In this paper, the effect of linker molecules on the solubilization capacity of an anionic surfactant system (sodium dihexyl sulfosuccinate) is studied. N-Alkyl alcohols are used as lipophilic linkers in middle-phase microemulsions of trichloroethylene, tetrachloroethylene, and hexane. The lipophilic linker effect increases the solubilization capacity of the anionic surfactant system. The solubilization parameter for both hydrocarbons and chlorinated hydrocarbon oil increases as a function of alcohol concentration. As the alkyl chain length of the alcohol linker molecule increases, the solubilization capacity increases. Moreover, the longer chain alcohol is more effective at linking oil molecules for hexane than for chlorinated hydrocarbon oils, indicating that the linker effect is more effective for higher equivilant alkane carbon number (EACN) oils. Sodium mono-and dimethylnaphthalenesulfonate is proposed as a hydrophilic linker to enhance the solubilization of lower EACN oils. The combination of lipophilic and hydrophilic linkers synergistically enhances the solubilization capacity of chlorinated hydrocarbon microemulsions.
In microemulsion formulations, linker molecules are additives that can enhance the surfactant-oil interaction (lipophilic linkers) or the surfactant-water interaction (hydrophilic linkers). In this paper, the role of the hydrophilic linker is elucidated through solubilization studies, interfacial tension studies, and by studying the partitioning of the hydrophilic linker into an optimum middle phase. This research used alkyl naphthalene sulfonates as the hydrophilic linkers, sodium dihexyl sulfosuccinate as the surfactant, and trichloroethylene as the oil phase. The hydrophilic linkers were found to have interfacial properties between a hydrotrope and a cosurfactant. More specifically, the data show that a hydrophilic linker is an amphiphile that coadsorbs with the surfactant at the oil/water interface but that has negligible interaction with the oil phase. The role of the hydrophilic linker can thus be interpreted as opening "holes" in the interface. Based on the characteristics of alkyl naphthalene linkers, carboxylic molecules were evaluated as hydrophilic linkers. For trichloroethylene microemulsions, sodium octanoate was found to be an alternative hydrophilic linker to sodium mono-and dimethyl naphthalene sulfonates.Paper no. S1269 in JSD 5, 151-157 (April 2002).Microemulsions are thermodynamically stable isotropic emulsions with nanometer-sized aggregates (1,2). A microemulsion can be oil solubilized in water (Winsor Type I), water in oil (Type II), or bicontinuously in oil and water (Winsor Type III middle phase). Microemulsions are characterized by ultralow interfacial tensions and a high capacity to dissolve oil and water into single clear phases. The transitions among the different types of microemulsions can be achieved under certain formulation conditions that change the hydrophilic/lipophilic balance (HLB) of the surfactant at the oil/water interface. More detailed descriptions of microemulsion systems can be found elsewhere (1-4). Figure 1 shows an example of the Winsor phase transitions discussed above, where salinity is used to produce the phase transition. By increasing salinity, the microemulsion phase changes from Type I to Type III (middle phase) to Type II. The point at which the same amount of oil and water are solubilized in Type III is known as the optimum formulation. At the optimum formulation, the interfacial tension between the excess oil and water reaches a minimal value. The solubilization and formulation results presented later will be for such optimum conditions. Microemulsion formulations often use additives such as hydrotropes, cosurfactants, cosolvents, and electrolytes to affect the HLB of the amphiphile at the interface. The spectrum of additives can range from very hydrophobic to very hydrophilic. Figure 2 illustrates the relative hydrophilic/ lipophilic character of different types of additives relative to their location at the oil/water interface. Graciaa et al. (5,6) introduced the lipophilic linker effect to characterize the role of long-chain alcohols in surfactant formu...
Water-in-Oil-type gel-emulsions (or highly concentrated emulsions) are spontaneously formed from oil-swollen micellar solutions or oil-in-water (O/W) microemulsions in a water/tetraoxyethylene dodecyl ether/oil system with an abrupt increase in temperature. The phase change occurs from water-continuous microemulsion to water-in-oil (W/O) gel-emulsion via a lamellar liquid crystal and a bicontinuous surfactant phase (L 3 phase). Hence, the spontaneous curvature of the surfactant layer is continuously changed with temperature change because the gel-emulsion consists of a reverse micellar solution and an excess water phase. In a narrow temperature range above the single L3-phase region, there is a two-phase region consisting of L3 and an excess water phase (W) in which emulsions are extremely unstable. The electroconductivity curve as a function of temperature monotonically decreases with increasing temperature when the final temperature is high and the temperature change is fast. If the temperature change is very slow, the electroconductivity has a maximum at the temperature where the L 3 + W region is found because excess water is separated. In this case, the water droplet size in the final concentrated emulsion is very large. Therefore, it is important to change the temperature quickly to form fine gel-emulsions.
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