Comparison of Pickering and network stabilization in water-in-oil emulsions

We compared the efficacy of Pickering crystals, a continuous phase crystal network, and a combination thereof against sedimentation and dispersed phase coalescence in water-in-oil (W/O) emulsions. Using 20 wt % water-in-canola oil emulsions as our model, glycerol monostearate (GMS) permitted Pickering-type stabilization, whereas simultaneous usage of hydrogenated canola oil (HCO) and glycerol monooleate (GMO) primarily led to network-stabilized emulsions. A minimum of 4 wt % GMS or 10 wt % HCO was required for long-term sedimentation stability. Although there were no significant differences between the two in mean droplet size with time, the free water content of the network-stabilized emulsions was higher than Pickering-stabilized emulsions, suggesting higher instability. Microscopy revealed the presence of crystal shells around the dispersed phase in the GMS-stabilized emulsions, whereas in the HCO-stabilized emulsion, spherulitic growth in the continuous phase and on the droplet surface occurred. The displacement energy (E(disp)) to detach crystals from the oil-water interface was ～10(4) kT, and was highest for GMS crystals. Thermal cycling to induce dispersed phase coalescence of the emulsions resulted in desorption of both GMS and GMO from the interface, which we ascribed to solute-solvent hydrogen bonding between the emulsifier molecules and the solvent oil, based on IR spectra. Overall, Pickering crystals were more effective than network crystals for emulsion stabilization. However, the thermal stability of all emulsions was hampered by the diffusion of the molten emulsifiers from the interface.     

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.     

Flocculation of dispersed phase droplets in water-in-oil emulsions: Experiment and mathematical simulation

The sedimentation stability of water-in-oil (W/O) emulsions is shown to drastically rise at dispersed phase fractions of higher than 0.25–0.30. According to electron microscopy data, in W/O emulsions, dispersed phase droplets can form both globular flocs and network structures, with the latter prevailing in sedimentation-stable emulsions. The fractal dimensions are determined for different types of flocs. The aggregation of dispersed phase droplets is mathematically simulated according to the models of the diffusion-limited cluster aggregation and Brownian dynamics. Both models are shown to predict the formation of aggregates with structures similar to flocs observed in micrographs. The value of the percolation threshold calculated for aggregates within the framework of the Brownian dynamics model coincides with the fraction of the dispersed phase in W/O emulsions at which the network structures begin to be formed.     

Water mass transfer in W/O emulsions

Water transportation through the oil phase in W/O emulsions and in W1/O/W2 systems (W/O emulsion in contact with water) was examined. Substance diffusion through interfaces led to interface instability and spontaneous emulsification which caused nanodispersion formation. The photomicrographs of Pt/C replicas of emulsions showed the presence in the continuous oil phase a lot of nanodispersion droplets with a diameter in the range 17-25 nm. Diffusion coefficient (D) of water calculated on the base of Lifshiz-Slezov-Wagner (LSW) equation was about 15 times lower than the coefficients of molecular diffusion. Since such emulsions were extremely unstable toward coalescence, the growth of water droplets took place through as Ostwald ripening as coalescence. In three-phase W1/O/W2 systems diffusion of water, Rhodamine C, and ethanol was studied. D calculated on the base of the equation of nonstationary diffusion were approximately 1000 times lower than molecular ones. It was assumed, that nanodispersion droplets were more likely water carriers in investigated W/O emulsions stabilized by sorbitan monooleate.     

High internal phase Pickering emulsions

Macroemulsions rendered stable by adsorbed colloidal particles are termed Pickering emulsions. If the volume fraction of dispersed phase exceeds around 0.75, the emulsions are named high internal phase Pickering emulsions abbreviated to HIPPEs, which present new properties and potential applications. We review here the recent progress in preparing and studying HIPPEs of both oil-in-water and water-in-oil types. This includes discussion of the range of solid particle emulsifiers, the choice of the two immiscible liquids and methods for their preparation. As a result of their high interfacial area and long-term stability, HIPPEs are being put to use in many potential applications including drug delivery, catalysis, and in the production of novel porous materials.     

