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ABSTRACT
Mixtures of anionic (SDS) and cationic (TTAB) surfactants exhibit a phase behavior which may be interpreted as a mixture of the equimolecular compound with the surfactant which is in excess.
The optimum formulation for three-phase behavior of anionic-cationic mixture-oil-brine-alcohol systems follows a linear mixing rule up to 25 % of any of the surfactants.
When both surfactants are in the concentration mid-range (25 to 75%), the equimolecular compound precipitates as a bilayer crystalline phase, which is insoluble in either oil, water or microemulsion, and may coexist with them at optimum.
A mixing rule model based on the linear combination of the equimolecular compound and the excess surfactant is proposed to interpret the results in the regions where no precipitation occurs, i.e., at less than 25% of any of the surfactant.