Microemulsion Formation and Phase Behavior of Anionic and Cationic Surfactants with Sodium Dodecyl Sulfate and Cetyltrimethylammonium Bromide.

The phase behavior and solubilization of multiphase microemulsions in mixed anionic-cationic surfactant systems were studied for fixed ratios of water-to-oil and surfactant-to-alcohol. In the mixed surfactants (sodium dodecyl sulfate + cetyltrimethylammonium bromide)/heptane/alcohol/water systems, microemulsion and birefringent phases are formed by adjusting the surfactant ratio epsilon and the cationic weight fraction delta. The bicontinuous (or w/o microemulsion) --> birefringent o/w microemulsion transition takes place and microemulsion domain enlarges with increasing epsilon. The optimum surfactant concentration gamma increases and the corresponding optimum delta decreases with increasing epsilon and both of them decrease with increasing the alcohol chain length from butanol to hexanol. The birefringent region shrinks rapidly with increasing alcohol and/or CTAB weight fractions in total surfactant concentration. Conductivity measurements have been performed in the single-phase region of the system containing mixed surfactants and alcohols at 25°C. The conductivity results indicate where a transition takes place and which of these different types of phase structures may be in the single-phase of the system containing anionic-cationic mixed surfactants.