Silica dissolution as a route to octaanionic silsesquioxanes

The octaanion, [OSiO_1.5]₈^8? (OA) is a low cost, discrete nano silica particle that can be made directly from high surface area, amorphous silica reacted with Me₄NOH in water alcohol mixtures. It would be ideal if Me₄NOH could be formed in situ from, for example, Me₄NCl and NaOH, as long known in the literature. This process would reduce costs and enable recycling of Me₄NCl produced in the functionalization of OA with chlorosilanes, RMe₂SiCl, to form [RMe₂SiOSiO_1.5]₈ organic/inorganic hybrid nanobuilding blocks. Kinetic studies were conducted to assess base‐promoted dissolution of fumed silica (25 m 2 /g) as a function of concentrations, times, etc., to form the octaanion [OSiO_1.5]₈^8? using Me₄NOH, NaOH and mixtures of NaOH/Me₄NCl. Surprisingly, we find that small amounts of Me₄NCl greatly inhibit the dissolution reaction for reasons that are as yet unknown. Copyright © 2005 John Wiley & Sons, Ltd.     

Relationship between the polymer/silica interaction and properties of silica composite materials

Cast film composites have been prepared from aqueous polymer solutions containing nanometric silica particles. The polymers were polyvinyl alcohol (PVA), hydroxypropylmethylcellulose (HPMC) and a blend of PVA‐HPMC polymers. In the aqueous dispersions, the polymer–silica interactions were studied through adsorption isotherms. These experiments indicated that HPMC has a high affinity for silica surfaces, and can adsorb at high coverage; conversely, low affinity and low coverage were found in the case of PVA. In the films, the organization of silica particles was investigated through transmission electron microscopy (TEM) and small‐angle neutron scattering (SANS). Both methods showed that the silica particles were well‐dispersed in the HPMC films and aggregated in the PVA films. The mechanical properties of the composite films were evaluated using tensile strength measurements. Both polymers were solid materials, with a high‐elastic modulus (65 MPa for HPMC and 291 for PVA) and a low‐maximum elongation at break (0.15 mm for HPMC and 4.12 mm for PVA). In HPMC films, the presence of silica particles led to an increase in the modulus and a decrease in the stress at break. In PVA films, the modulus decreased but the stress at break increased upon adding silica. Accordingly, the polymer/silica interaction can be used to tune the mechanical properties of such composite films. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1134–1146, 2006     

Grafting Titanocene on Hydrophilic Amorphous Silica:Synthesis of an Effective Catalyst for Olefin Epoxidation

Ti/silica catalysts were prepared by grafting titanocene dichloride (Cp₂TiCl₂) on hydrophilic amorphous silica with different Ti contents under mild conditions. The results of FT-IR, UV-vis and XPS analyses proved that titanium was successfully grafted in the form of Ti(IV) on amorphous silica and the maximum content of Ti grafted was found to be ca. 3 wt.%. When cyclohexene epoxidation with TBHP was carried out over these synthesized catalysts, both the activity and the selectivity to epoxide increased with the amount of Ti grafted. This amorphous Ti/silica catalyst showed a higher activity and a higher selectivity to epoxide than Ti-containing molecular sieves, Ti-MCM-41 and Ti-beta, with nearly the same Ti content. With Ti content larger than 3 wt.%, however, anatase phase (TiO₂) was formed to give a lower activity and selectivity to epoxide.     

Video-tape observation of the crystal growth and morphology of colloidal single crystals

Crystal growth and morphology of colloidal crystals of silica spheres (81.2 nm in diameter) are observed directly on a video-tape camera. Crystal growth from the round-shaped, small single crystals to the angular-shaped ones is clear. It is observable that the single crystals are packed densely and separated from each other with the grain boundaries. The morphology of colloidal crystals is quite similar to that of typical crystals such as metals, proteins, and ice.     

