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Comparison between membranes for use in cross flow membrane emulsification
Abstract
Emulsification is an important process, particularly in food production. Membrane emulsification attempts to improve on traditional emulsification methods by producing each droplet singly. Cross flow membrane emulsification uses the flow of the continuous phase to detach droplets from the membrane pores. Different factors were investigated, including the effects of cross flow velocity, trans-membrane pressure, and emulsifier concentration. The effects of using different membrane materials and morphologies were directly compared. The trans-membrane pressure was found to have a large effect on the diameter (D[4,3]) of the droplets, with similar results for all the studied membrane types. Increasing the pressure increases droplet sizes, and can be related to the different mechanisms of droplet detachment from the membrane. An increase in the linear cross flow velocity was shown to cause a decrease in the diameter of the produced droplets. The different membranes each have different characteristics.
... In a cross-flow membrane foaming (CFMF) system, the dispersed phase is injected into the continuous phase passing through a porous membrane matrix, and bubbles are detached at the membrane surface by the cross-flowing continuous phase. Compared to membrane emulsification, which has been substantially studied and reviewed [9][10][11][12][13], only few studies were conducted on membrane foaming and in most of these cases a high surfactant concentration (e.g., 10 wt % whey protein) was used and any short-term few studies were conducted on membrane foaming and in most of these cases a high surfactant concentration (e.g., 10 wt % whey protein) was used and any short-term destabilization process (in particular, coalescence) was thus not taken into account. The bubble properties and resulting foam stability were evaluated at the outlet of the foaming systems and were then explained in terms of the applied process parameters and/or the membrane properties such as pore size [5,8]. ...
... We first introduce the forces which dictate bubble formation. As was reported for cross-flow membrane emulsification systems, the bubble is subjected to four forces while growing at the membrane surface, which are the shear force imposed by the continuous phase flow rate and viscosity, the buoyancy force, the inertia force and the interfacial tension force [10,17]. For bubble formation in the present system, the interfacial tension force is the holding force, and scales as either γR 2 p R 0 or γR p , depending on the applicability of a force or torque balance (R p is the radius of the pore and R 0 is the in-line radius of bubble which grows towards D 0 ) [18]. ...
... Moreover, because the surface tension only can be decreased to a limited extent, bounded by the equilibrium surface tension of the air/water interface stabilized by whey proteins, the influence of protein adsorption on bubble size is moderate. Hence, bubble formation at the membrane surface falls into a regime where bubble snap-off is dominated by shear force, and under the conditions studied here, mainly controlled by the continuous phase flow rate [10]. The bubble size distribution varies with bubble size. ...
Membrane foaming is a promising alternative to conventional foaming methods to produce uniform bubbles. In this study, we provide a fundamental study of a cross-flow membrane foaming (CFMF) system to understand and control bubble formation for various process conditions and fluid properties. Observations with high spatial and temporal resolution allowed us to study bubble formation and bubble coalescence processes simultaneously. Bubble formation time and the snap-off bubble size (D0) were primarily controlled by the continuous phase flow rate (Qc); they decreased as Qc increased, from 1.64 to 0.13 ms and from 125 to 49 µm. Coalescence resulted in an increase in bubble size (Dcoal>D0), which can be strongly reduced by increasing either continuous phase viscosity or protein concentration—factors that only slightly influence D0. Particularly, in a 2.5 wt % whey protein system, coalescence could be suppressed with a coefficient of variation below 20%. The stabilizing effect is ascribed to the convective transport of proteins and the intersection of timescales (i.e., μs to ms) of bubble formation and protein adsorption. Our study provides insights into the membrane foaming process at relevant (micro-) length and time scales and paves the way for its further development and application.
... En revanche, si la porosité est très faible, le flux de la phase dispersée peut être insuffisant pour la production de gouttelettes [109]. Cela conduit à un détachement plus précoce des gouttes, d'où des tailles de gouttes plus petites [156]. Un effet contraire est observé dans le cas où le détachement est contrôlé par la force de tension superficielle. ...
... Par conséquent, plus les molécules des tensioactifs s'adsorbent rapidement à l'interface, plus la tension superficielle de la phase aqueuse diminue rapidement.Egalement, une concentration plus élevée en tensioactif entraîne une diminution plus rapide de la tension superficielle, formant des diamètres plus petits. Par conséquent, une tension superficielle plus faible signifie que la force de tension superficielle retenant la microbulle/gouttelette à la sortie du pore est plus faible, ainsi la microbulle/gouttelette se détache plus tôt, et des tailles plus petites sont obtenues[155,156]. L'adsorption rapide de tensioactifs permet également d'éviter le phénomène de coalescence des microbulles/gouttelettes formées provenant de deux pores adjacents, grâce à une rapide stabilisation de l'interface[126,144,156]. ...
... Par conséquent, une tension superficielle plus faible signifie que la force de tension superficielle retenant la microbulle/gouttelette à la sortie du pore est plus faible, ainsi la microbulle/gouttelette se détache plus tôt, et des tailles plus petites sont obtenues[155,156]. L'adsorption rapide de tensioactifs permet également d'éviter le phénomène de coalescence des microbulles/gouttelettes formées provenant de deux pores adjacents, grâce à une rapide stabilisation de l'interface[126,144,156]. Etant donné que la probabilité de coalescence des microbulles/gouttelettes à la surface de la membrane diminue, une distribution de diamètre plus étroite est ainsi obtenue. ...
Le développement de différentes formes colloïdales pour la thérapie et le diagnostic médical ultrasonore connait un intérêt croissant depuis de nombreuses années. En particulier, les microbulles de perfluorocarbone (PFC) sont des agents de contraste intéressants, car le gaz est un puissant réflecteur des ultrasons. Plus récemment, les gouttelettes de PFC ont été proposées pour de nouvelles applications acoustiques. Suite à une impulsion acoustique, les ultrasons induisent un changement de phase de l’état liquide à l’état gazeux. Ce phénomène est appelé la vaporisation acoustique de gouttelettes. Parallèlement à l’étude de nouvelles applications, le développement de nouvelles techniques de préparation offrant un meilleur contrôle lors de la production, reste un enjeu primordial. Ainsi, de nouvelles méthodes de préparation basées sur des dispositifs membranaires semblent être particulièrement intéressantes. L’objectif de la thèse porte donc sur le développement de nouvelles techniques à membrane pour la formulation de microbulles et de microgouttelettes de taille contrôlée pour des applications en imagerie et thérapie ultrasonore. Dans ce travail, l’émulsification membranaire directe avec un module membranaire de type cross-flow a été utilisé pour la préparation de microbulles stabilisées par des tensioactifs solubles, tandis qu’un module de type microkit a permis l’obtention de microbulles stabilisées par des phospholipides. Dans un second temps, l’émulsification membranaire par prémix a permis de formuler des microgouttelettes de PFC monodispersées. Pour les différentes formes colloïdales préparées, nous avons observé l’influence des paramètres du procédé (pression, débit et contrainte de cisaillement), des paramètres de formulation (molécules stabilisatrices, type de PFC de la phase dispersée) et des paramètres de la membrane (taille des pores) sur la formation des microbulles/ microgouttelettes. Par la suite, la caractérisation acoustique des microbulles/microgouttelettes a montré que ces systèmes présentent les propriétés nécessaires pour être utilisés comme agents de contraste ultrasonores
... As well as the membrane pore size, feed pressure (and thus oil flow rate) has a large effect in emulsion stability, since feed pressure influences the time necessary for drop formation and can also influence the drop shape. Differently from the present study, Hancocks et al. (2013) observed an increase in the mean droplet diameter of oil in water emulsions, when the transmembrane pressure was increased from 50 kPa to 300 kPa, using a ceramic membrane (pore size range 1 mme10 mm) and Tween ® 20 as surfactant. This different behavior of feed pressure can be due to the lower mean pore size of the membrane used in the present study. ...
