Continuous Oil Phase Research Articles

Preparation of biodegradable nanoporous microspherical aerogel based on alginate

Supercritical extraction of the solvent from organic gels, like polysaccharides, enables the production of highly porous biodegradable aerogel with a high surface area. The structural properties of the produced aerogel depend mainly on the preparation methods and the composition of the gel phase. This work presents a new method to produce biodegradable microspherical alginate aerogels particles using an emulsion technique. Water in oil (W/O) emulsion was produced by mixing a Na–alginate solution (dispersed phase) with oil (continuous phase) followed by cross-linking the dispersed phase to form the gel particles. The gelation parameters as well as the emulsion process parameters were investigated in order to control the form and the structural properties of the produced alginate aerogel. Alginate aerogel microspherical particles with a high surface area up to 680 m 2/g and relatively large pore volume up to 4.0 cm 3/g and different mean particle diameters ranging from 25 μm to few hundred microns were produced using the presented method.

Microemulsions based on virgin olive oil: A model biomimetic system for studying native oxidative enzymatic activities

Water-in-oil (w/o) microemulsions, composed of virgin olive oil as the continuous oil phase, a mixture of lecithin and propanol as emulsifiers and water as the dispersed phase, have been successfully used as model biomimetic media to carry out enzymatic reactions that may naturally occur in olive oil. To evaluate the size of the dispersed polar domains as a function of water content, dynamic light scattering (DLS) measurements were carried out and radii in the range of 60–130 nm were observed. The polarity of the interior of these polar domains was studied by electron paramagnetic resonance (EPR) spectroscopy using a polar spin probe. Up to certain water content (1% w/w) both polarity and spin probe's mobility were affected by water content of the system and then remained unaltered. Oxidative enzymatic reactions using a native olive peroxidase (POD) also present in virgin olive oil, and horseradish peroxidase (HRP) were carried out in w/o microemulsions using 2,2′-azino-bis[3-ethylbenzo-thiazoline-6-sulfonic acid] (ABTS) and gallic acid as substrates. Enzymatic activities were affected by the restricted environment of the nanoreactors and also the different polarity of the medium leading to comparable initial velocity values for both substrates.

In situ production of spherical aerogel microparticles

Due to their high surface area, low density, open pore structure and excellent insulation properties aerogels are intensively investigated since the past decades for a diverse range of applications. The current methods of silica aerogel production by supercritical extraction produce monolithic aerogels, where the sol is aged in molds and dried by extraction with supercritical CO2. Aerogels in the form of spherical microparticles would be beneficial for many applications, for instance, drug delivery for respiratory route; or as insulating materials. However, because of aerogel's mechanical properties, it is difficult, rather impossible, to obtain spherical microparticles by milling or crushing of the monolithic aerogels. This work presents a new method to produce biocompatible spherical aerogel microparticles using an emulsion technique (in situ production) followed by supercritical extraction of the resulted dispersion (gel–oil). Water in oil emulsion was produced by mixing the sol (dispersed phase) with a vegetable oil (continuous phase) followed by the gelation of the dispersed phase. The size distribution of the final gel particles was found to be influenced by agitation, surfactant concentration and sol:oil volume ratios. The gel–oil dispersion was subsequently extracted with supercritical CO2, Silica aerogel spherical microparticles with a surface area of 1100m2g−1, pore volume of 3.5cm3/g and different mean particle diameters ranging from 200μm to a few millimeters were produced using the presented method.

Highly sensitive trace analysis of paraquat using a surface-enhanced Raman scattering microdroplet sensor

We report a rapid and highly sensitive trace analysis of paraquat (PQ) in water using a surface-enhanced Raman scattering (SERS)-based microdroplet sensor. Aqueous samples of PQ, silver nanoparticles, and NaCl as the aggregation agent were introduced into a microfluidic channel and were encapsulated by a continuous oil phase to form a microdroplet. PQ molecules were adsorbed onto particle surfaces in isolated droplets by passing through the winding part of the channel. Memory effects, caused by the precipitation of nanoparticle aggregates on channel walls, were removed because the aqueous droplets were completely isolated by a continuous oil phase. The limit of detection (LOD) of PQ in water, determined by the SERS-based microdroplet sensor, was estimated to be below 2 × 10 −9 M, and this low detection limit was enhanced by one to two orders of magnitude compared to conventional analytical methods.

