Emulsion droplets mean sizes

The model system used by Mabille et al. [149, 150] was a set of monodisperse dilute (2.5 wt% of dispersed oil) emulsions of identical composition, whose mean size ranged from 4 p.m to 11 p.m. A sudden shear of 500 s was applied by means of a strain-controlled rheometer for durations ranging from 1 to 1500 s. All the resulting emulsions were also monodisperse. At such low oil droplet fraction, the emulsion viscosity was mainly determined by that of the continuous phase (it was checked that the droplet size had no effect on the emulsion viscosity). The viscosity ratio p = t]a/t]c = 0.4 and the interfacial tension yi t = 6 mN/m remained constant. 

In Eq. (5.6), co is defined as a coalescence frequency per unit surface area of the droplets. Considering Eqs. (5.5) and (5.6) and assuming that > is constant (independent of T>), it can be concluded that the mean size in the emulsion increases with time according the following law ... 

Flocculation. Flocculation means an aggregation of emulsion droplets but, in contrast to coalescence, the films of the continuous phase between the droplets survive. Hence, the process may be partially reversible. Both processes, flocculation and coalescence, speed up the creaming of an emulsion due to the increase of the drop size. The process of flocculation is even more important for dispersions of solids than for emulsions because in this case a coalescence is not possible. 

In the LASC-catalysed reactions, the formation of stable emulsions seemed to be essential for the efficient catalysis. We thus imdertook the observation of the emulsions by means of several tools. Optical microscopic observations of the emulsions revealed the formation of spherical emulsion droplets in water (Figure 13.1). The average size of the droplets formed from 3 in the presence of benzaldehyde in water was measured by dynamic light scattering, and proved to be ca. 1.1 pm in diameter. The shape and size of the emulsion droplets were also confirmed by transmission electron microscopy and atomic force microscopy. 

There are different ways of counting particles to obtain the distribution. One can simply count the number of particles smaller than a specific size. This means that in the curve, a droplet of 0.1 pm will count for one, just as a droplet of size 10 pm. However, the amount of oil present in the 10-pm droplet is 100, or one million times larger. Such a distribution therefore stresses the presence of small particles. Another way is to not use the number of particles but the total interfacial area present on the particles. Since the interfacial area of a small droplet is much smaller than that of larger droplets, smaller droplets are counted less extremely, and the resulting distribution is more realistic. Remember that the amount of surfactant needed to stabilize the interface is proportional to the total interfacial area present on the droplets, and the total energy needed to put into the emulsion is proportional to the total interfacial area. As a third option, one can use the volume of the particles smaller than a specific size. In this case the distribution gives that total amount of oil that is present in small, medium or larger droplets. It depends on the application which type of distribution should be used. 

Candau and co-workers were the first to address the issue of particle nu-cleation for the polymerization of AM [13, 14] in an inverse microemulsion stabilized by AOT. They found that the particle size of the final microlatex (d 20-40 nm) was much larger than that of the initial monomer-swollen droplets (d 5-10 nm). Moreover, each latex particle formed contained only one polymer chain on average. It is believed that nucleation of the polymer particle occurs for only a small fraction of the final nucleated droplets. The non-nucleated droplets also serve as monomer for the growing particles either by diffusion through the continuous phase and/or by collisions between droplets. But the enormous number of non-nucleated droplets means that some of the primary free radicals continuously generated in the system will still be captured by non-nucleated droplets. This means that polymer particle nucleation is a continuous process [ 14]. Consequently, each latex particle receives only one free radical, resulting in the formation of only one polymer chain. This is in contrast to the large number of polymer chains formed in each latex particle in conventional emulsion polymerization, which needs a much smaller amount of surfactant compared to microemulsion polymerization. 

The above mean is also referred to as the mean length diameter, dy, because it represents the sum of the length of the droplets divided by the total number of droplets. It is also possible to express the mean droplet size in a number of other ways (Table 2). Each of these mean sizes has dimensions of length (meters), but stresses a different physical aspect of the distribution, e.g., the average length, surface area, or volume. For example, the volume-surface mean diameter is related to the surface area of droplets exposed to the continuous phase per unit volume of emulsion, As ... 

