Hydrophobic Silica Particles Research Articles

Effectiveness of Emulsion-Templated Macroporous Polymer Micromixers Characterized by the Bourne Reaction

High porosity, interconnected macroporous polymers prepared by the polymerization of high internal phase emulsions (polyHIPEs) stabilized by either surfactants or a combination of surfactants and hydrophobized silica particles were used as micromixers for liquids. The polyHIPEs had permeabilities between 30 and 2550 mD. The extent of micromixing was characterized by a system of two competitive parallel reactions (fourth Bourne reaction) carried out within the polyHIPEs. These competitive reactions, an acid–base neutralization and acid-catalyzed hydrolysis, proceed at very different rates such that the extent of the hydrolysis reaction is dependent on the extent of micromixing in the system, with lower hydrolysis yields indicating more effective micromixing. It was found that the hydrolysis yields were up to 50% lower in macroporous polymers compared to those in an empty tube and also up to 45% lower than those in a conventional spiral static mixer at a flow rate of 1 mL/min. Macroporous polymers with smal...

Effect of alcohol frothing agents on the coalescence of bubbles coated with hydrophobized silica particles

The stability of capillary-pinned bubble pairs covered with hydrophobized particles in aqueous solutions of 1-pentanol or methyl isobutyl carbinol (MIBC) was studied using high-speed cinematography. Glass particles were first rendered hydrophobic by covalently bonding a linear alcohol onto the solid interface to achieve a specific hydrophobicity (i.e. contact angle of 43° measured with the captive bubble on a treated wafer) and effectively avoid the presence of any mobile hydrophobizing surfactant. The resistance to coalescence of the bubbles was measured at different frother concentrations and for various initial bubble interfacial areas covered by particles; with particle coverage not exceeding the contact region between the bubbles.Frother molecules were shown to delay the coalescence of bubbles whereas particles were not present in a sufficient quantity at the interface of the bubbles to provide steric stability. However, in some cases in the presence of MIBC, the particles were believed to act as means of transportation for the frother molecules to the surface of the bubbles thus forcing the local relaxation of the interface, which improved bubble stability.The coalescence of two bubbles released energy causing a rapid motion of the interface. This motion was sufficient to expel a fraction of the attached particles from the interface. The addition of frother, and of particles in some cases, increased the dampening of the oscillatory motion generated by bubble coalescence. In general, damped bubble oscillations were associated with a reduced quantity of particles detaching from the bubble. Although particles were observed to dampen the oscillation of the bubble, they were not as effective as the frother molecules in reducing the detachment of particles upon bubble coalescence. This finding is believed be of relevance for industrial applications such as froth flotation.

Tuning 3D topography on biomimetic surface for efficient self-cleaning and microfluidic manipulation

Currently, micro-/nanotopography on polymeric replica is generally limited to 2D when a mechanical demolding approach is applied. In this work, one-step replication of bio-inspired 3D topography is achieved using microinjection compression molding with novel dual-layer molds. Using a proposed flexible template, the replica topography and wettability are highly tunable during molding. Moreover, dual-scale topography on the mold is developed by coating the micropatterned insert with submicron silica particles. Contact angle and roll-off angle measurements indicate the lotus leaf, rose petal and rice leaf effects on biomimetic surfaces. Among the three kinds of surfaces, the petal-inspired surface possesses the superior performance in self-cleaning submicron contaminants and mechanical robustness, which is highly correlated to the low roughness-induced adhesive superhydrophobicity and the absence of fragile submicron-/nanostructure, respectively. Furthermore, a multi-layer mold structure is proposed for fabricating the open microfluidic devices. The embedment of the hydrophilic and hydrophobic silica particles in the microstructured open channel and the hydrophobic silica particles in the background area during replication renders the wettability contrast sharp, realizing the self-driven flow of microfluid confined within the open microchannel.