Influence of Model Oil with Surfactants and Amphiphilic Polymers on Cyclopentane Hydrate Adhesion Forces

Adhesion forces between cyclopentane hydrate particles were measured at atmospheric pressure and 3.2 °C using an improved micromechanical force apparatus. Because of the complexity of crude oil systems, a series of model oils was prepared by adding surface-active components to 200 cP mineral oil as analogues to crude oil systems. The addition of 1 wt % sorbitan monooleate (Span80, a commercial anti-agglomerant), 1 wt % polypropylene glycol (an amphiphilic polymer), and 0.6 wt % commercial naphthenic acid mixture, separately, to a mineral oil and cyclopentane continuous phase, reduced the average interparticle hydrate adhesion force by 37, 65, and 80%, respectively, compared to pure mineral oil and cyclopentane. The 95% confidence bounds of the Span80 and mineral oil data points overlap; therefore, we cannot conclude that Span80 was effective at reducing the adhesion force between hydrate particles. These results indicate that model amphiphilic polymers and commercial naphthenic acid mixtures may be surface-active on the hydrate particle, drastically reducing the agglomeration tendency between hydrate particles; naphthenic acids are found to be the most effective at lowering the adhesive force between particles. The structure of the additive plays a role in determining the extent of surface activity and effectiveness. Compounds with small hydrophilic groups can more efficiently adsorb to the hydrate surface, while additives that induce morphological changes to the hydrate surface may cause non-uniform growth and are more effective in preventing hydrate agglomeration.

Synthesis of superparamagnetic Fe3O4 nanocrystals in reverse microemulsion at room temperature

Superparamagnetic Fe3O4 nanocrystals were prepared via chemical precipitation in reverse microemulsion at room temperature. The water-in-oil reverse microemulsion system was composed of Triton X-100 as a surfactant, n-butanol as a co-surfactant, cyclohexane as a continuous oil phase, and aqueous phase. Powder X-ray diffraction and transmission electron microscopy results demonstrated that the nanoparticles were pure Fe3O4 with a polyhedral spinel structure and with an average size of 15 nm. Moreover, the sizes of Fe3O4 nanoparticles did not change as water/surfactant mole ratios (W0) increased from 5 to 60. From the magnetometer measurements data, the products exhibited superparamagnetic property with saturation magnetisations of 64·6 emu g−1 at room temperature. Such method was easy and mild, and could synthesise other metal oxide in such experiment situation.

Synergic Effect of the Mixed Cationic-Anionic Surfactants on the Interfacial Composition and Thermodynamics of W/O Microemulsions

The interfacial composition, the solubility of the alcohol in the oil phase, and the standard free energy change of transferring alcohol from the continuous oil phase to the interfacial layer in the W/O microemulsion stabilized by cationic-anionic surfactants sodium dodecyl sulfate and 1-hexadecyl-3-methylimidazolium bromide were studied with the dilution method. The synergic effect exists between cationic and anionic surfactants on the formation of W/O microemulsion. The effects of different alcohols, oils, and brine on the formation of W/O microemulsions stabilized by cationic-anionic surfactants were also investigated. The alcohols with longer chain length have higher efficiency to form the W/O microemulsion. Long-chain alcohol was easier to penetrate into the interfacial layer and less alcohol was needed to form the stable W/O microemulsion. The effects of decreasing the carbon chain length of the alkane molecules and increasing NaCl concentrations are equivalent to that of increasing the carbon chain length of the alcohol molecules.

Preparation of porous silk fibroin microparticles by a water‐in‐oil emulsification‐diffusion method

AbstractPorous silk fibroin (SF) microparticles were prepared by a simple one‐step water‐in‐oil emulsification‐diffusion method. Aqueous SF solution and organic phase solvent were used as water and oil phases, respectively. Four organic solvents with different water solubility were studied, i.e., ethyl acetate, diethyl ether, dichloromethane, and chloroform. Influences of organic phase solvent, water/oil volume ratio, and SF concentration on SF microparticle characteristics were investigated. It was found that shape of the SF microparticles depended upon the type of organic phase solvent. The SF microparticles with hollow or bowl‐like shape were fabricated when organic solvents with higher water solubility, which are ethyl acetate and diethyl ether, were used as the continuous oil phase. While using organic solvents with lower water solubility, i.e., dichloromethane and chloroform resulted in the spherical shape SF microspheres. The water/oil volume ratios and the SF concentrations did not affect on the bowl‐like and spherical shapes of SF microparticles. Results from scanning electron microscope shows that all the SF microparticle matrices were porous structures. Using ethyl acetate and diethyl ether as the oil phase gave larger SF microparticle sizes than using dichloromethane and chloroform. The influences of water/oil volume ratio and the SF concentration showed slightly effect on the sizes of SF microparticles. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Numerical simulation of the unsteady hydrodynamics of a water droplet in paraffin oil

