Demulsification of water-in-oil emulsions via filtration through a hydrophilic polymer membrane

It is known that hydrophobic microfiltration membranes can be used for demulsification of oil-in-water (o/w) emulsion due to coalescence of oil droplets in membrane pores. This study demonstrates that a hydrophilic polymer membrane can be used for the demulsification of surfactant-stabilized water-in-oil (w/o) emulsions. The success of demulsification is dependent on the type of emulsions and membrane used. Membrane pore size and transmembrane pressure were found to affect demulsification efficiency (DM), while other factors, such as membrane thickness and initial water content have slight or almost no effect. A coalescence mechanism of the demulsification phenomenon is also discussed. The separation process is not based on sieving effects due to a difference in membrane pore size, but is determined by droplet interactions with membrane surface.     

Removal of Mercury by Emulsion Liquid Membranes: Studies on Emulsion Stability and Scale Up

Emulsion liquid membranes (ELM) have received significant attention in the separation of various metal ions from industrial wastewater. Still efforts are needed to get the desired level of stability to overcome the hindrance in the application of ELM at industrial scale. In this paper, the effects of various parameters such as emulsification speed, concentration of cosurfactant, surfactant, carrier and impeller speed during extraction on the stability of an emulsion liquid membrane are studied. Dispersion destabilization of w/o emulsion is checked by Turbiscan. Drop size distribution and photomicrographs of the emulsions are also analyzed to evaluate stability of the emulsion. Instability of emulsion liquid membrane during extraction process is measured in terms of membrane breakage. A stable emulsion is used for the extraction of mercury from aqueous solution in small scale as well as in large scale.     

Water in oil (w/o) and double (w/o/w) emulsions prepared with spans: microstructure,stability, and rheology

The objective was to analyze the microstructure, stability, and rheology of model emulsions prepared with distilled water, refined sunflower oil, and different Spans (20, 40, 60, and 80) as emulsifiers. The effects of the water content and Span 60 concentration were studied. The lowest water contents led to w/o emulsions, whereas higher percentages gave w/o/w emulsions. Microscopy analysis showed that w/o/w emulsions of higher water contents had a lower number of internal water droplets. W/o emulsions were destabilized by coalescence and sedimentation, whereas creaming was observed in unstable w/o/w emulsions. In the last ones, the creaming stability decreased with increasing water content and enhanced with higher Span 60 concentration; the same effect was observed in their viscoelasticity: They were from unstable liquids to stable gels. Solid Spans (40 and 60) produced more consistent w/o/w emulsions at low water contents and more stable systems at high water percentages in comparison with liquid Spans (20 and 80).     

Effect of calcium salts and surfactant concentration on the stability of water-in-oil (w/o) emulsions prepared with polyglycerol polyricinoleate

The objective of this work was to obtain water-in-oil (w/o) emulsions with polyglycerol polyricinoleate (PGPR) as emulsifier and to study the effect of the addition of calcium in the dispersed aqueous phase on the stability of these systems. Emulsions were formulated with 0.2, 0.5 and 1.0% w/w PGPR and 10% w/w water containing calcium chloride at varied concentrations or other salts (calcium lactate or carbonate; sodium, magnesium or potassium chloride). The stability of these systems was studied with a vertical scan analyzer during 15 days; coalescence and sedimentation were observed as simultaneous destabilization processes. The increase of PGPR concentration and/or calcium chloride content gave more stable emulsions. The stabilizing effect of calcium salt was attributed to the diminution of the water droplets size, the decrease of the attractive force between water droplets and the increase of the adsorption density of the emulsifier. The viscoelastic parameters of the interfacial film were decreased with increasing calcium and PGPR concentrations. Calcium chloride produced a higher increase of stability than calcium salts with lower dissociation degree. The presence of any assayed salt in the aqueous phase also allowed the stabilization of w/o emulsions with higher water contents.     

