Effect of the type of the oil phase on stability of highly concentrated water-in-oil emulsions

The water-in-oil high internal phase emulsions were the subject of the study. The emulsions consisted of a super-cooled aqueous solution of inorganic salt as a dispersed phase and industrial grade oil as a continuous phase. The influence of the industrial grade oil type on a water-in-oil high internal phase emulsion stability was investigated. The stability of emulsions was considered in terms of the crystallization of the dispersed phase droplets (that are super-cooled aqueous salt solution) during ageing. The oils were divided into groups: one that highlighted the effect of oil/aqueous phase interfacial tension and another that investigated the effect of oil viscosity on the emulsion rheological properties and shelf-life. For a given set of experimental conditions the influence of oil viscosity for the emulsion stability as well as the oil/aqueous interfacial tension plays an important role. Within the frames of our experiment it was found that there are oil types characterized by optimal parameters: oil/aqueous phase interfacial tension being in the region of 19–24 mN/m and viscosity close to 3 mPa s; such oils produced the most stable high internal phase emulsions. It was assumed that the oil with optimal parameters kept the critical micelle concentration and surfactant diffusion rate at optimal levels allowing the formation of a strong emulsifier layer at the interface and at the same time creating enough emulsifier micelles in the inter-droplet layer to prevent the droplet crystallization.     

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.     

Fat crystals and water-in-oil emulsion stability

Products such as cosmetics, pharmaceuticals, and crude oil often exist as water-in-oil (W/O) emulsions during their processing or in final form. In many cases, their dispersed aqueous phase is encased in a crystal network and/or by interfacially-adsorbed (‘Pickering’) particles [paraffins, triacylglycerols, polymers, etc.] that promote emulsion kinetic stability by hindering droplet–droplet contact, coalescence and macroscopic phase separation. In processed foods, important questions remain regarding whether a continuous phase fat crystal network or Pickering crystal provides better stabilization. This review explores the following factors related to crystal-stabilized W/O emulsions: i) the key properties dictating fat crystal spatial distribution (at the interface or in the continuous phase); ii) how temperature and freeze–thaw emulsion destabilization are intimately linked with fat crystal spatial distribution, and; iii) why oil-soluble surfactant interactions with the continuous oil phase influence fat crystal wettability and emulsifier efficacy. It is shown that these parameters strongly govern W/O emulsion formation and stability.     

Thermoanalytical and microscopical investigation of the microstructure of emulsions containing polymeric emulsifier

Polymeric emulsifiers provide exceptional stability to oil-in-water, water-in-oil or multiple emulsions by their steric stabilization. Pemulens as polymeric emulsifiers are able to stabilize o/w type emulsions because their short lipophilic part integrates into the oil droplets while their long hydrophilic part forms a micro gel around the droplet. In our present study the microstructure and integration of the polymeric emulsifier at the water-oil interface was investigated with thermogravimetric and microscopical methods. It was established that depending on the amount of both of the polymeric emulsifier and added coemulsifier the microstructure of the system changes.     

Phase transitions and microstructure of emulsion systems prepared with acylglycerols/zinc stearate emulsifier

The emulsification processes, during which acylglycerols/zinc stearate emulsifier, water, and oil phase formed ternary systems, such as water-in-oil (W/O) emulsions, oil-in-water (O/W) dispersions, and unstable oil-water mixtures, were investigated in order to characterize the progressive transformations of the dispersed systems. The type, structure, and phase transitions of the systems were found to be determined by temperature and water phase content. Crystallization of the emulsifier caused the destabilization and subsequent phase inversion of the emulsions studied, at a temperature of 60-61 degrees C. The observed destabilization was temporary and led, at lower temperature, to W/O emulsions, "O/W + O" systems, or O/W dispersions, depending on the water content. Simultaneous emulsification and cooling of 20-50 wt % water systems resulted in the formation of stable W/O emulsions that contained a number of large water droplets with dispersed oil globules inside them ("W/O + O/W/O"). In water-rich systems (60-80 wt % of water), crystallization of the emulsifier was found to influence the formation of crystalline vesicle structures that coexisted, in the external water phase, with globules of crystallized oil phase. Results of calorimetric, rheological, and light scattering experiments, for the O/W dispersions obtained, indicate the possible transition of a monostearoylglycerol-based alpha-crystalline gel phase to a coagel state, in these multicomponent systems.     