... The decrease in the mean droplet size with the increase in surfactant concentration was also observed in other studies on membrane emulsification, and attributed to the decrease in interfacial tension, which results in detachment of smaller droplets from the pore openings and lower tendency of droplet coalescence (Kukizaki, 2009;Spyropoulos et al., 2011). According to Schr€ oder and Schubert (1999) and Hancocks et al. (2013), the flux of the dispersed phase flowing through the membrane is directly proportional to the transmembrane pressure, and the higher the pressure, the less time is taken to produce an emulsion. In addition, it is known that the dispersed phase flux increases with increasing pore size and pressure (Vladisavljevi c et al., 2004). ...
... Pressure ( definition of an optimal pressure for each process is necessary, mainly when considering the application of this technique in an industrial scale. Nevertheless, as droplet size or size distribution control is the aim of this study, it is important to keep the pressure below the point where the control of these parameters is drastically reduced, i.e. so that droplet detachment occurs due to pressure/ interfacial tension instead of streaming breakup (Hancocks et al., 2013). This allows membrane emulsification to be a useful process with the potential to produce a large range of emulsion droplet sizes and size distributions under controlled operational conditions. ...
Membrane emulsification has been drawing attention due to its manyapplication possibilities. This study
aimed to assess the preparation and stability of oil in water (O/W) emulsions by membrane emulsificationtechnique. Microporous ceramic membranes with mean pore size 0.2 mm and 0.8 mm were used
for producing sunflower oil in water emulsions, displaying mean droplet size ranging from 2.9 to 11.6 mm
and from 4.8 to 16.2 mm, respectively. The effect of the oil concentration (10%e20%), surfactant concentration
(1%e4%), tangential velocity (0.12 m s�1 to 0.24 m s�1), and feed pressure (100 kPae300 kPa)
on the process performance was investigated using an experimental design. All the parameters significantly
influenced the mean droplet size, the droplet size distribution, and the emulsion stability.
Depending on the emulsification conditions, monodisperse and polydisperse emulsions were obtained.
The use of a membrane with 0.2 mm mean pore size led to better results than those obtained with a
0.8 mm membrane, since they yielded emulsions displaying smaller droplet size and narrower size
distribution. The emulsions prepared using this membrane were stable up to 100 days.
... As well as the membrane pore size, feed pressure (and thus oil flow rate) has a large effect in emulsion stability, since feed pressure influences the time necessary for drop formation and can also influence the drop shape. Differently from the present study, Hancocks et al. (2013) observed an increase in the mean droplet diameter of oil in water emulsions, when the transmembrane pressure was increased from 50 kPa to 300 kPa, using a ceramic membrane (pore size range 1 mme10 mm) and Tween ® 20 as surfactant. This different behavior of feed pressure can be due to the lower mean pore size of the membrane used in the present study. ...
... The decrease in the mean droplet size with the increase in surfactant concentration was also observed in other studies on membrane emulsification, and attributed to the decrease in interfacial tension, which results in detachment of smaller droplets from the pore openings and lower tendency of droplet coalescence (Kukizaki, 2009;Spyropoulos et al., 2011). According to Schr€ oder and Schubert (1999) and Hancocks et al. (2013), the flux of the dispersed phase flowing through the membrane is directly proportional to the transmembrane pressure, and the higher the pressure, the less time is taken to produce an emulsion. In addition, it is known that the dispersed phase flux increases with increasing pore size and pressure (Vladisavljevi c et al., 2004). ...
... Pressure ( definition of an optimal pressure for each process is necessary, mainly when considering the application of this technique in an industrial scale. Nevertheless, as droplet size or size distribution control is the aim of this study, it is important to keep the pressure below the point where the control of these parameters is drastically reduced, i.e. so that droplet detachment occurs due to pressure/ interfacial tension instead of streaming breakup (Hancocks et al., 2013). This allows membrane emulsification to be a useful process with the potential to produce a large range of emulsion droplet sizes and size distributions under controlled operational conditions. ...
... For applications such relationships between pore size, liquid viscosity, transmembrane pressure and membrane type allow control over the droplet size [8]. A range of possible materials have been tested for use as membranes, including glass, ceramics, polycarbonates, microlaser-drilled steel and Teflon [11]. The most widely reported system is based on the SPG membranes. ...
... In principle, a droplet can grow from each of the pores directly into the continuous phase. The stability of a droplet on the membrane is controlled by a balance of the various forces acting on it; and the detachment point is usually controlled by an imbalance between the Laplace pressure and the shear stress caused by the cross-flowing continuous phase [11,14]. Earlier work has highlighted the importance of factors such as cross-flow rate, driving pressure, surface tension and droplet stabilization by surfactant adsorption during growth and detachment as important for controlling the ultimate droplet size and polydispersity of the end product [15]. ...
... Looking first at the emulsions produced with the smallest pore size membrane (0.2 µm), we can examine the effects of various operational parameters. (X)ME is known to be sensitive to the interplay between a wide range of operational and materials parameters, including: the crossflow shear field; the cross-membrane pressure; the wettability (of both phases) on the membrane; the viscosity of both phases; densities; and the interfacial tension [11,23]. Furthermore, the shape, spatial location and spacing of the pores can also have a significant influence [22]. ...
Accurate control of particle size at relatively narrow polydispersity remains a key challenge in the production of synthetic polymer particles at scale. A cross-flow membrane emulsification (XME) technique was used here in the preparation of poly(methyl methacrylate) microspheres at a 1–10 l h ⁻¹ scale, to demonstrate its application for such a manufacturing challenge. XME technology has previously been shown to provide good control over emulsion droplet sizes with careful choice of the operating conditions. We demonstrate here that, for an appropriate formulation, equivalent control can be gained for a precursor emulsion in a batch suspension polymerization process. We report here the influence of key parameters on the emulsification process; we also demonstrate the close correlation in size between the precursor emulsion and the final polymer particles. Two types of polymer particle were produced in this work: a solid microsphere and an oil-filled matrix microcapsule.
This article is part of the themed issue ‘Soft interfacial materials: from fundamentals to formulation’.
... In a cross-flow membrane foaming (CFMF) system, the dispersed phase is injected into the continuous phase passing through a porous membrane matrix, and bubbles are detached at the membrane surface by the cross-flowing continuous phase. Compared to membrane emulsification, which has been substantially studied and reviewed 78,[198][199][200][201] , only a few studies were conducted on membrane foaming and in most of these cases a high emulsifier concentration (e.g., 10% wt. whey protein) was used and any short-term destabilisation process (in particular, coalescence) was thus not taken into account. ...
... We first introduce the forces which dictate bubble formation. As was reported for cross-flow membrane emulsification systems, the bubble is subjected to four forces while growing at the membrane surface, which are the shear force imposed by the continuous phase flow rate and viscosity, the buoyancy force, the inertia force and the interfacial tension force 199,204 . For bubble formation in the present system, the interfacial tension force is the holding force, and scales as either properties of the used membrane -the pore size and the pore size distribution. ...
