Silica clusters

Gas-phase nucleation Flame synthesis of particles (e.g., carbon black, silica) cluster formation in chemical vapor deposition manufacture of high-purity silicon cluster structure and energetics plasma synthesis of refractory materials and coatings. 

Electron behavior, optical properties, catalytic properties, conductivity, and magnetic properties of nanocomposites were discussed in an extensive review pa-per. Complementary use of electron paramagnetic resonance and nuclear magnetic resonance helped to understand chain mobility in nanocomposites obtained from poly(ethylene oxide) encapped with triethoxy silicon. This nanocomposite is composed of PEO chains attached to silica clusters. It was found that chain fragments close to the silica clusters have hindered mobility due to the reduction of local free volume. The length of this hindered segment is estimated as three ethylene oxide units. 

Fig. 9. Photograph of monoliths made from sUica sols containing an ABC template as well as a latex dispersion. The latex, as the least dense component, floats on top of the centrifnge cell, whereas dense, larger silica clusters sediment to the bottom. The ABC-siUca hybrid phase remains between the two...

The size of silica clusters used in ab initio quantum chemical calculations of the activation energy plays an important role, for example, the use of a cluster with four tetrahedron Si04 on interaction of SiCU with the SiOH group through the SeI mechanism gives... 

Energy of Unimolecular and Bimolecular Reactions of Bound Phenyl-Methanol and Heptanol on Silica Cluster (PM3 Method)... 

Figure 9. Vibrational density of states of four-ring clusters. SSSS= pure silica cluster ASSS=one A1 substituted cluster A SSS=protonated A1 substituted cluster [16].

Keywords Silica Clusters / Meta-Silicon Acid / Quantum-Chemical Modeling / Semiempirical AMI and PM3 Approximations / Vibrational Spectroscopy... 

Product of reaction simulation between silica cluster 27Si02 and HCI (2) Product of reaction simulation between silica cluster 27Si02 and HCI with H2O (3) Product of reaction simulation between two silica clusters 27Si02+3-H20 and 27SiO2+6 H20 (4) Product of reaction simulation between two silica clusters... 

Hydrochloric acid is able to react with a hydroxyl-free silica surface without a barrier, with formation of hydroxyl groups and weakly bonded chlorine (Fig. 5( 1)). The distance between chlorine and silicon for the system under study is 2.417 A, compared to the experimental value of 2.017 A and calculated value 2.054 A for SiCU. The formal charge on chlorine for the system under study is -0.754 e, compared with -0.354 e for SiCU- The Wiberg index (bond order) for the chlorine-silicon bond for the system under study is 0.3231, compared to a value of 0.8468 for SiCU- A similar phenomenon is presented for systems like water and hydrochloric acid/water (Fig. 5(2)). These data show that the chlorine atom is bonded to the silica cluster mainly as an ion, and it is possible to understand the system under study as an ion-pair, which may dissociate into ions. As a result the chatted particles will be repelled electrostatically. 

Partially hydroxylated silica clusters are able to interact at long distances by dipole-dipole interactions, followed by attraction. At short distances, reaction may occure without any chemical reaction barrier of ordinary siloxane bonds, if silica particles contain one-coordinated oxygen and/or three-coordinated silicon atoms at their surfaces. In any other cases, without a coverage of chemically active groups, silica particles are able to interact by surface hydroxyl groups with formation of H- onds (Figs. 5(3) and 5(4)). 

Coordinal ion number Fig. 2. Typical silica clusters and their characteristics. 

Axford (1997) showed that the W value for silica cluster aggregation is affected by the ionic strength and cluster size. This information can be used to model the effect of ionic strength and cluster size on the aggregation time. 

Figure 9.6. Logarithm of the time needed for one half of the silica clusters to aggregate (log t, time in seconds) contoured in terms of the logarithm of the ionic strength (/) and logarithm of the cluster size N= number of 12 nm...

Ab initio quantum chemical calculations of a hydrated PG molecule in the free (Figure 1.75a) and adsorbed states (Figure 1.75b) show that the hydration shell changes upon the PG adsorption onto a silica cluster. 

---