Ethanol-in-oil emulsion (E/O) stabilized by polyglycerol polyricinoleate: A potential delivery system for ethanolic extract

This study evaluated how variations in polyglycerol polyricinoleate (PGPR) concentration and ethanol dispersed phase content affect the stability of ethanol-in-oil (E/O) emulsions. Results indicate that the stable 10?wt% E/O emulsions can be produced using 2?wt% PGPR. Increasing the ethanol dispersed phased content at constant PGPR concentration caused instability in emulsion. These emulsions remained stable to droplet flocculation and coalescence in the presence of Centella asiatica ethanol extract. PGPR does not greatly decrease the interfacial tension of the ethanol–oil interface. However, it adsorbed at the interface and stabilized the ethanol droplets in the emulsion via steric mechanism.     

Kinetics of the photoinitiated inverse microemulsion polymerization of 2-methacryloyl oxyethyl trimethyl ammonium chloride

The polymerization of inverse microemulsions of 2-methacryloyl oxyethyl trimethyl ammonium chloride stabilized by a blend of nonionic emulsifiers (a sorbitan sesquioleate and a sorbitan monooleate) and initiated by UV light in the presence of Azobis(isobutyronitrile) (AIBN) was investigated. The effect of initiator concentration, light intensity, emulsifier concentration, and dispersed phase weight fraction on the polymerization rate (R_p), number of polymer particles (N_p), and polymer molecular weight (M_w) was studied. The application of this process to tubular reactors is discussed. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 737–748, 1998     

Stabilization of Oil-in-Water Emulsions with SiO<Subscript>2</Subscript> and Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Nanoparticles

Stabilization of oil-in-water Pickering emulsions with SiO₂ and Fe₃O₄ nanoparticles has been studied. Emulsions containing three-dimensional gel networks formed by aggregated nanoparticles in the dispersion media have been shown to be stable with respect to flocculation, coalescence, and creaming. Concentration ranges in which emulsions are kinetically stable have been determined. Stabilization with mixed Ludox HS-30 and Ludox CL SiO₂ nanoparticles leads to the formation of stable emulsions at a weight ratio between the nanoparticles equal to 2 and pH 6.7. In the case of stabilization with Ludox CL and Fe₃O₄ nanoparticles, systems resistant to aggregation and sedimentation are obtained at pH 8. The use of mixed Ludox HS-30 and Fe₃O₄ nanoparticles has not resulted in the formation of emulsions stable with respect to creaming, with such emulsions appearing to be resistant only to coalescence at pH 2–6.     

A Three-Step Model for Submicron W/O Emulsion Formation in a Transitional-Phase Inversion Process

A three-step model of the transitional phase inversion (TPI) process for the formation of water-in-oil (W/O) emulsions is presented. Three types of emulsions exist in an emulsification process at different oil–water ratios and hydrophilic–lipophilic balance (HLB). A stable W/O emulsion was obtained using Sorbitan oleate (Span 80) and polyoxyethylenesorbitan monooleate (Tween 80) with a specified HLB and oil volume fraction. Oil was added into water, which contained the water-soluble surfactant, to dissolve the oil-soluble surfactant. This route allowed TPI to occur, and an interesting emulsification process was observed by varying the HLB, which corresponded to the change in the oil–water ratio. Two types of emulsions in the emulsification process were found: transition emulsion 1 (W/O/W high internal phase emulsion) and target emulsion 2 (W/O emulsion with low viscosity). This study describes the changes that occurred in the emulsification process.     