... Emulsions, which are dispersion systems of two immiscible liquid, hold tremendous promise in various industries including pharmaceutics and cosmetics, vaccine adjuvants, stimuli-response coatings, and high value-added biomolecule encapsulation [1][2][3]. Currently, many emulsion-related industries still adopt traditional methods using high shear stirring devices and high-pressure homogenizers, which results in technical bottlenecks in the production of monodispersed emulsion including high energy consumption, poor repeatability, and poor stability [4]. In order to solve the current technical problems, microporous membrane emulsification technology emerged to be a precise, low shear stress, lower energy consumption, and broadly applied strategy to promise greater control over both the emulsification process and finished products on a large scale [4,5]. ...
... Currently, many emulsion-related industries still adopt traditional methods using high shear stirring devices and high-pressure homogenizers, which results in technical bottlenecks in the production of monodispersed emulsion including high energy consumption, poor repeatability, and poor stability [4]. In order to solve the current technical problems, microporous membrane emulsification technology emerged to be a precise, low shear stress, lower energy consumption, and broadly applied strategy to promise greater control over both the emulsification process and finished products on a large scale [4,5]. The droplet size is controlled by appropriate membrane pore size selection instead of high shear stress, which is more beneficial to control the particle size and uniformity [6]. ...
The low percentage of pores at which droplets are formed results in lower production efficiency, which is one of the main issues hampering the application of microporous membrane emulsification on a large scale. The effect of microporous membrane pore activation on the droplet’s formation and emulsion performance was systemically investigated by changing both transmembrane pressures and membrane wettability during membrane emulsification. It indicated that the high transmembrane pressure increased the pore activation ratio (the percentage of pores at which droplets are formed) by the growing flux. These induced the enhancements of emulsification efficiency and the degradation of emulsion performance. Different wettability of membrane inflow and outflow influenced the pore activation speed (the speed which droplets are formed at pores). It indicated that the wettability of membrane outflow had a key effect on droplet formation and the high pore activation speed avoided splashing for high emulsion performance.
... The membrane dispersion microreactor reported in our previous works has been used in many heterogeneous systems to enhance the mixing and reaction processes [5][6][7] by preparing a microdroplet swarm. The microsieve membrane has shown great potential for the continuous mass production of microdroplets [1] owing to its morphology of regularly spaced and uniform pores, high mechanical strength, reproducibility, and durability [8,9]. However, in the absence of surfactants, the controllable production of microdroplets with narrow size distributions at sufficiently dispersed phase fluxes (to ensure the process is suitable for industrial application) is still a considerable challenge. ...
... Jetting is a considerable choice to reduce droplet size and realize droplet preparation on large scales [13], which meets the demand for continuous and highly efficient mixing of liquid-liquid heterogeneous systems; however, there are currently few applications utilizing membrane dispersion processes. In different flow patterns, the droplet size distribution is influenced by various factors, including the operating conditions of interfacial tension, flow rate, phase viscosities, and surfactant; together with the membrane properties of pore size, surface porosity, and wettability to liquids [9,[14][15][16]. As the droplets may touch and possibly coalesce during droplet formation with high surface porosity of the microsieve membrane, the CFD simulation of Abrahamse et al. demonstrated that the surface porosity should be less than 1.5% [17]. ...
A novel micronozzle array with grooves was specially designed for high-throughput microdroplet generation and high-efficiency mixing of heterogeneous systems without surfactants. The microgrooves, with a width of 25 μm between micropores, could significantly prevent the dispersed phase from spreading on the membrane surface and reduce the microdroplet size and polydispersity at sufficiently high fluxes of the dispersed phase. Through different oil-water systems, the effects of flow conditions, system physical properties, and flow patterns on droplet size were investigated in the micronozzle array device. Under the jetting flow pattern, the n-hexane, dichloromethane, and ethyl acetate droplets could obtain average diameters of 35 μm, 31 μm, and 24 μm, respectively, and the coefficient of variation was between 20% and 40%. The dripping and jetting flow regions were demarcated to fully understand the transition of the flow pattern. Furthermore, a droplet size prediction model was established based on force equilibrium. The micronozzle array proposed in this work has great potential for future practical applications.
... To the best of our knowledge, there is no report to reduce the tubular membrane thickness while increasing its mechanical stability for enhancing its practicality in number of applications such as, filtration, nanoseparation and nanoemulsion. In application such as oil-in-water nanoemulsion number of parameters have been investigated that affect the emulsion stability including, flow rate, transmembrane pressure, pore size, narrow pore size distribution, and membrane porosity [32][33][34][35][36]. The AAO tubular membrane could provide better result in all above parameters if it is given higher mechanical stability in smaller thickness. ...
... Nanoemulsion is a bi-phase system of immiscible liquids in which one liquid is dispersed as small droplets in the other one [35]. Number of parameters affects the emulsion stability such as membrane type, droplet size, transmembrane pressure, surfactant concentration, cross flow rate, viscosity, density, and concentration of disperse phase in continuous phase [36]. ...
... 'Interfacial tension (IFT) measurements of hexane/water were carried out at different surfactant concentrations (Fig. S4). Tween 20 was selected as the surfactant in the continuous phase as is a widely used in ME due to its non-ionic nature [35,36]. Four repetitions for each experimental condition were carried out at constant temperature of 18 ± 1°C. ...
... The results show that for Tween 20 concentrations higher than 1.0%, the equilibrium IFT has been reached (8.0 ± 0.1 mN/m). This is consistent with results for 1 µm SPG membranes where the minimum droplet size was achieved for Tween 20 concentrations above 0.4 %wt [35]. However, the slow adsorption kinetics of Tween 20 at the hexane/water interface, due to a combined effect of its high molecular weight (1228 g/mol) and its non-ionic nature [37], can result in broadening of average droplet size and widening of its distribution, especially for membranes with small interpore distance values (in the present case equal to 156 ± 6 nm). ...
... 16 In addition, Figure 2c shows the ratio of the droplet size to pore size, c, which is a common parameter to evaluate the performance of a ME setup under specific conditions. The c values of 2−3 obtained here for the highest crossflow velocities are indicative of the production of high-quality emulsions (i.e., with narrow size distribution), compared to the higher values reported in the literature for ceramic 38 and SPG membranes. 17 This is further confirmed by evaluating the Euler number (Eu), defined as the ratio of pressure to inertial forces, the former determined by the injection of the dispersed phase and the latter by the crossflow velocity of the continuous phase, v c ...
A continuous and scalable method to produce metal oxide nanoparticles (NPs) with control of both particle size and composition via membrane emulsification is reported for the first time using an oil-in-water emulsion and a tubular ceramic membrane (Dpore=100 nm). Using titania (TiO2) NPs as a model material, a systematic investigation of different process parameters allowed minimising the emulsion droplet size, yielding a low droplet diameter to membrane pore diameter ratio of less than 3, compared to literature values of up to 10. After calcination, TiO2 NPs as small as 10 ± 2 nm were obtained. The particles’ composition was changed via non-metal doping, with the incorporation of interstitial nitrogen and carbon in the TiO2 lattice, confirmed by FTIR and XPS. The TiO2 NPs showed to be active for the photocatalytic degradation of phenol under both UV and visible light. Productivity calculations showed that it is possible to obtain ~2 kg of NPs per hour per meter squared of membrane, opening the way to the large-scale production of NPs with fine control over their size and composition.
