Role of solvation forces in the gelation of fumed silica-alcohol suspensions

Aggregation and gelation kinetics of fumed silica were investigated by altering the solvent-surface interactions. Native and surface-modified (hydrophobic) fumed silica particles were dispersed in short-chain linear alcohols. Based on the kinetics of aggregation and gelation, we show that the solvent-surface interactions have a tremendous impact on the bulk suspension properties. The gelation kinetics were qualitatively similar in all of the fumed silica-alcohol samples, and the gel times for all the alcohols were captured on a master curve requiring two parameters. The two parameters, the stability ratio and critical volume fraction, describe the two regimes of gelation. At low concentrations, gelation occurs due to aggregation of the particles diffusing over a potential barrier (15-25 kT). The rate of aggregation and time to gelation then scales with the stability ratio. At high particle loadings, gelation occurs at a critical volume fraction due to localization in a secondary minimum with a depth of 3-4 kT. These observations are supported by evidence of hydrogen bonding between the solvent and the particle, creating oscillatory solvation forces that govern the magnitude of these two parameters.     

Dispersion stabilities and rheological properties of fumed silica suspensions

Herein, we have reviewed fumed silica suspensions in dispersing fluids, polymer melts, and polymer solutions, focusing on their dispersion stability and rheological properties as a function of the surface character of fumed silica powders and the silica volume fraction, ?. Hydrophilic fumed silica powders are well dispersed at ? < 0.01 in polar dispersing fluids or polar polymer melts, and their phase states change from sol to gel with increasing ?. Such changes should also be strongly related to the rheological responses of the hydrophilic fumed silica suspensions, which change from Newtonian flow behavior to gel-like elasticity with increasing ?. On the other hand, hydrophobic fumed silica powders are stabilized in both polar and nonpolar dispersing fluids, depending on the interactions between the surface hydrophobic moieties and the dispersing fluids, in addition to those between the residual surface silanol groups and dispersing fluid, except for the particle–particle interactions. Moreover, the effects of the adsorption and desorption of polymers, as well as of non-adsorbing polymers on the dispersion stability and rheological behavior of fumed silica suspensions are discussed, by taking account of their optical microscopic observation and SANS curves.     

Inversion of silica-stabilized emulsions induced by particle concentration

Emulsions of equal volumes of a cyclic silicone oil and water stabilized by fumed silica nanoparticles alone can be inverted from oil-in-water (o/w) to water-in-oil (w/o) by simply increasing the concentration of particles. The phenomenon is found to be crucially dependent both on the inherent hydrophobicity of the particles and on their initial location. Inversion only occurs in systems with particles of intermediate hydrophobicity when dispersed in oil; emulsions prepared from the same particles but initially dispersed in water remain o/w at all particle concentrations. The stability and drop size distributions in the different emulsions are compared. Various hypotheses are put forward and argued to explain this novel inversion route including adsorption of oil onto particle surfaces, hysteresis of contact angle affecting particle wettability in situ, and the structure of particle dispersions in oil or water prior to emulsification inferred from rheology and light scattering measurements. We propose that the tendency for particles to behave more hydrophobically at higher concentrations in oil is due to the reduction in the effective silanol content at their surfaces as a result of gel formation via silanol-silanol hydrogen bonds. In water, solvation of particle surfaces prevents this from occurring and particles behave as hydrophilic ones at all concentrations. A concentration-induced change in particle wettability is thus advanced.     

Water-in-carbon dioxide emulsions stabilized with hydrophobic silica particles

W/C emulsions were stabilized using hydrophobic silica particles adsorbed at the interface, resulting in average droplet diameters as low as 7.5 microm. A porous cross-linked shell was formed about a hydrophilic (colloidal and fumed) silica core with a trifunctional silylating agent, (heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethyoxysilane, to render the particles CO(2)-philic. The stability of emulsions comprising equal weights of CO(2) and water was assessed with visual observations of settling fronts and the degree of emulsion coalescence, and the average drop size was measured by optical microscopy. The effect of CO(2) density on both emulsion stability and droplet size was determined quantitatively. The major destabilizing mechanism of the emulsions was settling, whereas Ostwald ripening and coalescence were not visible at any density, even over 7 days. Flocculation of the settling droplets did not occur, although gelation of the emulsions through particle interactions resulted after longer periods of time. CO(2)-philic particles offer a new route to highly stable W/C emulsions, with particle energies of attachment on the order of 10(6)kT, even at CO(2) densities as low as 0.78 g ml(-1). At these low densities, surfactants rarely stabilize emulsions as the result of poor surfactant tail solvation.     

