Microemulsions system types

As described in the introduction, certain cosurfactants appear able to drive percolation transitions. Variations in the cosurfactant chemical potential, RT n (where is cosurfactant concentration or activity), holding other compositional features constant, provide the driving force for these percolation transitions. A water, toluene, and AOT microemulsion system using acrylamide as cosurfactant exhibited percolation type behavior for a variety of redox electron-transfer processes. The corresponding low-frequency electrical conductivity data for such a system is illustrated in Fig. 8, where the water, toluene, and AOT mole ratio (11.2 19.2 1.00) is held approximately constant, and the acrylamide concentration, is varied from 0 to 6% (w/w). At about = 1.2%, the arrow labeled in Fig. 8 indicates the onset of percolation in electrical conductivity. 

T. Sottmann and R. Strey Shape Similarities of Ultra-Low Interfacial Tension Curves in Ternary Microemulsion Systems of the Water-AUcane-CiEj Type. Ber. Bunsenges Phys. Chem. 100, 237 (1996). 

In this paper, a molecular thermodynamic approach is developed to predict the structural and compositional characteristics of microemulsions. The theory can be applied not only to oil-in-water and water-in-cil droplet-type microemulsions but also to bicontinuous microemulsions. This treatment constitutes an extension of our earlier approaches to micelles, mixed micelles, and solubilization but also takes into account the self-association of alcohol in the oil phase and the excluded-volume interactions among the droplets. Illustrative results are presented for an anionic surfactant (SDS) pentanol cyclohexane water NaCl system. Microstructur al features including the droplet radius, the thickness of the surfactant layer at the interface, the number of molecules of various species in a droplet, the size and composition dispersions of the droplets, and the distribution of the surfactant, oil, alcohol, and water molecules in the various microdomains are calculated. Further, the model allows the identification of the transition from a two-phase droplet-type microemulsion system to a three-phase microemulsion system involving a bicontinuous microemulsion. The persistence length of the bicontinuous microemulsion is also predicted by the model. Finally, the model permits the calculation of the interfacial tension between a microemulsion and the coexisting phase. 

The amounts of DBSA used were also found to affect the equilibrium position (Table 13.6). Each equilibrium position was confirmed by conducting both esterification of the carboxylic acid with the alcohol and hydrolysis of the ester. Table 13.6 clearly shows that increase of the amount of DBSA resulted in decrease of the yield of the ester at the equilibrium position. This result may be attributable to the size difference of the emulsion droplets that were formed by the hydrophobic substrates and the surfactant in water. As the amount of the surfactant-type catalyst increases, the size of each droplet may decrease, because the emulsion system may become a microemulsion system where the substrates are solubilized in water by a large amount of the surfactant. In fact, while 10 mol% DBSA gave the white turbid mixture, the reaction mixture was almost clear in the presence of 200 mol% DBSA, indicating that the size of the droplets became smaller. The smaller the droplets, the larger the sum of surface area of the droplets. As a result. 

Polymerization in microemulsion systems has recently gained some attention as a consequence of the numerous studies on microemulsions developed after the 1974 energy crisis (1,2). This new type of polymerization can be considered an extension of the well-known emulsion polymerization process (3). Hicroemulsions are thermodynamically stable and transparent colloidal dispersions, which have the capacity to solubilize large amounts of oil and water. Depending on the different components concentration, microemulsions can adopt various labile structural organizations -globular (w/o or o/w tyne), bicontinuous or even lamellar -Polymerization of monomers has been achieved in these different media (4-18),... 

Leong and Candau (18) obtained inverse latices of small size (<50nm) via photopolymerization of acrylamide in a microemulsion system of acrylamide, water, toluene and Aerosol OT. They observed that rapid polymerization and total conversion was achieved in less than 30 minutes. The microemulsions remained transparent and stable during polymerization. Candau et al. (19) also reported the results of a kinetic study of the polymerization of acrylamide in inverse microemulsions. Both oil soluble AIBN and water soluble potassium persulfate initiators were used. The rate was found to depend on the type of initiator, but in both cases neither autoacceleration nor dependence on initiator concentration was observed. An excellent review of microemulsion polymerization was published recently by Candau (20). 

