Hydrophobic Silica Particles Research Articles

Adsolubilization of 2-naphthol into adsorbed layers of triblock PEO–PPO–PEO copolymers on hydrophobic silica particles

Adsolubilization of 2-naphthol into an adsorbed layer of triblock poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO, Pluronics) copolymers on hydrophobically modified silica particles has been investigated. Four kinds of Pluronics (P103, P105, P123, and F108) were employed in order to understand the effect of the hydrophobicity of the surfactants on the adsolubilization. The amount of the Pluronics adsorbed of the maximum/saturation adsorption level was increased with a decrease in the HLB value, suggesting that the more hydrophobic Pluronics (P103 and P123) adsorb preferentially onto the hydrophobic silica surface over the more hydrophilic Pluronics (P105 and F108). The greater adsorbed amount of the more hydrophobic surfactants resulted in a greater amount of 2-naphthol adsolubilized into the adsorbed Pluronics layers. In the case of simultaneous addition of the Pluronics and 2-naphthol, the amount adsolubilized into the adsorbed P123 and P103 layers increased in their low-surfactant-concentration regime, reached a maximum, and then decreased with a further increase in the Pluronics concentration. On the other hand, for both the P105 and F108 copolymers, a decrease in the adsolubilized amount was not observed over the whole range of copolymer concentration investigated. This difference is attributed to a difference in the hydrophobicity of the micellar aggregates in solution and of the adsorbed layers on the hydrophobic surface. When 2-naphthol was added after replacement of the Pluronics supernatant by a surfactant-free solution, the final decrease in the adsolubilization was insignificant for all the Pluronics. Indeed, the maximum amount of adsolubilization was comparable to the corresponding amount obtained in the case of simultaneous addition.

Adsorption of Non-Ionic Surfactants on Hydrophobic Silica Particles and the Stability of the Corresponding Aqueous Dispersions

The temperature stability of aqueous dispersions of hydrophobic monodisperse silica particles stabilized with nonionic surfactants has been investigated. Adsorption isotherms in conjunction with surface tension measurements showed that the surfactant formed a monolayer on the surface of the particles, where the adsorbed amount depended on the molecular weight of the ethylene oxide headgroup. The temperature stability of these dispersions has been measured by a standard turbidimetric technique and visual observations in terms of their critical flocculation temperature (CFT). Parameters controlling the CFT of the individual dispersions stabilized with a monolayer of surfactant include the thickness of the steric layer, the particle size, and the volume fraction of the particles. Calculations show that the van der Waals attraction between the particles with adsorbed polymer layers increases as the temperature of the dispersion increases, and this largely accounts for the observed CFT behavior.

Thermal effects on pressure-induced infiltration of a nanoporous system

Thermal effects on the energy absorption efficiency of a system consisting of hydrophobic mesoporous silica particles immersed in water have been investigated. As the temperature increases, although the infiltration pressure decreases only slightly and the accessible pore volume remains nearly constant, the system recoverability can be significantly improved, primarily due to an increase in outflow pressure. The sequence of changing pressure and temperature has little influence on the system performance.

Structure and Stability of Silica Particle Monolayers at Horizontal and Vertical Octane−Water Interfaces

Monolayers of silica particles at horizontal and vertical octane-water interfaces have been studied by microscopy. It is found that their structure and stability depend strongly on the particle hydrophobicity. Very hydrophobic silica particles, with a contact angle of 152 degrees measured through the water, give well-ordered monolayers at interparticle distances larger than 5 particle diameters which are stable toward aggregation and sedimentation. In contrast, monolayers of less-hydrophobic particles are disordered and unstable. Two-dimensional particle sedimentation has been observed in the case of vertical monolayers. The results have been analyzed with a simple two-particle model considering the sedimentation equilibrium as a balance between the long-range electrostatic repulsion through the oil, the gravity force, and the capillary attraction due to deformation of the fluid interface around particles. The value of the charge density at the particle-octane interface, 14.1 muC/m(2), found for the most hydrophobic particles is reasonable. It drastically decreases for particles with lower hydrophobicity, which is consistent with the order-disorder transition in monolayer structure reported by us before. The pair interactions between particles at a horizontal octane-water interface have been analyzed including the capillary attraction due to undulated three-phase contact line caused by nonuniform wetting (the contact angle hysteresis). The results are in agreement with the great stability of very hydrophobic silica particle monolayers detected experimentally, even at low pH at the point of zero charge of the particle-water interface, and with the aggregated structure of hydrophilic particle monolayers.

