Critical microemulsion concentration

Recently, the phase equilibria of a microemulsion were reported. The phase behavior of a microemulsion formed with food-grade surfactant sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) was studied. Critical microemulsion concentration (cpc) was deduced from the dependence of the pressure of cloud points on the concentration of... 

PS- - poly(l,l-dihydroperfluorooctylacrylate) (FS-b-FFOA) by interfacial tension measurements and these micelles were also observed with neutron scattering[65] and dynamic light scattering measurements[10]. On the basis of interfacial tension measurements at the water-C02 interface, a critical microemulsion concentration was... 

In a recent report, the phase equilibria of a micromnulsion were reported (Birdi, 2010a). Phase behavior of a microemulsion formed with food grade surfactant sodium bis-(2-ethylhexyl) sulfosuc-cinate (AOT) was studied. Critical microemulsion concentration was deduced from the dependence of pressure on CP on the concentration of surfactant AOT at constant temperatme and water concentration. The results show that there are transition points on the CP curve in a very narrow range of concentration of surfactant AOT. The transition points were changed with the temperature and water concentration. These phenomena show that lower temperatme is suitable to forming microemulsion droplet and the microemulsion with high water concentration is likely to absorb more surfactants to structure the interface. 

To formulate a microemulsion, the interfacial tension (Y/) should be ultralow (<1 nnJ/m ) (Upadhyaya et al. 2006) and the interaction between the two phases (y ) must be high. Therefore, a certain amount of surfactant is needed to increase the interaction between the two phases to a level where a microemulsion is formed spontaneously. This concentration is called critical microemulsion concentration (cpc), which is the minimum surfactant concentration required to formulate a microemulsion (Aveyard et al. 1989). 

The surfactant concentration at the head of the fish, Yo, is a measure of the critical microemulsion concentration (cpc). At this surfactant concentration, the middle microemulsion phase forms first. Below the head of the fish (below yo), Or systems are observed where the surfactant is dissolved as monomers in the oil and water phases, and no mixing of oil and water is found. At yo, the three-phase region first appears indicating the formation of a middle microemulsion phase with an internal interface of surfactant separating oil and water microdomains. At yo, the concentration of surfactant in the excess water phase is identical to the critical micelle concentration (/water = cmc) (28). Thus, the amount of surfactant in the excess oil phase (/oil) is the overall surfactant concentration minus that dissolved... 

Figure 4.5. Schematic phase diagram of equal amounts of oil and water (a = 0.5) as a function of ethoxylated alcohol surfactant concentration (/) and temperature (T) yo denotes the critical microemulsion concentration , i.e. the surfactant concentration where the middle microemulsion phase first appears. In addition, T and Tu denote the temperature range of the three-phase region, while X, the point at which the tail and the body of the fisji meet, denotes the temperature and surfactant coordinates (T, y) for the most efficient formation of single microemulsion phases. The test tubes show the types of phase behaviour found in the various regions of the phase diagram. Reproduced by permission of the American Chemical Society (redrawn from Kahlweit and Strey (46))...

For enhanced oil recovery (EOR) and environmental remediation, an important property of any surfactant is its critical microemulsion concentration, or CfiC, because this is the minimum surfactant concentration needed to achieve ultralow interfacial tensions (<0.1 mN/m) [37, 38]. Recently, it has been determined that for rhamnolipids with a CMC of 10 mg/1, the C/rC is close to 100 mg/1 [39], which is approximately 10 times lower than the C/rC of anionic surfactants [38]. The study of the C/rC for rhamnolipids and other biosurfactants obtained from waste sources has not been performed yet. The work of Nguyen et al. [39] on the formulation of microemulsions with rhamnolipids also suggests that they are relatively hydrophilic (i.e. tend to form micelles but not reverse micelles), and that it is best to use them in combination with other surfactants. 

The issue of water in reverse micellar cores is important because water swollen reverse micelles (reverse microemulsions) provide means for carrying almost any water-soluble component into a predominantly oil-continuous solution (see discussions of microemulsions and micellar catalysis below). In tire absence of water it appears tliat premicellar aggregates (pairs, trimers etc.) are commonly found in surfactant-in-oil solutions [47]. Critical micelle concentrations do exist (witli some exceptions). 

The temperature (or salinity) at which optimal temperature (or optimal salinity), because at that temperature (or salinity) the oil—water interfacial tension is a minimum, which is optimum for oil recovery. For historical reasons, the optimal temperature is also known as the HLB (hydrophilic—lipophilic balance) temperature (42,43) or phase inversion temperature (PIT) (44). For most systems, all three tensions are very low for Tlc < T < Tuc, and the tensions of the middle-phase microemulsion with the other two phases can be in the range 10 5—10 7 N/m. These values are about three orders of magnitude smaller than the interfacial tensions produced by nonmicroemulsion surfactant solutions near the critical micelle concentration. Indeed, it is this huge reduction of interfacial tension which makes micellar-polymer EOR and its SEAR counterpart physically possible. 

Muller, N. Errors in micellization enthalpies from temperature dependence of critical micelle concentrations. In Micellization, solubilization, and microemulsions. Vol. l,p. 229, Mittal, K. L. (ed.). New York - London Plenum 1977... 

The fluorescent probe 4-aminophthalimide (63) was employed by several authors to study micelles. Samanta and coworkers187,188 used it to study micellization of common cationic (SDS), anionic (cetyltrimethylammonium bromide) and neutral (Triton-X 100) aqueous surfactants, and the same systems were also studied by Datta, Mandal and coworkers189 19°. The critical micelle concentration could be determined and it was found that the probe binds to the micelle water interface or to the cyclodextrin cavities that have also been studied187,188. Water-in-oil microemulsions of Triton-X 100 in a mixture of benzene and hexane showed190 the probe to reside in the water core of the reversed micelles, the polarity of which differs much from that of bulk water. 

Heterophase processes should be primarily distinguished based on their emulsion structure (oil-in-water or water-in-oil) and type of stability (kinetic or thermodynamic). This identifies four mutually independent polymerization regimes, each with unique colloidal and chemical behavior. L Macroemulsion, II. Inverse-Macroemulsion, HI. Microemulsion, IV. Inverse-Microemulsion. The macroemulsion and inverse-macroemulsion domains can be further subdivided into Suspension (la), Emulsion (lb), Inverse-suspension (Ha) and Inverse-emulsion (lib) subdomains based on a transition at the critical micelle concentration. 

Formation of microemulsion was carried out using a small quantity of ionic surfactant, CTAB (cetyltrimethylammonium bromide), and de-ionized water in excess dried toluene. Above critical micelle concentration (cmc) these three components form reversed micelles. As the size of such micelle system is related to the ratio of water/surfactant (W) added, tailored amount of water to surfactant ratios in order to control the sizes of the nano-composites were investigated. 

Table 1. Aqueous Phase Critical Micelle Concentrations (erne s), Limiting Surface Tensions yeme s and Microemulsion Stability Pressures for Fluorinated Surfactants.

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