This work reports on numerical simulations of the unsteady hydrodynamics occurring in droplets of pure water in a continuous paraffin oil phase. The considered droplets have varying initial position and are confined in a 1 cm × 1 cm square box. The systems are two-dimensional, submitted to gravity and surface tension effects are fully accounted for. Such systems are typical examples of two phases, unsteady, free interface problems. The full Navier–Stokes equation for incompressible fluids is solved and a volume of fluids technique without interface reconstruction is used. The time evolution of the droplets as well as their hydrodynamics is investigated. Influence of the initial condition and the bifurcations occurring in the velocity field inside the droplets are discussed. Besides droplets unsteady evolution, this work also aims to propose numerical techniques for the simulation of the hydrodynamics of complex fluids systems like emulsions.

Dielectric spectra of ionic water-in-oil microemulsions below percolation: Frequency dependence behavior

We have investigated the dielectric properties of water-in-oil microemulsions composed of sodium bis(2-ethyl-hexyl)sulfosuccinate, water, and decane, using radiofrequency impedance spectroscopy, below the percolation threshold, where the system behaves as surfactant-coated individual water droplets dispersed in a continuous oil phase. The analysis of the dielectric spectra has evidenced that the whole dielectric response below percolation is due to two different contributions, which give rise to two partially overlapping dielectric relaxations, approximately in the frequency range from 10 to 500 MHz. The first of these mechanisms is originated by the bulk polarization of counterions distributed in the electrical double layer of the droplet interior. The second mechanism is associated with a correlated motion of the anionic head groups SO3- at the surfactant-water interface. The introduction of this latter contribution allows us to justify the experimentally observed increase in the low-frequency permittivity as a function of temperature up to temperatures very close to percolation. The present study shows that deviations from the expected values on the basis of dielectric theories of heterogeneous systems (Maxwell-Wagner effect) observed when percolation is approaching can be accounted for, in a reasonable way, by the introduction of a further polarization mechanism, which involves the anionic surfactant groups. Only very close to percolation, when microemulsions undergo a scaling behavior, deviations of the permittivity (and electrical conductivity as well) are a print of the structural rearrangement of the whole system and models based on colloidal particle suspension theories fail. Even if the whole picture of the dielectric properties of microemulsion systems does not change in deep, nevertheless, the refinement introduced in this paper demonstrates how different polarization mechanisms could be simultaneously present in these rather complex systems and, above all, how the individual particle colloidal properties are maintained up to very close to the percolation threshold.

Nano- and Microstructure of High-Internal Phase Emulsions Under Shear

High-internal phase aqueous-in-oil emulsions of two surfactant concentrations were studied using small-angle neutron scattering (SANS) and simultaneous in situ rheology measurements. They contained a continuous oil phase with differing amounts of hexadecane and d-hexadecane (for contrast matching experiments), a deuteroaqueous phase almost saturated with ammonium nitrate, and an oil-soluble stabilizing polyisobutylene-based surfactant. The emulsions' macroscopic rheological behavior has been related to quantify changes in microscale and nanoscale structures observed in the SANS measurements. The emulsions are rheologically unexceptional and show, inter alia, refinement to higher viscosity after high shear, and shear thinning. These are explained by changes observed in the SANS model parameters. Shear thinning is explained by SANS-observed shear disruption of interdroplet bilayer links, causing deflocculation to more spherical, less linked, aqueous droplets. Refinement to higher viscosity is accompanied by droplet size reduction and loss of surfactant from the oil continuous phase. Refinement occurs because of shear-induced droplet anisotropy, which we have also observed in the SANS experiment. This observed anisotropy and the emulsion refinement cannot be reproduced by either isolated molecule or mean-field models but require a more detailed consideration of interdroplet forces in the sheared fluid.

Synthesis of Temperature-Responsive Poly(N-isopropyl acrylamide)/Poly(methyl methacrylate)/Silica Hybrid Capsules from Inverse Pickering Emulsion Polymerization and Their Application in Controlled Drug Release

Poly(N-isopropylacrylamide)/poly(methyl methacrylate)/silica hybrid capsules were prepared from inverse Pickering emulsion polymerization. A N-isopropylacrylamide aqueous solution was emulsified into an oil phase containing methyl methacrylate and divinyl benzene by sonication to obtain a W/O Pickering emulsion using modified silica nanoparticles as stabilizers. After the emulsion was polymerized, the hybrid capsules were obtained. The capsule wall contained two layers--a solid particle monolayer and a polymer layer--and the wall thickness could be controlled by adjusting the methyl methacrylate and divinyl benzene concentrations in the continuous oil phase before polymerization. The as-synthesized capsules exhibited temperature-responsive properties. The controlled release experiments showed that the release rate of a model drug from the hybrid capsules could be controlled by adjusting the wall thickness of the capsule or the temperature of the release medium.

