Emulsion dispersed-phase properties, determination

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

Emulsions are one of the most important structures prepared specifically for a given application. For example, a day cream (skin cream) has characteristics and ingredients different from that of a night cream. One of the main differences between emulsions is whether oil droplets are dispersed in the water phase, or water drops are dispersed in the oil phase. This can be determined by measuring their conductivity because conductivity is higher for the O/W than for the W/O emulsion. Another useful property is that O/W will dissolve water while W/O will not, thus showing that W/O or O/W may be chosen depending on the application area. Especially in the... 

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. 

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 ... 

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). 

This chapter outlines emulsion characterization techniques ranging from those commonly found infield environments to those in use in research laboratories. Techniques used in the determination of bulk emulsion properties, or simply the relative amount of oil, water, and solids present, are discussed, as well as those characterization methods that measure the size distribution of the dispersed phase, rheological behavior, and emulsion stability. A particular emphasis is placed on optical and scanning electron microscopy as methods of emulsion characterization. Most of the common and many of the less frequently used emulsion characterization techniques are outlined, along with their particular advantages and disadvantages. 

The characterization techniques that will be discussed here are used in field situations, on-line, and in the laboratory. In order to characterize an emulsion, it is necessary to determine the amount of each phase present, the nature of the dispersed and continuous phases, and the size distribution of the dispersed phase. The stability of an emulsion is another important property that can be monitored in a variety of ways, but most often, from a processing point of view, stability is measured in terms of the rate of phase separation over time. This phenomenological approach serves well in process situations in which emulsion formation and breaking problems can be very site specific. However, emulsion stability is ultimately related to the detailed chemistry and physics of the emulsion components and their interactions, and these details cannot be completely ignored. 

After all, the stability and size distribution of this phase determine most bulk emulsion properties. Fixed proportions of oil, water, and solids can be combined in various ways to produce emulsions having different size distributions of the dispersed phase, given only small differences in emulsifier or ion additions to the water or oil phases. These physical differences can lead to significantly different viscosity and stability in emulsions with nominally identical bulk composition. 

Aside from microscopy, the techniques for determining the size distribution of the dispersed phase in emulsion systems can be broadly divided into three categories techniques that depend upon the differences in electrical properties between the dispersed and continuous phases, those that effect a physical separation of the dispersed droplet sizes, and those that depend upon scattering phenomena due to the presence of the dispersed phase. Overviews of these types of techniques are found elsewhere 1-4,13, 46-49). 

Some of the more sophisticated techniques offer detailed information or levels of accuracy that are not required in day-to-day operations. However, when operational upsets cannot be handled by normal methods, details of the emulsion properties have to be understood. For example, subtle changes in the size distribution of the dispersed phase (while total oil, water, and solids remain constant) can be important in determining process performance. An oil-in-water or water-in-oil emulsion can invert during processing as one or the other phase is removed, and the point in the process when this inversion occurs can have implications for the efficiency of the operation. The addition of diluent to reduce oil-phase viscosity, for instance, is much more efficient if oil is the continuous phase. 

Fundamental mixing studies on simple two-component systems have provided insight into the effect of mixing parameters on critical emulsion properties such as particle size distribution. For example, Nagata [81] has shown the distribution of sizes of the dispersed liquid phase as a function of agitator speeds. As we might expect, a normal distribution occurs at higher speeds. In a similar study, the effect of surface tension was determined for several liquid dispersed phases from benzene to paraffin oil [82],... 

Volume fraction of dispersed phase q>, since it determines some essential properties, such as rheological ones. For an emulsion, the value desired can mostly be predetermined by the proportions of oil and water in the recipe. This is often different for foams, especially when made by beating the value of q> obtained then depends on several conditions (see the next section). In foams one often speaks of the overrun, i.e., the percentage increase in volume due to incorporation of gas. The relation is percentage overrun = 100

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Elastic deformation also reveals itself in foams and concentrated emulsions. The shear stress in this case is determined by an increase in the interfacial area due to the deformation of the system. Mechanical properties of solidified foams and other solid-like cellular structures are governed by the degree of dispersion, type of backbone structure and a combination of mechanical characteristics of dispersed phase and dispersion medium. 

The process can be described in terms of filtering of the disperse phase by a system of mesh electrodes. The primary goal is to determine the amount of residual water in the oil product at the filter exit. The volume concentration of water Woui at the exit depends on the volume concentration at the entrance Wi , disperse structure of the emulsion, electric field strength, construction parameters of the device, and physico-chemical properties of the emulsion. 

This entry addresses specific ion effects in thin films with thicknesses in the nano- to micrometer range and focuses on the effect of monovalent cations and anions on the structure of thin films. First, thin organic adsorbed films, so-called polyelectrolyte multilayers (PEMs) which are prepared by sequential adsorption of polyanions and polycations on a charged surface [10], are presented. Second, thin liquid (aqueous) films are discussed. These are thin layers of a continuous phase through which the dispersed phase (bubbles, droplets, solid particles) of colloidal dispersions such as foams, emulsions, and suspensions interacts. Both PEMs and liquid films have one thing in common The amount as well as the type of ions plays a central role in determining the properties of such thin films. 

Elastic deformation can also be observed in foams and concentrated emulsions. In such cases, the yield stress is determined by the increase in the interfacial area upon the deformation of the particles. The mechanical properties of solidified foams and other solid-like materials with a cellular structure are defined by the degree of their dispersion, their backbone structure, and the combination of the mechanical properties of dispersion medium and dispersed phase. 

Highly concentrated emulsions are interesting systems for the preparation of low-density macroporous materials by polymerization in the continuous phase of the emulsions followed by the removal of the dispersed phase components. Macroporous solid foams or aerogels produced by this method consist of intercormected spongelike macropores. The droplet size distribution of the highly concentrated emulsion, which can be controlled by choosing an appropriate emulsification method (e.g. spontaneous emulsification method), is a crucial factor in determining the properties of the macroporous monoliths. 

The performance of demulsifiers can be predicted by the relationship between the film pressure of the demulsifier and the normalized area and the solvent properties of the demulsifier [1632]. The surfactant activity of the demulsifier is dependent on the bulk phase behavior of the chemical when dispersed in the crude oil emulsions. This behavior can be monitored by determining the demulsifier pressure-area isotherms for adsorption at the crude oil-water interface. 

The number of the constituent phases of a disperse system can be higher than two. Many commercial multiphase pharmaceutical products cannot be categorized easily and should be classified as complex disperse systems. Examples include various types of multiple emulsions and suspensions in which solid particles are dispersed within an emulsion base. These complexities influence the physicochemical properties of the system, which, in turn, determine the overall characteristics of the dosage forms with which the formulators are concerned. 

A typical characteristic of many food products is that these are multi-phase products. The arrangement of the different phases leads to a microstructure that determines the properties of the product. Mayonnaise, for example, is an emulsion of about 80% oil in water, stabilized by egg yolk protein. The size of the oil droplets determines the rheology of the mayonnaise, and hence, the mouthfeel and the consumer liking. Ice cream is a product that consists of four phases. Figure 1 shows this structure schematically. Air bubbles are dispersed in a water matrix containing sugar molecules and ice crystals. The air bubbles are stabilized by partial coalesced fat droplets. The mouthfeel of ice cream is determined by a combination of the air bubble size, the fat droplet size and the ice crystal size. 

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