Stability of emulsions of water in mineral oils containing asphaltenes

Emulsions of water in mineral oils are stable if the oil phase contains asphaltenes which are near the condition of incipient flocculation. This condition is determined by the composition of the oil phase and by the nature of the asphaltenes. High aromaticity of the oil phase and the presence of deflocculants prevent flocculation of asphaltenes; the deflocculants may be interfacially active agents or asphaltene-like compounds with better solubility in the oil phase. Conditions of incipient flocculation of asphaltenes correlate very well with a considerable increase of rheological resistance of the interface between the oil phase and distilled water, determined according to the torsion oscillation method. Stabilization of the water-in-oil emulsions is therefore caused by the build-up of a coherent layer of asphaltenes in the water-oil interface in these cases. Deflocculants of asphaltenes in the oil phase destroy their stabilizing effect; however, the deflocculants themselves may stabilize the water-in-oil emulsions by adsorption on the water-oil interface and then the correlation between the condition of asphaltenes and emulsion stability does not hold, nor is the interfacial viscosity perceptibly increased. Under borderline conditions of emulsion stability a few percent of sodium chloride in the water phase counteracts the build-up of a stabilizing layer of asphaltenes in the water-oil interface and so do higher p_H values of a buffered water phase. At low p_H-values emulsion stability does not correlate with interfacial resistance. It can be concluded that asphaltenes stabilize water-in-oil emulsions if they accumulate on the water-oil interface. This interfacial layer may show a coherence, which is an indication of the presence of asphaltenes rather than a condition for stability of the emulsions.     

Emulsifying potency of new amino acid-type surfactant (II): stable water-in-oil (W/O) emulsions containing 85 wt% inner water phase

The ternary phase diagram for N-[3-lauryloxy-2-hydroxypropyl]-L-arginine L-glutamate (C12HEA-Glu), a new amino acid-type surfactant, /oleic acid (OA)/water system was established. The liquid crystal and gel complex formations between C12HEA-Glu and OA were applied to a preparation of water-in-oil (W/O) emulsions. Stable W/O emulsions containing liquid paraffin (LP) as the oil and a mixture of C12HEA-Glu and OA as the emulsifier were formed. The preparation of stable W/O emulsions containing 85 wt% water phase was also possible, in which water droplets would be polygonally transformed and closely packed, since the maximum percentage of inner phase is 74% assuming uniformly spherical droplets. Water droplets would be taken into the liquid crystalline phase (or the gel complex) and the immovable water droplets would stabilize the W/O emulsion system. The viscosity of emulsions abruptly increased above the 75 wt% water phase (dispersed phase). The stability of W/O emulsions with a lower weight ratio of OA to C12HEA-Glu and a higher ratio of water phase was greater. This unusual phenomenon may be related to the formation of a liquid crystalline phase between C12HEA-Glu and OA, and the stability of the liquid crystal at a lower ratio of oil (continuous phase). W/O and oil-in-water (O/W) emulsions containing LP were selectively prepared using a mixture of C12HEA-Glu and OA since the desirable hydrophile-lipophile balance (HLB) number for the emulsification was obtainable by mixing the two emulsifiers.     

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.     

Water-in-oil emulsion separation within a milli-fluidic device

A milli-fluidic method capable of continuous passive separation of water-in-oil emulsions with an efficiency of ～90% is described. Experiments used high oleic sunflower oil or mineral oil as the continuous phase and aqueous solutions of methylene blue, crystal violet, or dextran with methylene blue as the dispersed phase. To achieve continuous passive separation, a device with opposing channel walls of disparate hydrophobicity is used to stabilize cocurrent laminar flow of oil and water. The disparity in hydrophobicity of the channel walls is accomplished by defining one length of the channel with a hydrogel, in this case polyethylene glycol. Emulsion separation is facilitated by introducing the emulsion at the water/hydrogel interface. Advantages of performing separations at the milli-fluidic scale are presented.     

Emulsification of caraway essential oil in water by lecithin and β-lactoglobulin: emulsion stability and properties of the formed oil–aqueous interface

The stability and droplet size of protein and lipid stabilised emulsions of caraway essential oil as well as the amount of protein on the emulsion droplets have been investigated. The amount of added protein (β-lactoglobulin) and lipid (phosphatidylcholine from soybean (sb-PC)) were varied and the results compared with those obtained with emulsions of a purified olive oil. In general, emulsions with triglyceride oil proved to be more stable compared with those made with caraway essential oil as the dispersed phase. However, the stability of the emulsions can be improved considerably by adding sb-PC. An increase in the protein concentration also promoted emulsion stability. We will also present how ellipsometry can be used to study the adsorption of the lipid from the oil and the protein from the aqueous phase at the oil–water interface. Independently of the used concentration, close to monolayer coverage of sb-PC was observed at the caraway oil–aqueous interface. On the other hand, at the olive oil–aqueous interface, the presence of only a small amount of sb-PC lead to an exponential increase of the layer thickness with time beyond monolayer coverage. The amounts of β-lactoglobulin adsorbed at the caraway oil–aqueous interface and at the olive oil–aqueous interface were similar, corresponding roughly to a protein monolayer coverage.     

