Coalescence macroemulsions

Figure 14 shows a very interesting and an important correlation between the rate of coalescence in macroemulsions and the apparent viscosity in the flow through porous media. It was observed that a minimum in apparent viscosity for the flow of macroemulsions in porous media coincides with a minimum in phase separation time at the optimal salinity. This correlation between the phenomena occurring in the porous medium and outside the porous medium allows us to use coalescence measurements as a screening criterion for many oil recovery formulations for their possible behavior in porous media. It is. very likely that a rapidly coalescing macroemulsion may give a lower apparent viscosity for the flow in porous media (53). 

Freshly prepared macroemulsions change their properties with time. The time scale can vary from seconds (then it might not even be appropriate to talk about an emulsion) to many years. To understand the evolution of emulsions we have to take different effects into account. First, any reduction of the surface tension reduces the driving force of coalescence and stabilizes emulsions. Second, repulsive interfacial film and interdroplet forces can prevent droplet coalescence and delay demulsification. Here, all those forces discussed in Section 6.5.3 are relevant. Third, dynamic effects such as the diffusion of surfactants into and out of the interface can have a drastic effect. 

Emulsions made by agitation of pure immiscible liquids are usually very unstable and break within a short time. Therefore, a surfactant, mostly termed emulsifier, is necessary for stabilisation. Emulsifiers reduce the interfacial tension and, hence, the total free energy of the interface between two immiscible phases. Furthermore, they initiate a steric or an electrostatic repulsion between the droplets and, thus, prevent coalescence. So-called macroemulsions are in general opaque and have a drop size > 400 nm. In specific cases, two immiscible liquids form transparent systems with submicroscopic droplets, and these are termed microemulsions. Generally speaking a microemulsion is formed when a micellar solution is in contact with hydrocarbon or another oil which is spontaneously solubilised. Then the micelles transform into microemulsion droplets which are thermodynamically stable and their typical size lies in the range of 5-50 nm. Furthermore bicontinuous microemulsions are also known and, sometimes, blue-white emulsions with an intermediate drop size are named miniemulsions. In certain cases they can have a quite uniform drop size distribution and only a small content of surfactant. An interesting application of this emulsion type is the encapsulation of active substances after a polymerisation step [25, 26]. 

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. 

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. 

Figure 12. Effect of salinity on the phase separation or coalescence rate of hand-shaken and sonicated macroemulsions.

Figure 14. A correlation between the apparent viscosity and coalescence rate of sonicated macroemulsions.

The term stability, when applied to macroemulsions used for practical applications, usually refers to the resistence of emulsions to the coalescence of their dispersed droplets. The mere rising or settling of the droplets (creaming) because of a difference in density between them and the continuous phase is usually not considered instability. Flocculation or coagulation of the dispersed particles, without coalescence of the liquid interior of the particles, although a form of instability, is not considered as serious a sign of instability as coalescence or breaking of the... 

The rate of coalescence of the droplets in a macroemulsion is stated to be the only quantitative measure of its stability (Boyd, 1972). It can be measured by counting the number of droplets per unit volume of the emulsion as a function of time in a haemocytometer cell under a microscope (Sherman, 1968) or by means of a Coulter centrifugal photosedimentometer (Groves, 1964 Freshwater, 1966). 

The rate at which the droplets of a macroemulsion coalesce to form larger droplets and eventually break the emulsion has been found to depend on a number of factors (1) the physical nature of the interfacial film, (2) the existence of an electrical or steric barrier on the droplets, (3) the viscosity of the continuous phase, (4) the size distribution of the droplets, (5) the phase volume ratio, and (6) the temperature. 

Physical Nature of the Interfacial Film The droplets of dispersed liquid in an emulsion are in constant motion, and therefore there are frequent collisions between them. If, on collision, the interfacial film surrounding the two colliding droplets in a macroemulsion ruptures, the two droplets will coalesce to form a larger one, since this results in a decrease in the free energy of the system. If this process continues, the dispersed phase will separate from the emulsion, and it will break. The mechanical strength of the interfacial film is therefore one of the prime factors determining macroemulsion stability. 

Size Distribution of Droplets A factor influencing the rate of coalescence of the droplets is the size distribution. The smaller the range of sizes, the more stable the emulsion. Since larger particles have less interfacial surface per unit volume than smaller droplets, in macroemulsions they are thermodynamically more stable than the smaller droplets and tend to grow at the expense of the smaller ones. If this process continues, the emulsion eventually breaks. An emulsion with a fairly uniform size distribution is therefore more stable than one with the same average particle size having a wider distribution of sizes. 