Controllable synthesis of silica nanoparticle size and packing efficiency onto PVP-functionalized PMMA via a sol–gel method

This study aimed to investigate synthesis and adsorption behavior of silica nanoparticles onto polyvinylpyrrolidone (PVP)-functionalized poly(methyl methacrylate) under various conditions such as methanol/water ratio, ammonium hydroxide concentration, polymer contents, tetraethylorthosilicate contents, and total volume of solvent via sol–gel method. First, the copolymerization of methyl acrylate as a comonomer and 1-dodecanethiol as a chain transfer agent increased the thermal stability of the product; however, the uniformity of the PMMA particles decreased because of the chain transfer reaction. Second, the adsorption behavior and size of silica nanoparticles could be controlled by adjusting the silica synthesis conditions. The adsorbed silica particle size was greatly influenced by the ammonium hydroxide concentration and the addition of water further enhanced the size increase. However, increasing the water content reduced the packing efficiency of the adsorbed silica particles. Increasing the PVP-functionalized PMMA content at a fixed TEOS content linearly decreased the silica particle size. But TEOS concentration did not significantly affect the silica particle size. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 662–672     

Effects of solvent on TEOS hydrolysis kinetics and silica particle size under basic conditions

In-situ liquid-state ²⁹Si nuclear magnetic resonance was used to investigate the temporal concentration changes during ammonia-catalyzed initial hydrolysis of tetraethyl orthosilicate in different solvents (methanol, ethanol, n-propanol, iso-propanol and n-butanol). Dynamic light scattering was employed to monitor simultaneous changes in the average diameter of silica particles and atomic force microscopy was used to image the particles within this time frame. Solvent effects on initial hydrolysis kinetics, size and polydispersity of silica particles were discussed in terms of polarity and hydrogen-bonding characteristics of the solvents. Initial hydrolysis rate and average particle size increased with molecular weight of the primary alcohols. In comparison, lower hydrolysis rate and larger particle size were obtained in the secondary alcohol. Exceptionally, reactions in methanol exhibited the highest hydrolysis rate and the smallest particle size. Ultimately, our investigation elaborated, and hence confirmed, the influences of chemical structure and nature of the solvent on the formation and growth of the silica particles under applied conditions.     

Fumed silica synthesis: influence of small molecules on the particle formation process

Burning silicon tetrachloride in an oxygen‐hydrogen flame produces fumed silica. This process is known for at least 50 years [1‐5], but some important details are still uncertain. We would like to study several starting steps of fumed silica synthesis on the way from molecules to products. To do this we have performed quantum‐level simulations of protoparticle and primary particle formation, from silicon dioxide molecules. Additionally, we have simulated the behavior of silica clusters in the presence of small molecules like water and hydrochloric acid. The reaction of silicon dioxide molecules leads to a silica cluster, which is covered with chemicaly highly active sites of one‐coordinated oxygen atoms and three‐coordinated silicon atoms. These clusters interact together and produce silica bulk like quartz glass. Reaction with water terminates the silica particle surface and leads to a complicated structure of the particle surfaces. The hydroxyl shell protects the particle body against the increase in particle size, but leads to aggregate and agglomerate formation.     

Structural characteristics of pyrocarbon-fumed silica

Two series of pyrocarbon/fumed silica (CS) samples at different carbon concentrations C_C=0.5–64 wt.% (first series, CSI) and 2.6–53 wt.% (second series, CSII) synthesised by means of pyrolysis of CH₂Cl₂ at fumed silica substrate (S_BET=297 (CSI) and 232 (CSII) m² g⁻¹) under slightly different conditions were studied by using TEM, FTIR-PAS, DTG, and nitrogen adsorption–desorption methods. On methylene chloride carbonisation, disordered carbon deposits can form mainly in the inter-particle volume of secondary particles (aggregates of primary particles and agglomerates of aggregates) covering the surfaces of primary silica particles; therefore, marked reduction of the pore (gaps between primary particles) volume and the specific surface area is observed with increasing C_C. Estimation of distributions of the pore f_SCD(R_p) and particle f(a) sizes using a self-consistent method with binary regularisation with respect to both f_SCD(R_p) and f(a) shows that the average size of particles increases (silica particles are covered by pyrocarbon) and individual pyrocarbon particles (up to 50 nm according to TEM) also appear. Structural parameters of carbosils are characterised by nonlinear changes with increasing carbonisation time. Surface functionalities on CS samples correspond to aromatic and twinned CC bonds with contribution of oxygen-containing groups.     