Morphology and rheological properties of silica-filled poly(carbonate)/poly(methyl methacrylate) blends

The effect of silica nanoparticles on the morphology and the rheological properties was investigated in the immiscible polymer blend poly(carbonate)/poly(methyl methacrylate) (PC/PMMA). In the melt state, the linear viscoelastic properties of the nanocomposite showed a reduction effect of the silica nanoparticles on the mobility of one of the polymer which is related to the state of distribution of the silica nanoparticles. Hydrophilic and hydrophobic silica particles were used to study particle migration and their effects on the morphology and it was shown that the distribution of the nanoparticles depends on the balance of interactions between the surface of the particles and the polymer components. The effect on the coarsening kinetics was investigated in both hydrophilic and hydrophobic silica-filled blends. Compared to the hydrophilic silica, a better compatibilization can be obtained by introducing the hydrophobic silica particles at the PC/PMMA interface as the solid barrier. POLYM. ENG. SCI., 55:1951–1959, 2015. © 2014 Society of Plastics Engineers

Use of Hydrophobic Particles as Kinetic Promoters for Gas Hydrate Formation

Hydrophobic particles have been tested as kinetic promoters for gas hydrate formation. The experimental results obtained with stationary beds of sands show that methane (CH4) hydrate is more readily formed when the sand particles are hydrophobized by coating the surfaces with octadecyltrichlorosilane (OTS). The induction times decreased steadily with increasing water contact angles (θ) possibly due to the increased propensity of the water molecules in the vicinity of hydrophobic surfaces to form partial clathrates. The kinetics of CO2 hydrate formation has been studied using Teflon and hydrophobic silica particles to disperse either water in gas phase to obtain “dry water” or to disperse gas in water phase to obtain foam. The results show that hydrates are formed instantaneously due to the increased mass transfer rates at the gas/water interface and the formation of partial clathrates in the vicinity of hydrophobic surfaces.

Foaming and gas dispersion properties of non-ionic frothers in the presence of hydrophobized submicron particles

Foams represent an important area of research due to their relevance in many industrial processes. In many systems, particulates and surfactants co-exist and can largely define the stability of the foam. A typical example is froth flotation where hydrophobic particles and frothers are essential to maintain a stable froth. Grinding operations may yield a product containing submicron-size particles, which can affect the flotation process. The present study compares the foaming and gas dispersion properties of the non-ionic surfactants 1-pentanol, tri(propylene glycol) methyl ether, and poly(propylene glycol) 425 in the presence of hydrophobic colloidal silica particles in a column. In all cases, it has been found that the gas holdup increases proportionally with the increase in the concentration of particles in the system although an initial decrease may be observed. The effect of particles on the foaminess seems to be dependent upon the surfactant. With the 1-pentanol, the addition of particles systematically decreased the foaminess of the solutions. This is in contrast to poly(propylene glycol) solutions, which exhibited an increase. Interestingly, low concentrations of particles appeared to improve the foaminess of tri(propylene glycol) methyl ether; however, the froth became less stable with further increases in the number of particles in the column. Visual observation showed that the colloidal particles play a crucial role in defining the quality of the structure of the froth.

Influence of a mixed particle/surfactant emulsifier system on water-in-oil emulsion stability

Water-in-oil (W/O) emulsions consisting of water and paraffin oil were prepared and stabilized by using a dual emulsifier system including hydrophobic silica particles and non-ionic surfactant. The surfactant/particles synergistic interaction and more specifically the effect of surfactant concentration (from 0 to 1.8%, w/w) at fixed amount of particles (1.8%, w/w) on the emulsions properties were investigated. Interfacial tension, emulsion morphology, water droplet size distribution, stability with time, rheological stress–strain and oscillatory properties were determined to better understand the interaction and mechanism of emulsion stabilization by solid particles in presence of surfactant. The results showed the existence of strong interaction between surfactant molecules and silica particles. The critical micellar concentration for the mixed-emulsifier system was significantly higher than that of surfactant alone. In presence of a low amount of surfactant (0.1%, w/w), the emulsion was characterized by maximal stability (up to 21 days without any phase separation). This dependence was confirmed by rheological measurements which revealed a highest storage modulus (G′) value for this emulsion. Optical microscopy revealed the formation of closely packed aggregates of small droplets forming bridges between neighboring large drops, at low surfactant concentration. The possible explanation for this phenomenon could be the flocculation of small emulsion droplets in the presence of low amount of surfactant. The formation of flocs caused the thickening of the continuous oil phase, the increase in emulsion rigidity and consequently enhanced emulsion stability.