Aqueous Foams Stabilized Solely by Silica Nanoparticles

Foamability and foam stability: By using a novel dispersion method, suitably hydrophobic silica particles can be dispersed in water to form very stable foams through adsorption of particle aggregates at the surfaces of micrometer-sized bubbles. The optical microscope image shows air bubbles in pure water stabilized solely by partially hydrophobic silica nanoparticles (scale bar: 50 μm).

Conformational Changes of the Amyloid β‐Peptide (1–40) Adsorbed on Solid Surfaces

The conformational change of the 39-43 residues of the amyloid beta-peptide (Abeta) toward a beta-sheet enriched state promotes self-aggregation of the peptide molecules and constitutes the major peptide component of the amyloid plaques in Alzheimer patients. The crucial question behind the self-aggregation of Abeta is related to the different pathways the peptide may take after cleavage from the amyloid precursor proteins at cellular membranes. This work is aiming at determining the conformation of the Abeta (1-40) adsorbed on hydrophobic Teflon and hydrophilic silica particles, as model sorbent surfaces mimicking the apolar transmembrane environment and the polar, charged membrane surface, respectively. The mechanism by which the Abeta interacts with solid surfaces strongly depends on the hydrophobic/hydrophilic character of the particles. Hydrophobic and electrostatic interactions contribute differently in each case, causing a completely different conformational change of the adsorbed molecules on the two surfaces. When hydrophobic interactions between the peptide and the sorbent prevail, the adsorbed Abeta (1-40) mainly adopts an alpha-helix conformation due to H-bonding in the apolar part of the peptide that is oriented towards the surface. On the other hand, when the peptide adsorbs by electrostatic interactions beta-sheet formation is promoted due to intermolecular association between the apolar parts of the adsorbed peptide. Irrespective of the characteristics of the solid sorbent, crowding the surface results in intermolecular association between adsorbed molecules leading to a strong aggregation tendency of the Abeta (1-40). [Diagram: see text] CD spectra of Abeta (1-40) at pH 7: A) in solution ([Abeta]=0.2 mg.ml(-1)) freshly prepared (line) and after overnight incubation (symbols);B) on Teflon (Gamma=0.5 mg.m(-2)).

Improvement of recoverability of a nanoporous energy absorption system by using chemical admixture

In this paper, the behaviors of nanoporous energy absorption systems subjected to cyclic loadings are investigated. The systems consist of hydrophobic mesoporous silica particles immersed in aqueous solutions of sodium chloride (NaCl). In the first loading-unloading loop, increasing NaCl concentration, c, causes only slight increases in infiltration pressure and energy absorption efficiency. In the following loops, however, the system behaviors are strongly dependent on c. As c increases from 0%to25.9wt.%, the recoverability is improved by a factor of 3, indicating clearly that the addition of NaCl has a beneficial effect to promoting the outflow in the nanoenvironment.

Porous polystyrene microspheres having dimpled surface structures prepared within micellar assemblies of amphiphilic silica particles in water

Production of porous polystyrene microspheres having dimpled surface structures was demonstrated using amphiphilic and hydrophobic silica particles as structure-directing agents.

Structural Features of a Hyperthermostable Endo- β-1,3-glucanase in Solution and Adsorbed on “Invisible” Particles

Conformational characteristics and the adsorption behavior of endo- β-1,3-glucanase from the hyperthermophilic microorganism Pyrococcus furiosus were studied by circular dichroism, steady-state and time-resolved fluorescence spectroscopy, and calorimetry in solution and in the adsorbed state. The adsorption isotherms were determined on two types of surfaces: hydrophobic Teflon and hydrophilic silica particles were specially designed so that they do not interact with light and therefore do not interfere with spectroscopic measurements. We present the most straightforward method to study structural features of adsorbed macromolecules in situ using common spectroscopic techniques. The enzyme was irreversibly adsorbed and immobilized in the adsorbed state even at high temperatures. Adsorption offered further stabilization to the heat-stable enzyme and in the case of adsorption on Teflon its denaturation temperature was measured at 133°C, i.e., the highest experimentally determined for a protein. The maintenance of the active conformation and biological function particularly at high temperatures is important for applications in biocatalysis and biotechnology. With this study we also suggest that nature may employ adsorption as a complementary mode to maintain structural integrity of essential biomolecules at extreme conditions of temperature.