Separation profile of model water-in-oil emulsions followed by nuclear magnetic resonance (NMR) measurements: application range and comparison with a multiple-light scattering based apparatus

The application range and validity of two new NMR sequences (hereafter called sequence 1 and sequence 2) for the study of water-in-oil emulsions (w/o) has been assessed using model emulsions and comparison with results obtained by a commercial apparatus (Turbiscan). These new NMR sequences allow to determine the brine profile i.e. the vertical variations of the dispersed phase content (brine) in the NMR tube. Measuring these parameters as a function of time allows to monitor the separation (sedimentation and coalescence rate) between oil and water. The results obtained on model water-in-oil emulsions with both NMR sequences are consistent and meaningful for both stable and coalescing emulsions and are similar, even if not strictly identical, to the ones obtained with the Turbiscan. It also appears that the second NMR sequence is faster (30s to obtain a profile compared with 3 min for the 1st one in the conditions used in this article) and has a broader application range. Indeed, for these two methods, the oil phase must have a viscosity higher or equal than values which is around 5 mPas for the sequence 2 and 20-25 mPas for the method 1.     

Assessment of polymeric flocculants in oily water systems

Oil-in-water emulsions are usually formed during oil production and treatment. Before being discarded, such dispersions should be treated. In order to improve the oil–water separation process using physical processes (decantation, flotation, centrifugation, etc.) the particle size of the disperse phase should be increased. This may be achieved through flocculation, which consists in the agglomeration of various particles or drops, using, as flocculating agents, high molecular weight hydrophilic macromolecules. A few studies have been carried out on the flocculation of finely divided oil drops in brine with the aid of generally ionic polyelectrolytes. This work shows the results obtained using nonionic polymers as flocculants. Commercial samples of poly(ethylene oxide-b-propylene oxide) and poly(vinyl alcohol) were evaluated through flocculation–flotation tests as well as the drop size distribution. The performance of such additives as flocculants for oil–water dispersions is related to their structure, composition, molecular weight and hydrophilic–lipophilic balance. The composition of the produced water is also an important parameter when choosing the features of the flocculant additive.     

Differential scanning calorimetry characterization of water-in-oil emulsions from Mexican crude oils

A simplified equation relating water droplet size distribution to crystallization temperature, determined from differential scanning calorimetry (DSC) curves of aqueous emulsions of petroleum is reported in this article. A series of water-in-oil (W/O) emulsions was prepared by dispersion of water in different Mexican crude oils; in a classical DSC experiment, these emulsions were submitted to a regular heating and cooling cycle within temperatures including freezing and heating of dispersed water. The Z-average diameters of the water drops (D _dz) were estimated this way and correlated with petroleum composition.     

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.     

Vermicious thermo-responsive Pickering emulsifiers

Thermo-responsive vermicious (or worm-like) diblock copolymer nanoparticles prepared directly in n-dodecane via polymerisation-induced self-assembly (PISA) were used to stabilise water-in-oil Pickering emulsions. Mean droplet diameters could be tuned from 8 to 117 μm by varying the worm copolymer concentration and the water volume fraction and very high worm adsorption efficiencies (∼100%) could be obtained below a certain critical copolymer concentration (∼0.50%). Heating a worm dispersion up to 150 °C led to a worm-to-sphere transition, which proved to be irreversible if conducted at sufficiently low copolymer concentration. This affords a rare opportunity to directly compare the Pickering emulsifier performance of chemically identical worms and spheres. It is found that the former nanoparticles are markedly more efficient, since worm-stabilised water droplets are always smaller than the equivalent sphere-stabilised droplets prepared under identical conditions. Moreover, the latter emulsions are appreciably flocculated, whereas the former emulsions proved to be stable. SAXS studies indicate that the mean thickness of the adsorbed worm layer surrounding the water droplets is comparable to that of the worm cross-section diameter determined for non-adsorbed worms dispersed in the continuous phase. Thus the adsorbed worms form a monolayer shell around the water droplets, rather than ill-defined multilayers. Under certain conditions, demulsification occurs on heating as a result of a partial worm-to-sphere morphological transition.     