Molecularly imprinted polymer foams with well-defined open-cell structure derived from Pickering HIPEs and their enhanced recognition of λ-cyhalothrin

Abstract The water-in-oil Pickering high internal phase emulsions (HIPEs) were firstly stabilized by hydrophobic silica particles with adding small amounts of nonionic surfactant Hypermer 2296. Then molecularly imprinted polymer foams (MIPFs) derived from Pickering HIPEs were evaluated as adsorbents for the selective adsorption of λ-cyhalothrin. The results of characterization indicated that MIPFs possessed open-cell structure, interconnecting pores, thermal stability, moderate hydrophobicity and mechanical robust. The adsorption capacity for λ-cyhalothrin were closely related with the morphologies of the MIPFs, such as void diameter and interconnectivity, which could be easily tailored by varying the parameters of Pickering HIPEs. The batch mode experiments proved that as-prepared MIPFs exhibited the excellent adsorption capacity for λ-cyhalothrin (46.10 μmol g−1 at 25 °C) and enhanced recognition towards λ-cyhalothrin over fenvalerate and diethyl phthalate.

Influence of Nonionic Rosin Surfactants on Surface Activity of Silica Particles and Stability of Oil in Water Emulsions

AbstractRosin as a natural product has become a source for production of less toxic bio‐surfactants to produce emulsions which are widely used in various agriculture and food products, cosmetic, and pharmaceutical industries. In this respect, a nonionic surfactant was prepared from reaction of rosin acids and rosin maleic anhydride adduct with poly(ethylene glycol) monomethyl ether 750 (MPEG 750) to produce a rosin ester (RMPEG 750). The surface activity parameters of the prepared surfactants, such as surface excess concentration (Γmax), the area per molecule at interface (Amin), and the effectiveness of surface tension reduction, were measured to determine the micellization and adsorption characteristics of the prepared surfactants at the water/air interface. The adsorption of the prepared surfactants on the surface of either hydrophilic or hydrophobic silica particles was determined using a spectrophotometric method. Interfacial tension between water and toluene were measured to select the best condition to obtain toluene/water emulsion in the presence of modified solid silica particles. The effects of silica particle hydrophilicity and the surfactant concentrations on the surface, interfacial activity, and on the emulsion drop size were also investigated.

Development of new mineral oil-based antifoams containing size-controlled hydrophobic silica particles for gloss paints.

Water-based architectural paints commonly contain either mineral oil-based or silicone-based antifoams. Mineral oil-based antifoams generally reduce the gloss of paint films; thus, silicone-based antifoams are mainly used in the field of architectural paints. The relationship between the antifoaming performance and the particle size of hydrophobic silica for mineral oil-based antifoams was investigated and a novel mineral oil-based antifoam that provided a glossy surface to the paint films equivalent to the surface obtained with silicone-based antifoams and with excellent antifoaming performance compared to silicone-based antifoams was developed. The novel mineral oil-based antifoam exhibits better performance than silicon-based antifoam, and thus the former is a perfect alternative to the latter for use in architectural paints.

