Silica particles formation

Abdoul-Aribi, N. and Livage, J. (2005) Gelatine thin films as biomimetic surfaces for silica particles formation. Colloids and Sufaces B-Biointerfaces, 44, 191-196. 

Chang CL, Fogler HS (1996) Kinetics of silica particle formation in nonionic w/o microemulsions from TEOS. AIChE J 42 3153-3163... 

This chapter focuses on silica synthesis via the microemulsion-mediated alkoxide sol-gel process. The discussion begins with a brief introduction to the general principles underlying microemulsion-mediated silica synthesis. This is followed by a consideration of the main microemulsion characteristics believed to control particle formation. Included here is the influence of reactants and reaction products on the stability of the single-phase water-in-oil microemulsion region. This is an important issue since microemulsion-mediated synthesis relies on the availability of surfactant/ oil/water formulations that give stable microemulsions. Next is presented a survey of the available experimental results, with emphasis on synthesis protocols and particle characteristics. The kinetics of alkoxide hydrolysis in the microemulsion environment is then examined and its relationship to silica-particle formation mechanisms is discussed. Finally, some brief comments are offered concerning future directions of the microemulsion-based alkoxide sol-gel process for silica. 

Markowitz et al. developed a different approach, again in an attempt to overcome some of the inherent difficulties that arise when imprinted bulk materials are used as catalysts [82], Here, the authors used a template-directed method to imprint an a-chymotrypsin TSA at the surface of silica nanoparticles, prepared with a number of organically modified silanes as functional monomers. Silica particle formation was performed in a microemulsion, where a mixture of a non-ionic surfactant and... 

It was postulated that the silicon dioxide molecule is a first intermediate product in the route from silicon tetrachloride to fumed silica. Due to high temperatures in the flame the ortho-silicon acid or meta-silicon acid will not be the main product of silicon tetrachloride hydrolysis or/and oxidation. The silicon dioxide molecule is well known to be stable at high temperatures or in a matrix-isolated state. Therefore, we are able to use silicon dioxide molecules as initial silicon-containing substances to simulate silica particle formation. Meta-silicon acids are able to react with other meta-silicon acid molecules and produce bigger clusters. Three meta-silicon acid molecules form cyclic clusters with four-coordinated silicon atoms and two-coordinated oxygens (Fig. 1). At high temperatures this cluster may loose some water molecules from hydroxyl groups and... 

We have applied ab-initio molecular orbital methods in conjunction with reaction rate theory in a systematic procedure for calculating rates for chemical nucleation/reaction processes (12,13). An example of such an approach has been the calculation of the nucleation of SiO, a monomer to silica particle formation. The results of the computations indicate the rate constants are both pressure and temperature sensitive. This sensitivity results from the rate of energy transfer with the surrounding bath gas (M). Higher temperatures and lower pressures both lower the bath gas encounter rate with the growing cluster and therefore the nucleation rate. The individual kinetic steps in a nucleation event can be expressed as follows ... 

Mieszawska, A. J., Nadkami, L. D., Perry, C. C., and Kaplan, D. L. (2010). Nanoscale control of silica particle formation via silk-silica fusion proteins for bone regeneration. Chem. Mater. 22, 5780-5785. 

Bauer, C.A., Robinson, D.B. and Simmons, B.A. (2007) Silica particle formation in confined environments via bioinspired polyamine catalysis at near-neutral pH. Small, 3, 58-62. 

Two of the study systems, Lake Michigan and Pond 3513, exhibit cyclic behavior in their concentrations of Pu(V) (Figure 2 and 3). The cycle in Lake Michigan seems to be closely coupled with the formation in the summer and dissolution in the winter of calcium carbonate and silica particles, which are related to primary production cycles in the lake(25). The experimental knowledge that both Pu(IV) and Pu(V) adsorb on calcium carbonate precipitates(20) confirms the importance of carbonate formation in the reduction of plutonium concentrations in late summer. Whether oxidation-reduction is important in this process has not been determined. 

The surface of silica is covered by a layer of acidic silanol and siloxane groups. This highly polar and hydrophilic character of the filler surface results in a low compatibihty with the rather apolar polymer. Besides, highly attractive forces between silica particles result in strong agglomeration forces. The formation of a hydrophobic shell around the silica particle by the sUica-sUane reaction prevents the formation of a filler-filler network by reduction of the specific surface energy [3]. 

Figure 1c shows the spectrum of aerosil that has been slurried in water and then dried at 100 C. This treatment initiates gel formation, so that the sample is no longer a chain of silica particles held together by electrostatic forces, but a porous network held together by siloxane linkages. The most obvious features in this spectrum are an increase in the water adsorption features at 3400 cm"" and 1632 cm". In addition, a band at 976 cm" is evident, that was much less obvious in the spectra of the other two silica samples. This feature is due to siloxane bridges formed during gel formation (10,12). 

Figure 3.15. Formation of silica particle by polycondensation of silicic acid molecules (after LePage, 1987).

X-Ray irradiation of quartz or silica particles induces an electron-trap lattice defect accompanied by a parallel increase in cytotoxicity (Davies, 1968). Aluminosilicate zeolites and clays (Laszlo, 1987) have been shown by electron spin resonance (e.s.r.) studies to involve free-radical intermediates in their catalytic activity. Generation of free radicals in solids may also occur by physical scission of chemical bonds and the consequent formation of dangling bonds , as exemplified by the freshly fractured theory of silicosis (Wright, 1950 Fubini et al., 1991). The entrapment of long-lived metastable free radicals has been shown to occur in the tar of cigarette smoke (Pryor, 1987). 

Tleugabulova, D., Duft, A.M., Zhang, Z., Chen, Y., Brook, M.A. and Brennan, J.D. (2004) Evaluating formation and growth mechanisms of silica particles using fluorescence anisotropy decay analysis. Langmuir, 20, 5924—5932. 

Scheme 7.3 The three stages of the mechanism of formation of the PE silica particles. (Reproduced from ref. 8, with permission.)...

Figure 5.10. Images of an OAV emulsion containing 90 wt% of oil and 24.4 mg of hy-drophobically modified silica particles taken at different times since formation. The times are (a) 9 s, (b) 21 s, (c) 54 s, (d) 141 s and the scale bar = 7.5 mm. (Reproduced from [46], with permission.)...

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