Phase structure and rheological properties of a mixed zwitterionic/anionic surfactant system

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Abstract
The phase diagram of the mixed zwitterionic/anionic surfactant system hexadecyldimethyl-ammoniopropanesulphonate/sodium dodecyl sulphate/water has been determined in the aqueous region (90.0 % water) by optical microscopy and low angle X-ray scattering; the phases observed were an isotropic surfactant solution and a hexagonal liquid crystalline phase. Some aqueous solutions were found to be viscoelastic and the composition boundaries of solutions with these properties were parallel to the phase boundaries. N.m.r. was used additionally to study the structure of the viscoelastic solutions and the results are interpreted using a model which involves the existence of both normal spherical micelles and cylindrical micelles in equilibrium.