Demulsification of water-in-oil emulsions by permeation through Shirasu-porous-glass (SPG) membranes

To investigate the effect of the droplet/pore size ratio on membrane demulsification, water-in-oil (W/O) emulsions with uniform-sized droplets was demulsified by permeation through Shirasu-porous-glass (SPG) membranes with a narrow pore size distribution at mean droplet/pore diameter ratios of 0.52–5.75. At transmembrane pressures above a critical pressure, the water droplets larger than the membrane pore size were demulsified, where the SPG membrane acted as a coalescer because the hydrophilic membrane surface had a high affinity for the water droplets. By contrast, at transmembrane pressures below the critical pressure, the larger water droplets were all retained by the membrane due to the sieving effect of the uniform-sized pores. When a W/O emulsion with a mean droplet diameter of 2.30 μm was allowed to permeate through a membrane with a mean pore diameter of 0.86 μm, the demulsification efficiency increased with increasing transmembrane pressure, to a maximum value of 91% at a transmembrane pressure of 392 kPa, and then decreased, while the transmembrane flux increased almost linearly with increasing transmembrane pressure. The demulsification efficiency was higher for higher water phase content and lower concentration of the surfactant, tetraglycerin condensed ricinoleic acid ester, in the emulsions due to the reduction of the emulsion stability.     

Emulsification properties of chitosan

 The chitosans use as an emulsifier in food emulsions was explored. The properties of chitosan (air/solution surface activity, electrical conductivity, HLB) were studied. The obtained emulsions were stable multiple w/o/w emulsions, whose characteristics were explained on the basis of the emulsifier structure and solution properties. The reaction with an anionic surfactant, sodium dodecylsulfate, was also studied, giving a water-insoluble complex at a given surfactant/chitosan ratio. Received: 24 March 1998 Accepted: 13 July 1998     

Filtration of deformable emulsion droplets

Oil-in-water (o/w) emulsions of different droplet size were filtered on membranes of various pore sizes to investigate the growth and behaviour of o/w filter cakes. The cake desorptivity S and the filter membrane resistance R were measured at various filtration pressures P. The variation of S with P shows that filter cake oil droplets of radius a are effectively rigid for P < gamma/a and fully deformable for P > gamma/a, where gamma is the oil-water interfacial tension. For the largest P, when S became P-independent, the filter cake remained water-permeable as expected from theory.     

Emulsion stabilization and inversion using a pH- and temperature-sensitive amphiphilic copolymer

We describe how a versatile amphiphilic diblock copolymer can form oil-in-water (o/w) or water-in-oil (w/o) emulsions depending on pH and temperature. At high pH and temperature, this copolymer is mostly hydrophobic and forms w/o emulsions. Its spontaneous curvature is greatly increased upon pH and/or temperature lowering (due to protonation and/or hydration, respectively), which allows the formation of o/w emulsions. Conductivity measurements and confocal fluorescence micrographs evidence the two kinds of structures obtained over a wide range of pH and temperature. We also show how the emulsion type can be reversibly switched along a temperature scan under stirring. The lower stability of the w/o emulsions as compared to the o/w ones is attributed to a lack of electrostatic repulsion. The importance of the copolymer architecture and conformation with regards to droplet stability is discussed.     

