CATANIONIC SURFACTANTS - PHASE-BEHAVIOR AND MICROEMULSIONS

The phase equilibria of aqueous systems of catanionic (i.e. containing both a cationic and an anionic surfactant) amphiphiles were studied by classical and NMR techniques. Both binary amphiphile-water systems and ternary systems, with the third component being either a nonionic amphiphile or a hydrocarbon, were studied. In addition the effect of excess of one of the ionic surfactants was investigated. Microemulsions were studied with respect to microstructure by molecular self-diffusion (Fourier transform pulsed-gradient spin-echo NMR). It appears that the catanionic surfactant forms layers that are spontaneously curved toward water at high temperatures. The ability to incorporate water between the amphiphilic bilayers or in the core of a reversed micelle is determined by a short range force of the same type as the "hydration force" in a lecithin-water systems. Only small amounts of water may be incorporated in this type of system. The surfactant can be balanced by addition of ionic surfactant. Thus on increasing the amount of ionic surfactant the microemulsion phase region extends toward higher water contents and the microemulsion structure changes from being of the water-in-oil droplet type to some type of bicontinous structure.