... Emulsification is achieved by forcing a dispersed phase, usually an organic solution or a premixed coarse emulsion, into an aqueous continuous phase through a membrane of given porosity. The passage through the membrane produces homogeneous droplets, the size of which is determined by the membrane pore size and geometry, the droplet detachment regime from the membrane surface, and the flow shear resulting from the agitation method applied to the continuous phase (Hancocks et al. 2013). Additional attention should be taken in selecting the proper membrane wall material, depending on the polarity of the dispersed and continuous phases, as membrane wettability, charge and permeability influence droplet formation (Vladisavljević et al. 2012;Silva et al. 2017). ...
Background
Polylactides (PLA) and poly lactide-co-glycolides (PLGA) undoubtedly are among the major drivers in the pharmaceutical market. Their relevance in pharmaceutics and biomedicine is well established in light of their sustainability, safety, tunable biodegradability, and versatility. However, polymer degradability and plasticity can somehow restrain industrial developability of PLA and PLGA formulations, especially in the form of microparticles (MP).
Area covered
This review wants to deal with the known manufacturing issues of PLA/PLGA MP, debating the potential contribution of modern and cutting-edge manufacturing technologies to the solution of unmet production needs. Technological and regulatory aspects will be considered outlining the potential role of advanced manufacturing techniques in the advancement of PLA/PLGA MP production processes.
Expert opinion
The multifaceted complexity of PLA/PLGA MP manufacturing processes demands adequate standardization and updated guidelines covering the so far unmet industrialization requirements. Novel and evolving manufacturing technologies will surely support the future development of bench-to-production plant transfer for such products. Careful evaluation of production costs is demanded in order to ensure process sustainability and patient’s outreach.
... This could be due to the rapid depletion of emulsifier at low concentration of emulsifier that caused the rate of adsorption to decrease close to the membrane surface and thus coalescence of adjacent droplets resulted in larger droplet size and width distribution. It was argued that at low concentrations of emulsifier formulation is the limiting factor and not the processing conditions [43]. Indeed a substantial increase in the droplet size and span of Tween 20 stabilised emulsions (1% oil content) through RME was found for concentrations lower than 0.2 wt. ...
Producing stable particle-stabilised emulsions of small droplet sizes and high monodispersity via membrane emulsification approaches is hindered by the poor mixing environment during processing and the low diffusivity and minimal interfacial tension lowering capacity of colloidal particles. The present study investigates the co-stabilisation (particles and emulsifiers) of O/W emulsions formed by rotating membrane emulsification. Formulation aspects of the employed co-stabilisation strategy (type/concentration of emulsifiers and type/size of particles) were assessed at a fixed trans-membrane pressure (10 kPa) and rotational velocity (2000 rpm). Emulsion microstructure was shown to be affected by the occurrence of emulsifier/particle interactions. In formulations where these interactions are synergistic and emulsifier content is low, interfacial stabilisation is carried out by both species and resulting emulsions possess smaller droplet sizes, higher monodispersity indices and enhanced stability against coalescence, compared to systems stabilised by either of the two components alone. This work concludes that a carefully controlled co-stabilisation strategy can overcome the current challenges associated with the production of particle-stabilised emulsions via membrane emulsification methods.
... [33] Emulsions Buoyancy force assumed to be negligible because droplet is expected to be much smaller in magnitude. [34] iii) Evaporation under reduced pressure was done to remove water miscible solvent. [38][39] ...
This review provides a brief idea about the nanoemulsion formulation, methods, evaluation parameters and their applications in pharmaceuticals. Delivery of poorly water soluble drugs and hydrophobic nature of the new chemical entities takes the researchers mind to the nanoemulsions. As name suggests that nanoemulsion have nano sized particles and it can easily crosses the number of barriers and provides maximum drug absorption to the site. These are the stable isotropic system of two immiscible liquid phases mixed to form a single phase with the help of emulsifying agent or surfactants which helps in lowering the interfacial tension between the two liquids.
... The same procedure occurs when a premix is used to pass through the membrane (Candéa, 2013). According to some references (Candéa, 2013;Hancocks et al., 2013;Joscelyne and Trägårdh, 2000;Vladisavljevic et al., 2000), membrane emulsification may be more suitable for generating individual ingredients with carefully selected structures, permitting also the use of shear sensitive products; starch or proteins. ...
Nanoemulsions are considered as ideal vehicles for nanoencapsulation of food bioactive ingredients and nutraceuticals. They serve as nanocarriers in many different materials as they are capable of improving water solubility and preventing the degradation of bioactive food components.Oil in water emulsions (O/W) can be used for the encapsulation of lipophilic active agents; on the other hand, water in oil emulsions (W/O) can be also used to encapsulate hydrophilic compounds including most polyphenols. In the current chapter, both simple emulsions (nano- and microemulsions) and double emulsions, as a potential encapsulation matrix, will be discussed; for readability we will use the term nanoemulsions for all emulsions with a droplet size of less than one micron.
... La tendencia esperada de la disminución en el tamaño de gota con incrementos en la V FC (o con el esfuerzo de corte de la fase continua), reportada por: Peng y Williams, (1998), Joscelyne y Trägårdh (1999y 2000, Vladisavljević et al., (2004) y Hancocks et al., (2013, se observa en todos los métodos, con algunas excepciones. Según Hancocks (2013) ubicar un soporte (o en este caso malla) por encima de la membrana disminuye el efecto del esfuerzo de corte de la fase continua sobre la gota en formación. Sin embargo, el resultado de incluir la malla metálica en esta investigación fue favorable, ya que permitió mejorar el control de la diferencia de presión transmembrana y probablemente mejoró la distribución de la fase continua en el canal de flujo de la misma. ...
O/W Emulsions were prepared by using two emulsification methods with membranes and cross-flow: cross flow membrane emulsification (EMFC) and cross flow membrane emulsification with premix (EMPMFC). Flat hydrophilic PVDF membranes 0.45 μm were used. No previous reports for this application are known. An equipment was designed and built for this research. The effects of various operating conditions (transmembrane pressure differences and cross flow velocity) on the drop diameter, the width of the droplet size distribution and the disperse phase flux were evaluated. The EMPMFC technique allowed to obtain small droplet diameter emulsions (1.5> d50 (μm)> 2), with less droplet size distribution dispersion (span (dimensionless)≥ 0.7), high disperse phase flux (J ≥ 2.5 m3.m-2.h-1) and it was easier to control with the equipment.
... The droplet diameter depended on the pore size of porous membranes, and the coefficient of variation on size of the droplets produced was about 10%. The different types of ME system and emulsified case are shown in Table 5[41][42][43][44][45][46]. Membrane emulsification is applicable to O/W emulsions, W/O emulsions and multiple emulsions. ...