Double inversion of emulsions induced by salt concentration

The effects of salt on emulsions containing sorbitan oleate (Span 80) and Laponite particles were investigated. Surprisingly, a novel double phase inversion was induced by simply changing the salt concentration. At fixed concentration of Laponite particles in the aqueous phase and surfactant in paraffin oil, emulsions are oil in water (o/w) when the concentration of NaCl is lower than 5 mM. Emulsions of water in oil (w/o) are obtained when the NaCl concentration is between 5 and 20 mM. Then the emulsions invert to o/w when the salt concentration is higher than 50 mM. In this process, different emulsifiers dominate the composition of the interfacial layer, and the emulsion type is correspondingly controlled. When the salt concentration is low in the aqueous dispersion of Laponite, the particles are discrete and can move to the interface freely. Therefore, the emulsions are stabilized by particles and surfactant, and the type is o/w as particles are in domination. At intermediate salt concentrations, the aqueous dispersions of Laponite are gel-like, the viscosity is high, and the transition of the particles from the aqueous phase to the interface is inhibited. The emulsions are stabilized mainly by lipophilic surfactant, and w/o emulsions are obtained. For high salt concentration, flocculation occurs and the viscosity of the dispersion is reduced; thus, the adsorption of particles is promoted and the type of emulsions inverts to o/w. Laser-induced fluorescent confocal micrographs and cryo transmission electron microscopy clearly confirm the adsorption of Laponite particles on the surface of o/w emulsion droplets, whereas the accumulation of particles at the w/o emulsion droplet surfaces was not observed. This mechanism is also supported by the results of rheology and interfacial tension measurements.     

High internal phase emulsions: catastrophic phase inversion, stability, and triggered destabilization

We have investigated the formation, drop sizes, and stability of emulsions prepared by hand shaking in a closed vessel in which the emulsion is in contact with a single type of surface during its formation. The emulsions undergo catastrophic phase inversion from oil-in-water (o/w) to water-in-oil (w/o) as the oil volume fraction is increased. We find that the oil volume fraction required for catastrophic inversion exhibits a linear correlation with the oil-water-solid surface contact angle. W/o high internal phase emulsions (HIPEs) prepared in this way contain water drops of diameters in the range 10-100 μm; emulsion drop size depends on the surfactant concentration and method of preparation. W/o HIPEs with large water drops show water separation but w/o HIPEs with small water drops are stable with respect to water separation for more than 100 days. The destabilization of the w/o HIPEs can be triggered by either evaporation of the oil continuous phase or by contact the emulsion with a solid surface of the "wrong" wettability.     

Effect of Water Fraction on Rheological Properties of Waxy Crude Oil Emulsions

This article discusses the effect of water fraction on the rheological properties of waxy crude oil emulsions including gel point, yield stress, viscosity, and thixotropy. The experimental results reveal that the rheological behaviors of the w/o emulsion samples all intensify with the increase of water volume fraction within 60%. Of more significance is that a correlation for w/o emulsions between yield stress and water volume fraction is put forward with an average relative error of 6.75%. In addition, some mainstream viscosity prediction models of w/o emulsions are evaluated, and Elgibaly model is the best-fit for the emulsions in this study.     

Effect of Emulsification Method on the Properties of Lecithin‐ and PGPR‐Stabilized Water‐in‐Oil‐Emulsions

Water‐in‐oil (w/o) emulsions were prepared with phosphatidylcholine‐depleted lecithin or polyglycerol polyricinoleate (PGPR) as emulsifying agents. The effect of different laboratory emulsification devices and the effect of sodium chloride on particle size distribution, coalescence stability, and water droplet sedimentation were investigated. The properties of lecithin‐stabilized w/o emulsions were found to depend more strongly on the emulsifying method than those prepared with PGPR. The rotor‐stator system was not suitable for preparing stable w/o emulsions with lecithin. Whereas the addition of salt was essential to achieve coalescence‐stable emulsions prepared with PGPR, the presence of NaCl favored the coalescence of water droplets and phase separation in emulsions containing lecithin.     

Further Investigation into the Stability of Water-in-Crude Oil Emulsions Formed in Burgan Oilfield: Effect of Toluene,Resins to Asphaltenes Ratio,and Surfactant

Factors controlling the formation and stabilization of water-in-crude oil (w/o) emulsions in oil fields are of great concern to the petroleum industry for the economic development of underground oil reservoirs. Controlling and minimizing the formation of w/o emulsions and demulsification of water from emulsions are also important for environmental development. Because of its importance, the mechanisms, formation, and stability of w/o emulsions have received considerable attention. This article deals with some of the factors responsible for the formation and stabilization of w/o emulsions formed in Burgan oil field in Kuwait. Some of the factors investigated in this study are the naturally occurred surface active components of crude oils such as asphaltenes and resins. Stability of emulsion samples with resins to asphaltenes ratio (R/A) contents of 3, 5, 9, 12, and 20 has been studied. It was found that Emulsion tightness is correlated with resins to asphaltene content of the sample. As the R/content increases the emulsion becomes unstable. The effect of additives such as toluene and dodecyle benzene sulfonic acid (DBSA) on the stability of various emulsion samples collected from oil field are also reported. A 2 wt% of DBSA was found to resolve all the water from emulsion samples collected from Burgan oilfield.     

