Solubilization in monodisperse emulsions

The kinetics of oil solubilization into micelles from nearly monodisperse alkane-in-water emulsion droplets was investigated. Emulsions containing either hexadecane or tetradecane oils were fractionated to be narrowly distributed, using a method developed by Bibette [J. Bibette, J. Colloid Interface Sci. 147 (1991) 474]. These monodisperse emulsions were mixed with SDS or Tween 20 aqueous micellar solutions of various concentrations. Time-dependent solubilization was monitored using light scattering and a decrease in average droplet size over time was observed, in contrast to what has been observed previously with polydisperse emulsions. The rate at which the droplet size decreased was found to be independent of the initial droplet size. Turbidity measurements were also used to track the solubilization kinetics, and a population balance analysis used on both types of measurements to extract effective mass transfer coefficients. The dependence of these transfer coefficients on droplet size, alkane type, surfactant type and concentration provide insights into plausible mechanisms of emulsion droplet solubilization within micellar solutions.     

Water transfer in mixed water-in-oil emulsions studied by differential scanning calorimetry

Water transfers are observed within complex systems containing aqueous phases separated by a membrane or an oil phase, such as biological cells or multiple emulsions. In order to better understand water transfer mechanism, a system made of a mixed water-in-oil (W/O) emulsion containing two kinds of aqueous droplets — pure water and a 30 % urea solution — was developed. Water transfer from pure water droplets to urea solution droplets was evidenced by Differential Scanning Calorimetry (DSC). Finally the mixed emulsion contains one kind of droplets made of a diluted urea solution which composition is in agreement with formulation and data obtained from experiments performed on single W/O emusions which dispersed phase is a diluted urea solution of the same composition. These mixed emulsions have been pictured as a three-fluid phases system containing two aqueous phases separated by a plane oil membrane. From a homogeneous solubility-diffusion model applied to a quasi-stationnary regime, the water intra-diffusion coefficient has been obtained and compared to the value calculated from the Stokes-Einstein equation. A factor ten makes the discrepancy between the two values, the value deduced from the model being the highest. A possible influence of the emulsifier molecules has been evoked.     

Interface composition of multiple emulsions: rheology as a probe

We have investigated the dynamic rheological properties of concentrated multiple emulsions to characterize their amphiphile composition at interfaces. Multiple emulsions (W1/O/W2) consist of water droplets (W1) dispersed into oil globules (O), which are redispersed in an external aqueous phase (W2). A small-molecule surfactant and an amphiphilic polymer were used to stabilize the inverse emulsion (W1 in oil globules) and the inverse emulsion (oil globules in W2), respectively. Rheological and interfacial tension measurements show that the polymeric surfactant adsorbed at the globule interface does not migrate to the droplet interfaces through the oil phase. This explains, at least partly, the stability improvement of multiple emulsions as polymeric surfactants are used instead of small-molecule surfactants.     

Ripening Phenomena in Emulsions — A Calorimetry Investigation

Ripening phenomena occurring within different kinds of emulsions have been studied. The emulsions concerned are simple water-in-oil (W/O) or oil-in-water (O/W) emulsions, mixed emulsions obtained by the mixture of two simple emulsions, and multiple emulsions water-in-oil-in-water (W/O/W) or oil-in-water-in-oil (O/W/O) emulsions. Composition ripening due to a mass transfer and solid ripening due to the formation of solid particles from the undercooled droplets or due to the formation of solid hydrate around the droplets have been pointed out on using a suitable calorimetric technique. For that purpose a non-diluted emulsion sample is submitted to a cooling and heating cycle during which solidification and melting temperatures and energies of the different phases are analyzed. It has been shown that correlations between these quantities and the properties of the dispersed phase permit one to get information about the ripening phenomena under study. The solution-diffusion model used for mass transfer is in good agreement with the experimental results. From the shell model used for the hydrate formation, it has been possible to deduce the formation energy and the influence of salt upon the temperature of formation.     