Emulsions are dispersions of at least two immiscible liquids, one of which is dispersed as droplets in the other liquid, and stabilized by an emulsifier, such as oil and water. Different types of emulsions can be formulated according to different applications. Emulsions are categorized as simple or multiple type. Oil-in-water (O/W) and water-in-oil (W/O) are the simple emulsions, while water-in-oilin-water (W/O/W), and oil-in-water-in-oil (O/W/O) emulsions are known as multiple emulsions. Droplets of different sizes and the size distribution patterns formed based on different emulsification processes can affect their physicochemical properties. Hence, the droplet size and its distribution may determine the shelf-life stability, rheological properties, color and taste of food emulsions. Micro/ nano-emulsions have been increasingly utilized in the food industry as delivery system carriers that can encapsulate, protect, and deliver lipophilic functional food components, such as bioactive lipids, oil-soluble flavors, vitamins, and nutraceuticals. The application of micro/nano-emulsions has potential advantages in increasing the bioavailability of lipophilic functional food components, modulating the product texture and improving the stability of droplets against aggregation. This article provides an overview of the current status of micro/nano-emulsion containing functional food components, preparations and characterizations, and presents the applications of micro/nanoemulsions in food industry. © 2015, Japan Society for Food Engineering. All rights reserved.
Continuous ceramic membrane emulsification is a promising and scalable technique to prepare water‐in‐heavy oil (W/O) emulsions. The droplet size of W/O emulsions is comprehensively influenced by phase parameters, operational parameters, and membrane parameters, which collectively impact the forces acting on water droplets. In this work, a droplet size prediction model involving multiple factors is established. The forces are analyzed by considering the influence of transmembrane pressure and the viscosity ratio between the dispersed and continuous phases, which are not well considered by current researchers. Additionally, the effects of pore size, crossflow velocity, temperature, and transmembrane pressure were experimentally verified. The experimental results show a high degree of agreement with the predictions. Also, based on the relaxation time difference in oil and water, magnetic resonance imaging was used for the first time to assess the stability of W/O emulsions which was found to be stable for 4 months.
Background
Membrane emulsification (ME) is an attractive membrane process to produce various kind of simple or multiple emulsions. The membrane utilized in ME process should have high dispersed phase flux, narrow pore size distribution and excellent antifouling property.
Methods
This work proposed a novel near-field electrospun (NFES) poly(tetrafluoroethylene) (PTFE) membranes with ordered rectangular pore geometry. Compared with other membranes, the order pore geometry of NFES PTFE membrane was helpful to obtain a more homogeneous and more stable water-in-oil (W/O) emulsion.
Significant findings
The droplet size was simply controlled by the NFES PTFE membrane pore size. And NFES PTFE membrane was more conducive to the formation of droplets with narrow droplet size distribution because of its unique straight rectangular membrane pore geometry. In addition, W/O emulsion droplet size be regulated in coordination with operating parameters and phase parameters. Results indicated that a smaller rectangular pore area of PTFE membrane would lead to a smaller droplet size. Introducing of shear stress in the continuous phase was contributed to the generation of monodisperse droplets. When the emulsifier (Span 80) content was 1wt%, the smallest droplet size (∼103.6 nm) was obtained.
An important role of the scientific society has been to search for tools to minimize the harmful impact of human activities on the natural environment. In the agricultural field, the development of sustainable alternatives to control pathogens, pests, and weeds in different cultures fall within such scope. One of the alternatives is the use of fermented media containing metabolites produced by microorganisms, especially by the group of endophytic fungi. Many species of fungi belonging to the genus Phoma produce a large variety of metabolites that have demonstrated herbicidal action. Considering this important subject in the biological control area, this chapter intends to present a critical and constructive discussion about current findings and future perspectives that justify the use of metabolites produced by fungi for different purposes. Commonly, purification of fermented media, characterization, and their application in bioindicators are some indispensable tasks for having strong knowledge of the action of metabolites. Besides, suitable formulations can provide the final product with stability, wettability, spreadability, and penetration, which can improve its action.
Traditional thermal and freezing processing techniques have been effective in maintaining a safe high quality food supply. However, increasing energy costs and the desire to purchase environmentally responsible products have been a stimulus for the development of alternative technologies. Furthermore, some products can undergo quality loss at high temperatures or freezing, which can be avoided by many alternative processing methods.
This second edition of Alternatives to Conventional Food Processing provides a review of the current major technologies that reduce energy cost and reduce environmental impact while maintaining food safety and quality. New technologies have been added and relevant legal issues have been updated. Each major technology available to the food industry is discussed by leading international experts who outline the main principles and applications of each. The degree to which they are already in commercial use and developments needed to extend their use further are addressed.
This updated reference will be of interest to academic and industrial scientists and engineers across disciplines in the global food industry and in research, and to those needing information in greener or more sustainable technologies.
Commercial products of miniemulsion polymerization are still scarce, and one of the major challenges for industrial implementation is the miniemulsification step. The aim of this work is to introduce the membrane emulsification technology as a robust and low energy alternative. The proposed system allows the production of monomer-in-water miniemulsions with droplet sizes of around 300 nm and moderate distributions for up to 30 wt% of methyl methacrylate phase in a quick and simple way. Thus, the membrane emulsification technology can become an excellent approach for the production of miniemulsions for polymerization. The emulsifying power of four pumps, used in the membrane emulsification setups, was also studied and it was shown that in most cases, with the single use of the pumps, emulsions with broad distributions were produced.
In this work, oil-in-water emulsions (O/W) were prepared successfully by membrane emulsification with 0.5 μm pore size membrane. Sunflower oil was emulsified in aqueous Tween80 solution with a simple crossflow apparatus equipped with ceramic tube membrane. In order to increase the shear-stress near the membrane wall, a helical-shaped reducer was installed within the lumen side of the tube membrane. This method allows the reduction of continuous phase flow and the increase of dispersed phase flux, for cost effective production. Results were compared with the conventional cross-flow membrane emulsification method. Monodisperse O/W emulsions were obtained using tubular membrane with droplet size in the range 3.3-4.6 μm corresponded to the membrane pore diameter of 0.5 μm. The final aim of this study is to obtain O/W emulsions by simple membrane emulsification method without reducer and compare the results obtained by membrane equipped with helix shaped reducer. To indicate the results statistical methods, 3p type full factorial experimental designs were evaluated, using software called STATISTICA. For prediction of the flux, droplet size and PDI a mathematical model was set up which can describe well the dependent variables in the studied range, namely the run of the flux and the mean droplet diameter and the effects of operating parameters. The results suggested that polynomial model is adequate for representation of selected responses.
The effects of using different membrane materials and morphologies in the membrane emulsification process were observed using similar operating parameters and system geometry, allowing a direct comparison of not only the membranes themselves but also between both a stationary cross-flow membrane emulsification device and a rotated membrane emulsification device. Each membrane type tested had distinct characteristics, and the droplet sizes produced responded differently to changes in operating conditions.
The rotating membrane produced similar droplet sizes to the cross flow membrane system, but at a much lower shear rate. This suggests that the detachment of the droplets occurs sooner due to the additional centrifugal force and system vibration. The Shirasu porous glass (SPG) membrane produced the smallest droplet sizes (<1 µm from a 1 µm membrane), however the stainless steel membrane produced the lowest droplet size to pore size ratio (~0.5:1) due to its cylindrical pore geometry as opposed to the tortuous geometries of the other membranes used. The droplet sizes produced at different pressures are similar between rotated and cross-flow membrane emulsification, with increases in pressure increasing droplet size and size distribution. The viscosity of the continuous phase has an effect on the droplet size; increasing the viscosity decreases the droplet size by increasing the applied shear, allowing fine tailoring of the size produced, with a more viscous continuous phase reducing the droplet size from ~4 µm to ~1 µm with an increase in viscosity of 100 mPa s.