Force measurements between Teflon AF and colloidal silica particles in electrolyte solutions

The interaction force between a very hydrophobic polymer surface and colloidal silica particles with a roughness of 10–15 nm has been measured in aqueous solutions of KOH and KCl using an atomic force microscope. The interaction can be described according to the DLVO theory by an electrical double-layer force that is repulsive at long distances and attractive at short distances and an attractive van der Waals force. The electrical double-layer potentials are compared to the zeta potentials of Teflon AF and the silica spheres. The roughness of the silica particles leads to an underestimation of the short-range attraction and the surface potential. Both KCl and KOH solutions affect the potential of the interacting surfaces. OH − ions that adsorb preferentially to the Teflon AF surface create higher potentials than Cl − ions. Range and strength of the attractive interaction are not affected by KCl solutions but reduced by addition of KOH. This can be explained by decreasing potential differences between the silica sphere and Teflon AF with increasing KOH concentration. In addition, the preferential adsorption of OH − ions may lead to a reduction of the van der Waals interaction. The presence of nanobubbles, too, might play a role.

Flow electrification in nonaqueous colloidal suspensions, studied with video microscopy.

Flow electrification in nonaqueous suspensions has been scarcely reported in the literature but can significantly affect colloidal stability and (phase) behavior, perhaps even without being recognized. We have observed it in shear flow experiments on concentrated binary suspensions of hydrophobized silica particles in chloroform. In this low-polarity solvent, electrical charges on the large-particles' surfaces manifest themselves via long-ranged forces, because hardly any screening can take place through counterions. By shearing the suspension for a prolonged time, we could demonstrate that the effective interactions between the large particles change from weakly attractive (due to the small particles) to strongly repulsive (due to acquired Coulomb interactions). One of the conditions required for flow electrification was the presence of a glass surface in the shear cell. A spectacular manifestation of the phenomenon was observed with confocal video microscopy. First, the formation of large-particle aggregates was seen, while subsequently (over a long shearing time) the aggregates disintegrated into small entities, mostly primary particles. The spatial distribution of these entities in the quiescent state after stopping the flow showed evidence for acquired long-range repulsion. The occurrence of flow electrification was further corroborated by control experiments, where no flow was imposed, antistatic agent was added, or the glass bottom was coated with a conducting (indium tin oxide, ITO) layer: here, the aggregates kept growing until they became very large. To further diagnose the phenomenon, we have also done experiments in which an external electric field was applied (via the ITO layer) to an aggregated suspension. When the lower electrode was given the lowest potential, the aggregates were found to move away from the bottom and disintegrate. The qualitative similarity hereof with the flow electrification experiment suggests that in the latter, the glass acquired negative charges. After prolonged application of an external electric field, we observed segregation into regions enriched in large particles and regions completely depleted of them. In the quiescent fluid these regions exist as isolated units, but in shear flow they merge into bands, a behavior which resembles shear banding.

Integrated Microanalytical System Coupling Permeation Liquid Membrane and Voltammetry for Trace Metal Speciation. Technical Description and Optimization

AbstractA new minicell coupling the liquid‐liquid extraction technique called permeation liquid membrane (PLM) with an integrated Ir‐based Hg‐plated microelectrode array for voltammetric detection has been developed for the speciation of heavy metals in natural waters. Lead and cadmium have been used as model compounds. The PLM consists of a carrier (0.1 M 22DD+0.1 M lauric acid) dissolved in 1 : 1 mixture of toluene/phenylhexane held in the small pores (30 nm) of a hydrophobic polypropylene membrane (Celgard 2500). One side of this membrane is in contact with a flowing source solution, containing the metal ions of interest. An acceptor or strip solution (pyrophosphate) is placed on the other side of the PLM with the microelectrode array placed at 480 μm of the PLM. The analyte is transported by the carrier from the source solution to the strip solution. The originality of the new minicell is that accumulation in the strip solution is voltammetrically followed by the integrated microelectrode array in real time, and at low concentration level, using square‐wave anodic stripping voltammetry (SWASV). In order to protect the Hg microelectrodes from the adsorption of the hydrophobic carrier, the microelectrodes are embedded in a thin gel layer (280 μm) of 1.5% LGL agarose gel containing 10% of hydrophobic silica particles C18. The choice of optimum conditions is discussed in details in this article. Due to the very small effective strip volume of the new cell (less than 1 μL), high enrichment factor can be obtained (e.g., 330 for Pb) after 2 hours of accumulation. No deaeration of the solutions is required for SWASV measurements. Detection limits under these conditions are 2 pM and 75 pM for Pb and Cd, respectively, using a voltammetric deposition time of 5 min. In addition, no fouling effects were observed with natural water samples.