Polydopamine Nanocluster Embedded Nanofibrous Membrane via Blow Spinning for Separation of Oil/Water Emulsions

Developing a porous separation membrane that can efficiently separate oil–water emulsions still represents a challenge. In this study, nanofiber membranes with polydopamine clusters polymerized and embedded on the surface were successfully constructed using a solution blow-spinning process. The hierarchical surface structure enhanced the selective wettability, superhydrophilicity in air (≈0°), and underwater oleophobicity (≈160.2°) of the membrane. This membrane can effectively separate oil–water emulsions, achieving an excellent permeation flux (1552 Lm⁻² h⁻¹) and high separation efficiency (~99.86%) while operating only under the force of gravity. When the external driving pressure was increased to 20 kPa, the separation efficiency hardly changed (99.81%). However, the permeation flux significantly increased to 5894 Lm⁻² h⁻¹. These results show that the as-prepared polydopamine nanocluster-embedded nanofiber membrane has an excellent potential for oily wastewater treatment applications.     

Microgels as stimuli-responsive stabilizers for emulsions

Temperature- and pH-sensitive microgels from cross-linked poly(N-isopropylacrylamide)-co-methacrylic acid are utilized for emulsion stabilization. The pH- and temperature-dependent stability of the prepared emulsion was characterized. Stable emulsions are obtained at high pH and room temperature. Emulsions with polar oils, like 1-octanol, can be broken by either addition of acid or an increase of temperature, whereas emulsions with unpolar oils do not break upon these stimuli. However, complete phase separation, independent of oil polarity, can be achieved by successive acid addition and heating. This procedure also offers a way to recover and recycle the microgel from the sample. Interfacial dilatational rheology data correlate with the stimuli sensitivity of the emulsion, and a strong dependence of the interfacial elastic and loss moduli on pH and temperature was found. The influence of the preparation method on the type of emulsion is demonstrated. The mean droplet size of the emulsions is characterized by means of flow particle image analysis. The type of emulsion [water in oil (w/o) or oil in water (o/w)] depends on the preparation technique as well as on the microgel content. Emulsification with high shear rates allows preparation of both w/o and o/w emulsions, whereas with low shear rates o/w emulsions are the preferred type. The emulsions are stable at high pH and low temperature, but instable at low pH and high temperature. Therefore, we conclude that poly(N-isopropylacrylamide)-co-methacrylic acid microgels can be used as stimuli-sensitive stabilizers for emulsions. This offers a new and unique way to control emulsion stability.     

Flow cytometry: a new method to investigate the properties of water-in-oil-in-water emulsions

We present a new and facile method to evaluate w/o/w emulsions containing fluorescent markers by flow cytometry. Flow cytometry allows simultaneous measurement of w/o/w emulsion droplets "marked" with a fluorescent marker or "blank" without the need for complicated sample preparation. The yield of preparation of the w/o/w emulsion and the release rate of the fluorescent marker FITC-BSA were investigated by this new method. The release fraction (after 24 h) of FITC-BSA from the w/o/w emulsion decreased with increasing concentration of FITC-BSA inside the internal phase, just like the release fraction of NaCl as marker from the w/o/w emulsion. Flow cytometry results show that the yield and release behavior in w/o/w emulsions are in agreement with results reported by more complicated methods.     