Composition Ripening in Mixed Water-in-Oil Emulsions Stabilized with Solid Particles

Water and oil transport in emulsified systems is far from being elucidated. Calorimetric analysis has proved to be an appropriate technique to study composition ripening in mixed water in oil emulsions. In this article, the role of the stabilizing agent is studied and particular attention is given to emulsions stabilized solely with solid particles. Mixed emulsions are prepared by mixing two simple water-in-oil (W/O) emulsions, one with pure water droplets and one with droplets containing an aqueous urea solution. At different time intervals, a sample is introduced in a calorimeter cell and submitted to successive cooling and heating cycles. During the cooling phase, the aqueous internal phase solidifies at a temperature which depends on its composition. Just after the mixed emulsion was prepared, the calorimetric experiment identified two solidification peaks, one corresponding to pure water droplets, and the other one to urea solutions. After a long enough stabilization time, just one peak was observed, showing that the systems evolved toward one type of droplets characterized by a unique composition, due to water transfer between the two aqueous phases. The effect of emulsion stabilizing agent (particles or nonionic emulsifier) on the kinetics of water transfer was investigated.     

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.     

Phase inversion of the Pickering emulsions stabilized by plate-shaped clay particles

We investigated the phase inversion of Pickering emulsions stabilized by plate-shaped clay particles. Addition of water induced a phase inversion from a water-in-oil (W/O) emulsion to an oil-in-water (O/W) emulsion when the amount of the oil phase exceeded a limiting amount of oil absorption to solid particles. On the other hand, a phase inversion from a powdery state to an O/W emulsion state through an oil-separated state is observed when the amount of an oil phase is less than the limiting amount of the oil absorption. Interestingly, the oil separated is re-dispersed as emulsion droplets into the O/W emulsion phase. This type of phase inversion, which is a feature of the Pickering emulsions stabilized by the clay particles, is caused by a change in the aggregate structures of particles.     

Polyols,High Pressure,and Refractive Indices Equalization for Improved Stability of W/O Emulsions for Food Applications

Mixtures of polyols (glycerol, propylene glycol, glucose) and water were emulsified in oil (isopropyl myristate (IPM), medium chain triglycerides (MCT), long chain triglycerides (LCT), and d-limonene) under elevated pressures and homogenization, in the presence of polyglycerol polyricinoleate (PGPR), glycerol monooleate (GMO), and their mixture as emulsifiers to form water-in-oil emulsions. High pressures was applied to: a) the emulsion, b) the aqueous phase and c) the oil phase in the presence of the emulsifiers (PGPR and GMO). Under optimal pressure (2000 atms) applied to the ready-made emulsion or to the aqueous phase prior to its emulsification, and with optimal composition (30wt% polyol in the aqueous phase and MCT as the oil phase), the aqueous droplets were stable for months and submicron in size (0.1 μm). Moreover, due to equalization of the oil and the aqueous phases refractive indices, the emulsions were almost transparent. Pressure and polyols have synergistic effects on the emulsions stability. During preparation, surface tensions and interfacial tensions were dramatically reduced until an optimal water/polyols ratio was achieved, which allows rupturing of the droplets to submicronal size (0.1 μm) without recoalescence and fast diffusion to the interface. These unique W/O emulsions are suitable for preparing W/O/W double emulsions for sustained release of active materials for food applications.     