Ultrafiltration of triglyceride from biodiesel using the phase diagram of oil–FAME–MeOH

To continuously obtain biodiesel of high purity, a membrane separator integrated with liquid–liquid extraction for the oil–FAME–MeOH system is studied. The liquid–liquid phase equilibrium data for the oil–FAME–MeOH are determined experimentally and compared with the general prediction of the modified UNIFAC. The tie line test demonstrates that composition of the methanol-rich phase is free of TG at 20 °C. Using the continuous cross-flow ultrafiltration, the oil-rich phase can be rejected by the ceramic membranes while the methanol-rich phase permeates through the membranes. When the feed bulk composition is controlled within the two-phase zone, such as the oil:FAME:MeOH of 20:30:50 wt.%, the permeate is found to be free of oil while the obtained permeate flux is higher than 300 kg/m² h under the transmembrane pressure of 600 mmHg and the inlet flow rate of 300 ml/min at 20 °C. By contrast, it shows almost no separation when the inlet concentration of oil–FAME–MeOH locates on its boundary line or within the single-phase zone. The quantitative filtration tests show that the compositions in the two liquid phases and the operating parameters are also considered simultaneously to screen the origin oil and get the FAME product of high purity.     

Multi-Walled Carbon Nanotubes as a Cosurfactant for Highly Concentrated Emulsions

The role of multi-wall carbon nanotubes (MWCNT) as a solid surfactant in highly concentrated water-in-oil emulsions was investigated. MWCNT were dispersed in the oil phase. These suspensions are viscoplastic fluids with the yield stress increasing by more than 1000 times with addition of 2% MWCNT, which demonstrates intensive “structurizing” ability. After emulsion preparation, MWCNT were concentrated at the interface, stabilizing emulsions. The dependence of the inversion point on MWCNT concentration was found. Emulsions containing up to 94 wt% of the aqueous phase can be prepared only when MWCNT is combined with conventional surfactant. Rheological properties of such compositions were measured. It was established that emulsions stabilized by a combined surfactant were more stable in comparison to conventional surfactant stabilized emulsion.     

Optimization of Phytosterols Dispersion in an Oil/Water Emulsion Using Mixture Design Approach

Major problems related to enrichment of products with phytosterols are high melting temperature, chalky taste and low solubility in water phase. Dispersion of phytosterols in an emulsion was optimized using a mixture design with four components (phytosterols, emulsifier, soy oil, and water). It was found that the particle size of the dispersed phase decreased with the increase in emulsifier concentration. The appearance viscosity was increased with decreasing particle size. The stability of these emulsions could be correlated with the decrease in surface tension and particle size by using oil and emulsifier as components of oil phase.     

STABILITY OF HATER IN OIL EMULSIONS USING HIGH MOLECULAR WEIGHT EMULSIFIERS

Excellent stability of water-in-oil emulsions could be obtained by partial crosslinking of the fatty chain in several polyglycerol fatty esters. Such products were capable of emulsifying and stabilizing up to 50 wt% water in vegetable oils at a level of 3-5% emulsifier per total emulsion weight. The corresponding non-crosslinked products require at least 20-25% emulsifier to give the sane level of stability, with much higher viscosity.

Degree of polymerization, molecular weight distribution, viscosity, dielectric constant and refractive index of the emulsifier were correlated to the emulsion stability. The most remarkable result is a clear correlation between the molecular weight of the emulsifier and emulsion stability; best emulsions were prepared with polymeric emulsifier with MW of Ca. 40000.     

Na-caseinate/oil/water systems: emulsion morphology diagrams

The concentrated (dispersed phase 50-70 wt%) composition space of Na-caseinate, a family of milk proteins, stabilised emulsions was investigated for three different oils: soybean oil, palm olein and tetradecane with pH 6.8 phosphate buffer continuous phase. The variation of emulsion stability and microstructure were explored using static light scattering, diffusion nuclear magnetic resonance, cryo-scanning electron microscopy, rheology and the time varying macroscopic phase separation of the emulsions. For soybean oil and palm olein a rich diversity of emulsion microstructures and stabilities are realised. Five emulsion domains, each having a different microstructure and macroscopic stability have been identified within the composition space probed. For the lowest concentrations of emulsifier bridging flocculation is evident and emulsions are of low stability. Increasing Na-caseinate concentration leads to an increased stability and the existence of distinct individual oil droplets, visualised using cryo-scanning electron microscopy. Further increases in Na-caseinate concentration reduce emulsion stability due to depletion flocculation. Na-caseinate self-assembly is then initiated. At sufficiently high Na-caseinate and/or oil concentrations the continuous phase of the emulsion is a three-dimensional protein network and emulsion stability is again enhanced. At the limits of the emulsion composition space a gel-like paste is formed. The diversity of emulsion microstructure is reduced when tetradecane is the discrete phase. Na-caseinate self-assembly is limited and there is no evidence for formation of a protein network.