Surface-modified silica particles

Philipse, A.P. van Bruggen, M.P.B. Path-mannanharon, C. (1994) Magnetite silica dispersions. Preparation and stability of surface modified silica particles with a magnetic core. Langmuir 10 92—99... 

TTHE SYNTHESIS OF UNCOATED AND SURFACE-MODIFIED silica particles via the hydrolysis of tetraethoxysilane (TEOS) in a homogeneous alcoholic solution of water and ammonia is well documented in the literature (1-7). TEOS and other metal alkoxides (including those of titanium and iron) can... 

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].)...

Beside alkaline non-surface-modified colloidal silica there are also deionized, or salt-free, silica dispersions, which are zero charged at about pH 2 [6]. Such dispersions are not as long-term stable as alkali-stabilized coUoidal silica dispersions, but can stiU be stable for years if stored at low temperatures [7]. In general, the colloidal stability for dispersions of non-surface-modified silica particles is dependent on pH, salt concentration, counter ion valence... 

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... 

Figure 3. Vitamins C and E molecules on the surface of a modified silica particle modeled by a cluster with TMS and silanol groups.

The grafting to method was used to anchor polymer chains onto the surface of silica particles and silicon wafers (Scheme 1). The synthetic procedure starts with covalent grafting of GPS to the surface. A self-assembled monolayer of GPS on silicon wafer surfaces was prepared according to the procedure suggested by Luzinov [32], For this, Si wafers were immersed in a 1% GPS toluene solution for 15 h under dry Ar atmosphere (< 1 ppm H2O). After treatment, GPS modified wafers were washed 3 times in dry toluene under dry Ar atmosphere to avoid polymerisation of non-grafted GPS and precipitation of particles. Afterwards, the silanized wafers were washed 2 times with ethanol in ultrasonic bath for 5 min followed by drying with nitrogen flux. The thickness of the GPS layer was determined by null ellipsometiy. 

Pellicular or controlled surface porosity particles were introduced in the late 1960s these have a solid inert impervious spherical core with a thin outer layer of interactive stationary phase, 1-2 pm thick [13]. Originally, the inner sphere was a glass bead, 35-50 pm i.d., with a thin active polymer film or a layer of sintered modified silica particles on its surface. Such particles were not very stable, had very low sample load capacities because of low surface areas and are not used any more. Nowadays, this type of material is available as micropellicular silica or polymer-based particles of size 1.5 to 2.5 pm [14]. Micropellicular particles are usually packed in short columns and because of fast mass-transfer kinetics have outstanding efficiency for the separation of macromolecules. Because the solutes are eluted as very sharp narrow peaks, such columns require a chromatograph designed to minimise the extra-column contributions to band broadening. 

Burow and Minoura performed a similar kind of investigation to prepare protein imprinted polymers [48]. They used methacrylate modified silica particles as the carrier matrix on which imprinted sites were created. Using acrylic acid as the functional monomer and A,TV -1,2-diethylene bisacrylamide as the cross-linker, template polymerisation was carried out in the presence of glucose oxidase. This approach led to formation of a thin layer of cross-linked polymer film on the silica surface. After removing the template protein, substrate selectivity of the polymer was tested. Preferential affinity of the polymer for its template suggests the formation of substrate-selective binding sites in the polymer matrix. 

Comparing PDMS/HMDS- and HMDS-modified silica particles, a lower phase shift of up to 70° has been observed for the PDMS/HMDS silicas, and for the HMDS silicas a higher phase shift of up to 90° has been found. Differences in phase shift values indicate the impact of surface modification on the local hardness of the silica particles. PDMS modification leads to a softer, polymer-like grafting, whereas pure HMDS modification oidy increases the hydrophobicity by a hard monolayer formation of trimethylsiloxy groups. HMDS-treated silicas seemed to show a weaker interaction with the toner resin surfaces. In contrast, PDMS/HMDS-treated silicas show stronger adhesion to the toner resin surfeces, so they can easily be imaged at high resolution. 

To simulate the toner-silica interactions, PS/PMMA blends were used as a model for polystyrene/acrylic toners. Silicon tips modified either with HMDS or with PDMS were applied to model surface-treated silica particles. The Pulsed Force Mode images of PS/PMMA films displayed... 

Silica aggregates are visible on the surface of the CPT as clusters with a diameter up to 500 nm. In the phase image these aggregates show the highest phase shill, indicating that they are the hardest part of the surface. Around the particles an area with very low phase shift can be found. The thickness of this soft area extends to 100 nm around the particles. Similarly to the PDMS-modified silica particles on the polyester toner, the silylated surface interacts with the toner surface. The large extension of the soft area indicates that the interaction is stronger than for the polyester resin. 

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.

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]. 

One goal is to modify the surfaces of silica particles to improve bonding with PDMS, for example, with vinylethoxysilane or by si-lanization. Similarly, tetraphenyl-modified fumed silica has been used to increase PDMS radiation resistance. Such materials can be difficult to characterize quantitatively. For example, in some cases fumed silica particles in PDMS formed secondary domain structures that made it difficult to characterize nanoparticle formation by tapping-mode atomic force microscopy. Ultra small angle x-ray and neutron scatterings are useful for characterization of such complex morphologies. 

Suzuki et al. [36] modified silica particles (SiP) with a 2-bromoisobutyryl group-carrying silane coupling reagent, and a polymer brush of carboxy methylbetaine, poly[l-carboxy-N,N-dimethyl-N-(2- methacryloyloxyethyl) methanaminium inner salt] (PolyCMB), was generated onto the surface of the particles using SI-ATRP. 

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