Fabrication of macroporous foam and microspheres of polystyrene by Pickering emulsion polymerization

Poly(styrene-co-methacrylic acid) (PS-co-MAA) particles were synthesized via surfactant-free emulsion polymerization and then used as particulate emulsifiers for preparation of Pickering emulsions. Our results showed that adjusting the solution pH can tune the wettability of PS-co-MAA particles to stabilize either water-in-oil (W/O) or oil-in-water (O/W) Pickering emulsions. Stable W/O emulsions were obtained with PS-co-MAA particles at low pH values due to their better affinity to the dispersed oil phase. In contrast, increasing the pH value significantly changed the stabilizing behavior of the PS-co-MAA particles, leading to the phase inversion and formation of stable O/W emulsions. We found that the oil/water ratio had a significant influence on pH value of the phase inversion. It decreased with decreasing the oil/water ratio, and no phase inversion occurred when the styrene volume fraction reduced to 10 %. Additionally, macroporous polystyrene (PS) foam and PS microspheres were obtained via polymerization of Pickering high internal phase emulsion (Pickering HIPE) and O/W Pickering emulsion, respectively.     

Three-dimensional molecular mapping of a multiple emulsion by means of CARS microscopy

Multiple emulsions consisting of water droplets dispersed in an oil phase containing emulsifier which is emulsified in an outer water phase (W/O/W) are of great interest in pharmacology for developing new drugs, in the nutrition sciences for designing functional food, and in biology as model systems for cell organelles such as liposomes. In the food industry multiple emulsions with high sugar content in the aqueous phase can be used for the production of sweets, because the high sugar content prevents deterioration. However, for these emulsions the refractive indexes of oil and aqueous phase are very similar. This seriously impedes the analysis of these emulsions, e.g., for process monitoring, because microscopic techniques based on transmission or reflection do not provide sufficient contrast. We have characterized the inner dispersed phase of concentrated W/O/W emulsions with the same refractive index of the three phases by micro Raman spectroscopy and investigated the composition and molecular distribution in water-oil-water emulsions by means of three-dimensional laser scanning CARS (coherent anti-Stokes Raman scattering) microscopy. CARS microscopy has been used to study water droplets dispersed in oil droplets at different Raman resonances to visualize different molecular species. Water droplets with a diameter of about 700 nm could clearly be visualized. The advantages of CARS microscopy for studying this particular system are emphasized by comparing this microscopic technique with conventional confocal reflection and transmission microscopies.     

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.     

Effect of oil soluble surfactant in emulsions stabilised by clay particles

Although surfactants and particles are often mixed together in emulsions, the contribution of each species to the stabilisation of the oil-water interface is poorly understood. We report the results of investigations into the formation of emulsions from solutions of surfactant in oil and aqueous suspensions of laponite. Depending on the salt concentration in the aqueous suspensions, the laponite dispersed as individual disc-shaped particles, 30 nm in diameter, or flocculated into aggregates tens of micrometres in diameter. At the concentrations studied, the flocculated particles alone stabilized oil-in-water emulsions. Synergistic interactions between the particles and octadecylamine at the oil-water interface reduced the average emulsion drop size, while antagonistic interactions with octadecanoic acid enhanced coalescence processes in the emulsions. The state of particle dispersion had dramatic effects on the emulsions formed. Measurements of the oil-water interfacial tension revealed the origins of the interactions between the surfactants and particles.     

Modelization of the release from a tetradecane/water/hexadecane multiple emulsion: evidence of significant micellar diffusion

The release of tetradecane from a multiple emulsion of the type tetradecane/water/hexadecane was studied experimentally using the differential scanning calorimetry technique. The kinetics of the tetradecane release was measured for three formulations containing different concentrations of hydrophilic surfactant (2%, 4%, and 7%). A new mass transfer model derived from the shrinking core model was developed. The values of the model parameters deduced from the least-squares fittings led to the determination of the tetradecane diffusivity. Thus, the preponderant mechanism of mass transfer was proved to be micellar diffusion and not molecular diffusion. This conclusion was confirmed by considering the effect of the change in the hydrophilic surfactant concentration.     