Effects of particle volume fraction on distortion of particle-arrayed structure during immobilization of colloidal crystals formed by poly(methyl methacrylate)-grafted silica in acetonitrile

Changes of particle array structure with particle volume fraction during immobilization of colloidal crystals, formed by poly(methyl methacrylate)-grafted silica in acetonitrile, were investigated. Immobilization of colloidal crystals formed in acetonitrile was carried out by two-step photo-radical copolymerization of methyl methacrylate and ethylene dimethacrylate to make organogel, followed by solidification after exchanging the solvent with methyl methacrylate. Crystallite size in colloidal crystals formed in acetonitrile was mostly unchanged with particle volume fraction in the range of 0.11–0.18, while the size and number of single crystals decreased during gelation. Disordering in particle array in immobilized colloidal crystals in gel and poly(methyl methacrylate) matrix was observed to decrease with increasing particle volume fraction less than 0.18 due to strong electrostatic repulsion between particles.     

Influence of morphology and composition of fumed oxides on changes in their structural and adsorptive characteristics on hydrothermal treatment in steam phase at different temperatures

A series of fumed oxides such as silica, titania, alumina, silica/alumina (SA), silica/titania (ST), and alumina/silica/titania (AST), initial and hydrothermally treated (HTT) in the steam phase at T(HTT)=150, 250, and 350 degrees C was studied by adsorption, AFM, XRD, FTIR, and theoretical methods. Diminution of the size of primary particles (corresponding to increasing S(BET)) of initial silica and mixed oxides results in enhancement of their structural changes on HTT with elevating T(HTT) and increasing density of packing of primary particles in the secondary structures. Relative changes in the texture of treated fumed silicas are smaller than those of mesoporous silica gels occurring under similar HTT conditions. On HTT, aggregates of primary particles and their agglomerates become denser but their surface layers become looser because of transfer of silica fragments from one particle to another, and the smaller the initial primary particles, the greater the relative diminution of the specific surface area S(BET) for the same type of primary particle packing in aggregates. Relative changes in the pore volume V(p) (or V(BJHd)) on HTT are more complex than that of S(BET), as for many samples the V(p) value increases especially at T(HTT)=150 degrees C. Alumina and titania partially inhibit structural changes on HTT, which decrease in the series silica > SA > AST approximately ST.     

Effect of lithium salt on the stability of dispersions of fumed silica in the ionic liquid BMImBF4

We have investigated the stability and interactions in dispersions of colloidal fumed silica, Aerosil 200, and the ionic liquid 1-butyl-3-methylimidazolium tetraflouroborate (BMImBF(4)) as a function of the Li salt concentration (LiBF(4)). Photon correlation spectroscopy was used to study the aggregation behavior at low silica concentrations, and Raman spectroscopy was used to investigate the interactions in the ionic liquid and with the silica surface. We find that the addition of LiBF(4) increases the stability of the dispersions, with smaller agglomerates of silica particles and higher gelation concentrations in the presence of Li salt. The increased stability with the addition of Li salt is explained by the formation of a more stable solvation layer, where Li ions accumulate on the surface. This leads to an increased interaction between lithium ions and the BF(4)(-) anions in the solvation layer, as seen by Raman spectroscopy. Upon gelation, the Li ions are expelled from the surface because hydrogen bonding between the silica particles are formed. For both neat BMImBF(4) and Li-salt-doped BMImBF(4)/silica dispersions, a weak gel phase was found preceding the formation of a strong gel at slightly higher silica concentrations.     