Tailoring of the external and internal morphology of poly-3-hydroxy butyrate microparticles

Some microencapsulation procedures such as oil-in-water (o/w) and water-in-oil-in-water (w/o/w) emulsions were selected in an attempt to produce tailored poly-3-hydroxybutyrate (PHB) microparticles. The effects of several processing parameters such as polymer precipitation, surfactant, solvent, stirring and solvent evaporation rates were also considered. As a rule, low stirring rates at 500 rpm yielded particles ranging between 100 to 250 μm and at rates over 8000 rpm, diameters around 5–10 μm. The surfaces of the bigger particles, observed by scanning electron microscopy (SEM), were rough and the smaller ones were even rougher, irregular, cauliflower like. The extraction of the chloroform under low pressure or to the open atmosphere did not produce any appreciable change in the morphology for either type of particle. Transmission electron microscopy (TEM) micrographs suggest that microparticles obtained by o/w emulsions are monolithics but those obtained by w/o/w emulsions are of capsule-like structure. Microencapsulation of a peptide material such as follicle stimulating hormone was carried out with success using a double emulsion technique. This biomaterial, dissolved in the inner aqueous phase, was able to stabilize the primary emulsion without using a surfactant.     

Emulsion Inversion in an Oil‐Surfactant‐Water System Based on Model Naphthenic Acids under Alkaline Conditions

Emulsion inversion has been studied in a system based on oil (toluene/heptane), 5β‐cholanic acid, and an alkaline brine solution by varying the concentration of sodium hydroxide. At an intermediate pH w/o emulsions were formed, and in the high pH region o/w emulsions were formed. Emulsion inversion occurred in the pH range 8.5–10. The w/o emulsions were consistently more stable compared to the o/w emulsions. Increasing the amount of acid enhanced the stability of the emulsions. Maximum stability was observed close to pH 8, where the ratio between the undissociated and dissociated acid was approximately 1.5. From light microscopy, it can be seen that the emulsions are stabilized by a liquid gel phase. At equilibrium the system consists of an oil phase, a liquid gel phase, and an aqueous phase. Increasing the oil fraction eventually gave only w/o emulsions in the pH range between 7 and 14. For these emulsions, no obvious difference in stability was observed at pH 8, while the stability of the emulsions in the high pH region was significantly enhanced. An increase of the ratio between toluene and heptane gave no obvious difference in either stability or type of emulsion while varying the pH. Use of a less lipophilic acid, such as 4‐octylbenzoic acid, gave very unstable w/o emulsions in the intermediate pH region, while stable o/w emulsions were found in the high pH region.     

Microgels at droplet interfaces of water-in-oil emulsions—challenges and progress

Most of the commonly used microgels (MGs) are hydrophilic and tend to form oil-in-water emulsions. In this review, we discuss the function of MGs at the droplet interface in order to stabilize water-in-oil (w/o) emulsions. This topic addresses both interesting questions in fundamental research and a significant impact for applications, where the w/o emulsion type is of essential importance. Promising approaches to stabilize w/o emulsions with MGs are highlighted from different perspectives, ranging from novel MG modifications to assisted co-stabilization by adding soft or hard particles. We summarize the accumulated knowledge, evaluate the challenges and solutions, and highlight future research trends.     

Materials based on solid-stabilized emulsions

Solid-stabilized emulsions are obtained by shearing a mixture of oil, water, and solid colloidal particles. In this article, we present a large variety of materials, resulting from a limited coalescence process. Direct (o/w), inverse (w/o), and multiple (w/o/w) emulsions that are surfactant-free and monodisperse were produced in a very wide droplet size range, from micrometers to centimeters. These materials exhibit original properties compared with surfactant-stabilized emulsions: outstanding stability with respect to coalescence and unusual rheological behavior. For such systems, the elastic storage modulus G' is not controlled by interfacial tension but by the interfacial elasticity resulting from the strong adhesion between the solid particles adsorbed at the oil-water interface. Due to the wide accessible droplet size range, concentrated emulsions can be extremely fluid while emulsions with low droplet volume fraction can behave as solids.