A quantitative expression for the rate of coalescence of droplets in a macroemulsion, which includes most of the factors discussed previously, was developed by Davies and Rideal (1963), based on the von Smoluchowski (1916) theory of the coagulation of colloids. 

Baldauf LM, Schechter RS, Wade WH, Graciaa A. The relationship between surfactant phase behavior and the creaming and coalescence of macroemulsions. J Colloid Interface Sci 1982 85 187-197. 

In macroemulsions the DSD is determined by a droplet breakup-coalescence process. Breakup occurs only in the region very near the impeller, while coalescence occurs in the rest of the reactor, which is used to recirculate material back to the impeller [37]. Microemulsions, on the other hand, are thermodynamically (permanently) stable. Stability of miniemulsions lies in between those of macroemulsions and microemulsions. 

Figure 20.1 illustrates in a generalized fashion the relationship between the droplet size and the ernulsion stability for the three types of emulsions. Macroemulsions are the coarser of the three in that the droplet size is relatively large (1-100 pm) and the stability is limited to minutes. Phase separation is rapid unless the system is well mixed. Elroplets continuously collide and coalesce, and are broken by the shear exerted on the system. The droplet size is dependent on the system components (oil, stabilizer, phase ratio) and the mixing characteristics (impeller type and speed). Microemulsicms, on the other hand. 

The main difference between emulsions and microemulsions lies in the size and shape of the droplets of dispersed phase, which causes the differences in the thermodynamic stability of the two systems. Emulsions allow the drug to be administered as a dispersed oil solution and thus are kinetically stable but thermodynamically unstable. After storage or aging, droplets will coalesce and the two phases separate. Unlike emulsions, microemulsions are thermodynamically stable and phases do not separate on storage. Another important difference between the two systems is their appearance emulsions have a cloudy appearance, while microemulsions are transparent because of the lower dispersed phase size than macroemulsions. 

Emulsified systems can be classified aceording to their thermodynamie stability and their droplets size. Macroemulsions (or simply emulsions) are metastable systems, i.e., the system is not in thermodynamic equilibrium, and it will breakdown into two distinct phases if suffieient time is allowed. However, emulsions that keep their kinetic stability for periods of months or years ean be prepared by using appropriate components and amounts (McClements et al., 2007). This is the most common type of emulsion, and it is found in many food systems such as milk and salad dressing. Macroemulsions are usually polydisperse, with droplet sizes in the range of 1-100 pm. The main destabilization mechanisms in macroemulsions are droplets creaming, flocculation, and coalescence. 

Nevertheless, some physicochemical tricks such as the triggering of instability in oversaturated systems can produce a very fine dispersion of one phase in the other, i.e., a macroemulsion made up of two immiscible phases but with extremely fine droplets, e.g., in the 10 nm range. Such extremely fine macroemulsions, which could be called miniemulsions, are not thermodynamically stable, but they can exhibit a stable appearance because of the large entropy term (-TAS) contribution that decreases their free energy and because of the presence of retardation phenomena in the kinetics of coalescence. It is worth remarking that a considerable number of such miniemulsions are called microemulsions in the scientific and technical literature. In any case, such miniemulsions can be diluted with their external phase, as is done with cutting oil concentrates, without losing their stability. On the other hand, microemulsions cannot be diluted indefinitely. 

In distinction from macroemulsions, where the kinetic stability is the manifestation of droplet-droplet hydrodynamic interaction and droplet deformation, in miniemulsions the kinetic stability is the manifestation of the interplay between surface forces and Brownian movement (23). As the molecular forces of attraction decrease linearly with decreasing droplet dimension, namely, approximately 10 times at the transition from macroemulsions to miniemulsions, the potential minimum of droplet-droplet interaction (secondary minimum) decreases, and for miniemulsions this depth can be evaluated as 1—5 kT (12, 37). At this low energy. Brownian movement causes droplet doublet disaggregation after a short time (the doublet fragmentation time,7j). If this time is shorter than the lifetime of the thin film, rapid decrease in the total droplet concentration (t.d.c.) is prevented (restricted by the coalescence time, r ), i.e., stability is achieved due to this kinetic mechanism (23). 

A large disparity exists between knowledge concerning kinetic stability and thermodynamic stability. The main attention has been paid to kinetic stability for both macroemulsions (16-22) and miniemulsions (23-30). As a result, the droplet-droplet interaction and the collective processes in dilute emulsions are quantified (38, 39) and important experimental investigations are made (27, 28, 40). Some models are elaborated for the entire process of coalescence in concentrated emulsions as well (41, 42). Given thermodynamic stability, a thin interdroplet film can be metastable. 

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