Rotating membrane emulsification has properties with potential to produce shear sensitive emulsion microstructures with small droplet sizes. Emulsion microstructures such as duplex emulsions, core/shell structures beads etc. can be used in the production of novel food structures.
Modern emulsion processing technology is strongly influenced by the market demands for products that are microstructure-driven and possess precisely controlled properties. Novel cost effective processing techniques, such as membrane emulsification, have been explored and customised in search for better control over the microstructure, and subsequently the quality of the final product. Part A of this review reports on the state-of-the-art in membrane emulsification techniques, focusing on novel membrane materials and proof of concept experimental set-ups. Engineering advantages and limitations of a range of membrane techniques are critically discussed and linked to a variety of simple and complex structures (e.g. foams, particulates, liposomes etc.) produced specifically using those techniques.
Membrane emulsification is a promising process for formulating emulsions and particulates. It offers many advantages over conventional 'high-shear' processes with narrower size distribution products, higher batch repeatability and lower energy consumption commonly demonstrated at a small scale. Since the process was first introduced around 25 years ago, understanding of the underlining mechanisms involved during microstructure formation has advanced significantly leading to the development of modelling approaches that predict processing output; e.g. emulsion droplet size and throughput. The accuracy and ease of application of these models is important to allow for the development of design equations which can potentially facilitate scale-up of the process and meet the manufacturer's specific requirements. Part B of this review considers the advantages and disadvantages of a variety of models developed to predict droplet size, flow behaviour and other phenomena (namely droplet-droplet interactions), with presentation of the appropriate formulae where necessary. Furthermore, the advancement of the process towards an industrial scale is also highlighted with additional recommendations by the authors for future work.
Food grade duplex W1/O/W2 emulsions were prepared using three different techniques: SPG cross-flow membrane, SPG rotating membrane and high-shear mixer. The primary W1/O emulsion had sodium chloride encapsulated in the inner aqueous droplets as a marker compound. Duplex emulsion droplet size and salt encapsulation were both investigated by modifying the emulsification conditions inherent for each technique; cross-flow velocity (CFV) and trans-membrane pressure (TMP) for the cross-flow membrane, rotational velocity (RV) and TMP for the rotating membrane, and mixing time for the high-shear mixer.Emulsion droplet size was shown to increase with TMP and to decrease with both CFV and RV. Minimum droplet size obtained (∼12 μm) was similar for all three emulsifying techniques, which suggests that at high shear stresses, the minimum droplet size is determined primarily by the decrease in the interfacial tension.It was also shown that the amount of salt released during storage depends on the emulsification technique (8–20% for the cross-flow membrane, ∼13% for the high-shear mixer and ∼8% for the rotating membrane). The differences in salt release were explained in terms of emulsions droplet size and interfacial properties of adsorbed surfactant molecules. The unexpected high amount of salt released by duplex emulsions produced by the cross-flow membrane was associated with the magnitude and duration of shear forces, which act on duplex droplets during semi-batch emulsification.
Amethod for manufacturing emulsions based on cross flow membrane emulsification has been studied. This involves the formation of emulsions by breaking up the discontinuous phase into droplets in a controlled manner without the use of turbulent eddies. This is achieved by passing the discontinuous phase through a suitable microporous medium and injecting the droplets so formed directly into a moving continuous phase. This paper summarizes the development of this technology. Experimental data obtained using a single pore (capillary tube) are presented here for the production of model oil-in-water emulsions. A high speed video camera was used to measure droplet growth and detachment processes from the pore as a function of process parameters such as transmembrane pressure drop, continuous phase crossflow velocity etc. A phenomenological model is developed and tested to predict droplet size and production rate.
A new reflectance technique has been developed to measure the droplet size evolution during the process of emulsification in real-time. This has been used to investigate the dynamic behaviour of 50% oil-in-water emulsions. The performed experiments were designed in order to investigate the occurring droplet break-up and droplet coalescence phenomena individually, by carefully creating processing conditions where each of those events is dominant. The effect of emulsifier (Tween 20) concentration and different hydrodynamic conditions on the droplet break-up and coalescence phenomena and the emulsion droplet size evolution during processing were all investigated. The concentration of Tween 20 was shown to be a key parameter affecting the droplet size of the emulsion at the early stages of processing (within the first 3min). However, during the later stages of processing, hydrodynamic conditions have a more pronounced effect on determining the final droplet size. Unlike droplet break-up, droplet coalescence rate decreases by intensifying the hydrodynamic conditions of the process as a consequence of the high capillary pressure of the smaller droplets being produced.
Emulsification is usually performed using high-pressure homogenizers and rotor/stator systems. In the dispersing zone of these machines high shear stresses are applied to deform and disrupt large droplets of a premix. Membrane emulsification is a new emulsification technology based on the use of a microporous membrane. In this process, the disperse phase is pressed through the pores into the continuous phase where the droplets are formed. The droplets reaching a critical diameter detach from the membrane surface under the influence of shear forces caused by the flow of the continuous phase. In this investigation, polypropylene hollow fibers with 0.4 μm pores and 1.7 mm inside diameter were used to produce water-in-oil (W/O) emulsions consisting of demineralized water as the dispersed phase, mineral oil Velocite no. 3 as the continuous phase, and polyglycerol polyricinoleate (PGPR 90) as the emulsifier. The size of water droplets in the prepared emulsions and the droplet size distribution strongly depended on the membrane pretreatment procedure, the transmembrane pressure, the dispersed phase content and the emulsifier concentration. The emulsion droplets with a mean Sauter diameter of about 0.3 μm and a span value between 1.1 and 1.6 were produced using 10 wt.% emulsifier at a transmembrane pressure below 50 kPa. Droplet sizes smaller than pore size were obtained; this effect is explained by existing of an oil film inside the pores reducing the effective pore size. This effect could provide a new possibility for producing small droplets of uniform distribution.
In emulsions prepared using a membrane emulsification system, dispersion droplet diameter depends basically upon membrane pore diameter. For practical applications, it is necessary to select the appropriate type and concentration of emulsifiers, U and Je.For practical applications in the food industry, where large volume production is conducted, it is especially important to increase Je. When preparing an O/W emulsion, this can be carried out by adding emulsifier to the oil phase, and Je can be increased up to 10 times.In the preparation of a W/O emulsion, increasing Je can be accomplished using a hydrophilic membrane pre-treated by immersion in the oil phase. The dispersion droplet diameter is roughly controlled and Je can be increased about 100 times in comparison to using a hydrophobic membrane. This makes a membrane emulsification system practical for large-scale food application of a W/O emulsion. Without adding preservatives, a low fat spread with a fat content of 25 wt% was created using this system.
Emulsification with membranes is a new technology for producing emulsions. The apparent shear stress is low compared with that in conventional emulsification systems, because the droplets of the disperse phase are produced in a different way. The disperse phase is pressed through the pores of a membrane into the continuous phase flowing alongside the membrane surface, where droplets form. Droplet formation and detachment from membrane pores depend on various process parameters (e.g. transmembrane pressure, shear stress of the continuous phase acting on the membrane surface), membrane material and structure. Emulsification results for o/w emulsions produced using ceramic membranes are presented in terms of droplet size and disperse phase flux as a function of the main parameters of the process, e.g. dynamic interfacial tension of the emulsifier, transmembrane pressure and wall shear stress.Experimental results show that shear stress at the membrane surface is very low. Shear stress does not have to exceed 30Pa. This is approximately 10 times less than that for droplet disruption in laminar flow. The membrane emulsification process is therefore advantageous with respect to shear-sensitive ingredients. Furthermore it is less energy consuming than conventional emulsifying systems.
