Monodisperse Silica Particles Research Articles

Synthesis of Nanosize Silica in a Nonionic Water-in-Oil Microemulsion: Effects of the Water/Surfactant Molar Ratio and Ammonia Concentration

The effect of ammonia concentration on the region of existence of single-phase water-in-oil microemulsions has been investigated for the system polyoxyethylene (5) nonylphenyl ether (NP-5)/cyclohexane/ammonium hydroxide. The presence of ammonia decreases the size of the microemulsion region. A minimum concentration of surfactant (estimated at about 1.1 wt%) is required for solubilization of the aqueous phase; this value is not significantly affected by ammonia concentration. As indicated by fluorescence spectral data, the transition between bound and free water occurs when the water-to-surfactant molar ratio is about 1 and the presence of ammonium hydroxide does not appear to have a significant effect on this. Ultrafine (30–70 nm diameter), monodisperse silica particles produced by hydrolysis of tetraethoxysilane (TEOS) in the microemulsion show a complex dependence of the particle size on the water-to-surfactant molar ratio (R) and on the concentration of ammonium hydroxide. At relatively low ammonia concentration in the aqueous pseudophase (1.6 wt% NH3) the particle size decreases monotonically with increase in R. However, for higher ammonia concentrations (6.3–29.6 wt% NH3) a minimum in particle size occurs as R is increased. These trends are rationalized in terms of (a) the effects of the concentration, structure, and dynamics of the NP-5 reverse micelles on the hydrolysis and condensation reactions of TEOS, and (b) the effects of ammonia concentration on the stability of the microemulsion phase, the hydrolysis/condensation reactions of TEOS, and the depolymerization of siloxane bonds.

Non-linear magnetorheological behaviour of an inverse ferrofluid

The non-linear magnetorheological behaviour is studied of a model system consisting of monodisperse silica particles suspended in a ferrofluid. The stress/strain curve as well as the flow curve was measured as a function of volume fraction silica particles and field strength, using a home-made magnetorheometer. Both curves were found to scale linearly with the volume fraction and quadratically with the magnetic moment of the silica particles. Affine deformation cannot lead to the observed yield stresses. Instead, a bead–rod model is described that gives results closer to what is observed experimentally. A master curve for the viscosity was obtained by plotting the dimensionless viscosity versus the Mason number, Mn. The free single chain model of Martin and Anderson does not give a good description of the measured curves. This may be due to the simplifications with respect to the microstructure of and processes in the suspension.

Rheo-optical Behaviors and Stability of a Silica Particle Suspension Coated with Silane Coupling Agents

The rheo-optical behaviors and suspension stability of silica particles coated with silane coupling agents were investigated experimentally. Mono-dispersed silica particles were synthesized by the sol–gel method and the particles were coated with silane coupling agents such as vinyltriethoxy silane (VTES) and γ-methacryloxypropyl triethoxy silane (MPTES). Although all the suspensions of identical particle volume fraction exhibited similar rheological behaviors at high shear rates, only the stabilized suspensions coated with either VTES or MPTES displayed smooth shear thinning rather than abrupt change in the shear viscosity, as is typical of suspensions with no surface treatment. The present study showed that the MPTES coating was very effective in enhancing the phase stability compared to the VTES coating. The flow-induced dichroism for the MPTES-coated suspensions did not experience sign change, while those of the non-stabilized suspensions changed sign as the shear rate was increased. The VTES-coated suspensions underwent a transition from a stable to an unstable state as the particle volume fraction increased. Specifically, the rheological behaviors of the VTES-coated suspensions of particle volume fraction 0.35 were similar to those of the MPTES-coated suspensions. When the volume fraction exceeded 0.45, however, the effect of VTES coating diminished. Finally, the stress-optical rule proposed by Bender and Wagner was adopted to model the stabilized suspensions considered here. The present results indicated that the stress-optical coefficient could be predicted successfully by the proposed stress-optical rule if the contribution from the hydrodynamic interaction is considered separately from the thermodynamic contribution.