Highly concentrated (gel) emulsions,versatile reaction media

Highly concentrated (gel) emulsions are characterised by dispersed phase volume fractions exceeding 0.74, the critical value for the most compact packing of monodispersed undistorted spheres. Their structure consists of polyhedral droplets separated by thin films of continuous phase, a structure resembling gas–liquid foams. Their rheological properties vary from elastic to viscoelastic having a gel appearance. One of the most promising applications is their use as reaction media. The recent advances in the preparation of low-density polymeric materials (solid foams, aerogels) are reviewed and new applications are described. These include the preparation of dual meso/macroporous inorganic oxide materials and the use of gel emulsions as alternative to conventional solvent media in chemical and enzyme-catalysed reactions.     

Effects of pH and salt concentration on oil-in-water emulsions stabilized solely by nanocomposite microgel particles

Aqueous dispersions of lightly cross-linked poly(4-vinylpyridine)/silica nanocomposite microgel particles are used as a sole emulsifier of methyl myristate and water (1:1 by volume) at various pH values and salt concentrations at 20 degrees C. These particles become swollen at low pH with the hydrodynamic diameter increasing from 250 nm at pH 8.8 to 630 nm at pH 2.7. For batch emulsions prepared at pH 3.4, oil-in-water (o/w) emulsions are formed that are stable to coalescence but exhibit creaming. Below pH 3.3, however, these emulsions are very unstable to coalescence and rapid phase separation occurs just after homogenization (pH-dependent). The pH for 50% ionization of the pyridine groups in the particles in the bulk (pK(a)) was determined to be 3.4 by acid titration measurements of the aqueous dispersion. Thus, the charged swollen particles no longer adsorb at the oil-water interface. For continuous emulsions (prepared at high pH with the pH then decreased abruptly or progressively), demulsification takes place rapidly below pH 3.3, implying that particles adsorbed at the oil-water interface can become charged (protonated) and detached from the interface in situ (pH-responsive). Furthermore, at a fixed pH of 4.0, addition of sodium chloride to the aqueous dispersion increases the degree of ionization of the particles and batch emulsions are significantly unstable to coalescence at a salt concentration of 0.24 mol kg(-1). The degree of ionization of such microgel particles is a critical factor in controlling the coalescence stability of o/w emulsions stabilized by them.     

Emulsion pathways to composite polymeric membranes for separation processes

A novel method for the preparation of selective composite membranes from emulsions is suggested. The dispersed phase is chosen to yield a polymer soluble in those components for which the membrane should be selective; the continuous phase, on the other hand, is selected to yield a polymer that is insoluble in any of the components of the mixture. Conventional emulsions (which have a maximum dispersed phase volume fraction of 0.74) or microemulsions can be employed to generate composites. However, concentrated emulsions which allow volume fractions as large as 0.99 are most suitable as precursors to selective and efficient membranes. These concentrated emulsions have the appearance of gels with a structure similar to that of foams. The relatively high permeabilities obtained with the resultant membranes are due to the small thickness of the films of the continuous phase. A concentrated emulsion of a hydrophobic (hydrophilic) monomer dispersed in a hydrophilic (hydrophobic) continuous phase is first prepared at room temperature, with suitable initiators in each phase for later polymerization, and with an appropriate dispersant in the continuous phase. To ensure the stability of the emulsion, the hydrophilic monomer is, in general, replaced by monomer plus water. On heating the gel at 50 °C, polymerization occurs in both phases and the emulsion transforms into a composite polymer membrane. As examples, composite membranes containing polystyrene as the dispersed phase and polyacrylamide as the continuous phase are used to separate toluene from cyclohexane, while other composite membranes containing acrylamide as the dispersed phase and a crosslinked polystyrene as the continuous phase are used to separate water from ethanol.Lecture presented at the Colburn Symposium, University of Delaware, October 19, 1988.     

反相浓乳液方法制备聚苯乙烯/二乙烯基苯结构型泡孔聚合物

通过反相浓乳液聚合方法制备了系列聚苯乙烯/二乙烯基苯(PS/DVB)泡孔聚合物.水作为分散相,其分散相体积分数可达90%;苯乙烯单体作为连续相,聚合后构成PS/DVB泡孔聚合物的结构骨架.用扫描电镜系统研究了乳化剂的浓度、分散相体积分数、添加不同沸点的溶剂等对泡孔聚合物断面形态的影响,并考察了泡孔聚合物对水和柴油的吸附情况.结果表明,不同工艺条件下可以制备出不同孔径的泡孔聚合物,加入不同沸点溶剂使得泡孔壁也形成了多孔结构.     