Soo and Radke (11) also studied the effect of average droplet size of emulsion on the flow behavior in porous media. The droplet size distribution of the emulsions that were prepared with surfactants and NaOH in a blender are shown in Figure 12. These droplet size distributions were found to be log-normal distributions. Others (9, 27) have also observed that the size of emulsion droplets was log-normally distributed. Soo and Radke (11) conducted experiments with emulsions having different average mean diameter in fine Ottawa water-wet sand packs. Their results of the reduced permeability, k/ko, and reduced effluent volume concentration as a function of the pore volume of oil (in the emulsion) injected are shown in Figure 13. All emulsions were of 0.5% quality, and the initial permeability, ko, was 1170 mD (millidarcies). The lines in the figure represent results of flow theory (12,13) based on deep-bed filtration principles. 

From Fig. 4.18, which showed log normal as weU as cumulative internal droplet size distribution, it can be observed that emulsion prepared using 4000 rpm impeUer speed resulted in a narrow size distribution with internal droplets mean diameter 0.024 pm. In contrast, emulsion prepared using 8000 rpm impeUer speed yields broader droplet size distribution with lower mean diameter of 0.015 pm. 

In most cases d 2 (the volume/surface average or Sauter mean) is used, while the width of the size distribution can be given as the variation coefficient. The latter is the standard deviation of the distribution weighted with d divided by the corresponding average d. Generally C2 will be used which corresponds to d 2 An alternative way to describe the emulsion quahty is to use the specific surface area A (the surface area of all emulsion droplets per unit volume of emulsion). 

In the chemical industry (on the mega- as well as the micro-scale) fine emulsions have many useful applications in, e.g., extraction processes or phase transfer catalysis. Additionally, they are of interest for the pharmaceutical and cosmetic industry for the preparation of creams and ointments. Micromixers based on the principle of multilamination have been found to be particularly suitable for the generation of emulsions with narrow size distributions [33]. Haverkamp et al. showed the use of micromixers for the production of fine emulsions with well-defined droplet diameters for dermal applications [38]. Bayer et al. [39] reported on a study of silicon oil and water emulsion in micromixers and compared the results with those obtained in a stirred tank. They found similar droplet size distributions for both systems. However, the specific energy required to achieve a certain Sauter mean diameter was 3-1 Ox larger for the macrotool at diameters exceeding 100 pm. In addition, the micromixer was able to produce distributions with a mean as low as 3 pm, whereas the turbine stirrer ended up with around 30 pm. Based on energy considerations, the intensification factor for the microstirrer appears to be 3-10. 

Another technique to increase sedimentation stability of emulsions is viscosity control of the disperse phase. It is noted in [31—32, 35] that the viscosity of the disperse phase influences the stability of o/w emulsions. The effect of the disperse phase viscosity of various nature on the mean size d of the emulsion droplets is clearly seen from Fig. 6.9. The change of the nature of high-viscosity hydrocarbon component (decane, toluene, MS-20 oil) has no substantial effect... 

As pointed out in Sec. II the echo attenuation curve for the PFG experiment, when applied to molecules entrapped in an emulsion droplet, is a signature of the size of the emulsion droplet, is a signatiue of the size of the emulsion droplet (cf Fig. 2). As a conse quence, droplet sizes can be determined by means of the PFG experiment. 

When applied to a real emulsion one has to eonsider the fact that the emulsion droplets in most cases are polydis-perse in size. This effect can be accounted for if the molecules confined to the droplets are in a slow exchange situation, meaning that their lifetime in the droplet must be longer than A. For such a case, the echo attenuation is given by ... 

We have shown that the NMR self-diffusion method is sensitive to the mean displacement of a molecule of interest on the time scale of the NMR experiment (A). This fact allows us to measure molecular transport inside the emulsion droplets, as in the case of determination of droplet sizes, and the exchange between the emulsion droplets, as in the case of highly concentrated emulsions. In more complex systems the NMR self-diffusion method is sensitive to the molecular exchange between the emulsion droplets and the continuous phase, as in the case of multiple emulsions. Many emulsion systems are currently used as carriers for drugs or other bioactive substances, such as pesticides. The selective measurement of the diffusivity of the individual components within flic emulsion system is therefore of theoretical and practical relevance. The NMR self-diffusion technique is an appropriate tool to study the drug release from emulsion droplets. This useful information may be obtained in a rapid and nondestructive way. 

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