Effect of Particle Properties on the Break Up of Nanoparticle Clusters Using an In‐Line Rotor‐Stator

This study was carried out to investigate the break up of nanoparticle clusters in a liquid using an in‐line rotor stator. Two types of fumed silica particles were dispersed in distilled water: Aerosil 200 V, which is hydrophilic, has a primary particle size of 12 nm and Aerosil R816 which is based on Aerosil 200 V and surface modified to render it hydrophobic. The article reports on the rheology of the dispersions, particle size analysis, the effect of concentration, and processing conditions such as the rotor speed, that is, the specific power input, and flow rate, that is, the residence time.     

Rheological Properties of Silica Particle Suspensions in Mineral Oil

The rheological properties of particles suspended in a non‐polar mineral oil have been investigated as a function of volume fraction of particles, particle size, surface properties and shear rate. Three different types of particles were investigated; glass microspheres, monodisperse silica particles and fumed silica. The suspensions showed shear thinning behavior at higher volume fractions, and the viscosity increased with decreasing particle size. The hydrophobic particles display lass shear thinning effects. The relative viscosity of all the suspensions was well fitted to the Krieger and Dougherty model.     

Effect of synthesis parameters on the particle size, composition and colloid stability of polypyrrole–silica nanocomposite particles

 The effect of varying the oxidant, monomer and silica sol concentrations, silica sol diameter, polymerization temperature, stirring rate and oxidant type, on the particle size, polypyrrole content and conductivity of the resulting polypyrrole– silica colloidal nanocomposites has been studied. Surprisingly, nanocomposite formation appears to be relatively insensitive to most of the above synthesis parameters. One synthesis parameter which does have a significant and reproducible effect is the stirring rate: smaller, more monodisperse nanocomposite particles are obtained from rapidly stirred reaction solutions. However, this effect is only observed for the (NH₄)₂S₂O₈ oxidant. An alternative oxidant, H₂O₂/Fe³⁺, was found to give nanocomposites of similar particle size, polypyrrole content and conductivity to those obtained using the (NH₄)₂S₂O₈ oxidant. The colloid stability of these polypyrrole–silica nanocomposite particles depends on their silica content. The colloid stability of a silica-rich nanocomposite prepared using the (NH₄)₂S₂O₈ oxidant in the presence of electrolyte was comparable to that of a silica sol, whereas a polypyrrole-rich nanocomposite prepared using FeCl₃ had markedly poorer colloid stability under these conditions. These observations are consistent with a charge stabilization mechanism for these nanocomposite particles. Received: 5 March 1998 Accepted: 27 April 1998     

Effect of particle hydrophobicity on the properties of silica particle layers at the air-water interface

This article describes a study of fumed silica particle layers adsorbed at the air-water interface. We have performed surface pressure, ellipsometry, and Brewster angle microscopy measurements. These determinations were complemented by surface viscoelasticity studies, using capillary waves to measure the compression moduli and an oscillating disc to measure the shear moduli. Our results show a strong influence of the particle hydrophobicity and surface density on the properties of the layers. Under compression-expansion, the particle layers rearrange quasi-instantaneously, and at high density, they buckle and/or collapse. Shear measurements show a transition from viscous to elastic behavior for particles with contact angles close to 90 degrees. The surface compression moduli are quite small and most likely not related to the stability of the foams made with these particles, in contrast to the case of more common surfactant foams.     

Effect of various inorganic cations (Li,Na, K and Cs) and silica sources on the synthesis of the silica analogue of zeolite NCL-1 (Si-NCL-1)

The role of various inorganic cations such as Li, Na, K and Cs has been studied in the hydrothermal synthesis of the silica polymorph of zeolite NCL-1 (Si-NCL-1). The crystallization time decreases in the order (Cs + Na) > K > Na> (Li + Na) under otherwise identical gel composition and crystallization conditions. Further, the synthesis has been standardized using different silica sources including various fumed silicas with different surface area and particle size, through optimizing the OH⁻/Si molar ratio by adding the required amount of alkali hydroxide in the synthesis gel. Among different silica sources (fumed silicas, tetraethyl orthosilicate, silica sol etc.), fumed silicas with smaller particles (~0.007 μn) like Sigma S-5005, S-5130, and Cab-O-Sil-M5 were found to be quite effective and preferable for the synthesis of Si-NCL-1 molecular sieves. These materials were characterized through XRD, IR, SEM, sorption and surface area measurements.     