Addition of halloysite nanotubes prevents cracking in drying latex films

Investigating the process of film drying from aqueous dispersions containing a polymer latex as well as halloysite nanotubes (HNTs), we found that composite films could be formed without cracking under conditions where films of the pure polymer would always crack. Scanning electron micrographs showed that the HNTs were well dispersed and, further, that the distribution of fiber orientations was close to isotropic. The pendulum hardness of films formed from acrylate dispersions strongly increased upon addition of the inorganic phase. The pencil hardness, on the other hand, was poor, which presumably goes back to insufficient coupling between the organic and the inorganic phase. All films were white in appearance. For fiber concentrations higher than 10 vol %, the final films were porous.     

Invert and double emulsions as a base for microheterogeneous matrices for transdermal delivery of lipophilic drugs

Stable invert emulsions water/oil and double emulsions oil₁/water/oil₂ containing polyisobuthylene as pressure-sensitive polymeric adhesive are prepared. The dispersed phase of the invert emulsions contained micelles of the surfactant Tween 80 (Tw) with solubilized drug (Felodipine) and a skin permeation enhancer (glycerol monooleate, GMO). The active components (Felodipine and GMO) of the double emulsions were incorporated into the internal dispersed phase, while the intermediate water layer contained Tw and hydroxypropylcellulose. Ultradispersed polymer films with good adhesion to skin were prepared from both types of emulsions. The films based on double emulsions demonstrate the ability to release Felodipine at therapeutically effective levels and maintain these levels during the first 24 h to attain a therapeutically required dose. The invert and double emulsions were for the first time utilized as bases for microheterogeneous matrices for delivery of a lipophilic drug in bioavailable form.     

Langevin-dynamics simulation of flocculation in water-in-oil emulsions

Aggregation of internal phase droplets in water-in-oil emulsions has been simulated by the Langevin-dynamics method. At initial stages, the process rate obeys the Smoluchowski equations. The concentration ranges have been determined in which percolation clusters are formed. It has been shown that the examined emulsions may be divided into three groups, which enable one to predict their sedimentation stability. Percolation clusters are not formed in emulsions with internal-phase fractions of ≤0.1; therefore, their sedimentation rate is high. In the range of internal-phase fractions from 0.1 to 0.3–0.4, the percolation clusters are formed within a few fractions of a second; however, the rate of subsequent droplet addition to the clusters is low. The flocculation of internal-phase droplets results in the formation of a weak network structure followed by its densification and the separation of a dispersion medium with time. In emulsions with internalphase fractions of >0.3–0.4, almost all droplets unite into percolation clusters within a few fractions of a second. This structure is slowly densified; therefore, the rate of dispersion medium separation is low. The results obtained have been confirmed by experimental data on the sedimentation stability of the water-in-oil emulsions.     

Drop Coalescence during Emulsion Formation in a High-Pressure Homogenizer for Tetradecane-in-Water Emulsion Stabilized by Sodium Dodecyl Sulfate

