Surfactant Phases with Bilayer Structures and Their Rheological Properties

Abstract

The ternary phase diagram Dodecyldimethylaminoxide/Hexanol/Water is reported for low volume fractions of surfactant. Besides the well known L1 Lα and L3 phases, three more single phase regions are observed in the phase diagram and called L1*, L3* and Lαl. The L1* and the L3*-phases are optically isotropic, slightly turbid and the Lαal-phase is birefringent and highly viscous. With increasing cosurfactant concentration, the sequence of the phases is L1 L1*, Lαal, L3*, Lαh and L3. The last five phases have all a bilayer type structure. The L1* and the L3* phase consist of single and multilamellar vesicles. The L3*-phase disappears at higher surfactant concentration and the two Lαa- phases border then on each other but seem to be separated by a two phase region. At low volume fractions of about one % both Lα-phases show iridescent colours which are due to interference of white light from the ordered bilayers. The colours of the Lαh-phase are much brighter than the colours of the Lαl- phase. The bilayers of the two Lα phases have probably a different thickness. The L1*, Lαl and the L3-phases are viscoelastic phases with very long structural relaxation times. For all three phases both, the storage and the loss moduli are about the same and independent of the oscillating frequency over several decades. The Lαh and the L3-phase have a low viscosity. For the L3-phase the zero shear viscosity is explained on the basis of a simple network model in which η0 is the product of the shear modulus GO and the orientation time τs of the bilayers which have a persistence length of the size of the interlamellar spacing of the Lαh-phase.KeywordsStorage ModulusLoss ModulusComplex ViscosityPersistence LengthInterlamellar SpacingThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.