Effect of added salt and poly(ethylene glycol) on the phase behavior of a balanced AOT-water-oil system

The phase behavior of AOT-water-NaCl-poly(ethylene glycol) (PEG; M-w = 20 000) - isooctane mixtures at equal volumes of water and oil has been systematically explored as a function of AOT, NaCl, and PEG concentrations. The effect of adding PEG is in many respects similar to that of adding salt although the changes are less pronounced. Each additive, taken separately, shifts the phase behavior to higher temperature, promotes the formation of bicontinuous microemulsions, and induces a lamellar-to-bicontinuous microemulsion phase transition. The simultaneous presence of both additives enhances the effect of salt. These phase behavior results suggest that the two variables salt and PEG act in the same direction on the spontaneous curvature C-0, the bending rigidity kappa, and the Gaussian bending modulus <(kappa)over bar> of the AOT monolayer.