Highly monodisperse and sub-nano silver particles synthesis via microemulsion technique

Highly monodisperse silver nanoparticles were synthesized by chemical reduction of silver nitrate dissolved in water core of one microemulsion and sodium borohydride (reducing agent) dissolved in water core of another microemulsion. The surfactant used in the study was di-octyl sodium sulfosuccinate or aerosol OT (AOT), which is an effective surfactant for synthesizing silver nanoparticles in microemulsion. In the present work, influence of water-to-surfactant mole ratio ( ω), molar ratio of reducing agent-to-silver nitrate ( R′) and stirring effect on silver nanoparticles size and size distribution were studied. In the present work cyclohexane was chosen as continuous oil phase. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) were used for elucidating the structure, size and size distribution of the synthesized silver nanoparticles. TEM micrographs show that the synthesized nanoparticles were having average diameter of 5.5 nm, which increased ω. Isooctane was also used as a continuous oil phase to study the influence of water-to-surfactant ratio. The results showed a highly monodisperse and sub-nano size silver particles formation. The surface area of the synthesized particles was calculated to be 1.46 × 10 10 mm 2/g. An increase in R′ from 0.66 to 1 resulted in slightly smaller nanoparticles production, but on further increase of R′ led to bigger particles formation.

Evolution of water-in-oil emulsions stabilized with solid particles: Influence of added emulsifier

Water-in-oil emulsions made of water droplets dispersed in a continuous paraffin oil phase were prepared and stabilized with hydrophobic silica particles alone. Differential scanning calorimetry (DSC) experiments were carried out to characterize the water droplets freezing transitions and their evolution with time. Water droplet size distribution, oil–water interfacial tension, and rheological stress–strain properties were determined alongside to better understand the role of particles in the formation and stabilization of emulsions. The results obtained were compared to the properties of emulsions prepared with a non-ionic emulsifier, sorbitan monooleate. No interfacial tension reduction was observed at the oil/water interface in presence of particles. As a consequence, the fragmentation of water into droplets required more energy in absence of surface-active emulsifiers. The resulting emulsion droplet size distribution is polydisperse and contains large droplets. Rheology measurements showed that the stability of the emulsion prepared with particles originates from the formation of a 3D network of particles in the continuous oil phase. In emulsion samples containing sorbitan monooleate, calorimetry experiments revealed a progressive displacement of the water droplet freezing transition due to a change in ice nucleation mechanism. The interpretation points out a possible reorganization of the emulsifier film at the water/oil interface, which could modify the conditions of crystallization of dispersed water droplets and affect the emulsion long-term stability. Calorimetry, when used together with other techniques, such as laser light scattering, provides complementary information on the evolution with time of the structure of the emulsion.

Structural and Magnetic Properties of Nanosized Barium Hexaferrite Powders Obtained by Microemulsion Technique

Thin hexagonal barium hexaferrite particles synthesized using the microemulsion technique were studied. A water-in-oil reverse microemulsion system with cetyltrimethylammonium bromide (CTAB) as a cationic surfactant, n-butanol as a co-surfactant, n-hexanol as a continuous oil phase, and an aqueous phase were used. The microstructural and magnetic properties were investigated. The particles obtained were mono-domain with average particle size 280 nm. The magnetic properties of the powder were investigated at 4.2 K and at room temperature. The saturation magnetization was 48.86 emu/g and the coercivity, 2.4 x 105 A/m at room temperature. The anisotropy field Ha and magneto-crystalline anisotropy K1 were 1.4 x 106 A/m and 2.37 x 105 J/m3, respectively.

Cryo-TEM imaging of a novel microemulsion system of silicone oil with an anionic/nonionic surfactant mixture