Preparation characteristics of water-in-oil-in-water multiple emulsions using microchannel emulsification

Microchannel (MC) emulsification is a novel technique for preparing monodispersed emulsions. This study demonstrates preparing water-in-oil-in-water (W/O/W) emulsions using MC emulsification. The W/O/W emulsions were prepared by a two-step emulsification process employing MC emulsification as the second step. We investigated the behavior of internal water droplets penetrating the MCs. Using decane, ethyl oleate, and medium-chain triglyceride (MCT) as oil phases, we observed successful MC emulsification and prepared monodispersed oil droplets that contained small water droplets. MC emulsification was possible using triolein as the oil phase, but polydispersed oil droplets were formed from some of the channels. No leakage of the internal water phase was observed during the MC emulsification process. The internal water droplets penetrated the MC without disruption, even though the internal water droplets were larger than the resulting W/O/W emulsion droplets. The W/O/W emulsion entrapment yield was measured fluorometrically and found to be 91%. The mild action of droplet formation based on spontaneous transformation led to a high entrapment yield during MC emulsification.     

Stabilization of Water‐Soluble Antioxidant in Water‐in‐Oil‐in‐Water Double Emulsions

Abstract

In this study, we are introducing a method that can effectively stabilize antioxidants in water‐in‐oil‐in‐water (W/O/W) double emulsions. Preliminarily, stable W/O/W double emulsions were produced by manipulating the characteristics of internal aqueous phase via two‐stage emulsification, resulting consequently in the formation of fine internal water droplets in the dispersed oil droplets. From conductivity measurements that can determine the elution amount of internal aqueous phase, it was confirmed that the double emulsion stability could be improved by treating the internal aqueous phase with a hydroxypropyl‐beta‐cyclodextrin. In this study, kojic acid, 5‐hydroxy‐2‐(hydroxymethyl)‐4‐pyrone was selected as a model antioxidant. The stabilization of kojic acid was attempted by locating it in the internal water droplets of the stable W/O/W double emulsions. The stability of kojic acid in the double emulsion system could be maintained at 90% for 10 weeks at high temperature. We believe that these stable W/O/W double emulsions could be used meaningfully as a carrier for many unstable antioxidants.     

W/O/W多重乳液中水传递的控制

建立了简化的W/O/W(水/油/水)多重乳液乳珠模型——统计平均半径模型, 预测出当W/O/W多重乳液内水相水滴之间以及内外水相之间均达到水传递平衡时的内外水相中盐的浓度, 从而实现对水传递的控制, 以维持W/O/W多重乳液的稳定. 按理论预测制备出了不同稳定态的W/O/W多重乳液, 利用差分扫描量热仪(DSC)检测了多重乳液中水的传递过程, 确定体系在实验状态下的稳定程度, 实验结果与理论预测基本吻合.     

Differential scanning calorimetry (DSC) in multiple W/O/W emulsions

Multiple water-in-oil-in-water (W/O/W) emulsions offer a huge potential as encapsulation systems in different food, cosmetic, and pharmaceutical applications. Because of their complex structure, however, it is difficult to characterize these systems. Typical measurement techniques to determine the size and stability of the inner water droplets encapsulated in the oil droplets show limitations and inaccuracies. Determining the total amount of water in the inner droplets is most often done by indirect methods to date. We describe an analytical method based on differential scanning calorimetry (DSC) for characterizing the total amount of encapsulated water droplets and their stability in W/O/W multiple emulsions. It uses the possibility to directly determine the latent heat of freezing of water droplets of the same size and composition as in the multiple emulsions. The amount of water in the inner droplets of a W/O/W emulsion can thus be calculated very accurately. It is shown that this method enables furthermore detecting multi-modalities in the size distribution of inner water droplets in W/O/W emulsions. Changes in droplet size distribution of the inner droplets occurring during the second emulsification step of processing or during storage can be detected. DSC thus offers a powerful tool to characterize the structure of multiple W/O/W emulsions.     

