Membrane emulsification devices

Emulsification devices where the membrane is immersed in a stirred vessel containing the continuous phase, so as to obtain a batch emulsification device operating in deadend emulsification mode, have also been developed (Figure 21.13). Both flat-sheet and tubular membranes are used. In this membrane emulsification device, the continuous phase kept in motion creates the shear stress at the membrane surface that detaches the forming droplets. In a different operation mode, that is, when the continuous phase is not stirred, droplet formation in quiescent conditions is obtained. 

Figure 21.1 Schematic representation (a) of membrane emulsification, where the membrane works as a high-throughput device to form droplets with regular dimensions (b) photo of an o/w emulsion...

The various membrane emulsification procedure can be practised by using appropriate membranes and devices configuration. 

Figure 21.13 Emulsification devices where the membrane is immersed in a stirred vessel containing the continuous phase. Transmembrane pressure applied from (a) external or shell side, and (b) internal or lumen side.

Each type of device has specific advantages and disadvantages. The batch emulsification is suitable for laboratory-scale investigations. The construction of the device is simple and handling during emulsification as well as for cleaning. Crossflow membrane emulsification is used when it is important that a proper adjustment of all process parameters and larger amounts of emulsion have to be produced. 

Miaogel preparation by polymerization of monomers can also be realized in aqueous droplets prepared by using membrane emulsification or miaofluidic devices. By this approach, one can obtain monodisperse micron-sized microgels by thermally induced or photoinduced polymerization. ... 

A few devices have been proposed that resemble cross-flow membrane emulsification, where a phase is induced to disperse through the small openings of a membrane in a flow of continuous phase [3]. Such processes are more energy efficient than conventional approaches such as colloid mills and homogenizers [142]. A microstructured analogue of membrane emulsification was presented by de... 

DEs (e.g. W/OAV or OAV/0) were prepared by a two-step method applying an additional rotational membrane process (ROME, ETH Ziiiich/Kinematica AG Luzem, CH) and using the same emulsifiers and stabilizers applied for SE (Table 23.1). At first, the SE was prepared by the rotor-stator device at high rotational speed (up to 10,000 rpm) for tK, 15-20 min at constant temperature (22-23 °C) as described before. The primary droplet size was adjusted between 1 and 2 pm, by the rpm of the rotor-stator device and treatment duration. This SE was then used as the disperse phase in the rotary membrane emulsification (ROME) device. The gentle rotational membrane emulsification/foaming process had been developed at the Laboratory of Food Process Engineering (ETH Zurich) within the past decade and reported elsewhere [29,52,39]. The membrane emulsification has... 

Principles and Operating Methods for Membrane Emulsification 124 Single-Emulsion Production Using Membranes 129 Single-Emulsion Production Using Microfluidic Devices 133 Production of Double Emulsions Using Membranes and Microchannels 136... 

Microfluidic devices can produce droplets with a very tight size distribution, but to date, only at small production scales and droplets of at least a few microns in diameter. Scale-up with membrane emulsification tends to be simpler and more versatile. 

Membrane and microfiuidic devices have also been adopted for the precision manufacture of solids from double-emulsion templates. To date, several different types of particles have been successfully produced by incorporating use of various membrane and microfiuidic devices in processes of polymerization, gel formation, crystallization, and molecular or particle self-assembly. Membrane emulsification is more suited to the fabrication of less sophisticated particulates, such as solid lipid micro-Znanoparticles, gel microbeads, coherent polymeric microspheres, and inorganic particles such as silica microparticles. Microfiuidic devices allow more sophisticated particle designs to be created, such as colloidosomes, polymerosomes, 3D colloidal assemblies, asymmetric vesicles, core-shell polymer particles, and bichromal particles. 

Yamazaki N, Naganuma K, Nagai M, Ma GH, Omi S. 2003. Preparation of W/O (water-in-oil) emulsions using a PTFE (polytetrafluoroethylene) membrane A new emulsification device. / Dispersion Sci Technol 24 249-257. 

The next chapter by G. T. Vladisavljevic and R. A. Williams is a comprehensive and systematic review of new techniques of preparation of multiple emulsions, emulsions, and microparticles. The authors envisage the ways to form multiple droplets by using membrane emulsification processes and microchan-nel and microcapUlary devices. They also pay special attention to the preparation of solid microparticles via a double emulsion emulsification method using membrane emulsification and microfiuidic devices. 