Surfacial, liquid sorption and monolayer-forming properties of hydrophilic and hydrophobic St�ber silica particles

The surface of silica spheres with a diameter of 500 nm was modified with ethoxysilane. Hydrophilic and partially hydrophobic silica spheres were obtained, suitable for the preparation of two-dimensional monoparticle films at the liquid-air interface. The tendency of these particles to self-assemble is basically dependent on surface hydrophobicity. Liquid sorption excess isotherms were studied in ethanol-cyclohexane and ethanol-chloroform mixtures with the aim of characterizing the adsorption capacity of the particles. Specific surface area and porosity were measured by nitrogen adsorption. The specific surface area determined by liquid sorption was considerably larger than determined by gas adsorption. This is ascribed to penetration of ethanol into the pores and the swelling of the silica particles in ethanol. Surface modification of hydrophilic particles changed the film-forming properties of the particles. The compressibility and the lift-off area of the monolayer films of hydrophobic particles on water were higher than for the films of hydrophilic particles.

Temperature-dependent stability of water-in-undecanol emulsions

Abstract The stability of water-in-undecanol emulsions to sedimentation and coalescence as a function of temperature is described. By monitoring the release of water and oil by the emulsions, the stability of emulsions prepared in the absence of stabilisers and in the presence of either hydrophobic silica particles or octanoic acid molecules is characterised. We show that the destabilisation phenomena observed for the solid-stabilised emulsions reflects the breakdown of emulsions prepared in the absence of stabilisers. The extent of coalescence in the solid-stabilised emulsions undergoes a transition at a temperature about 10 °C above the melting point of undecanol and the stability of the emulsion to sedimentation passes through a maximum at about the same temperature. We attribute the low stability of these emulsions to coalescence at temperatures up to 10 °C above the melting point of undecanol to the formation of a solid-like layer of alcohol at the drop surface, similar to the surface crystallisation phenomena of medium chain length alcohols at planar oil–water interfaces. The addition of octanoic acid to these emulsions does not affect the surface crystallisation.

Yield stress and wall slip phenomena in colloidal silica gels

Evidence of wall slip and magnitude of yield stress are examined for colloidal gels consisting of hydrophobic silica, polyether, and lithium salts using geometries with serrated, smooth, hydrophilic and hydrophobic surfaces. Serrated plates, which provide minimal wall slip, are used to compare different methods of measuring yield stress: conventional extrapolation of shear stress in steady shear experiments and dynamic experiments at large strain amplitudes. In the latter, the yield stress is denoted by the maximum in the elastic stress, the product of the elastic modulus and strain (G′γ), when plotted as a function of strain amplitude. Although excellent agreement is observed in the yield stress values using both these techniques, the dynamic method seems preferable considering its experimental ease, accuracy, and lack of extrapolation. In the presence of smooth geometries, the silica gels show evidence of wall slip with a concomitant decrease in yield stress. Using underestimation of yield stress as a measure of wall slip, we find slip to be unaffected by changes in the gel modulus obtained through incorporation of additional silica or salts. The use of smooth surfaces compared to serrated surfaces leads to approximately a 60% reduction in yield stress for all such samples. Finally, control of wall slip is attempted using plates modified to have different surface energies. Hydrophobic plates reduce slip significantly and produce data comparable to those with the serrated plates. In contrast, hydrophilic plates have minimal effect on slip and produce data analogous to those obtained using smooth plates. These results can be explained based on the fact that the particle-lean layer, responsible for slip, remains so with hydrophilic plates as it repels the hydrophobic silica particles in favor of the polar solvent. In contrast, the hydrophobic silica interacts with the hydrophobic plates, thus reducing slip.

Outstanding Stability of Particle-Stabilized Bubbles

Methods have been used to generate air bubbles beneath a planar air−water interface, stabilized by partially hydrophobic quasi-spherical silica particles (primary diameter of 20 nm) in pure water. Particles tended to aggregate at the planar interface, and all the silica dispersions had low foamability. However, those bubbles that were formed (with radii of 5−200 μm) were completely stable to disproportionation for several days, in contrast to similar bubbles stabilized by the best protein foam stabilizers, which typically shrank and disappeared in 1 or 2 h.

