SURFACTANT-OIL-WATER SYSTEMS NEAR THE AFFINITY INVERSION .9. OPTIMUM FORMULATION AND PHASE-BEHAVIOR OF MIXED ANIONIC-CATIONIC SYSTEMS

Mixtures of anionic (SDS) and cationic (TTAB) surfactants exhibit a ph ase behavior which may be interpreted as a mixture of the equimolecula r compound with the surfactant which is in excess. The optimum formula tion for three-phase behavior of anionic-cationic mixture-oil-brine-al cohol systems follows a linear mixing rule up to 25 % of any of the su rfactants. When both surfactants are in the concentration mid-range (2 5 to 75%), the equimolecular compound precipitates as a bilayer crysta lline phase, which is insoluble in either oil, water or microemulsion, and may coexist with them at optimum. A mixing rule model based on th e linear combination of the equimolecular compound and the excess surf actant is proposed to interpret the results in the regions where no pr ecipitation occurs, i.e., at less than 25% of any of the surfactant.