Fabrication of Tunable Janus Microspheres with Dual Anisotropy of Porosity and Magnetism

This work presents a facile approach to produce a novel type of Janus microspheres with dual anisotropy of porosity and magnetism based on Pickering-type double emulsion templates. A stable aqueous Fe3O4 dispersion-in-oil-in-water (WF/O/W) double Pickering emulsion is first generated by using hydrophobic silica and hydrophilic mesoporous silica particles as stabilizers. Janus microspheres with multihollow structure possessing magnetite nanoparticles concentrated on one side of the microspheres are obtained after polymerization of the middle oil phase of the double emulsion under a magnetic field. The resultant Janus microspheres are characterized by optical microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray analysis (EDX). Moreover, we have systematically investigated the influences of Fe3O4 particle concentration, hydrophobic silica particle content, and volume ratio of the inner water phase to middle oil phase (WF/O) on the double emulsion formation and consequently on the structure of the resulting Janus microspheres. Our results show that the distribution of the multihollow structures within the prepared microspheres can be accurately tailored by adjusting the ratio of WF/O. In addition, the obtained Janus microsphere can be fairly orientated under a magnetic field, making them a potential candidate for synthesizing Janus membrane.

Rheological Model for a Nanoporous-Elasto-Hydrodynamic Composite Material

This work proposes a rheological model for a nanoporous-elasto-hydrodynamic composite material (NPEHDCM), which can be obtained by mixing a colloid, consisted of water and water-repellent nanoporous silica micro-particles, with an adequate jellification agent. Hydrogel is modeled as a biphasic mixture consisted of a nanoporous hydrophilic isotropic and linear elastic solid matrix, and a liquid phase (water). At dynamic pressurization, water molecules exude from the hydrogel matrix and forcedly penetrate the nanopores of hydrophobic silica particles. Based on the proposed rheological model, the NPEHDCM can be suitably designed.

Anomalous chain diffusion in unentangled model polymer nanocomposites

We studied unentangled poly(ethylene-alt-propylene) (PEP) in a composite with hydrophobic silica particles as a function of the filler concentration. Our neutron spin echo (NSE) experiments cover both the internal dynamics as well as the center of mass diffusion beyond the Rouse time. The key experimental results are (i) all of the chains are equally mobile, (ii) the basic segmental (Rouse) relaxation rate is unaffected even at highest filler concentrations, and (iii) apparently the obstacles reduce significantly the translational center of mass motion. This happens, even in the case when the particles do not significantly confine the polymer. (iv) A transition from regular to anomalous diffusion in the Rouse regime at the highest particle fractions is clearly evidenced. In order to understand the microscopic mechanisms underlying the experimental observations, we performed coarse grained simulations. We demonstrate that the geometrical confinement only affects the dynamics at a long time scale outside the experimental window and therefore it is not able to explain the results found in the NSE experiments. The consideration of inter-chain interactions, however, results in a significant influence even at shorter times and a quantitative agreement between the experiments and simulations was found. The simulations clearly demonstrate that the interfaces cause a deceleration of the chains in their close vicinity. Then the inter-chain interactions carry this slowing down to the other chains at a time-scale of the Rouse relaxation time. Hence, in the experimental datasets an overall slowing down is observed.

Effect of silica particles on stability of highly concentrated water-in-oil emulsions with non-ionic surfactant

