Influence of salt concentration and surfactant concentration on the microstructure and rheology of lamellar liquid crystalline phases

The microstructures and linear rheological properties of onion phases prepa red from sodium dodecyl-benzenesulfonate, a C13-15 ethoxylated alcohol with on average 7 EO groups at a fixed weight ratio of 7:3, sodium citrate and water were investigated over a broad concentration range of surfactant and salt. At low salt concentration a lamellar phase in equilibrium with a mice llar phase is found. When more salt is added (9-15%), a colloidally stable, swollen onion phase is found. The repulsion between the surfactant bilayer s is caused by thermal undulations. The effect of the undulations can be ob served on electron microscope photographs. At salt concentrations between a pproximately 15 and 20% the appearance of the onion phase changes from tran slucent to milky white and the onions are (weakly) flocculated. The hydrati on of the surfactant molecules, the bilayer repeat distance, and, as a cons equence of this, the obtained volume fraction of onions decrease rapidly. T hermal undulation of the bilayers is no longer observed because of stiffeni ng of the layers. At even higher salt concentrations between 20 and 30% sev ere flocculation of the onions occurs. For the linear rheological behavior of the colloidally stable onion dispersions at 9.5 and 12.7% salt, a model taking into account the undulation potential as given by Helfrich and the l ength scale of the affine deformation is derived. The model satisfactorily explains the magnitude of the found elastic modulus G ' at low frequencies, suggesting affine motion at a length scale larger than the size of the oni ons. The curved parts of the onions are assumed to be relaxed. The volume f raction dependence of G ' of the onion dispersions at 16.9 and 28% salt is similar to that of a flocculated dispersion. At salt concentrations of 9.5 and 12.7% and onion volume fractions of 0.6 and above a broad relaxation tr ansition in G " is observed. The strength of the relaxation (AG) can be exp lained if we assume that the transition is caused by the inclusion of the c urved part of the onions. At constant salt level, the characteristic time ( tau) of the transition and AG vary proportionally, showing that the viscosi ty (Delta eta) associated with the transition is constant. Going from 9.5 t o 12.7% salt Delta eta increases by a factor of 2.5. Some possible explanat ions for this effect are offered.