Macroemulsions formation

Macroemulsions have been known for thousands of years. The survey of ancient literature reveals that the emulsification of beeswax was first recorded in the second century by the Greek physician, Galen (1). Macroemulsions are mixtures of two immiscible liquids, one of them being dispersed in the form of fine droplets with diameter greater than 0.1 ym in the other liquid. Such systems are turbid, milky in color and thermodynamically unstable (i.e. the macroemulsion will ultimately separate into two original immiscible liquids with time). Since the early 1890s, extensive and careful studies have been carried out on macroemulsions and several excellent books have been written on various aspects of formation and stability of these systems (2,10). In addition, several theories and methods of macroemulsion formation have been discussed in the recent articles (13 ... 

The mechanics of microemulsion formation differ from those of macroemulsion formation. The most important difference Hes in the fact that exerting more effort in producing a macroemulsion, or increasing the emulsifier, usually improves... 

The method developed originally for microemulsion formulation (Section II above) has been adapted (Salager, 1983, 2000) to macroemulsion formation. In this method, the value of the left-hand side of equation 8.10 or 8.11 is called the hydrophilic-lipophilic deviation (HLD). When the value equals zero, as in Section II, a microemulsion is formed when the value is positive, a W/O macroemulsion is preferentially formed when it is negative, an O/W macroemulsion is preferentially formed. The HLD is similar in nature to the Winsor R ratio (equation 5.2) in that when the HLD is larger than, smaller than, or equal to 0, R is larger than, smaller than, or equal to 1. The value of the HLD method is that, on a qualitative basis, it takes into consideration the other components of the system (salinity, cosurfactant, alkane chain length, temperature, and hydrophilic and hydrophobic groups of the surfactant). On the other hand, on a quantitative basis, it requires the experimental evaluation of a number of empirical constants. 

Flow properties of macroemulsions are different from those of non-emulsified phases 19,44). When water droplets are dispersed in a non-wetting oil phase, the relative permeability of the formation to the non-wetting phase decreases. Viscous energy must be expended to deform the emulsified water droplets so that they will pass through pore throats. If viscous forces are insufficient to overcome the capillary forces which hold the water droplet within the pore body, flow channels will become blocked with persistent, non-draining water droplets. As a result, the flow of oil to the wellbore will also be blocked. 

The above-mentioned artificial microbubble surfactant mixtures, and other successful mixtures found for stable microbubble production (ref. 544-546), all contain saturated glycerides (with acyl chain lengths greater than 10 carbons) combined with cholesterol and cholesterol derivatives (cf. Chapters 9 and 10, and ref. 544). As described earlier, long chain lengths in nonionic (or even unionized) surfactants are known to favor the formation of both large, rodlike micelles (as opposed to small spherical micelles) and macroemulsions (as opposed to microemulsions) (see... 

The competition for oligomeric radicals also includes particles that have been created. In miniemulsion polymerizations, the nucleation of one droplet results in the formation of one particle of equal surface area. Therefore, nucleation therein has little effect on competition for radicals. This is not so with macroemulsions, since both micellar and homogeneous nucleation result in a large shift in the surface area from micelles to particles as the particles are created and grow. 

Observation (i) above can be understood in terms of droplet nucleation and the lack of competition between nucleation and growth. A mechanistic understanding of observation (ii) above was provided by Samer and Schork [64]. Nomura and Harada [136] quantified the differences in particle nucleation behavior for macroemulsion polymerization between a CSTR and a batch reactor. They started with the rate of particle formation in a CSTR and included an expression for the rate of particle nucleation based on Smith Ewart theory. In macroemulsion, a surfactant balance is used to constrain the micelle concentration, given the surfactant concentration and surface area of existing particles. Therefore, they found a relation between the number of polymer particles and the residence time (reactor volume divided by volumetric flowrate). They compared this relation to a similar equation for particle formation in a batch reactor, and concluded that a CSTR will produce no more than 57% of the number of particles produced in a batch reactor. This is due mainly to the fact that particle formation and growth occur simultaneously in a CSTR, as suggested earlier. 

The formation of a surfactant film around droplets facilitates the emulsification process and also tends to minimize the coalescence of droplets. Macroemulsion stability in terms of short and long range interactions has been discussed. For surfactant stabilized macroemulsions, the energy barrier obtained experimentally is very high, which prevents the occurrence of flocculation in primary minimum. Several mechanisms of microemulsion formation have been described. Based on thermodynamic approach to these systems, it has been shown that interfacial tension between oil and water of the order of 10- dynes/cm is needed for spontaneous formation of microemulsions. The distinction between the cosolubilized and microemulsion systems has been emphasized. 

In spite of this progress, we still do not have good predictive methods for the formation or breaking macroemulsions. For the formation of a stable macroemulsion from two immiscible liquids, there is no reliable predictive method for selecting the emulsifier or... 

Transient Processes. There are several transient processes such as formation and coalescence of oil drops as well as their flow through porous media, that are likely to occur during the flooding process. Figure 12 shows the coalescence or phase separation time for hand-shaken and sonicated macroemulsions as a function of salinity. It is evident ithat a minimum in phase separation time or the fastest coalescence rate occurs at the optimal salinity (53). The rapid coalescence could contribute significantly to the formation of an oil bank from the mobilized oil ganglia. This also suggests that at the optimal salinity of the system, the interfacial viscosity must be very low to promote the rapid coalescence. 

The droplet deformation increases with increases in the Weber number which means that, in order to produce small droplets, high stresses (i.e., high shear rates) are require. In other words, the production of nanoemulsions costs more energy than does the production of macroemulsions [4]. The role of surfactants in emulsion formation has been described in detail in Chapter 10, and the same principles apply to the formation of nanoemulsions. Thus, it is important to consider the effects of surfactants on the interfacial tension, interfacial elasticity, and interfacial tension gradients. 

The formulation of microemulsions or micellar solutions, like that of conventional macroemulsions, is still an art. In spite of exact theories that have explained the formation of microemulsions and their thermodynamic stabihty, the science of microemulsion formulation has not advanced to a point where an accurate prediction can be made as to what might happen when the various components are mixed. The very much higher ratio of emulsifier to disperse phase which differentiates microemulsions from macroemulsions appears at a first sight that the appHcation of various techniques for formulation to be less critical. However, in the final stages of the formulation it can be realised immediately that the requirements are critical due to the greater number of parameters involved. 

Figure 10.12 Schematic of the variation of the phase behaviour during the degreasing process. In the short float the ultra-low interfacial tension between water and oil ensures efficient degreasing. Upon reducing the salt mass fraction the phase behaviour shifts to higher temperatures. At the degreasing temperature now an oil-in-water microemulsion coexists with an oil-excess phase. Shearing induces the formation of a stable macroemulsion that prevents the depositing of the fat on the skin and ensures the transport of the fat away from the skin. Note that only the Gibbs triangles correspond to the real experimental conditions. The T-y cuts are shown for clarity.

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