Abstract
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 ϵ and the cationic weight fraction δ. The bicontinuous (or w/o microemulsion) → birefringent o/w microemulsion transition takes place and microemulsion domain enlarges with increasing ϵ. The optimum surfactant concentration γ increases and the corresponding optimum δ decreases with increasing ϵ 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.
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