The present study investigates the effects of homogenizer pressure, surfactant concentration, ionic strength, and dispersed phase fraction on the coalescence rate of tetradecane-in-water emulsions during their formation in a high-pressure homogenizer. Experiments were conducted in a recirculating system consisting of a Rannie laboratory-scale single-stage homogenizer and a stirred vessel for tetradecane-in-water emulsions stabilized by sodium dodecyl sulfate (SDS). The initial evolution of the number concentration of droplets in the stirred tank was measured when subjected to a negative stepchange in the homogenizer pressure. The average drop coalescence rate constant in the homogenizer was inferred by fitting the experimental evolution of the number concentration of drops to a simple model accounting for the coalescence in the homogenizer under the assumption of a quasi steady state in the homogenizer. The residence time of the emulsion in the homogenizer was evaluated from the analysis of radial turbulent flow between disks. The step down homogenizer pressure was varied in the range 20.7-48.3 MPa, the drop size in the range 174-209 nm, the dispersed phase fraction in the range 5%-15%, SDS concentration in the range 0.0033-0.25 wt%, and ionic strength in the range 0.01-0.1 M. The coalescence rate constants were found to be in the range from 3.34x10(-17) to 2.43x10(-16) m(3) s(-1). The coalescence rate constant was found to be higher for higher homogenizer pressures, smaller drop sizes, lower dispersed phase fractions, and lower SDS concentrations and was insensitive to variations in ionic strength. Copyright 2001 Academic Press.     

聚合物共混相态形成过程及其理论研究进展

聚合物共混过程中相态的形成及变化规律是近年来研究热点问题,这一动态过程将决定静态材料的结构与性能.聚合物共混形态结构发展的机理有初期破碎、中期分散及后期归并等机理.在共混初期,分散相尺寸较大,以细化为主.随着混合时间的增加,分散相逐渐细化,尺寸越来越小,细化也越来越难,在应力场的作用下,分散相粒子可能因碰撞而发生归并,归并与细化将逐渐达到平衡而形成稳定的结构.在混炼后期,剪切力减小,粒子发生有效碰撞归并概率增大.     

Concentrated CO(2)-in-Water Emulsions with Nonionic Polymeric Surfactants

Concentrated CO(2)-in-water (C/W) emulsions are reported for amphiphiles containing alkylene oxide-, siloxane-, and fluorocarbon-based tails as a function of temperature and salinity. Poly(ethylene oxide)-b-poly(butylene oxide) (EO(15)-b-BO(12)) can emulsify up to 70% CO(2) with droplet sizes from 2 to 4 &mgr;m in diameter, as determined by video-enhanced microscopy. This emulsion is stable over 48 h against both flocculation and coalescence. In contrast, it is extremely difficult to form concentrated water-in-CO(2) (W/C) emulsions with surfactants containing alkylene oxide moieties due to limited solvation of such tails by CO(2). In several cases, C/W emulsions are formed even when the surfactant prefers CO(2). This violation of Bancroft's rule may be attributed in part to the low viscosity of the compressed CO(2), which governs several mass and momentum transport mechanisms relevant to emulsion formation and stabilization. For the first time, W/C microemulsions are observed in a system with a nonionic amphiphile, namely F(CF(2)CF(2))(3-8)CH(2)CH(2)O(CH(2)CH(2)O)(10-15)H. For the same system, the emulsion morphology changes from C/W to W/C as the temperature increases. The electrical conductivity of C/W emulsions is predicted successfully as a function of the dispersed phase volume fraction of CO(2) with Maxwell's theory for inhomogeneous systems. Copyright 2001 Academic Press.     

Phase behavior, self-assembly, and emulsification of tween 80/water mixtures with limonene and perfluoromethyldecalin

The phase behavior, microstructure, and emulsification of polyoxyethylene (20) sorbitan monooleate (Tween 80), water, and d-limonene (LM) or perfluoromethyldecalin (PFMD) has been studied by small-angle X-ray scattering and polarizing optical microscopy. In the Tween 80/water binary system, a micellar solution (L(1)), a hexagonal (H(1)) phase, and a water-swellable isotropic surfactant liquid (L(2)) phase are successively formed at 25 °C. LM can be solubilized into all of the phases formed by Tween 80/water mixtures, whereas no solubilization of PFMD occurs. The L(2) phase was found by small-angle neutron scattering to be bicontinuous with low interfacial curvature. Added water swells and amplifies the pre-existing amphiphilic structure. The stability of oil-in-H(1) complex emulsions is found to be sensitive to changes in structure that accompany solubilization.