The top-down approach involves size reduction by the application of three main types of force — compression, impact and shear. In the case of colloids, the small entities produced are subsequently kinetically stabilized against coalescence with the assistance of ingredients such as emulsifiers and stabilizers (Dickinson, 2003a). In this approach the ultimate particle size is dependent on factors such as the number of passes through the device (microfluidization), the time of emulsification (ultrasonics), the energy dissipation rate (homogenization pressure or shear-rate), the type and pore size of any membranes, the concentrations of emulsifiers and stabilizers, the dispersed phase volume fraction, the charge on the particles, and so on. To date, the top-down approach is the one that has been mainly involved in commercial scale production of nanomaterials. For example, the approach has been used to produce submicron liposomes for the delivery of ferrous sulfate, ascorbic acid, and other poorly absorbed hydrophilic compounds (Vuillemard, 1991 ... 

More than 100 micro structured devices are listed on the homepage of the pChemTec consortium [24]. The devices cover physical applications such as flow distribution, mixing, heat transfer, phase transfer, emulsification and suspension, as well as chemical applications such as chemical and biochemical processing. Some separation units such as membrane separation and capillary electrophoresis are also offered. Control devices such as valves, micro pumps for product analysis and mass flow controllers supplement the catalog. 

As a new option, for the bioconversion of poorly soluble substrates the classical EMR-concept can be extended to an Emulsion Membrane Reactor , comprising a separate chamber for emulsification (with a hydrophilic ultrafiltration membrane), an EMR-Ioop with a normal ultrafiltration module, and a circulation pump. This approach has been successfully demonstrated for the enzymatic reduction of poorly soluble ketones [107]. Using this device, e.g., for the enantioselective reduction of 2-octanone to (S)-2-octanol (e.e. >99.5%) with a carbonyl reductase from Candida parapsilosis under NADH-regeneration with FDH/for-mate, the total turnover number was increased by a factor 9 as compared with the classical EMR. 

Some preparation methods specific to the formation of nanoparticle suspensions are provided in References [20,62,63]. Many such methods are simply conventional colloidal suspension preparation methods that have been extended to produce smaller particle sizes, but others involve novel approaches. Some ofthese involve making nanoemulsions as a first step. For example, membrane, microfluidic and nanofluidic devices have been used to make nanoscale emulsions of all kinds, as already noted earlier, and the emulsion droplets so generated can be used in turn to make sohd microparticles and nanoparticles. If the nanoparticles are intended to encapsulate other materials, then a double emulsification technique can be used, at elevated temperature, to prepare a multiple emulsion (i.e. 

Microfluidic devices can be used for either premix emulsification (a method in which a coarse emulsion is broken up by passing it through a geometry) or direct emulsification (a method in which oil and water are introduced separately in the device and the emulsion is formed at their point of contact). Depending on the surface properties of the microfluidic device or other microstructured devices (e.g., membrane) either oil in water (hydrophilic device) or water in oil (hydrophobic device), emulsions are formed. Also related products, such as double emulsions, particles, and capsules, are reported in literature. Eor an extensive description of the construction of various microfluidic devices for emulsion preparation, and the various products that have... 

In this chapter, we describe the new technologies that can produce various double emulsions of controlled sizes, structures and compositions. We also explain the emerging new applications of the monodisperse double emulsions prepared using those technologies. Section 21.2 describes the use of porous materials for emulsification membranes. Section 21.3 describes the use of a channel array or through-holes fabricated on a silicon substrate. Section 21.4 describes the use of microfiuidic channels on a planar substrate. Section 21.5 explains coaxial microcapillary devices. Section 21.6 describes the applications of monodisperse multiple emulsions to a new class of functional materials. 

A most important additional aspect of such devices is that, as long as the phase interfaces are immobilized via appropriate pressure/wetting conditions, one can have a very wide range of flow rate ratios between the two phases. There is no need for any density difference between the phases. The issue of flooding does not arise, emulsification is unlikely to arise, and the need for coalescence is absent However, surfactant impurities, if present, could interfere with interface immobilization. Further, the solvents must not swell the membrane very much. Therefore the compatibility of the membrane with the solvents to be used should be checked. Smaller pore membranes will lead to a broader range of pressure difference between the two phases for nondispersive operation. The value of Kta for such devices can be larger than conventional devices by 5-50 times. 

It has been stated that the development of emulsification methods for production of mono-sized droplets must be rooted in one of two possible manufacturing approaches (Williams et al., 2001a) (1) reduction of process length scales of the turbulent perturbations and enhancement of their uniformity in the mixing processes that rupture the liquids, and (2) the creation of droplets individually (drop by drop). The production of emulsions using membrane and microfluidic devices represents a typical example of the second approach. This chapter aims to introduce the latest development on the utilization of the membrane and microfluidic emulsification techniques for the preparation of double emulsions, as well as micro- and nanoparticles from double-emulsion precursors. 

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