Ceramic membranes were used to produce oil-in-water (O/W) emulsions consisting of vegetable oil as the dispersed phase and skim milk as the dispersion medium. The purpose of the work was to find operating conditions suitable for producing small emulsion droplets, a small size being important for emulsion stability. The main parameters investigated were the effect of wall shear stress, emulsifier concentration and membrane pore size. Formation of small droplets was favoured at higher emulsifier concentrations and for a high wall shear stress using a membrane with a small nominal pore size. Submicron particles were produced at an 8% emulsifier concentration for a wall shear stress of 135 Pa using a 0.1 μm pore size membrane. Under these conditions the flux was >100 kg m−2 h−1. A high flux is important for industrial-scale production of food emulsions using membrane emulsification.
Conventional devices used in industrial emulsification processes disperse the inner phase by droplet disruption of high energetic laminar or turbulent flow. Membrane emulsification is different because small droplets are directly formed at the surface of a microporous membrane. Energy consumption of the process is lower, and the stresses on the system at the membrane surface and inside the pores are smaller. This allows processing of shear-sensitive substances. The result of the emulsification process can be described by the mean droplet size and the flux of the disperse phase. Among other parameters, pore size of the membrane, pressure of the disperse phase, and adsorption kinetics of the emulsifier influence the results of emulsification. The faster the emulsifier molecules adsorb at newly formed interfaces, the smaller the droplets of the emulsion produced. Transmembrane pressure greatly influences the flux but causes little change in droplet size.
The membrane emulsification process is attracting great interest in many industrial fields. For optimization and scaling up of the emulsification process, controlling the emulsion uniformity using monitoring process and membrane parameters is especially important. In this study, the effects of phase physical property (viscosity, interfacial tension), and operation conditions (trans-membrane pressure shear stress), on droplet size distribution of an oil-in-water (o/w) emulsion were systemically investigated by membrane emulsification experiments with a cross-flowing continuous phase. Inspired by the idea that droplet spontaneous formation is one of the most important mechanisms to form a uniform emulsion, a simple model based on torque balance equations by describing the variable force torques on droplet formation process was proposed to predict experimental tendencies. The experiment phenomena showed a good coincidence with model prediction. The following experiment conditions were found to facilitate the production of uniform droplets: (1) low cross-flow velocity of the continuous phase, (2) low transmembrane pressure, (3) high viscosity of the dispersed phase, and (4) an emulsifier with great ability and rapid rate to decrease interfacial tension.
Conventional emulsification processes are usually coupled with a high energy input and high shear rates. Not only in pharmaceutical industry but also in food industry, temperature and shear can mutate or destroy sensitive product components. Therefore membrane emulsification processes can be used. The droplets are formed at pores and pressed into the continuous phase. For small ratios of pore distance to pore size, droplet coalescence on the membrane surface can occur. Membranes are usually used static up to now and the detaching force is limited with the overflow velocity of the continuous phase. A method to resolve this limitation is designed and described as follows. To avoid coalescence on the membrane surface a controlled pore distance membrane was developed combined with a special treatment of the surface. A measuring cell provides insight into the phenomena on the membrane surface. The drop detachment is monitored by two cameras from the top and the side view. In order to decouple the detaching force from the overflow of the continuous phase, the membrane is fixed on a cylinder and can be rotated in a variable gap. Up to now the influence of the rotational speed, the gap size and the volume ratio of the two phases were tested for W/O emulsions. The higher the rotational speed the smaller are the droplets and the narrower is the size distribution. The phase ratio does not influence the results, which is an advantage compared to a static membrane set-up. The gap width has a significant influence on the emulsification process and is coupled with formation of Taylor vortices.
Microporous glass (MPG) membrane emulsification was used for micro encapsulation of Lactoba-cillus casei YIT 9018. Several process parameters of membrane emulsification were investigated for producing a stable emulsion. The droplet dia in the emulsion depended upon the membrane pore size. The monodispersed emulsion obtained by this technique resulted in well-formed microcapsules with a narrow particle size distribution. For artificial gastric acid and bile, the viable count of encapsulated cells was constant through the incubation time, while the count of nonencapsulated cells was significantly decreased. A storage stability test at different temperatures resulted in a viability of encapsulated cells 3 to 5 log cycles higher than the viability of nonencapsulated cells.
Oil-in-water emulsions (30∶70, vol/vol) were formulated with sunflower lecithin to characterize the destabilization processes
and the vesicles formed. Dispersions containing levels of 0.1% lecithin were more stable against coalescence than the control
system. When the lecithin concentration was increased to 0.5%, the presence of spherical structures, such as vesicles, was
recorded that occluded the emulsion inside. Vesicles underwent a creaming process, and a narrow coalescence zone was detected
in the upper layers of the samples. As the lecithin concentration was increased, more vesicles were formed, representing as
much as 80% of the system volume. A reduction in the average size of vesicles was observed at high lecithin concentrations
(2.5 and 5.0%). The vesicle size distribution changed as a function of lecithin concentration, decreasing the ratio of large
to small particles in the same way. Coalescence took place in zones where large-volume vesicles were in contact in the upper
portion of the tube sample. The results obtained suggest that sunflower lecithins present interesting emulsifying properties
that may prove useful in food technology.
Emulsification with high porosity micro-engineered membranes leads to stable emulsions with a low droplet span when, besides a surfactant in the continuous phase, an additional, suitable surfactant is used in the dispersed phase. This surfactant should exhibit relatively fast adsorption dynamics, which is more critical when the surfactant in the continuous phase has slower dynamics. Dispersed-phase fluxes of up to 92.5 × 10−6 m3/m2s could be achieved, which is an order of magnitude higher than previously reported for SPG membrane-based cross-flow emulsification.
Crossflow membrane emulsification is a promising method to achieve very small and uniform emulsions. The droplet size produced is controlled mainly by the choice of membrane. Using microengineering technology it is currently possible to produce membranes with precision defined parameters (uniform pore size, shape and inter-pore distance). In the work presented here, individual pore behaviour was studied using micromachined membranes with wider inter-pore distances (100 μm). It was found that the diameter of droplets increased during an initial period of operation. Also, interaction between droplets formed at adjacent pores was observed to enhance the reduction of mean droplet size and negatively correlated with inter-pore distance. A ‘push-to-detach’ mechanism was proposed to explain the behaviour observed. It was demonstrated that a micromachined membrane with pore diameter of 2 μm and inter-pore distance of 20 μm produced smaller droplets than for membranes with larger inter-pore distances. To facilitate the droplet detachment from the membrane and provide additional control over droplet detachment, the effects of membrane vibration were investigated. Preliminary results showed that smaller droplets could be produced by introducing low frequency (0–100 Hz) membrane vibrations without increasing their size distribution.