Rheological behavior of thermoreversible kappa-carrageenan/nanosilica gels

The rheological behavior of silica/kappa-carrageenan nanocomposites has been investigated as a function of silica particle size and load. The addition of silica nanoparticles was observed to invariably impair the gelation process, as viewed by the reduction of gel strength and decrease of gelation and melting temperatures. This weakening effect is seen, for the lowest particle size, to become slightly more marked as silica concentration (or load) is increased and at the lowest load as particle size is increased. These results suggest that, under these conditions, the particles act as physical barriers to polysaccharide chain aggregation and, hence, gelation. However, for larger particle sizes and higher loads, gel strength does not weaken with size or concentration but, rather, becomes relatively stronger for intermediate particles sizes, or remains unchanged for the largest particles, as a function of load. This indicates that larger particles in higher number do not seem to increasingly disrupt the gel, as expected, but rather promote the formation of stable gel network of intermediate strength. The possibility of this being caused by the larger negative surface charge found for the larger particles is discussed. This may impede further approximation of neighboring particles thus leaving enough inter-particle space for gel formation, taking advantage of a high local polysaccharide concentration due to the higher total space occupied by large particles at higher loads.     

Effects of the degree of surface modification on the rheological responses of precipitated silica suspensions in benzyl alcohol

Two types of precipitated silica powders modified by poly (dimethylsiloxane) (PDMS) were suspended in benzyl alcohol and their rheological properties were investigated as a function of silica volume fraction, φ. The suspensions were classified into sol, pre-gel, and gel states based on the increase in φ. An increase in the degree of surface modification by PDMS caused gelation at higher φ. Plots of apparent shear viscosity against shear rate in the sol and pre-gel states of highly modified silica suspensions showed weak shear thickening behavior, while the same plots for silica suspensions with a low modification level exhibited shear thinning behavior. The dynamic moduli of hydrophobic suspensions in the pre-gel and gel states were dependent on the surface modification: the storage modulus G′ was larger than the loss modulus G″ in the linear region and these moduli increased with increasing φ, irrespective of the silica powder. The linear region of the φ range for the precipitated silica suspensions was wider than that for the fumed silica powders modified by PDMS suspended in benzyl alcohol, while the G′ value in the linear region for the precipitated silica suspensions was less than those for the fumed silica suspensions.     

Aggregation and charging of colloidal silica particles: effect of particle size

We studied systematically aqueous suspensions of amorphous well-characterized silica particles by potentiometric titration, electrophoretic mobility, and time-resolved light scattering. Their charging behavior and aggregation rate constants were measured as a function of pH and ionic strength in KCl electrolytes for three types of particles of approximately 30, 50, and 80 nm in diameter. The charging behavior was consistent with the basic Stern model; the silica particles carry a negative charge, and its magnitude gradually increases with increasing pH and ionic strength. On the other hand, their early-stage aggregation (or coagulation) behavior is complex. The aggregation of the largest particles shows features resembling predictions of the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory. On one hand, the rate constant decreases sharply with increasing pH at low ionic strengths and attains fast aggregation conditions at high ionic strengths. On the other hand, we observe a characteristic slowing down of the aggregation at low pH and high ionic strengths. This feature becomes very pronounced for the medium and the small particles, leading to a complete stabilization at low pH for the latter. Stabilization is also observed at higher pH for the medium and the small particles. From these aggregation measurements we infer the existence of an additional repulsive force. Its origin is tentatively explained by postulating hairy layers of consisting of poly(silicilic acid) chains on the particle surface.     

Preparation of large monodispersed spherical silica particles using seed particle growth

To obtain large-sized, monodispersed spherical particles of silica by sol precipitation, a seed particle growth method was attempted. The formation of secondary particles during seed particle growth causing a multimodal distribution of particle size was suppressed via fine adjustment of the reaction conditions, such as TEOS, ammonia, and water concentrations, as well as operational conditions such as feeding time and agitation speed. Among the reaction conditions, an increase of TEOS concentration promoted secondary particle formation, resulting in bimodal particle distribution. However, secondary particle formation was depressed with increasing ammonia and water concentrations. In addition, long feeding time (low feed flow rate) and rigorous agitation significantly reduced secondary particle formation because they contributed to the slow generation of supersaturation and rapid seed particle growth, respectively.