Thixotropic destruction and secondary cluster growth of supramolecular crystals in suspensions of monodisperse spherical silica particles

Supramolecular crystals (SC) which appear in concentrated suspensions of monodisperse spherical silica particles (MSSP) can be mechanically destroyed. This destruction occurs because the bonding of structural units (SU) in SC is weak and the interaction forces of SU become locally unbalanced. The SC are destroyed into microblocks with the initial SC structure. The nascent secondary suspension consists of these blocks (clusters). The secondary crystals grow from the clusters and when these clusters are large, the growth is of a dendritic character.     

Controlling stages of supramolecular crystallization in suspensions of monodispersed spherical silica particles

Supramolecular crystallization in alcohol suspensions of monodispersed spherical silica particles at pH ~ 7-7.8 is controlled by sedimentation of structural units. At pH ~ 7.8-10.5 or in the presence of indifferent (inert) electrolytes, the crystallization is controlled by the linear growth of supramolecular crystals. The peculiarities of crystallization and nucleation in both cases are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Supramolecular crystal growth in concentrated suspensions of charged monodisperse spherical silica particles

Concentrated suspensions of charged monodisperse spherical silica particles (MSSP) stabilized by alkalis or ammonia are able to crystallize at a certain destabilization. Crystal structures with the particles fixed at certain distances from each other show an isotropic normal mechanism of continuous growth with a rough phase boundary. The crystallization is determined by three parameters,víz. the concentration of particles, temperature, the thickness of the ion atmosphere around the particles and the concentration of counterions. The crystallization of MSSP suspensions is considered as a model of the supramolecular crystallization in the field of synthesis of mesoporous structures.     

Mechanism of structural ordering of monodisperse spherical silica particles in concentrated alcohol suspensions

Structural ordering of monodispersed spherical silica particles (MSSP) occurs in ammonia stabilized concentrated suspensions obtained by tetraethoxysilane (TEOS) hydrolysis in alcohol-aqueous solutions in the ammonia concentration range from 0.0001 to 0.0008 mol/L. MSSP interaction follows the DLFO (Deryagin, Landau, Ferway, and Overbeck) mechanism when electrostatic repulsive forces between the particles predominate, and the structural ordering requires straightened conditions, which are provided by suspension concentrating through MSSP gravitational precipitation.     

Kinetics of the Growth of Supramolecular Crystals in a Suspension of Monodisperse Spherical Silica Particles

Kinetics of the supramolecular crystallization of monodisperse spherical silica particles (MSSP) suspended in diethyl ether with a concentration of structural units close to the phase transition point has been studied for the first time. It was found that the linear crystal growth rate depends on temperature and the process activation energy is 25 kcal/mol. The linear law of crystal growth is associated with diffusion of the structural units on the interface.     

Mechanism of the growth of monodispersed spherical silica particles to the submicron size

Summary Monodispersed spherical submicron silica particles were obtained by the precipitation of soluble silica on the surface of preliminary obtained smaller particles. Silica was added into the system at low concentrations to prevent both its polymerization in the solution and the formation of new particles. The kinetics of the particle growth is controlled by the diffusion of soluble silica through the double diffusion layer.     

Energy dissipation in concentrated monodisperse colloidal suspensions of silica particles in polyethylene glycol

Highly concentrated colloidal suspensions exhibit a discontinuous shear-thickening behaviour. The transition from a low to a high viscosity state is associated to a large energy dissipation. This effect could find applications in structural damping while the viscosity increase brings added stiffness. In the present work, highly concentrated suspensions of monodisperse spherical silica particles in polyethylene glycol were selected for their strong thickening at low critical shear rates. Their damping properties were characterized by measuring the energy dissipated per cycle at low frequency (below 2 Hz) during oscillatory tests using a rheometer. The influence of parameters such as particle concentration, size and frequency was investigated. Damping was found to overcome that of benchmark elastomeric materials only in high frequencies and high strain domains.     

Parametric Analysis of Kinetic Models. Xiii. Catalytic Trigger in a Flow System

The thickness of the concentrated layer in the MSSP suspensions, which is sufficient to initiate phase transition into supramolecular crystals, has been experimentally determined and calculated. When the electrostatic repulsion predominates over molecular attraction, the transition occurs only if gravitational forces, which virtually act as molecular attracting forces, are involved.     

