Direct membrane emulsification

Table 21.1 List of most common membranes used in direct membrane emulsification. 

Both crossflow and deadend systems can be used in premix and direct membrane emulsification. In the crossflow premix system the coarse emulsion is diluted by permeation into pure continuous phase/diluted emulsion recirculating at the low-pressure side of the membrane. In the deadend system the fine emulsion is withdrawn as a product after passing through the membrane, without any recirculation and/or dilution with the continuous phase. In this process, the fine emulsion can... 

Droplet formation during direct membrane emulsification and in particular in crossflow emulsification has been described using models different in the scale and in the considered mathematical and physical phenomena, such as ... 

Although the premix membrane emulsification can yield larger fluxes with respect to direct membrane emulsification neither methods using surface-energy minimization nor microfluid dynamics approaches have been until now reported on the theoretical treatment of the premix membrane emulsification. 

By direct membrane emulsification (conventional membrane emulsification), the phase to be dispersed has to be pressed through a microporous membrane. Small droplets are formed and detached from the membrane by a flow of the continuous phase (Figure 13.5). For an appropriate droplet formation, the surface of the membrane has to be wetted by the continuous phase, for example a hydrophilic membrane has to be used to produce an o/w emulsion. 

Figure 13.5 Droplet formation by direct membrane emulsification process.

Whereas only a few pores are active at a disperse phase pressure of about the capillary pressure, the volume flow through the pores as well as the fraction of active pores is increased at higher pressures. Since the probability for neighboring pores forming droplets at the same time rises, the risk of coalescence is higher at a high fraction of active pores. Consequently, low membrane porosity can decrease the risk of coalescence at the membrane surface. Absence of coalescence, due either to a low porosity or a disperse phase pressure of about the capillary pressure, enables one to produce emulsions with narrow droplet size distributions with direct membrane emulsification [8]. 

The key factor for the success of this process is the wettability of the membrane surface. A rule of thumb states that the continuous phase of the resulting fine emulsion has to wet the surface of the membrane as for direct membrane emulsification. Thus, for the production of an o/w emulsion, the membrane surface has to be hydrophilic, regardless whether the process is combined with a phase inversion or not [2]. 

In the early 1990s, Nakashima et al. [2] introduced membrane technology in emulsions preparation by a direct emulsification method, whereas, in the late 1990s, Suzuki et al. used premix membrane emulsification to obtain production rates higher than other membrane emulsification methods [11]. 

Among the applications of membrane emulsification, dmg-delivery system (DDS) is one of the most attractive fields. W/o/w emulsions have been prepared to transport and deliver anticancer drug [4, 63-65]. The emulsion was directly administered into the liver using a catheter into the hepatic artery. In this way, it was possible to suppress the strong side effects of the anticancer drug and also concentrate the dosage selectively to focus on the cancer. The clinical study showed that the texture of the cancer rapidly contracted and its volume decreased to a quarter of its initial size. 

In bounded flow other relationships hold for example, if the smallest dimension of the part of the apparatus in which the droplets are disrapted (e.g., a slit) is comparable to the droplet size, the flow will always be laminar. A different regime prevails, however, if the droplets are injected directly through a narrow capillary into the continuous phase (injection regime), namely membrane emulsification. 

Vladisavljevic, G.T., Shimizu, M., and Nakashima, T., Direct Observation of Droplet Formation in Membrane Emulsification, Proceedings of the DDS Seminar on Development of Lipid Microcarriers and Their Appheation to Drug Dehvery Systems, Sadowara, Japan, August, 2003. 

A different regime prevails if the droplets are directly injected through a narrow capillary into the continuous phase (injection regime), e.g. membrane emulsification. 

Membrane emulsification processes can be directly visualized by microscope as vell as by the use of high-speed cameras. In this case, information can be obtained about droplet disruption [10, 11] and fouling of the membrane. An indirect characterization method is the (inline) measurement of the emulsion characteristics. The emulsion is mainly characterized by its droplet size and droplet size distribution [2]. These infiuence important product characteristics like structure, mouthfeel, color and appearance, texture and viscosity [12, 13]. 

Figure 6.1 Membrane emulsification methods (a) Direct ME (Nakashima et al., 1991a) (b) premix ME (Suzuki et al., 1996) (c) premix ME with phase inversion (Suzuki et al., 1999).

Both direct and premix emulsification can be obtained with a continuous phase flowing along the membrane surface (i.e., crossflow, stirring) (Figure 21.2(b)). However, it is important to distinguish between the droplet-formation mechanism and the macroscopic operation procedure. In other terms, often, in the literature, the... 

Although capsule membrane PTC is not suitable for direct scale-up to industrial level due to the inconveniences of working with capsules, the principles can be exploited in membrane reactors, with the PT catalyst immobilized on the membrane surface. This would not only enable easy recovery of both aqueous and organic phases after reaction without any problems of emulsification, but also ensure that the PT catalyst does not contaminate the product in the organic phase. Using a membrane reactor will also ensure high mass-transfer rates due to high interfacial areas per unit volume of reactor. More importantly, it will open up possibilities for continuous operation. 

In this paper, an accurate method for determining the concentration of manganese and other metals in gasoline and diesel fuel by ICP without the use of chilled spray chamber, direct injection nebulizer, ultrasonic nebulizer with micro-porous membrane desolvator, thermostated condenser, or emulsification is discussed. 

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... 

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