Microemulsions droplet coalescence

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 term microemulsion, which implies a close relationship to ordinary emulsions, is misleading because the microemulsion state embraces a number of different microstructures, most of which have little in common with ordinary emulsions. Although microemulsions may be composed of dispersed droplets of either oil or water, it is now accepted that they are essentially stable, single-phase swollen micellar solutions rather than unstable two-phase dispersions. Microemulsions are readily distinguished from normal emulsions by their transparency, their low viscosity, and more fundamentally their thermodynamic stability and ability to form spontaneously. The dividing line, however, between the size of a swollen micelle ( 10-140 nm) and a fine emulsion droplet ( 100-600 nm) is not well defined, although microemulsions are very labile systems and a microemulsion droplet may disappear within a fraction of a second whilst another droplet forms spontaneously elsewhere in the system. In contrast, ordinary emulsion droplets, however small, exist as individual entities until coalescence or Ostwald ripening occurs. 

The most reasonable explanation for the increase in apparent hydrodynamic diameter measured by DLS is the enhanced micelle-micelle interactions as the boundary of a two-phase system is approached (i.e., the pressure is lowered). Figure 4 illustrates this concept of micelle-micelle interactions, which is manifested as aggregation (or clustering) of the reverse micelle or microemulsion droplets. Since the solvent environment is essentially unchanged by this "macromolecular aggregation" (Ui) we exclude the possibility of (other than transitory) micelle-micelle coalescence to form stable, larger micelles. The micelles may coalesce briefly to form transitional species (which might be a "dumbbell" or more cylindrical structures), in which the water cores collide and intermix. 

Garcia et al. [98] conducted coulometric initiation of acrylamide polymerization in oil-continuous AOT-toluene-water microemulsions using platinum/Nafion solid polymer electrodes (SPEs). The SPE served to separate the microemulsion from an aqueous electrolyte phase. Polymerization was initiated at room temperature by constant-potential electrolytic reduction of potassium persulfate initiator solubilized in the microemulsion droplets. Acrylamide monomer behaved as a cosurfactant and was required for the redox process. Latex particles and solid polyacrylamide were obtained. The kinetics of electroinitiated polymerization was slower than observed with UV or thermal initiation. Latex stability results suggest that coalescence is the primary mechanism for particle growth. 

Table 1 shows the effect of the addition of isobutanol on various properties of oil/brine/surfactant systems for TRS 10-410 and TRS 10-80. Because the same IFT values were obtained for the systems with and without IBA (Table 1), the observed differences in oil recovery cannot be explained in terms of any change in IFT. The presence of alcohol did not significantly influence the partition coefficient of surfactant in n-dodecane or n-octane. It is important to emphasize that the partition coefficient changes sharply near the ultralow IFT region (19). Thus, the partition coefficient does not appear to correlate with the oil displacement efficiency. However, the presence of isobutanol decreases the interfacial viscosity and markedly influences the flattening time of the oil droplets. It has been suggested (18) that a rigid potassium oleate film at the oil/water interface can be liquefied by the penetration of the hexanol molecules in order to produce spherical microemulsion droplets. It has been shown (14) also that for a commercial petroleum sulfonate-crude oil system, the oil droplets with the alcohol coalesce much faster than the ones without alcohol. For the systems studied here, IBA is believed to have penetrated the petroleum sulfonate film as seen by the decrease in IFV. The decrease in interfacial viscosity would presumably promote the coalescence in porous media. 

It is evident from the DLS measurements that the microemulsions are monodis-perse and the size of aggregates in all the microemulsions decreases with increasing temperature. This indicates the noninteracting hard sphere nature of the aggregates in RTILs/[C mim][AOT]/benzene microemulsions at R = 1.0 without droplet coalescing [62, 92, 96]. The microemulsions retain their structural integrity aeross the temperature range used in the study. The temperature effect on the... 

Figure 6.4. A schematic model for the nucleation and growth of latex particles in the acrylamide microemulsion polymerization stabilized by sodium bis(2-ethylhexyl)sulfosuccinate. (I) The initial condition of the polymerization system consists of a very large population of the acrylamide/ water-swollen micelles ( 6nm in diameter) dispersed in the continuous oily phase. Nucleation of particle nuclei occurs when free radicals are absorbed by the microemulsion droplets. (II) Growth of latex particles are achieved by (a) collision and then coalescence between two particles and (b) diffusion of monomer molecules from the microemulsion droplets through the continuous oily phase and then into the particles, (c) The polymerization system comprises water-swollen polyacrylamide particles ( 40nm in diameter) and acrylamide/water-swollen micelles ( 3nm in diameter) dispersed in the continuous oily phase at the end of polymerization [81].

