The Process of Emulsification

Chapter 4 will clearly show that particle preparation from macroemulsions involves essentially the W/O variety. This, of course, makes the initial selection of the emulsifier rather easy, i.e. relatively oil-soluble surfactants with low HLB values (the recommended range is 3-6) would generally serve the purpose. 

It must be pointed out here that the relative solubilities of the hydrophilic and lipophilic parts of an emulsifier (surfactant molecule) change with temperature as a result, the HLB value also changes. This phenomenon is very prominent for non-ionic emulsifiers containing polyethylene oxide (PEO)-type hydrophilic groups. At higher temperatures, such emulsifiers become less soluble in water and hence, stabilize W/O emulsions, and vice versa (see also Section 1.3). Ionic surfactants are more likely to stabilize O/W emulsions at high temperatures [38]. 

The phase inversion temperature (PIT) of an emulsifier designates a point in the temperature scale where its hydrophilic and lipophilic properties exactly balance. This can serve as a measure of the hydrophile-lipophile balance especially for the non-ionic surfactants facile experimental determination of the PIT is obviously an advantage [38]. 

It has been shown that micelles forming just above CMC take a spherical or near-spherical shape note that this occurs within the bulk of the continuous phase. As the amphiphile concentration increases, the architecture of self-aggregation changes and follows more complex geometries than simple spheres [ 3,41]. When in addition a dispersed phase is available in the system (which now changes from 

As already discussed, only a very unstable emulsion is generally obtained by mechanical agitation of various water - oil mixtures unless the emulsion is very dilute, which is of course economically non-remunerative from the viewpoint of particle preparation. Small quantities of surfactants (below CMC) impart working stability to the emulsion by reducing the interfacial tension, but the latter still remains thermodynamically unstable. However, even such emulsions have their utility in particle synthesis, as will be seen in Chapter 4. In the simplest procedure, unstable emulsions suitable for particle synthesis, i.e. W/O type systems are produced through essentially three steps  

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High pressure homogenization may also be used to form microemulsions but the process of emulsification is generally inefficient (due to the dissipation of heat) and extremely limited as the water-oil-surfactant mixture may be highly viscous prior to microemulsion formation. ... 

Emulsifiers also have an important role in the process of emulsification. Surfactant emulsifiers reduce interfacial tensions during emulsification, making droplets easier to break up as well as reducing the tendency for recombination. Other emulsifiers such as... 

The various methods of agitation to produce emulsions have been described recently (18). In addition, the emulsions of smaller droplets can be produced by applying more intense agitation to disrupt the larger droplets. Therefore, the liquid motion during the process of emulsification is generally turbulent (9) except for high viscosity liquids. 

The production of small (submicron) droplets requires the application of large amounts of energy, as the process of emulsification is generally very inefficient (as illustrated below). 

The formation of emulsions is an important event in an oil spill. First, and most importantly, it substantially increases the actual volume of the spill. Emulsions of all types contain about 70% water and thus, when emulsions are formed, the volume of the oil spill more than triples. Even more significantly, the viscosity of the oil increases by as much as 1000 times, depending on the type of emulsion formed. For example, an oil that has the viscosity of a motor oil can triple in volume and become almost solid through the process of emulsification. 

There are three similarities between macroemulsions and foams (1) They both consist of a dispersion of an immiscible state of matter in a liquid phase. Foams are dispersions of a gas in a liquid emulsions are dispersions of a liquid in a second immiscible liquid. (2) The tension y7 at the relevant interface is always greater than zero, and since there is a marked increase in interfacial area AA during the process (of emulsification or foaming), the minimum work involved is the product of the interfacial tension and the increase in interfacial area (Vkmm = AA x y7. (3) The system will spontaneously revert to two bulk phases unless there is an interfacial... 

NaOH and 1% NaCl (Caustic System). The process of emulsification was observed through the Nikon LKe Interference Phase Microscope (magnifications of 400 and 1,000). The action was captured by high speed cine-photomicrography (64 frames/second) for the caustic system and normal speed (24 frames/second) for the sulfonate system. 

P Becher. The process of emulsification a computer model. Langmuir 7 1325,1991. 

In order to produce an emulsion from two immiscible liquids, one requires a third component, i.e. an emulsifier. The role of the latter can be clearly understood from a consideration of the process of emulsification, as illustrated in Figure 4.3. 

Nearly the same results were obtained for CTAB solution-tetradecane system (curve 3, Fig. 28). However, emulsification begins to show up here at somewhat lower CTAB concentration. The process of emulsification is induced by formation after a retreating meniscus of a dynamic wetting film whose thickness is proportional, according to Derjaguin s theory [7], to the parameter r(vj//yo) - Decrease of interface tension with CTAB concentration promotes formation of thicker dynamic wetting films. Capillary instability of such films gives rise to the process of undulation and subsequent coalescence... 

The process of emulsification of other fats is probably initiated by the churning motion of the stomach and is greatly facilitated when the chyme passes into the duodenum and is mixed with the bile and pancreatic juice. These secretions, which are both alkaline, neutralize the acid from the stomach so that conditions become suitable for the action of pancreatic lipase. Bile contains no enzymes but owes its effects to the presence of the bile salts, sodium glycocholate and sodium taurocholate, which are powerful detergents. The bile salts are derived from cholic acid, a sterol, which is joined by a peptide linkage either to glycine or to taurine. 

The effect of surfactants and/or polymers on the droplet size spectrum of a spray can be, to a first approximation, described in terms of their effect on the surface tension. Since surfactants lower the surface tension of water, one would expect that their presence in the spray solution would result in the formation of smaller droplets. This is similar to the process of emulsification described in Chapter 6. Owing to the low surface tension in the presence of surfactants, the total surface energy of the droplets produced on atomization is lower than that in the absence of surfactants. This implies that less mechanical energy is required to form the droplets when a surfactant is present. This leads to smaller droplets at the same energy input. However, the actual situation is not simple since one is dealing with... 

The Laplace pressure in the process of emulsification is what causes an emulsion to become thermodynamically inefficient. For an emulsion to form the small, highly curved droplets, extra energy is required to overcome the large pressure that exists in the droplets. 

There were several attempts to interpret Bancroft s rule in terms of the damping of surface corrugations by the Gibbs elasticity or in terms of the rate of thinning of emulsion films prior to coalescence. Another interpretation is concerned not with the stability of an already formed emulsion, but with the process of emulsification. ... 

Figure 11.13 The process of emulsification of a W/0 gel emulsion by the spontaneous emulsification method. An oil-in-water (0/W) microemulsion at 7°C (a) is rapidly heated to 40 °C and a gel emulsion is spontaneously formed after 38 s (b). At this stage, the emulsion does not flow if the container is turned upside down (c)...

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