothermal synthesis conditions of SBA-15. Silica templating block copolymers and silica precursors were added to PVP-protected Pt nanoparticle solutions and subjected to the standard SBA-15 silica synthesis conditions. Neutral, rather than acidic pH conditions were employed to prevent particle aggregation and amorphous silica formation [16j. (Reprinted from Ref. [16], 2006, with permission from American Chemical Society.)...

Common to all encapsulation methods is the provision for the passage of reagents and products through or past the walls of the compartment. In zeolites and mesoporous materials, this is enabled by their open porous structure. It is not surprising, then, that porous silica has been used as a material for encapsulation processes, which has already been seen in LbL methods [43], Moreover, ship-in-a-bottle approaches have been well documented, whereby the encapsulation of individual molecules, molecular clusters, and small metal particles is achieved within zeolites [67]. There is a wealth of literature on the immobilization of catalysts on silica or other inorganic materials [68-72], but this is beyond the scope of this chapter. However, these methods potentially provide another method to avoid a situation where one catalyst interferes with another, or to allow the use of a catalyst in a system limited by the reaction conditions. For example, the increased stability of a catalyst may allow a reaction to run at a desired higher temperature, or allow for the use of an otherwise insoluble catalyst [73]. 

Ow et al. (2005) developed an improved method of incorporating fluorescent molecules into silica particles using a modified Stober synthesis, which resulted in both enhanced fluorescence and photostability of the encapsulated dyes. In this two-stage procedure, reactive organic dyes... 

Adsorption behavior and the effect on colloid stability of water soluble polymers with a lower critical solution temperature(LCST) have been studied using polystyrene latices plus hydroxy propyl cellulose(HPC). Saturated adsorption(As) of HPC depended significantly on the adsorption temperature and the As obtained at the LCST was 1.5 times as large as the value at room temperature. The high As value obtained at the LCST remained for a long time at room temperature, and the dense adsorption layer formed on the latex particles showed strong protective action against salt and temperature. Furthermore, the dense adsorption layer of HPC on silica particles was very effective in the encapsulation process with polystyrene via emulsion polymerization in which the HPC-coated silica particles were used as seed. 

All these results indicate that the dense adsorption layer of HPC formed on silica particles at the LCST plays a very important role in the area of particle encapsulation. 

Sol-gel microencapsulation in silica particles shares the versatility of the sol-gel molecular encapsulation process, with further unique advantages. Sol-gel controlled release formulations are often more stable, potent and tolerable than currently available formulations. The benefits of microencapsulation can be customized to deliver the maximum set of benefits for each active ingredient. Overall, these new and stable combinations of active pharmaceutical ingredients (APIs) result in improved efficacy and usability. 

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