We have studied aqueous mixtures of tetradecyldimethylaminoxid (C14DMAO) and calciumdodecyl sulfate Ca(DS)2. Surface tension measurements show a stronger synergism for these mixtures than for the combination of C14DMAO and sodiumdodecyl sulfate SDS. With an increasing mole fraction of dodecyl sulfate XDS, for 100 mM solutions one observes a viscoelastic L1-phase, a birefringent Lα-phase, and a viscous L1-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 Lα-phase is composed of densely packed multilamellar vesicles. For concentrations above 100 mM, the Lα-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 ∼800 Å 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.

An isotropic, clear phase at high alcohol content is found in a system composed of calcium dodecyl sulfate, water, and an alcohol of medium chain length, like pentanol, hexanol, heptanol, or octanol. This phase is located in that region of the phase diagram in which L3 phases are classically formed. Most ionic surfactants, however, only form sponge phases when the ionic charge of the surfactant molecules is sufficiently shielded by excess salt. At 5% surfactant and octanol as cosurfactant, the L3 phase disappears if more than 5% of the calcium ions are substituted by sodium. Calcium dodecyl sulfate, in some aspects, behaves like a nonionic or double-chain surfactant. The phase discussed in this article is completely transparent, has low viscosity, and exhibits neither birefringence nor yield stress. Viscosity, electrical conductivity, and freeze-fracture electron microscopy indicate the sponge structure of the phase. In contrast to typical L3 phases, small-angle neutron-scattering experiments show a peak the position of which is nearly unshifted compared with that of the neighboring lamellar phase.

Calcium dodecyl sulfate (CDS) is insoluble both in water and in alcohols at room temperature. However, it can be dissolved up to high concentrations in mixtures of water and ethanol, propanol, or butanol. No liquid-crystalline phases are formed with ethanol and propanol at 25 °C but are observed with butanol at higher concentrations of CDS. With pentanol and hexanol, lamellar phases are observed, which can be swollen largely. Neutron scattering measurements of phases with a weight ratio CDS:hexanol ) 1:1 show stiff lamellae in the concentration range of 1.5-25 wt % CDS. The repeat distance grows linearly with dilution. The effective thickness for the hydrocarbon part of the lamellae decreases to some extent at high CDS concentrations.

The swelling behavior of calcium dodecyl sulfate (CDS)/water with heptanol and octanol was investigated at 25°C. Both systems form a condensed lamellar phase at a high content of surfactant and cosurfactant and a highly swollen lamellar phase at a low surfactant content. There is no single-phase channel between both lamellar regions. A dilute L3 phase exists in the heptanol system as well as in the octanol system. This L3 phase is water-like transparent and shows no streaming birefringence. On dilution of samples with an appropriate composition of water, iridescent phases can be obtained which display bright blue to green colors. In contrast to CDS with pentanol or hexanol, the lamellar region is broken into a dilute phase and a condensed phase because of decreasing undulation forces of systems containing longer chain alcohols.

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