Novel vesicles from single-chain surfactants

We have studied aqueous mixtures of tetradecyldimethylaminoxid (Cl(4)DMAO) and calciumdodecyl sulfate Ca(DS)(2). Surface tension measurements show a stronger synergism for these mixtures than for the combination of C(14)DMAO and sodiumdodecyl sulfate SDS. With an increasing mole fraction of dodecyl sulfate X-DS, for 100 mM solutions one observes a viscoelastic L-alpha-phase, a birefringent L-alpha-phase, and a viscous L-1-phase on the dodecyl side. The maximum of the viscosity in the first L1-phase occurs around X-DS = 0.28. At this composition, the viscosity is more than 4 orders of magnitude higher than the viscosity of the single-component solutions. Rheological measurements show that the birefringent L-alpha-phase is composed of densely packed multilamellar vesicles. For concentrations above 100 mM, the L-alpha-phase has a yield stress value that is large enough to suspend small dispersed air bubbles. SANS measurements show that the interlamellar spacing between the bilayers at 100 mM is similar to 800 Angstrom and is independent of X-DS within the existence region of the liquid crystalline phase. To our knowledge, this is the first time that a vesicle phase from combination of a single-chain zwitterionic surfactant and a Ca salt of a single-chain anionic surfactant has been reported. The large synergism between the two surfactants is likely to be the result of strong binding of Ca ions to two dodecyl sulfate ions. The Ca ions act as links for two ionic surfactants, and by doing so, the Ca(DS)(2) behaves like a double-chain surfactant, In addition, it is conceivable that the Ca ions coordinate to the polar oxygen of the aminoxide.