Influence of temperature on the aggregation and sedimentation rate of monodisperse spherical silica particles

The rise in temperature of alcohol suspensions of monodisperse silica particles to 50-100^oC results in an increase in the number of coagulated particles. This phenomenon results from an increase in the degree of electrolyte dissociation, which is equivalent to the rise in its concentration under normal conditions. The suspensions with the aggregates settle at elevated temperatures with the rates that are considerably higher than it is conditioned by the decrease in the viscosity of the suspension. The aggregates and free particles settle jointly, indicating that the non-aggregated particles are held in the field of the long-range molecular attraction of aggregates. This revised version was published online in June 2006 with corrections to the Cover Date.     

Synthesis and characterization of monodisperse porous polymer particles

Monodisperse porous polymer particles in the size range of 10 μm in diameter were prepared via seeded emulsion polymerization. Linear polymer (polystyrene seed) or a mixture of linear polymer and solvent or nonsolvent were used as inert diluents. The pore diameters of these porous polymer particles were on the order of 1000 Å with pore volumes up to 0.9 mL/g and specific surface areas up to 200 m²/g. The physical features of the porous polymer particles depended on the diluent type and the crosslinker content, as well as the molecular weight of polymer seed particles. By varying the molecular weight of the linear polymer, monodisperse porous polymer particles with different pore size distribution could be synthesized. Polymer seed with a low degree of crosslinking instead of linear polymer could also be used to prepare monodisperse porous polymer particles with smaller pore volume and pore size.     

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.     

Capabilities of polymer-modified monodisperse colloidal silica particles as biomaterial carrier

 Polymer modification of monodispersed colloidal silica (0.5 μm) with poly(maleic anhydride-co-styrene) (P(MA-ST)) and poly (maleic anhydride-co-methyl methacrylate) (P(MA-MMA)) and application of the composite particles to biomaterial carriers were investigated. The reaction of bovine serum albumin(BSA)-immobilized P(MA-MMA)/SiO₂ with the anti-BSA antibody showed higher sensitivity in immunological agglutination test than BSA–P(MA-ST)/SiO₂, though immobilization efficiency of BSA on P(MA-MMA)/SiO₂ was lower than that on P(MA-ST)/SiO₂. Alkaline phosphatase and glucose oxidase immobilized on the composite particles exhibited extremely low activities, but α-chymotrypsin immobilized on P(MA-MMA)/SiO₂ and its derivative particles showed the relative activity of 12.5% and 16.1% to the native enzyme, respectively. Grafting of a hydrophilic polymer of poly(acrylic acid) to P(MA-ST)/SiO₂ let to an increase of the immobilized α-chymotrypsin activity to give the maximum relative activity of 55.5%. Received: 23 August 1996 Accepted: 16 October 1996     

Complex shear modulus of concentrated suspensions of solid spherical particles

Starting from the complex shear modulus equation for a dilute suspension system, three new equations are developed for the complex shear modulus of concentrated suspensions of solid spheres. The continuous phase (matrix) and the dispersed particles are treated as viscoelastic materials in the derivation. Complex shear modulus data on suspensions of spherical glass beads in polymeric liquid were obtained experimentally and compared with the predictions of the proposed equations. The proposed equations describe the experimental data reasonably well.     

Preparation of micron-size monodisperse poly(2-hydroxyethyl methacrylate) particles by dispersion polymerization

Dispersion polymerization of 2-hydroxyethyl methacrylate using four categories of polymeric stabilizers in a mixture of good and poor solvents was performed to produce polymeric particles. The stabilizers employed were methyl methacrylate and styrene homopolymers, methacryloyl-terminated poly(methyl methacrylate) and polystyrene macromonomers, an amphiphilic poly(methyl methacrylate-co-methacrylic acid-graft-styrene), and polybutadiene derivatives containing reactive vinyl groups. Dispersion copolymerization with a small amount of the macromonomer gave micron-size particles with relatively narrow size distribution. The amphiphilic graft copolymer and the polybutadiene derivatives also afforded monodisperse particles. The mixed ratio between good and poor solvents greatly affected the particle size and size distribution. © 1996 John Wiley & Sons, Inc.     

