Coalescence and solubilization kinetics in linker-modified microemulsions and related systems

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Previously, we reported on formulating microemulsions with combined linker molecules. These linker molecules enhance the interaction of the dynamic surfactant membrane with water, in the case of hydrophilic linkers, or oil, in the case of lipophilic linkers, thereby yielding microemulsions with desirable properties. In this paper we evaluate the coalescence and solubilization kinetics of trichloroethylene emulsions and microemulsions using sodium dihexyl sulfosuccinate and different linker formulations. Sodium mono- and dimethylnapthalenesulfonate (SMDNS) was used as the hydrophilic linker, and dodecanol was used as the lipophilic linker. The interfacial properties (interfacial thickness/tension/rigidity) of these linker-based microemulsions were also studied. The turbidity curves of optimum middle phase microemulsions are fitted with a second-order kinetic equation, with the coalescence activation energy being a function of the interfacial rigidity of the systems. It was found that the addition of lipophilic linkers tends to increase the interfacial thickness and the interfacial rigidity and tends to decrease the coalescence rate. In contrast, hydrophilic linkers showed the opposite effect to lipophilic linkers. A combination of both linkers shows an intermediate effect. Finally, the solubilization of TCE in surfactant solution and in surfactant and hydrophilic linker is tracked turbidity, with the SMDNS-based formulation showing a faster solubilization rate than the surfactant alone formulation.

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