Effect of poly(acrylic acid) on the preparation of thick silica films by electrophoretic sol–gel deposition of re-dispersed silica particles

Thick silica films were prepared by the electrophoretic sol–gel deposition technique in the presence of poly(acrylic acid) (PAA) using monodispersed silica particles; the particles were prepared by the sol–gel method, pre-heat treated and then re-dispersed in the mixture of H2O and ethanol. The weight of deposited silica films was maximized when 0.2 mass % of PAA against the whole amount of sol was added. The particles constructing the thick silica films were packed densely when the amount of added PAA was less than 0.2 mass%. The weight of the film increased with decrease in the content of H2O in the sol when a fixed amount of PAA was added. After the heat treatment of deposited films at 800 °C, crack-free silica films of about 30 μm thickness were prepared. © 1998 Chapman & Hall

Preparation of Narrow Size Distribution Silica Particles Using Microemulsions

Monodisperse silica particles were prepared in microemulsion media using block copolymers and nonionic surfactants. The silica particles were obtained by hydrolysis and condensation of tetraethoxys...

Adsorption of Cationic Polyacrylamide onto Monodisperse Colloidal Silica from Aqueous Electrolyte Solutions

Adsorption of a low linear charge density cationic polyacrylamide (CPAM) onto monodisperse silica particles has been studied over a wide range of KCl concentration (10−3–1M) and pH (4–9). In parallel the surface charge density of silica particles with and without adsorbed polyelectrolyte has been determined. The adsorbed amount (Γ) as a function of electrolyte concentration (cs) exhibits a maximum atcs≈ 0.1M.As pH increases from 4 to 6, Γ increases and then either reaches a plateau (cs≤ 0.1M) or decreases (cs> 0.1M). These features can be rationalized in terms of the interplay between the different factors that govern polyelectrolyte adsorption. The effect of polyelectrolyte adsorption on the surface charge density of silica is largest at lowcs. It decreases with increase of the salt content and vanishes atcs> 0.1M.This suggests that at electrolyte concentrations lower than 0.1M,the polyelectrolyte plays a dominant in surface charge regulation. The ratio of the adsorbed polyelectrolyte charge to total surface charge is close to unity at high pH. It increases drastically with decreasing pH and increasing salt. A large overcompensation of the substrate charge by polyelectrolyte at these conditions is established in the present work.

Preparation of monodisperse silica particles in emulsion media

The main aim of this work was to prepare monodisperse silica particles with specific properties suitable for chromatographic adsorbents. Silica particles were obtained in W/O emulsions by reaction of tetraethyl orthosilicate, using either NH 3 or HCl as catalysts. Emulsions were prepared by physicochemical, low energy emulsification methods. Monodisperse particles were obtained by using an appropriate emulsification method and by controlling emulsion stability during reaction. Silica particles, with mean sizes from 2 μm to 1000 μm, were prepared using HCl as a catalyst while NH 3 produced particles with mean sizes from 0.5 μm to 10 μm. The size was controlled by the mixing ratio between two different type of surfactants. Silica particles were characterised by optical and electron microscopy, nitrogen sorption, FTIR, small angle X-ray scattering and thin layer chromatography. The results showed that monodisperse spherical particles with a high specific surface, appropriate for chromatographic purposes, can be prepared in emulsion media using HCl as a catalyst.

Multivariate analysis of the size dependence of monodisperse silica particles prepared according to the sol-gel technique

The dependence of the size of the silica particles produced by a sol-gel process on constituent concentrations, reaction temperature and time was studied using reduced factorial design 2 6-2. Much information is obtained with a few experiments. The data obtained are interpreted using multiple regression analysis. To account for the experimental results two models based on particle diameter and the other on total particle area are suggested. The full model with cross terms based on particle diameter explains 97.5% of the variation in particle size, while the model including the experimental variables explains only 18.4%. The full model based on total particle area explains 98.4% of the variance, while the model including only the main variables (tetraethyl orthosilicate and water concentration) explains 90%. The models are non-linear, suggesting that at least one of the variables is non-linear. The effects of changed levels of variables are explained on the basis of changed reaction rates.

The Size Dependence of Growth Rate of Monodisperse Silica Particles from Tetraalkoxysilane

The size-dependence of growth rate of monodisperse silica particles was investigated by analyzing the competitive growth of a dispersion of silica spheres with a bimodal and trimodal size distribution. It was found that under the conditions that no new particles could be formed, particles with different size grew at the same rate, demonstrating that the growth was surface reaction limited process. Under conditions of formation of new particles, however, smaller particles grew faster than larger particles, indicating that the growth proceeded through diffusion limited process.