We report the nanostructures in a novel microemulsion system of silicone oil, water and a surfactant mixture of an anionic and a nonionic surfactant. The phase diagram of the investigated system exhibits two isotropic single-phase channels for constant temperature. The upper channel, that is the channel with the higher mass fraction of nonionic surfactant, starts with an L3-phase at the water side, and passes through a minimum continuously to the oil side. The channel with the lower mass fraction of nonionic surfactant starts with an L1-phase at the water side, and passes with increasing oil content and increasing mass fraction of nonionic surfactant to the middle of the phase diagram and ends there. No connection between the two channels was detected at a surfactant concentration of 15%. The two channels are separated by a single Lα-phase and multiphase regions. In contrast to the results from microemulsions with nonionic surfactants, cryo-TEM micrographs on this system show that the upper phase channel has a bicontinuous structure from zero to only about 35% of oil. At higher oil content the channel contains water droplets in a continuous oil phase. At a water/oil ratio of 1 : 1, the structure looks like a polyhedral foam structure or a high internal phase emulsion (HIPE) structure, and not like the usual bicontinuous structure, as generally assumed. Nevertheless, the dimensions of the imaged bicontinuous and water-in-oil-structures were consistent with the theoretical consideration for nanostructures in microemulsions. The lower channel with its o/w-structure could not well be imaged with the cryo-TEM method. Instead of small droplets, small vesicles were imaged, that obviously were formed by the loss of oil in the thin film during the specimen preparation process for cryo-TEM.

Droplet Based Microfluidics for Synthesis of Mesoporous Silica Microspheres

AbstractHerein we present methods for synthesizing monodisperse mesoporous silica particles and silica particles with bimodal porosity by templating with surfactant micelle and microemulsion phases. The fabrication of monodisperse mesoporous silica particles is based on the formation of well-defined equally sized emulsion droplets using a microfluidic approach. The droplets contain the silica precursor/surfactant solution and are suspended in hexadecane as the continuous oil phase. The solvent is then expelled from the droplets, leading to concentration and micellization of the surfactant. At the same time, the silica solidifies around the surfactant structures, forming equally sized mesoporous particles. We show that hierarchically bimodal porous structures can be obtained by templating silica microparticles with a specially designed surfactant micelle/microemulsion mixture. Oil, water, and surfactant liquid mixtures exhibit very complex phase behavior. Depending on the conditions, such mixtures give rise to highly organized structures. A proper selection of the type and concentration of surfactants determines the structuring at the nanoscale level. Tuning the phase state by adjusting the surfactant composition and concentration allows for the controlled design of a system where microemulsion droplets coexist with smaller surfactant micellar structures. The microemulsion droplet and micellar dimensions determine the two types of pore sizes.

Ferrofluid-in-oil two-phase flow patterns in a flow-focusing microchannel

This study investigates the two-phase flow formation process of water-based Fe3O4 ferrofluid (dispersed phase) in a silicon oil (continuous phase) flow in the microfluidic flow-focusing microchannel under various operational conditions. With transparent PDMS chip and optical microscope, four main two-phase flow patterns as droplet flow, slug flow, ring flow and churn flow are observed. The droplet shape, size, and formation mechanism were also investigated under different Ca numbers and intended to find out the empirical relations. The paper marks an original flow pattern map of the ferrofluid-in-oil flows in the microfluidic flow-focusing microchannels. The flow pattern transiting from droplet flow to slug flow appears for an operational conditions of QR<1 and Lf/W<1. The power law index that related Lf/W to QR was 0.36 in present device.

Flow of two immiscible liquids with low viscosity in Y shaped microfluidic systems: effect of geometry

This article investigates the formation of albumin droplets in fatty esters by means of a flow focussing geometry where the continuous oil phase is introduced in the two lateral branches of a Y junction. The effect of the geometry is investigated in order to clarify the scales controlling the droplet generation with this type of fluid couple. The transition from regular droplet flow to stratified flow is identified from the experiments. It is found that the droplet size varies linearly with the flow rate ratio between the disperse and continuous phases. This is similar to what is found in T junctions microfluidic systems for low capillary numbers.

Aqueous dispersions of silica shell/water-core microcapsules

The preparation is described of water-core/silica-based shell particles, from W/O emulsion droplets, by adding alkoxysilanes to the oil-continuous phase, to form the shell by an interfacial condensation reaction at the W/O interface. In order to form relatively thick (and hence stronger) shells, it is found necessary to use a mixture of tetraethoxysilane (TEOS) and diethoxydimethylsilane (DEODMS), rather than TEOS alone. It is suggested that, in the former case, trans-shell diffusion of the alkoxysilane monomers (from the oil side) and water molecules (from the aqueous side) can continue, as a result of the higher permeability of the shells to these small molecules, thus allowing the interfacial condensation reaction to continue, even when the reaction would have ceased for a harder shell, having a much lower permeability, as occurs when TEOS alone is used. A successful method of transferring the water-core/silica-based shell particles from oil into water is described, based on the direct centrifugation of the particles from an upper oil phase (containing the dispersed particles initially) into a lower aqueous phase placed beneath, which contains a surfactant capable of adsorbing onto the particles and making them water-wetted.