Polymerization of styrene in each of the three types of microemulsions was performed using a water soluble initiator, potassium persulfate (K2S208), as well as an oil-soluble initiator, AIBN. As desired, solid polymeric materials were obtained instead of latex particles. In the anionic system, the cosolvent 2-pentanol or butyl cellosolve separates out during polymerization. Three phases are always obtmned after polymerization. The solid polymer was obtained in the middle with excess phases at the top and bottom. GC analysis of the upper phase indicates more than 80% 2-pentanol, while Karl-Fisher analysis indicated more than 94% water in the lower phase. Some of the initial microemulsion systems have either an excess organic phase on top or an excess water phase as the bottom layer. GC analysis showed the organic phase to be rich in 2-pentanol. However, the volume of the excess phase is much less in the initial system than in the polymerized system. 

Microemulsions. The structure of microemulsion systems has been reviewed (22). Both bicontinuous and droplet-type structures, among others, can occur in microemulsions. The droplet-type structure is conceptually more simple and is an extension of the emulsion structure that occurs at relatively high values of IFT. In this case, very small thermodynamically stable droplets occur, typically smaller than 10 nm (7). Each droplet is separated from the continuous phase by a monolayer of surfactant. Bicontinuous microemulsions are those in which oil and water layers in the microemulsion may be only a few molecules thick, separated by a monolayer of surfactant. Each layer may extend over a macroscopic distance, with many layers making up the microemulsion. 

Double Layer Interactions and Interfacial Charge. Schulman et al (42) have proposed that the phase continuity can be controlled readily by interfacial charge. If the concentration of the counterions for the ionic surfactant is higher and the diffuse electrical double layer at the interface is compressed, water-in-oil microemulsions are formed. If the concentration of the counterions is sufficiently decreased to produce a charge at the oil-water interface, the system presumably inverts to an oil-in-water type microemulsion. It was also proposed that for the droplets of spherical shape, the resulting microemulsions are isotropic and exhibit Newtonian flow behavior with one diffused band in X-ray diffraction pattern. Moreover, for droplets of cylindrical shape, the resulting microemulsions are optically anisotropic and non-Newtonian flow behavior with two di-fused bands in X-ray diffraction (9). The concept of molecular interactions at the oil-water interface for the formation of microemulsions was further extended by Prince (49). Prince (50) also discussed the differences in solubilization in micellar and microemulsion systems. 

Both experimental approaches are supposed suitable to a) charade terize the microemulsion in the two different states liquid and solid b) identify the presence of a "free water" fraction in microemulsion systems and the concentration at which the latter becomes detect able c) distinguish between different types of w/o dispersions depending on whether they possess a free water content. 

Hence, the exiplex has a sandwich structure which promotes efficient back e transfer at the water pool, and the ion yield is very small. However, a sandwich reactant pair of this sort is not formed on a micelle surface and back reaction is slower than the escape of the cation from the surface. Hie swollen micelle and microemulsion systems lead to both randomly organised ionic products and sandwich pairs, to varying extents, which are reflected in the observed yield of ions, with polar derivatives of pyrene, e.g. pyrene sulfonic acid, etc., the reactants are kept on the assembly surface where reaction occurs, giving rise to ions from a non-sandwiched type of configuration. In the reverse micellar system, these ions although they are formed, nevertheless have a short lifetime, as they cannot escape to any great distance in the small water pool. Huts, micelles are far superior to microemulsions in various aspects of... 

The phase behavior of microemulsions is complex and depends on a number of parameters, including the types and concentrations of surfactants, cosolvents, hydrocarbons, brine salinity, temperature, and to a much lesser degree, pressure. There is no universal equation of state even for a simple microemulsion. Therefore, phase behavior for a particular microemulsion system has to be measured experimentally. The phase behavior of microemulsions is typically presented using a ternary diagram and empirical correlations such as Hand s rule. 