Growth mechanism of large monodispersed silica particles prepared from tetraethoxysilane in the presence of sodium dodecyl sulfate

Monodispersed silica particles up to ca. 1.2 μm in diameter were prepared by hydrolysis of tetraethoxysilane in the presence of sodium dodecyl sulfate (SDS). The particle size was increased with an increase of SDS added. The geometrical standard deviation of the particles was decreased with an increase of SDS. In the earlier reaction stage, double spherical particles by the coalescence of the particles were frequently observed when large amounts of SDS were added. Particle size was gradually increased after the coalescence occurred and spherical particles were finally obtained. The results of Nielsen’s chronomal analysis suggest that the polynuclear layer growth took place after the coalescence of the particles in the presence of larger amount of SDS.

Characteristic Aspects of Formation of New Particles during the Growth of Monosize Silica Seeds

To prepare large monodisperse silica particles, it is necessary to grow silica seeds without the formation of new silica particles. Described in this paper are investigations of the characteristic aspects of formation of new particles during the growth of silica seeds through the hydrolysis and condensation of tetraethylorthosilicate. Experimental results indicated that with silica particles as seeds, under a given set of reaction conditions and total external surface area of seeds, the smaller the seed particles, the fewer the newly formed particles during seed growth. The larger the total surface area of the seeds, the fewer the newly formed particles during growth with the same diameter particles as seeds. The critical surface area of nucleation increases linearly with the diameter of seeds. Addition of a small amount of NaCl to the breeding solution can dramatically decrease the number of newly formed particles. From these experimental findings, it is believed that under the conditions of formation of new particles, seed growth during the early stage is controlled by diffusion of colloid-unstable condensed species carrying electric charges capable of overcoming electrostatic repulsion.

A method for the preparation of PMMA-SiO2 nanocomposites with high homogeneity

It is possible to optimize the performance of the inorganic-organic composites dispersing the inorganic component in the organic matrix on a nanomiter length scale. If dry the inorganic phase cannot be intimately dispersed during the incorporation in the matrix. When the particle surface is organically modified, and the incorporation is made starting from a liquid dispersion (particles in polymer solution), the resulting composites exhibit an excellent homogeneity. Here, monolithic [poly(methyl methacrylate)/monodisperse silica particles] nanocomposites have been prepared and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS), micro-hardness, and differential scanning calorimetry (DSC).

The effect of surface characteristics of silica particles on electrorheology

The effect of the surface characteristics of silica particles on electrorheology (ER) was investigated with a suspension that contained monodispersed silica particles in a silicon oil. Two types of adsorbed water exist at the silica surface; one is dehydrated below 200°C (free water) and the other above 200°C (bound water). It was found that the free water was essential for the ER effect and there was an appropriate amount of free water for an optimum ER effect. The silica surface was treated with cetanol. Cetanol-treated silica has less bound water than untreated silica. The amount of free water on the cetanol-treated silica was controlled by vacuum drying and exposure to saturated water vapor. The cetanol treatment of silica lowered the ER effect. With less bound water on the cetanol-treated silica, the control of free water did not change the ER effect. A direct relationship was not found between the ER effect and current density.

Preparation of monodispersed polymer-modified colloidal silica particles of less than 50 nm diameter

The preparation of monodisperse, ultrafine polymermodified colloidal silica, 11 or 42 nm in diameter, were studied. The reaction of the colloidal silica with polymeric silane coupler in 1,2-dimethoxyethane and removal of the unreacted polymer with granular silica gel (10–20 mesh) in acetone gave a suspension of monodisperse composite particles.

Tailoring of Thermomechanical Properties of Thermoplastic Nanocomposites by Surface Modification of Nanoscale Silica Particles

Thermoplastic nanocomposites based on linear polymethacrylates as matrix materials and spherical silica particles as fillers have been synthesized using the in situ free radical polymerization technique of methacrylate monomers in presence of specially functionalized SiO2 nanoparticulate fillers. Uncoated monodisperse silica particles with particle sizes 100 nm and 10 nm were used as reference fillers. For surface modification, the alcoholic dispersions of the fillers were treated with appropriate amounts of methacryloxypropyltrimethoxysilane (MPTS) and acetoxypropyltrimethoxysilane (APTS). Transmission electron microscopy (TEM) was used to investigate dispersion behaviour in dependence on surface modification. Dynamic mechanical properties were measured by dynamic mechanical thermal analysis (DMTA).