Surface Modification of Nanometer Silica by N, N''''-dicyclohexyl- carbodiimide Mediated Amidation

Nanometer silica has gained ever more attention for its superior properties over conventional micrometer particles and has been widely used as a filler in the manufacture of paints, rubber, plastics and so on1. However it is difficult to get silica mono- dispersed in the matrix due to heavy agglomeration. Surface modification through organic reactions is an efficient way to improve the dispersibility and compatibility with the matrix, thus improving the mechanical properties of the composite…     

Rigidity of colloidal alloys as studied by reflection spectroscopy in sedimentation equilibrium

Rigidities of colloidal alloys of binary mixtures of colloidal silica spheres (CS82; 103 nm in diameter) with larger silica spheres (CS91; 110 nm, CS121; 136 nm and CS161; 184 nm) have been measured by reflection spectroscopy in sedimentation equilibrium. Substitutional-solid-solution-type alloy structures are formed for mixtures of CS82 and CS91 and for CS82 and CS121. A superlattice, probably MgCu₂ type, is formed for CS82 and CS161 mixtures. The rigidities of the colloidal crystals of the single component of the spheres increase as the sphere size increases at the same number density of spheres. The rigidities of the colloidal alloys decrease when a comparatively small number of the larger spheres are mixed with the small spheres at the same total sphere number density. Received: 14 June 2000 Accepted: 3 November 2000     

Estimation of the diffusion coefficient of water evolved during the non-isothermal dehydration of Australian sedimentary opal

The dehydration of an opal specimen was investigated by thermogravimetric analysis (TG) in powder and bulk forms. The change in geometry resulted in a significant difference in the temperature range in which dehydration occurred with peak temperatures in the differential TG (DTG) curve for the hand ground opal at 203°C and for the bulk opal at 340°C. This difference was attributed to time taken for diffusion of free water in the bulk opal to the specimen surface prior to evolution as a registered mass loss. A model was proposed to account for the diffusion of water and was used to estimate the diffusion coefficient.     

Tailoring 3D shapes of polyhedral milliparticles by adjusting orthogonal projection in a microfluidic channel

Polyhedral milliparticles (PMPs) have promising applications in self-assembly, tissue engineering, mechanical engineering, and photonics. The shapes and sizes of the PMPs have great impact on their functions. To date, the reported methods for adjusting the shapes and sizes of PMPs are still limited. Especially, it remains a challenge to fabricate the PMPs with high asymmetry. Here, we present a facile and efficient approach that focuses on the alteration of the relative positions between the microchannel and the UV light to tailor the shaping process of the PMPs. By tuning the rotation angle and translation displacement, a variety of monodisperse PMPs with both centrosymmetric and noncentrosymmetrical structures are synthesized, including tetrahedra, pentahedra, hexahedra, and so forth. Moreover, the polymeric PMPs are loaded with silica nanoparticles and further sintered into silica. The obtained silica PMPs hold great potential for the applications such as machining tools, abrasives, and electronics.     

Enantiodivergent Chemoenzymatic Dynamic Kinetic Resolution: Conversion of Racemic Propargyl Alcohols into Both Enantiomers

Natural lipases typically recognize enantiomers of alcohols based on the size differences of substituents near the carbinol moiety and selectively react with the R enantiomers of secondary alcohols. Therefore, lipase-catalyzed dynamic kinetic resolution (DKR) of racemic secondary alcohols produces only R enantiomers. We report herein a method for obtaining S enantiomers by DKR of secondary 3-(trialkylsilyl)propargyl alcohols by using a well-known R-selective Pseudomonas fluorescens lipase in combination with a racemization catalyst VMPS4, in which the silyl group reverses the size relationship of substituents near the carbinol moiety. We have already reported R-selective DKR of the corresponding propargyl alcohols without substituents on the ethynyl terminal carbon, and the presence of an easily removable silyl group has enabled us to produce both enantiomers of propargyl alcohols in high chemical yields and with high enantiomeric excess. In addition, immobilization of the lipase on Celite was found to be important for achieving a high efficiency of the DKR.