Hamaker. The contribution of the surface extension energy and/or bending elasticity to the pair interaction potential is also included. The extension of the drop surface upon the deformation corresponds to a soft interdroplet repulsion. All the remaining possible interactions (electrostatic, steric, depletion, etc.) can usually be treated in the framework of Deqaguin s approximation, which allows one to account for the two contributions of the total interaction energy (i) across the flat film and (ii) between the spherical surfaces surrounding the fllm. " Combined with relevant expressions for the hydrodynamic interactions, this approach could be used for studying the coalescence of Brownian emulsion and microemulsion droplets. ... 

Water-in-oil microemulsions (w/o-MEs), also known as reverse micelles, provide what appears to be a very unique and well-suited medium for solubilizing proteins, amino acids, and other biological molecules in a nonpolar medium. The medium consists of small aqueous-polar nanodroplets dispersed in an apolar bulk phase by surfactants (Fig. 1). Moreover, the droplet size is on the same order of magnitude as the encapsulated enzyme molecules. Typically, the medium is quite dynamic, with droplets spontaneously coalescing, exchanging materials, and reforming on the order of microseconds. Such small droplets yield a large amount of interfacial area. For many surfactants, the size of the dispersed aqueous nanodroplets is directly proportional to the water-surfactant mole ratio, also known as w. Several reviews have been written which provide more detailed discussion of the physical properties of microemulsions [1-3]. 

Microemulsions are dynamic systems in which droplets continually collide, coalesce, and reform in the nanosecond to millisecond time scale. These droplet interactions result in a continuous exchange of solubilizates. The composition of the microemulsion phase determines the exchange rate through its effect on the elasticity of the surfactant film surrounding the aqueous microdomains. Compared with nonionic surfactant-based microemulsions, AOT reverse micelles have a more rigid... 

Oil and water do not mix, but on addition of a suitable surfaetant a microemulsion can be formed depending on the relative concentrations of the three components. Microemulsions (i.e. surfactant/water/oil mixtures) can also be used as reaction media see references [859-862] for reviews. Microemulsions are isotropic and optically clear, thermodynamically stable, macroscopically homogeneous, but microscopically heterogeneous dispersions of oil-in-water (O/W) or water-in-oil (W/O), where oil is usually a hydrocarbon. The name microemulsion, introduced by Schulman et al. in 1959 [863], derives from the fact that oil droplets in O/W systems or water droplets in W/O systems are very small (ca. 10... 100 nm nanodroplets). Unlike conventional emulsions, microemulsion domains fluctuate in size and shape with spontaneous coalescence and breakup. The oil/water interface is covered with surfactant molecules and this area can amount to as much as 10 m per litre ( ) of microemulsion. 

For reactions in inverse microemulsions that involve the total confinement of the reactant species within the dispersed water droplets, the exchange of reactants by the coalescence of the two droplets take place prior to their chemical reaction. The chemical reaction produces an (almost) insoluble product. The reaction medium is first saturated with this product. When the saturation exceeds a critical limit, nucleation occurs. Then the nuclei start to grow rapidly and consume the reaction product leading to a decline in the supersaturation. As soon as the supersaturation falls below the critical level, no further nucleation occurs, so only the existing particles grow beyond this point. If the time period of nucleation is short in comparision to the growth period, rather monodisperse particles are obtained. 

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 type I microemulsion, the emulsified oil droplets are carried forward and are coalesced with the oil ahead to form an oil bank. In type II microemulsion, it is easy for the external oil to merge with residual oil to form an oil bank. 

Stabilization of emulsions by powders can be viewed as a simple example of structural- mechanical barrier, which is a strong factor of stabilization of colloid dispersions (see Chapter VIII, 5). The stabilization of relatively large droplets by microemulsions, which can be formed upon the transfer of surfactant molecules through the interface with low a (Fig. VII-10), is a phenomenon of similar nature. The surfactant adsorption layers, especially those of surface active polymers, are also capable of generating strong structural mechanical barrier at interfaces in emulsions. Many natural polymers, such as gelatin, proteins, saccharides and their derivatives, are all effective emulsifiers for direct emulsions. It was shown by Izmailova et al [49-52]. that the gel-alike structured layer that is formed by these substances at the surface of droplets may completely prevent coalescence of emulsion drops. 

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