Effect of Emulsifier Type on the Characterization of O/W Emulsions Using a Spectroscopy Technique

The droplet size distribution (DSD) of emulsions is the result of two competitive effects that take place during emulsification process, i.e., drop breakup and drop coalescence, and it is influenced by the formulation and composition variables, i.e., nature and amount of emulsifier, mixing characteristics, and emulsion preparation, all of which affect the emulsion stability. The aim of this study is to characterize oil-in-water (O/W) emulsions (droplet size and stability) in terms of surfactant concentration and surfactant composition (sodium dodecyl benzene sulphonate (SDBS)/Tween 80 mixture). Ultraviolet-visible (UV-vis) transmission spectroscopy has been applied to obtain droplet size and stability of the emulsions and the verification of emulsion stability with the relative cleared volume technique (time required for a certain amount of emulsion to separate as a cleared phase). It is demonstrated that the DSD of the emulsions is a function of the oil concentration and the surfactant composition with higher stability for emulsions prepared with higher SDBS ratio and lower relative cleared volume with the time. Results also show that smaller oil droplets are generated with increasing Tween 80 ratio and emulsifier concentration.     

Quantification of unadsorbed protein and surfactant emulsifiers in oil-in-water emulsions

Unadsorbed emulsifiers affect the physical and chemical behaviour of oil-in-water (O/W) emulsions. A simple methodology to quantify unadsorbed emulsifiers in the aqueous phase of O/W emulsions has been developed. Emulsions were centrifuged and filtered to separate the aqueous phase from the oil droplets and the concentration of unadsorbed emulsifiers in the aqueous phase determined. The quantification of unadsorbed surfactants based on the direct transesterification of their fatty acids was validated for Tween 20, Tween 80, citric acid ester (Citrem), Span 20 and monolauroyl glycerol. To determine unadsorbed proteins, results obtained with Folin-Ciocalteu reagent or UV-spectrophotometry were compared on emulsions stabilized by β-lactoglobulin (BLG), β-casein (BCN) or bovine serum albumin (BSA). The first method gave more accurate results especially during aging of emulsions in oxidative conditions. The whole methodology was applied to emulsions stabilized with single or mixed emulsifiers. This approach enables optimization of emulsion formulations and could be useful to follow changes in the levels of unadsorbed emulsifiers during physical or chemical aging processes.     

Characterization of fluorocarbon-in-water emulsions with added triglyceride

Fluorocarbon-in-water emulsions are being explored clinically as synthetic oxygen carriers in general surgery. Stabilizing fluorocarbon emulsions against coarsening is critical in maintaining the biocompatibility of the formulation following intravenous administration. It has been purported that the addition of a small percentage of long-chain triglyceride results in stabilization of fluorocarbon emulsions via formation of a three-phase emulsion. In a three-phase emulsion, the triglyceride forms a layer around the dispersed fluorocarbon, thereby improving the adhesion of the phospholipid surfactant to the dispersed phase. In the present study, we examined the effect of triglyceride addition on the physicochemical characteristics of the resulting complex dispersion. In particular, we examined the particle composition and stability of the dispersed particles using a method which first fractionates (classifies) the different particles prior to sizing (i.e., sedimentation field-flow fractionation). It was determined that the addition of a long-chain triglyceride (soybean oil) results in oil demixing and two distinct populations of emulsion droplets. The presence of the two types of emulsion droplets is not observed via light scattering techniques, since the triglyceride droplets dominate the scattering due to a large difference in the refractive index between the particles and the medium as compared to fluorocarbon droplets. The growth of the fractionated fluorocarbon emulsion droplets was followed over time, and it was found that there was no difference in growth rates with and without added triglyceride. In contrast, addition of medium-chain-triglyceride (MCT) oils results in a single population of emulsion droplets (i.e., a three-phase emulsion). These emulsions are not stable to droplet coalescence, however, as significant penetration of MCT into the phospholipid lipid interfacial layer results in a negative increment in the monolayer spontaneous curvature, thereby favoring water-in-oil emulsions and resulting in destabilization of the emulsion to the effects of terminal heat sterilization or mechanical stress.