Emulsion determining dispersed phase

Equation (9.21) has ben used to determine droplet sizes in emulsions and disperse phases using the PGSE NMR method [132, 133]. 

Classification of the many different encapsulation processes is usehil. Previous schemes employing the categories chemical or physical are unsatisfactory because many so-called chemical processes involve exclusively physical phenomena, whereas so-called physical processes can utilize chemical phenomena. An alternative approach is to classify all encapsulation processes as either Type A or Type B processes. Type A processes are defined as those in which capsule formation occurs entirely in a Hquid-filled stirred tank or tubular reactor. Emulsion and dispersion stabiUty play a key role in determining the success of such processes. Type B processes are processes in which capsule formation occurs because a coating is sprayed or deposited in some manner onto the surface of a Hquid or soHd core material dispersed in a gas phase or vacuum. This category also includes processes in which Hquid droplets containing core material are sprayed into a gas phase and subsequentiy solidified to produce microcapsules. Emulsion and dispersion stabilization can play a key role in the success of Type B processes also. 

The immense interfacial area separating dispersed globules from the dispersion phase is of critical Importance in determining their stability. For example, it is estimated that a typical emulsion has approximately 7 X 10 cra interfacial area per liter (3 ). Thus, those factors controlling the properties of the interfacial membrane are extremely Important in determining the stability of the emulsion. 

Such reactions can take place predominantly in either the continuous or disperse phase or in both phases or mainly at the interface. Mutual solubilities, distribution coefficients, and the amount of interfadal surface are factors that determine the overall rate of conversion. Stirred tanks with power inputs of 5-10 HP/1000 gal or extraction-type equipment of various kinds are used to enhance mass transfer. Horizontal TFRs usually are impractical unless sufficiently stable emulsions can be formed, but mixing baffles at intervals are helpful if there are strong reasons for using such equipment. Multistage stirred chambers in a single shell are used for example in butene-isobutane alkylation with sulfuric acid catalyst. Other liquid-liquid processes listed in Table 17.1 are numbers 8, 27, 45, 78, and 90. 

The protocol described here is based on the relationship between the density of the dispersed phase, the density of the continuous phase, the emulsion density, and the volume fraction, which is a measure of the oil droplet concentration. An essential part of the measurement is the precise determination of density. The method introduced below is the most inexpensive method to accurately determine density, but has the drawback that a relatively large sample volume is required. 

Figure 13. Ultrasonic determination of creaming profiles. is the disperse phase volume fraction, t is the time and x is the height of the emulsion.

To understand the mechanism of polyblending, experiments have been carried out with polymeric solution. W. Borchard and G. Rehage mixed two partially miscible polymer solutions, measured the temperature dependence of the viscosity, and determined the critical point of precipitation. When two incompatible polymers, dissolved in a common solvent, are intimately mixed, a polymeric oil-in-oil emulsion is formed. Droplet size of the dispersed phase and its surface chemistry, along with viscosity of the continuous phase, determine the stability of the emulsion. Droplet deformation arising from agitation has been measured on a dispersion of a polyurethane solution with a polyacrylonitrile solution by H. L. Doppert and W. S. Overdiep, who calculated the relationship between viscosity and composition. 

The inner phase volume fraction determines many properties of an emulsion. One example is the viscosity r/em. For small volume fractions one can often regard the disperse phase as consisting of rigid, spherical particles instead of liquid, flexible drops. Then we can apply Einstein s3 equation [541], with rj being the viscosity of the pure dispersing agent ... 

There can even be more complex emulsion types [2] Figure 1.2 shows an example of a crude oil W/O/W/O emulsion. The type of emulsion that is formed depends upon a number of factors. If the ratio of phase volumes is very large or very small then the phase having the smaller volume is frequently the dispersed phase. If the ratio is closer to one then other factors determine the outcome. See Chapter 11 (especially Table 11.1) for examples of petroleum emulsion types. 

If the dispersed phase concentration is not too high, and the species are very small, light-scattering can yield size information. The theory underlying the determination of size distribution for a colloidal dispersion is quite involved [13,75,76], When a beam of light enters a suspension some light is absorbed, some is scattered, and some is transmitted. Many dilute, fine emulsions and suspensions show a noticeable turbidity given by,... 