Special Hysteresis Effects in N2-Sorption and Mercury-Porosimetry Measurements

AbstractModel pore structures were prepared from dispersions of submicron monodispersed silica particles by a sedimentation process. Ordered dense sphere packing structures were observedwith scanning electron microscopy. Nitrogen sorption- as well as Hg-porosimetry measurements confirmed the calculated values of the pore openings in those structures. In Hgporosimetry measurements a two step extrusion curve was observed, when the pore system was only partially filled during the intrusion process. This two step curve was not observed in case the pore system was filled with mercury to more than 95% during the intrusion run. The mercury porosimetry results can be interpreted in terms of the coexistence of octahedral and tetrahedral voids (pores) in the examined sphere packing structure and their special arrangement within the structure (connectivity). Two models will be described to explain thegeneral occurrence of hysteresis in Hg-porosimetry. The actual pore geometry is shown to have a profound influence on the hysteresis shape as well as a change in the contact angle (constant within each measurement) can result in totally different hysteresis curves. Nitrogen adsorption and desorption measurements on the same powders did not reveal any fine structure within the hysteresis range.

Coagulation rate of silica dispersions investigated by single-particle optical sizing

Abstract Using a single-particle optical sizer, the kinetics of the rapid coagulation of monodisperse silica particles of radius 265 nm in KCl and CaCl2 solutions was studied. The rate constant k11 of the process is 1.7 × 10−12 cm3 s−1, which is three times smaller than the Smoluchowski value for the overall rate constant. This discrepancy is explained by the retardation effect of viscous interactions and the existence of long-range attractive forces between particles. For suspensions with an initial particle concentration between 2.5 × 108 and 2.4 × 109 cm−3, the k11 values are independent of the pH value (in the range 7–10) and of the particle number concentration. The ratio between the coagulation concentrations of monovalent and bivalent counterions is 50, under both perikinetic and orthokinetic conditions, which indicates that this silica suspension is electrostatically stabilized. This is in contrast to literature data for very small SiO2 particles. We propose that the so-called “structural” forces are much less important for large silica particles.

Preparation of thick silica films in the presence of poly(acrylic acid) by using electrophoretic sol-gel deposition

Thick silica films were fabricated by electrophoretic sol-gel deposition of silica particles on a stainless steel sheet. Using sols prepared by the sol-gel method with poly(acrylic acid) (PAA) films of ca. 25 μm in thickness were prepared with no cracks. The films were shown to be agglomerates of monodispersed silica particles with PAA. The size of the silica particles decreased with an increase in the added amount of PAA. The deposited weight was considerably larger for the films with PAA than that of the films without PAA.

Diffraction of Visible Light by Ordered Monodisperse Silica−Poly(methyl acrylate) Composite Films

Composites of ordered colloidal crystals of amorphous monodisperse silica particles in amorphous poly(methyl acrylate) (PMA) films selectively Bragg diffract visible light. The 153 nm diameter colloidal silica particles coated with 3-(trimethoxysilyl)propyl methacrylate form colloidal crystals in methyl acrylate (MA) monomer, and the crystal order is locked into place by polymerization of the MA. Bragg diffraction from the films is detected spectrophotometrically as narrow peaks of low percent transmission of visible light normal to the film plane. The diffraction wavelength is tuned by varying the d spacing of the crystal lattice and by varying the Bragg angle. Variation in the lattice spacing is achieved via the particle size or particle concentration, uniaxial stretching of the composite, and swelling the composite with monomers such as styrene or MA followed by photopolymerization of the imbibed monomers. Films that diffract from 434 to 632 nm have been prepared.

Preparation of Micron-Size Monodisperse Silica Particles by Seed Growth Method.

テトラエトキシシラン (TEOS) の半回分操作による加水分解および重縮合反応を行い, ミクロンオーダーの単分散シリカ微粒子を効率よく得るためのシード粒子成長法を検討した.その結果, TEOSの添加により反応を開始させた場合は, 添加速度によらずいずれの場合も新たな微小粒子が発生し, 単分散性は損なわれてしまうことがわかった。しかし, 単分散性を維持する回分操作の条件により, 反応を開始させた後に, TEOSおよび水をさらに連続添加することでミクロンオーダーの単分散微粒子が効率よく得られることも判明した.また, この半回分操作のシード粒子成長法における粒子径が精度よく推算可能であることを示した.