Solute Effect on Connectivity of Water-in-Oil Microemulsions

The effect on microstructure of solubilizing a solute in a ternary microemulsion has been examined. We used the water−didodecyldimethylammonium bromide (DDAB)−dodecane ternary solution as a model for microemulsions with a rigid interfacial film. The apolar model solutes chosen are lindane, which is an organochloride pesticide (γ-isomer of hexachlorocyclohexane), and phenol as a semipolar molecule. The curvature variation of the surfactant film induced by the solute has been determined using conductivity and small-angle X-ray scattering experiments. This curvature effect is rationalized by means of the DOC cylinders model, combining fixed area per molecule, volume fraction, a strongly preferred spontaneous curvature constraint, and a low flexibility of the interfacial film ( k c ≫ k B T ). Scattering and conductivity properties examined together demonstrate that the shift in the surfactant packing parameter induced by solubilization corresponds to an increase of surfactant layer volume combined with a slight decrease of the area per headgroup.