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 (gamma-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) much greater than 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.