Sols of Silica Particles with Modified Surfaces

Sols of Silica Particles with Modified Surfaces 

SOLS OF SILICA PARTICLES WITH MODIFIED SURFACES 

Once silica particles have been formed, the surface can be modified by the attachment of different atoms or groups to modify physical and chemical behavior. If the silica surface is completely covered with a layer of alumina even as thin as one or two molecules in thickness, for example, it then acts as though it were a solid alumina particle, bearing a positive charge and stable at low pH. 

A monolayer of chemisorbed hydrocarbon groups makes the particle act as though it were a large hydrocarbon molecule. 

---

Hirai et al [365] reported fabrication of silica-CdS composites by first adding 3-mercaptopropyltrimethoxysilane into freshly prepared CdS nanoparticles in a two -microemulsion system (AOT/isooctane/aqueous solution of cadmium nitrate and sodium sulfide). The surface modified nanoparticles were collected, washed in hexane, and dispersed in tetramethyl orthosilicate, dimethyl formamide, dichloromethane, chloroform etc. When selected dispersions were added to silica sols and properly processed, 100 nm silica particles with CdS core could be prepared. In an earlier work [366], silica particles were first obtained by precipitation in a microemulsion containing Igepal CO-520 i.e. poly(oxyethylene)nonylphenyl ether or Triton N-101 with a similar chemical structure, cyclohexane, hexanol (for the Triton surfactant) and ammonium hydroxide solution. The source of silica was TEOS which was injected into the reverse microemulsion. After this injection, two microemulsions of similar compositions but containing Cd(N03)2 or (NH4)2S in the aqueous phase were simultaneously injected into the microemulsion prepared for silica synthesis. After several hours, the hydrolysis-condensation product of TEOS grew into particles of size 35-50 nm depending on experimental conditions, with uniformly dispersed, 10 mol % CdS nanoparticles (size about 2.5 nm) incorporated in them. Zinc-doped, alkanedithiol-modified silica particles obtained by hydrolysis of TEOS were also used for immobilization of CdS from a reverse micelle system. The general motivation was the development of photocatalysts [367]. 

Figure 39.5 Impact residual impressions of sol-gel silica coatings reinforced with (a) unmodified silica particles and (b) surface-modified silica particles after impact testing (1 kg from 1 m height). (Reproduced from Ref. [44].)...

The key effect of oxide supports on the catalytic activities of metal particles is exerted through the interface between oxides and metal particles. The key objective of this study is to develop synthesis methodologies for tailoring this interface. Here, an SSG approach was introduced to modify the surface of mesoporous silica materials with ultrathin films of titanium oxide so that the uniform deposition of gold precursors on ordered mesoporous silica materials by DP could be achieved without the constraint of the low lEP of silica. The surface sol-gel process was originally developed by Kunitake and coworkers.This novel technology enables molecular-scale control of film thickness over a large 2-D substrate area and can be viewed as a solution-based... 

In an interesting illustration of the first strategy, Sakaguchi and coworkers covalently attached hemoglobin to an aminopropyl silica particle and then polymerized organoalkoxysilanes on the surface of the hemoglobin-modified silica particle.85 The template was removed via treatment with oxalic acid.85 In more recent work, Zhang and coworkers utilized a similar approach. In their case, the sphere was made from the functionalized biopolymer, chitosan.86 The model template protein, bovine serum albumin, was covalently attached to the chitosan microsphere and then coated with a composite sol prepared from TEOS and an aminosilane.86... 

The industrial development of silica sol manufacturing methods is reviewed. Primary attention is focused on the preparation of monodispersed sols from water glass by the ion-exchange method. Details are given for variations of manufacturing process and for the characteristics of both the processes and sols obtained. Furthermore, the following surface modifications of particles are demonstrated silica sols stabilized with ammonia, amine, and quaternary ammonium hydroxide aluminum-modified or cation-coated silica sol and lithium silicate. Finally, future trends in silica sol manufacturing are discussed from the viewpoint of not only raw materials and improvement of the procedures but also the function of the silica sols and their particle shape. 

Scheme V, silica particles are brought into contact with a cation-exchange resin, and if necessary, with an anion-exchange resin, to obtain an acidic silica sols of pH 2-4. This acidic sol is stable because it is negatively charged even at pH 2-4, according to zeta-potential measurements. Starting from such acidic silica sol, surface-modified silica sols are manufactured.

Positive silica sols and alumina modified silica sols wherein the ultimate silica particles have been modified and/or made electrically positive by partially or completely coating the particle surface with aluminum compounds can also be used in the present invention as a source of amorphous silica. Such sols are described for example by G. B. Alexander and G. H. Bolt in U.S. Pat. No. 3,007,878 and... 

One of the advantages of starting with a silica sol is that when such colloidal solutions are dried, the surface of the silica particles becomes coated with the metal or metal oxide or their soluble precursors, in molecular dimensions, and little, if any, excess metal modifier will be present in locations other than on said particle surfaces. Thus, the metal modifier is located in such a manner as to exert its maximum influence on catalytic activities and harmful excesses are not concentrated in isolated areas. 

Porous silica particles can be prepared by the sol/gel process with a wide range of pore volumes and radii, which can be chosen to be close to the approximate size of template molecules of interest. The inner surface of the silica can be modified such... 

If efficient active sites shall be created by stabilizing the molecular species at the surface of a well-defined phase, as in the case of V0x/Ti02 - anatase catalysts, the better activity in NO SCR was achieved by cocondensation of the dopant alkoxide with the surface groups of preformed colloidal particles of the matrix precursor, followed by gelation [76]. A similar approach of modifying the matrix precursor sol with dopant molecules yielded well-dispersed tantalum species at the silica surface, efficient in selective oxidation of pyrimidine thioether [77]. 

In order to obtain hybrid coatings that combine both the advantages of today s organic materials and the advantages of inorganic materials, sol-gel chemistry is performed in situ on the organic resin. Fumed silica particles (Aerosil ) are modified via condensation of the hydrolyzed organofunctional silane with the Si-OH-rich surface. Scheme 1 describes the condensation of the... 

PDMS is relatively weak and stands to benefit most from reinforcement. PDMS is easily absorbs the precursor materials generally used in the sol-gel process. Nearly monodisperse silica microparticles can be obtained using siloxane elastomer mixtures. In some cases, the PDMS has been part of a copolymer, with ureas, imides, amideimides, ° and diani-lines. In other approaches, the particle surface is modified, for example, with a polysiloxane. Siloxane/silica nanocomposites have also been used as culture-stone-protective materials. ... 

Her (324) prepared a series of sols modified with various amounts of aluminate. One series was made from an unmodified sol of 22 nm particles (Ludox TM) and another of 14 nm particles (Ludox HS). The coverage by aluminate ions ranged from 1.8 to 25% of the total silanol groups on the surface (assuming 8 SiOH nm ). Stabilization as an aluminate ion in the particle surface requires that each aluminum atom be surrounded by three oxygens linked to silicon, which means that no more than 25% of the surface silicon sites can be replaced by aluminum atoms. This assumes, of course, that the underlying silica surface is nonporous and not accessible to reaction with Al(OH)4" ions. 

---