Water-in-oil, high internal phase emulsion made of super-cooled aqueous solution containing a mixture of inorganic salts and stabilized with non-ionic surfactant (sorbitan monooleate) alone was investigated. It was not possible to produce a highly concentrated emulsion (with aqueous phase fraction = 94 wt %), stabilized with surface-treated silica, solely: we were able to form an emulsion with a maximal aqueous phase mass fraction of 85 wt % (emulsion inverts/breaks above this concentration). The inversion point is dependent on the silica particle concentration, presence of salt in the aqueous phase, and does not depend on the pH of the dispersed phase. All emulsions stabilized by the nanoparticles solely were unstable to shear. So, the rheological properties and stability of the emulsions containing super-cooled dispersed phase, with regards to crystallization, were determined for an emulsion stabilized by non-ionic surfactant only. The results were compared to the properties obtained for emulsions stabilized by surface treated (relatively hydrophobic) silica nanoparticles as a co-surfactant to sorbitan monooleate. The influence of the particle concentration, type of silica surface treatment, particle/surfactant ratio on emulsification and emulsion rheological properties was studied. The presence of the particles as a co-stabilizer increases the stability of all emulsions. Also, it was found that the particle/surfactant ratio is important since the most stable emulsions are those where particles dominate over the surfactant, when the surfactant’s role is to create bridging flocculation of the particles. The combination of the two types of hydrophobic silica particles as co-surfactants is: one that resides at the water/oil interface and provides a steric boundary and another that remains in the oil phase creating a 3D-network throughout the oil phase, which is even more beneficiary in terms of the emulsion stability.

Does the hydrophobic attraction contribute to the interaction between colloidal silica spheres coagulated by an adsorbing cationic surfactant?

The character of the attractive interaction between hydrophobic surfaces, which is beyond the realm of the DLVO theory, is still under debate. The strength and extension of this non-DLVO, hydrophobic attraction (HA) between fixed macroscopic surfaces have been found to vary depending on the substrate type and, especially, the procedure adopted to render its surface hydrophobic. A related question is how HA manifests between hydrophobic colloidal particles in disperse systems. Here, the way of the particles’ hydrophobization is expected to be of utmost importance again. In the present article we compare experimentally determined absolute aggregation rate constants of uniform silica spheres at once hydrophobized and aggregated in water by a cationic surfactant (hexadecyltrimethylammonium bromide, CTAB) with their theoretical counterparts calculated from the DLVO theory in combination with hydrodynamic interactions and without any adjustable parameters. It has been found that both constants become identical when approaching a limiting magnitude in the specific situation of CTA+ cations adsorbed on the surface of silica colloids to such an extent that they compensate the negative charge and at the same time maximally hydrophobize the surface. Moreover, the limiting magnitude of the rate constants is comparable with that measured and predicted for silica colloids coagulated by K+ cations (KCl). It is concluded that the aggregation of hydrophobic silica particles initiated by the surfactant adsorption can be considered as the electrolytic coagulation in the fast (diffusion-limited) regime with no energy barrier due to the electrostatic repulsion, driven solely by the van der Walls attraction. Consequently, HA does not seem to influence the coagulation process at least in this colloidal system. Possible reasons of the disagreement of this finding with previous results of direct force experiments on similar surfactant-hydrophobized macroscopic systems are discussed.

Kinetics and Properties of Asphaltene Adsorption on Surfaces

A quartz crystal microbalance (QCM) was used to probe asphaltene adsorption on gold surfaces and hydrophilic and hydrophobic silica particles. The adsorption studies were conducted with solutions of various ratios of pentane solvent to model oil (S/O) and asphaltene concentrations. The adsorption of asphaltenes on solid surfaces at different S/O showed multilayer deposition without reaching equilibrium after 16 h. Adsorption of asphaltenes and other components on the surfaces were detectable well below the onset of precipitation. The amount of material adsorbed increased significantly after the onset of precipitation. Adsorption was more pronounced on the gold surface than on the silica particles. The swelling of the asphaltene aggregates on the surface of the quartz crystal led to restructuring of the adsorbed material on the surfaces, depending on the S/O. For S/O > 0, the swelling effect suggested open nanoaggregate or polymer brush structure. The adsorbed asphaltenes demonstrated more viscoelastic behavior as S/O was increased. The kinetics of the adsorption of asphaltenes on gold at room temperature suggested the formation of very large, slowly diffusing aggregates, even in toluene. These results were not consistent with optical microscopy or other methods.