The argument to use a two-phase model to represent surfactant phase behavior without type III microemulsion is that experiments (Seethepalli et al., 2004 Zhang et al., 2006 Liu et al., 2008) indicate that the volume of type III microemulsion phase is small if the overall surfactant concentration is low (<0.1 wt.%). In the cases of low surfactant concentration, a type III microemulsion system was not observed by Salager et al. (1979b). The reason is that if we cannot make a sufficient number of salinity scans, and the volume of the type III microemulsion phase is small, the equilibrium phase behavior... 

A microemulsion can exist in three types of systems—type II(-), type III, or type II(+)—depending on salinity. Below a certain salinity Cs i, the system is type II(-). Above a certain salinity Cse , the system is type II(+). If the salinity is between Csei and Cseu, the system is type III. In a type III system, the interfacial tension (HT) of microemulsion/brine is lower than that in a type II(-i-) system, and the IFT of microemulsion/oil is lower than that in a type II(-) system. Thus, both IFTs are collectively low. At optimum salinity, which is defined as the middle of Cjei and Cse , the two IFTs are equal. IFT is a very important parameter, with a lower value resulting in a higher capillary number (Nc). A higher capillary number will lead to lower residual oil saturation, thus higher oil recovery. Therefore, optimum salinity seems to be an obvious choice. Another area of contention is whether a type II(-) or type II(-i-) system is better for oil recovery (Larson, 1979). 

From the previons sensitivity resnlts and discnssions, we can see that phase type is very important in determining the hnal oil recovery. Table 8.16 lists some advantages and disadvantages of three types of microemulsion systems. The highest oil recovery conld be from a type II(-), type III, or type II(+) system. Not only IFT, bnt many parameters, especially relative permeabilities, individnally or in combination, may make any of type II(-), type HI, and type II(+) microemnlsion systems the optimnm type. This is different from the conventional approach that focnses on interfacial tension as the determining parameter and conseqnently that the optimum phase type is, necessarily, type III. 

The main controlling parameters are relative permeabihty curves and types of microemulsion systems. Relative permeability curves control the multiphase flow, and the types of microemulsion systems dictate which relative permeability curves are sensitive. 

As oil saturation is decreased (water saturation is increased), the acid content in the oil is decreased. Consequently, the soap molar fraction X oap is decreased, as Table 12.6 shows. As X oap is decreased, type 111 salinity limits are closer to those of surfactant. Thus, the limits are increased, and the optimum salinity is increased as well. The system is changed from type III to type I. This transition from type III to type I is exactly the salinity gradient we need. In practical alkaline-surfactant flooding, water saturation will be increased from the flood front to the upstream, and the microemulsion system will change from type III... 

NMR measurements could be used to distinguish clearly between the two types of microemulsion system. 

The above-mentioned reaction between sodium phenoxide and 1-bromooctane to synthesise 1 -phenoxyoctane has been carried out in different types of microemulsion systems, all based on the same non-ionic surfactant, Triton X-100 (an octylphenol ethoxylate), the same surfactant concentration (20 wt.%), the same oil to water ratio (2 3) but different hydrocarbons as oil component [28]. This results in different phase volume ratios for the different hydrocarbons. A one-phase microemulsion is only obtained with toluene as oil component. The more hydrophobic oils, i.e. cumene, isooctane, hexadecane and paraffin oil, all give a microemulsion in equilibrium with an excess oil phase, i.e. a Winsor I system. With the more hydrophilic chlorobenzene as oil a microemulsion coexisting with an excess water phase, i.e. a Winsor II system, is obtained. As is also shown in Fig. 5.4, the reactivity is highest in the chlorobenzene- and the paraffin oil-based microemulsions, i.e. in the systems... 

Whereas Winsor III systems exhibit ultra-low interfacial tensions between the three phases and also very high solubilisation capacity, Winsor I systems have higher interfacial tensions and much lower solubilising power. At the transition between the two types of microemulsion systems, an intermediate behaviour can be found which is called supersolubilisation [47,70]. The uptake of oils into surfactant aggregates is usually enhanced by one to two orders of magnitude compared to effective micellar systems, but interfacial tension reduction is still moderate. The transition point can be adjusted by varying the salinity or organic components. 

Binks BP. Emulsion type below and above the cmc in AOT microemulsion systems. Colloids Surfaces A Physicochem Eng Aspects 1993 71 167-172. 

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