Particle size distribution multiple emulsions

Let us consider a shallow fluidized bed combustor with multiple coal feeders which are used to reduce the lateral concentration gradient of coal (11). For simplicity, let us assume that the bed can be divided into N similar cylinders of radius R, each with a single feed point in the center. The assumption allows us to use the symmetrical properties of a cylindrical coordinate system and thus greatly reduce the difficulty of computation. The model proposed is based on the two phase theory of fluidization. Both diffusion and reaction resistances in combustion are considered, and the particle size distribution of coal is taken into account also. The assumptions of the model are (a) The bed consists of two phases, namely, the bubble and emulsion phases. The voidage of emulsion phase remains constant and is equal to that at incipient fluidization, and the flow of gas through the bed in excess of minimum fluidization passes through the bed in the form of bubbles (12). (b) The emulsion phase is well mixed in the axial... 

Figure 1. Particle size distribution of a) type A, b) type B and type C v/o/v emulsions (where 0=isopropyl myristate) just after preparation as described in the text. Key x=multiple oil drops o=simple or empty oil drops and internal aqueous droplets.

As well as the setting rate, a further important characteristic of the emulsion is its viscosity. The choice of emulsifier can influence the viscosity of the emulsion, through an influence on the particle size distribution, or on the extent of multiple-phase droplets that are formed. 

Figure 6.9 W/O/W emulsion prepared by SPG membrane emulsification for trans-catheter arterial injection chemotherapy of hepatocellular carcinoma (HCC) (a) Microscopic view of multiple emulsion droplets. Internal water droplets containing anticancer drug are visible as black dots, (b) Particle size distribution of oil droplets immediately after preparation and 40 days after preparation (Nakashima et al., 2000 Higashi et al., 1995).

The sol-gel reaction during the formation of silica particles in the multiple emulsion system started in the external oil phase containing the precursor alkoxide type (tetraethyl orthosilicate, TEOS), as shown in Figure 7.25. Under stirring, the TEOS molecules can penetrate the surfactant layer surrounding the aqueous phase, and then hydrolysis can start. As hydrolysis proceeds, the Si-OH based molecules diffuse and dissolve in the aqueous phase. A gel network is formed by condensation, yielding the insoluble hydrated silica encapsulating the retinol molecules. The water content in the multiple emulsion was demonstrated to impart the final shape and size distribution of the particles. 

Methotrexate loaded multiple emulsion was prepared by using bovine serum albumin as stabilizer in internal phase to avoid internal droplet coalescence. By microwave technique the albumin in internal aqueous phase was sohdified to form a microspheres-in-oil-in-water emulsion (S/O/W). The formulation and process variables were optimized and evaluated for physicochemical characteristics, such as microscopic structure, electrical charge, particle size distribution, rheological behavior, yield, entrapment efficiency, drug release and stability. The emulsions were found to be stable and showed prolonged release in vitro (Tao et al., 1992). 

For the reasons described above, the droplet size distribution of the same emulsion measured on different laser diffraction instruments can be significantly different, depending on the precise design of the optical system and the mathematical theory used to interpret the diffraction pattern. It should be noted, however, that the most common source of error in particle size analysis is incorrect operation of the instrument by the user. Common sources of user error are introduction of air bubbles into the sample, use of the wrong refractive index, insufficient dilution of emulsion to prevent multiple scattering. and use of an unclean optical system. 

With the advent of advanced characterization techniques such as multiple detector liquid exclusion chromatography and - C Fourier transform nuclear magnetic resonance spectroscopy, the study of structure/property relationships in polymers has become technically feasible (l -(5). Understanding the relationship between structure and properties alone does not always allow for the solution of problems encountered in commercial polymer synthesis. Certain processes, of which emulsion polymerization is one, are controlled by variables which exert a large influence on polymer infrastructure (sequence distribution, tacticity, branching, enchainment) and hence properties. In addition, because the emulsion polymerization takes place in an heterophase system and because the product is an aqueous dispersion, it is important to understand which performance characteristics are influended by the colloidal state, (i.e., particle size and size distribution) and which by the polymer infrastructure. 

For the successful preparation of emulsions, the wetting conditions on the membrane surface are crucial. It is necessary that the membrane surface is only wetted by the liquid that forms the continuous phase. The droplet size correlates with the membrane pore size by a simple relation, Dd = /Dm where / is a value typically between 2 and 8 (35). Droplets can be produced with diameters in the pm-, as well as in the sub-micrometre range. This technique has been successfully applied to produce monodisperse emulsions and multiple emulsions, as well as to carry out polymerizations leading to polymer particle in the pm size range with narrow size distributions (36, 37). Further advantages (38) are as follows the droplet size is controllable and generally a quite narrow DSD can be achieved, the method is reproducible and the scale-up is easy just by increasing the number of membrane modules, the characteristic features are independent of scale-up, batch as well as continuous operations modes are possible, the continuous phase is exposed to a lower stress. 

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