Particle Stabilization of Oil‐in‐Water‐in‐Air Materials: Powdered Emulsions

Drops of a particle-stabilised oil-in-water emulsion are encapsulated in air by using hydrophobic silica particles. Such systems, termed oil-in-water-in-air powdered emulsions, show free-flowing behavior. Successful preparation of the material is achieved by preventing oil droplets from creaming and coalescing.

Effect of pH on monolayer properties of colloidal silica particles at the air/water interface

Monodisperse colloidal silica particles were prepared by the Stober method and hydrophobized by grafting a silane coupling agent, octadecyltrimethoxysilane. Two different types of silica particles, i.e., hydrophilic and hydrophobic silica particles were spread at the air/water interface to form the Langmuir monolayers. Monolayer properties of those particles were investigated by measuring surface pressure–area (π–A) isotherms at different subphase pH. At pH above the isoelectric point (IEP) of silica, as pH increased the π–A isotherms for the hydrophobic particles slightly shifted to larger surface area whereas those for the hydrophilic particles showed a reverse trend. At pH below the IEP, the π–A isotherms for both types of particles shifted to much larger surface area with different shapes. In order to analyze the π–A isotherm results further, the time dependence of π was examined. When pH is above the IEP, the π for the hydrophilic particles significantly decreased with increasing time and it did more at higher pH. On the other hand, the decrease in π for the hydrophobic particles was insignificant regardless of pH. For both types of silica particles, the decrease in π was minimal at pH below the IEP. These results were discussed in terms of particle desorption into the water subphase and interparticle electrostatic repulsion which is directly influenced by zeta potential.

Comparative Study of Stabilizing Agents for Water-in-Oil Emulsions

The mechanism and ability to stabilize water-in-oil (w/o) emulsions have been compared for different emulsifiers, including asphaltenes extracted from a crude oil sample, hydrophobic silica particles, and a model acidic surfactant, N-(1-hexylheptyl)-N′-(5-carbonylicpentyl)perylene-3,4,9,10-tetracarboxylic bisimide (C5Pe). The results indicated that C5Pe describes the behavior of asphaltenes relatively well, although the interfacial activity was substantially higher than for the real asphaltenes at high pH. The silica particles produced rather different results. The particles did not appear to desorb from the oil–water interface, leading to a time-independent situation without any visible signs of coalescence. Additionally, the transition from unstable to stable emulsions happened at a “critical” particle concentration. For asphaltenes and C5Pe, the influence of other parameters has been investigated, such as solvent composition and pH. In both cases, the interfacial activity increased when a poor solvent was added, because the new oil phase made it more energetically favorable for the molecules to assemble at the interface. C5Pe became a better emulsion stabilizer at high pH, because of dissociation of the carboxylic acid groups and enhanced interfacial activity of the molecule. The asphaltenes had the lowest ability to stabilize around pH 6, while their ability increased for both higher and lower pH values because of ionization of acids and bases. Finally, the asphaltene concentration in the residual oil phase was measured by ultraviolet spectroscopy, to compare to results obtained by other authors. All of the results suggested that only a subfraction of the asphaltenes is responsible for emulsion stabilization.

Dry Water: a Prospective Material for Methane Storage via Clathrate Hydrate Form

Dry water (DW) is a powder-like, solidified form of water, where water droplets are surrounded by silica nanoparticles. DW is considered to be an effective medium for gas storage because it can slurp up gases which combine the water molecules to form clathrate hydrate. In this paper, DW was prepared by mixing water and hydrophobic silica particles (H18) at high speeds, and based on which, dry water methane hydrates (DW-MH) were synthesized under certain conditions. The characteristics of DW and DW-MH were investigated by Raman spectroscopy. The results show that DW-MH is a typetical sI clathrate hydrate with hydration number around 5.9. The storage capacity in clathrate hydrate prepared with methane and four different materials, i.e. DW, powder ice, Sodium Dodecyl Sulfate (SDS) solution and liquid water, was measured by volumetric method. The results showed that DW is prospective medium for gas storage that can increase much higher of the storage capacity compared to other materials.