Synthesis of monodisperse spherical nanocrystals

Much progress has been made over the past ten years on the synthesis of monodisperse spherical nanocrystals. Mechanistic studies have shown that monodisperse nanocrystals are produced when the burst of nucleation that enables separation of the nucleation and growth processes is combined with the subsequent diffusion-controlled growth process through which the crystal size is determined. Several chemical methods have been used to synthesize uniform nanocrystals of metals, metal oxides, and metal chalcogenides. Monodisperse nanocrystals of CdSe, Co, and other materials have been generated in surfactant solution by nucleation induced at high temperature, and subsequent aging and size selection. Monodisperse nanocrystals of many metals and metal oxides, including magnetic ferrites, have been synthesized directly by thermal decomposition of metal-surfactant complexes prepared from the metal precursors and surfactants. Nonhydrolytic sol-gel reactions have been used to synthesize various transition-metal-oxide nanocrystals. Monodisperse gold nanocrystals have been obtained from polydisperse samples by digestive-ripening processes. Uniform-sized nanocrystals of gold, silver, platinum, and palladium have been synthesized by polyol processes in which metal salts are reduced by alcohols in the presence of appropriate surfactants.     

Supramolecular Crystallization of Monodispersed Silica Spherical Particles (MSSP) in Concentrated Suspensions

The supramolecular crystallization of MSSP concentrated suspensions in alcohol and ethyl ether with various ammonia concentrations is found to occur as a phase transition, with appearence of a pre-transitional state. The temperature dependence of the time, it takes for this state to be established, evidences for the presence of processes of nucleation and activation character.     

Dynamic electrophoretic mobility of spherical colloidal particles in salt-free concentrated suspensions

In this contribution, the dynamic electrophoretic mobility of spherical colloidal particles in a salt-free concentrated suspension subjected to an oscillating electric field is studied theoretically using a cell model approach. Previous calculations focusing the analysis on cases of very low or very high particle surface charge are analyzed and extended to arbitrary conditions regarding particle surface charge, particle radius, volume fraction, counterion properties, and frequency of the applied electric field (sub-GHz range). Because no limit is imposed on the volume fractions of solids considered, the overlap of double layers of adjacent particles is accounted for. Our results display not only the so-called counterion condensation effect for high particle charge, previously described in the literature, but also its relative influence on the dynamic electrophoretic mobility throughout the whole frequency spectrum. Furthermore, we observe a competition between different relaxation processes related to the complex electric dipole moment induced on the particles by the field, as well as the influence of particle inertia at the high-frequency range. In addition, the influences of volume fraction, particle charge, particle radius, and ionic drag coefficient on the dynamic electrophoretic mobility as a function of frequency are extensively analyzed.     

Two-dimensional ordered arrays of monodisperse silica particles grafted with concentrated polymer brushes

Hybrid particles which have a core of monodisperse silica particle and a shell of well-defined poly(methyl methacrylate) chains end-grafted on the core surface with a surface density as high as 0.8 chains/nm² were prepared by surface-initiated atom transfer radical polymerization of methyl methacrylate with an initiator-fixed silica particle. Monolayers of the hybrid particles were formed at the air-water interface by depositing a defined amount of the particle suspension onto water surface. Transmission electron microscopic and atomic force microscopic observations of these monolayers showed that the hybrid particles formed a two-dimensional hexagonally ordered lattice with a wide controllability of interparticle distance. This lattice structure was utilized as a template for the fabrication of a negatively patterned surface of poly(dimethylsiloxane) elastomer.     

Synthesis of spherical submicron-sized magnetite/silica nanocomposite particles

This paper describes a method for fabricating spherical submicron-sized silica particles that contained magnetite nanoparticles (magnetite/silica composite particles). The magnetite nanoparticles with a size of ca. 10 nm were prepared according to the Massart method, and were surface-modified with carboxyethylsilanetriol. The fabrication of magnetite/silica composite particles was performed in water/ethanol solution of tetraethoxyorthosilicate with ammonia catalyst in the presence of the surface-modified magnetite nanoparticles. The magnetite/silica composite particles with a size of ca. 100 nm were successfully prepared at 0.05 M TEOS, 15 M water, and 0.8 M ammonia with injection of the magnetite nanoparticle colloid at 2 min after the initiation of hydrolysis reaction of TEOS. Magnetite concentration in the composite particles could be raised to 17.3 wt.% by adjustment of the injected amount of the magnetite colloid, which brought about the saturation magnetization of 7.5 emu/g for the magnetite/silica composite particles.