Silylation of Partially Aggregated Colloidal Silica Nanoparticles for Reinforcement in Silicone Elastomers

AbstractAqueous suspensions of colloidal silicas are readily silylated with chlorosilanes in the presence of acid and isopropyl alcohol without aggregation of the silica particles. By using chlorosilanes, spherical nanoparticles with controlled functionality can be made and transferred to an organic phase to provide stable, water free suspensions. The hydrophobic colloidal silica particles readily disperse into silicone polymers. In order to provide mechanical reinforcement comparable to traditional fumed silicas, a high filler loading of ca. 60 wt % mono-dispersed colloidal silicas need to be used.Recently a unique pearl necklaced-shaped colloidal silicas become commercially available. These nanoparticles are successfully silylated and they maintain the original structures as evidenced by transmission electron microscopy (TEM). They are compounded into silicone polymers at ca. 30 wt % and they provide excellent viscosities and mechanical properties, especially the tear strengths. In this paper, we present the silylation, characterization and mechanical properties of these partially aggregated colloidal silica nanoparticles in silicone elastomers.

Adhesion between silica particles in an alcohol medium

The destruction times of the sediment column structures of hydrophobic and hydrophilic silica particles were measured using a simple device. Experiments were carried out for different fractions of both silica particles in alcohols ranging from ethanol to decanol. On the basis of linear relationships between the reciprocal of the destruction times of the silica sediment column structures and average diameters of the silica fractions, the density of alcohols and the work of cohesion of alcohols, and the critical values of these parameters were determined. For the silica particle/alcohol/silica particle system having a critical value of the average diameter of the particle (or the critical density of alcohol, or the critical work of cohesion of alcohol), the detachment force of one silica particle from another was found to be equal to the attachment force between them. The values of the detachment force were found to decrease with increasing length of the hydrocarbon chain of the alcohols. Using these force values and the critical work of cohesion of alcohol, the radii of the contact planes between two silica particles and then the attachment forces were calculated. The attachment force values increased with increasing length of the hydrocarbon chain of the alcohols. It has also been found that the critical parameters (diameter of silica particles, density and work of cohesion of alcohols) depend on the surface properties of the solid. Hydrophilization of the silica surface caused an increase of the average critical diameter and the critical work of cohesion and a decrease of the critical density and, as a consequence, an increase of the attachment force. The increase of the destruction time with increasing length of the hydrocarbon chain of alcohols was caused, on one hand, by the decrease of the detachment forces and the perimeter of the contact planes, and, on the other, by the increase of the attachment force.

Synthesis and Use of Colloidal Silica for Reinforcement in Silicone Elastomers

Abstract Aqueous suspensions of colloidal silicas are readily silylated with either chlorosilanes or disiloxanes in the presence of acid and isopropyl alcohol without aggregation of the silica particle. By using a mixture of chlorosilanes or disiloxanes, spherical nanoparticles with controlled functionality can be made and transferred to an organic phase to provide stable, water free suspensions. The hydrophobic silica particles readily disperse into silicone polymers. At sufficient loading levels, they provide mechanical reinforcement comparable to traditional fillers but with improved clarity and lower viscosities. Modulus and durometer control in the cured elastomer is possible by varying the ratio of the vinyl concentration on the filler particle to the vinyl concentration in the polymer phase.

Catastrophic Phase Inversion of Water-in-Oil Emulsions Stabilized by Hydrophobic Silica

A short review of the experimental findings concerning the stabilization of emulsions by solid particles is given. We then describe the preparation and properties of water-in-oil (w/o) emulsions stabilized by nanometer-sized hydrophobic silica particles alone. Emulsions of median diameter equal to 0.6 μm are completely stable to coalescence as a result of an adsorbed layer of particles at the oil−water interface. Their stability to sedimentation increases with particle concentration due to network formation of the particles in the continuous oil phase. The w/o emulsions catastrophically invert, without hysteresis, to oil-in-water (o/w) at volume fractions of water around 0.7, i.e., as soon as the drops begin to deform. The drops in o/w emulsions are larger (100 μm) and cream rapidly but remain stable to coalescence. We demonstrate that for emulsions stabilized by hydrophilic silica particles, phase inversion from o/w to w/o occurs at the same dispersed phase volume fraction as above. It is therefore suggested that the system hydrophile−lipophile balance is determined by the particle wettability. Comparisons with the behavior of surfactant-stabilized emulsions are given throughout.