Cross-flow membrane emulsification has great potential to produce monodisperse emulsions and emulsions with shear sensitive components. However, until now, only low disperse phase fluxes were obtained. A low flux may be a limiting factor for emulsion production on a commercial scale. Therefore, the effects of membrane parameters on the disperse phase flux are estimated. Besides, the effects of these parameters on the droplet size and droplet size distribution are qualitatively described. Wetting properties, pore size and porosity mainly determine the droplet size (distribution). Membrane morphology largely determines the disperse phase flux. As an example, industrial-scale production of culinary cream was chosen to evaluate the required membrane area of different types of membranes: an SPG membrane, an α-Al2O3 membrane and a microsieve. Due to the totally different morphologies of these membranes, the fraction of active pores is 1 for a microsieve and is very low for the other membranes. The choice of the optimal membrane did not depend on the production strategy: either to produce large quantities or to produce monodisperse emulsions, the best suitable was a microsieve with an area requirement of around 1 m2. In general, the total membrane resistance should be low to obtain a large disperse phase flux. In contrast, the membrane resistance should be high to obtain monodisperse emulsions when using membranes with a high porosity.
We investigate the process of membrane emulsification in the presence of the nonionic surfactant Tween 20, and the milk proteins Na-caseinate and beta-lactoglobulin (BLG). Our goal is to examine the factors which control the drop-size distribution in the formed emulsions. The drops are produced at the outer surface of a cylindrical microporous glass membrane, so that the process of their formation and detachment can be directly observed by an optical microscope. In the case of 2 wt.% aqueous solution of Tween 20 we obtain a relatively fine and monodisperse oil-in-water emulsion with a mean drop diameter about three times that of the pore. The microscopic observations show that in this case the oil drops intensively pop out of separate pores. In contrast, for the lower concentrations of Tween 20, as well as for the investigated solutions of Na-caseinate and BLG, we observe that the membrane is covered by a layer of growing attached emulsion drops, which are polydisperse, with a relatively large mean drop size. This fact can be explained with a greater dynamic contact angle solid-water-oil. In such a case, after a drop protrudes from an opening, it does not immediately detach, but instead, the contact area drop/membrane expands over several pore openings. The smaller drop size in the emulsions stabilized by BLG, in comparison with those stabilized by Na-caseinate, is related to the circumstance that BLG adsorbs faster at the oil-water interface than Na-caseinate. In the investigated emulsions we did not observe any pronounced coalescence of oil drops. Hence, the generation of larger and polydisperse oil drops in some of the studied solutions is attributed mostly to the effect of expansion of the drop contact line and formation of hydrophobized domains on the membrane surface. Therefore, any factor, which leads to decrease of the dynamic three-phase contact angle, and thus prevents the contact-line expansion, facilitates the production of fine and monodisperse emulsions.
Membrane emulsification is a simple method that has received increasing attention over the last 10 years, with potential applications in many fields. Experimental studies which have focused mainly on investigations of process parameters such as, membrane type, average pore size and porosity, crossflow velocity, transmembrane pressure and emulsifier, are reviewed. By careful choice of these parameters, emulsions with narrow emulsion droplet size distributions have been produced, with average droplet sizes ranging between 2 and 10 times the nominal membrane pore diameter. The effects of individual parameters are reasonably well understood, particularly at a qualitative level. Results can be explained by a direct influence of the membrane pore size, diameter and distribution. Interfacial tension and the action of wall shear stress are also important.In comparison with conventional turbulence based methods, such as homogenization and rotor-stator systems, less energy is needed to produce droplets of a given size using membrane emulsification. However, one of the main limiting factors with regard to industrial scale-up can be the often low level of dispersed phase flux through the membrane, especially for small submicron droplets.
Iodinated poppy-seed oil (IPSO) accumulates selectively in hepatocellular carcinoma (HCC) when injected into the hepatic artery. This virtue has been applied to the hepatic arterial injection chemotherapy for the disease. We invented a new water-in-oil-in-water emulsion (W/O/W), in which IPSO microdroplets, 70 μm in diameter, were suspended in physiological saline enclosing numerous vesicles of an aqueous solution of epirubicin with remarkable stability. After hepatic arterial injection, the microdroplets accumulated only in HCC tissue and remained in the tissue for more than 3 weeks affecting tumor cells. Efficacy of the W/O/W has been fully proved clinically; the 6-year cumulative survival rate for 24 patients bearing HCC nodules recurrent after hepatectomy, including even 12 patients with four or more nodules, though prognosis of these patients is recognized very poor, was 24%.
Particle-size control of emulsion is very important for maintaining stability and giving emulsions new functional roles. Porous glass membrane, prepared by phase separation of a glass composition, is available as an emulsifying element, from which, one can obtain monodispersed emulsion with different particle sizes, and useful water/oil/water (W/O/W) emulsion in very high yield. The authors have called this new technology 'membrane emulsification'. Applications of membrane emulsification technology to drug delivery systems were carried out under cooperative research with Miyazaki Medical College. It was found that the clinical administration of a W/O/W drug emulsion that encapsulated an anticancer drug in its inner droplets was surprisingly effective for both terminal and multiple nodules of hepatocellular carcinoma when the drug was injected to damaged liver through a catheter inserted in the hepatic artery. Other applications have been tried and developed elsewhere.
A systematic experimental study of the effect of several factors on the mean drop diameter, d32, during emulsification, is performed with soybean oil-in-water emulsions. These factors are (1) type of used emulsifier; (2) emulsifier concentration, CS; and (3) ionic strength of the aqueous solution. Three different types of emulsifier, anionic (sodium dodecyl sulfate, SDS), nonionic (polyoxyethylene-20 cetyl ether, Brij 58), and protein (whey protein concentrate), are studied. For all of the studied systems, two well-defined regions are observed in the dependence of d32 on CS: at low surfactant concentration, d32 increases significantly with the decrease of CS (region 1), whereas d32 does not depend on CS at high surfactant concentration (region 2). The model, proposed by Tcholakova et al. (Langmuir 2003, 19, 5640), is found to describe well the dependence of d32 on CS in region 1 for the nonionic surfactant and for the protein emulsifier at high electrolyte concentration, 150 mM NaCl. According to this model, a well defined minimal surfactant adsorption (close to that of the dense adsorption monolayer) is needed for obtaining an emulsion. On the other hand, this model is found inapplicable to emulsions stabilized by the ionic surfactant, SDS, and by the nonionic surfactant, Brij 58, at low electrolyte concentration. The performed theoretical analysis of drop-drop interactions, in the emulsification equipment, shows that a strong electrostatic repulsion between the colliding drops impedes the drop-drop coalescence in the latter systems, so that smaller emulsion drops are obtained in comparison with the theoretically predicted ones. The results for SDS-stabilized emulsions in region 1 are explained by a quantitative consideration of this electrostatic repulsion. The drop size in region 2 (surfactant-rich regime) is described very well by the Kolmogorov-Hinze theory of turbulent emulsification.
Membrane emulsification is a promising and relatively new technique for producing emulsions. The purpose of this study was to better understand the influence of interfacial tension on droplet formation during membrane emulsification. Droplet formation experiments were carried out with a microengineered membrane; the droplet diameter and droplet formation time were studied as a function of the surfactant concentration in the continuous phase. These experiments confirm that the interfacial tension influences the process of droplet formation; higher surfactant concentrations lead to smaller droplets and shorter droplet formation times (until 10 ms). From drop volume tensiometer experiments we can predict the interfacial tension during droplet formation. However, the strong influence of the rate of flow of the to-be-dispersed phase on the droplet size cannot be explained by the predicted values. This large influence of the oil rate of flow is clarified by the hypothesis that snap-off is rather slow in the studied regime of very fast droplet formation.