Not all emulsions exhibit the classical milky opaqueness with which they are usually associated. A tremendous range of appearances is possible, depending upon the droplet sizes and the difference in refractive indices between the phases. An emulsion can be transparent if either the refractive index of each phase is the same, or alternatively, if the dispersed phase is made up of droplets that are sufficiently small compared with the wavelength of the illuminating light. Thus an O/W microemulsion of even a crude oil in water may be transparent. If the droplets are of the order of 1 pm diameter a dilute O/W emulsion will take on a somewhat milky-blue cast if the droplets are very much larger then the oil phase will become quite distinguishable and apparent. Physically the nature of the simple emulsion types can be determined by methods such as [95] ... 

Droplet-size distribution and disperse-phase percentage determine the emulsion properties characterizing the final formulation for an intended use. 

Colloidal interactions between emulsion droplets play a primary role in determining emulsion rheology. If attractions predominate over repulsive forces, flocculation can occur, which leads to an increase in the effective volume fraction of the dispersed phase and thus increases viscosity (McCle-ments, 1999). Clustering of milk fat globules due to cold agglutination increases the effective volume fraction of the milk fat globules, thereby increasing viscosity (Prentice, 1992). 

Liquid emulsions are inherently unstable to a varying degree. It is important to understand, therefore, the mechanisms that contribute to emulsion stability. Before the solidification step, instability of an emulsion can arise due to either phase separation or phase inversion (Mulder and Walstra, 1974). It is evident that the likelihood of phase inversion will increase as the fraction of dispersed phase increases. The vast majority of literature references are concerned with the stability to phase separation as coalescence or creaming in oil-in-water emulsions (Hailing, 1981 Jaynes, 1983). In addition, a method for determining the stability of water-in-oil emulsions to inversion has not been reported. It is usually assumed that certain aspects of oil-in-water emulsion theory apply in reverse to water-in-oil emulsions. 

Cross-linked polystyrene porous particles (with 21 mol% DVB) have been prepared by the concentrated emulsion polymerization method, using either toluene or decane as the porogen and an aqueous solution of SDS as the continuous phase. Since toluene is a good solvent for polystyrene while decane is a nonsolvent , the morphologies obtained in the two cases were different. The particles based on toluene (with a volume fraction of dispersed phase of 78%) have very small pores which could not be detected in the SEM pictures. The pore size distribution, which has sizes between 20 and 50 A and was determined with an adsorption analyzer, almost coincides with that in a previous study [49] in which porous polystyrene beads have been prepared by suspension polymerization. In contrast, the porous particles based on decane have pore sizes as large as 0.1-0.3 pm, which could be detected in the SEM pictures [44a], and also larger surface areas (47 m2 g ) than those based on toluene (25 m2 g ). The main difference between the concentrated emulsion polymerization and the suspension polymerization consists of the much smaller volume fraction of continuous phase used in the former procedure. The gel-like emulsion that constitutes the precursor in the former case contains polyhedral cells separated by thin films of continuous phase. The polymerization of the cells does not... 

Comparison of published tota on vinylacetate polymerization kinetics, for which a steady-state period is typical (13), to the kinetics of variation of the number of particles which decreases during the process up to a factor of 40 (14), permits us to conclude that there is no correlatlonbetween the rate and the number of particles. This conclusion was supported by Medvedev et al. ( ) in the case of emulsion polymerization of methylm hacrylate. We deduce from the above data that the emulsifier concentration itself does not determine either the total surface of the disperse phase or the mmber of particles during polymerization of polar monomers. 

In non-scattering systems, ultrasonic properties and the volume fraction of the disperse phase are related in a simple manner. In practice, many emulsions and suspensions behave like non-scattering systems under certain conditions (e.g. when thermal and visco-inertial scattering are not significant). In these systems, it is simple to use ultrasonic measurements to determine 0 once the ultrasonic properties of the component phases are known. Alternatively, if the ultrasonic properties of the continuous phase, 0and p2 are known, the adiabatic compressibility of the dispersed phase can be determined by measuring the ultrasonic velocity. This is particularly useful for materials where it is difficult to measure jc directly in the bulk form (e.g. powders, granular materials, blood cells). 

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