Johannes Zipfel

Influence of sodium dodecyl sulfate on structure and rheology of aqueous solutions of the nonionic surfactant tetraethyleneglycol-monododecyl ether (C12E4)

Abstract The influence of the ionic surfactant sodium dodecylsulfate (SDS) on structure and rheology of the lamellar phase of the nonionic surfactant tetraethyleneglycol-monododecyl ether (C 12 E 4 ) at intermediate concentrations was investigated by means of small angle light and neutron scattering (SALS, SANS) and rheometry. The long range orientational order increased with added SDS as expected and the Caille parameter was obtained from SANS data. The shear viscosity increased when SDS was added. Rheo SALS experiments revealed the shear induced formation of multilamellar vesicles (MLV or ‘onions’) and the MLV size decreased not only in shear thinning but also in the shear thickening region. The increase in Bragg scattering intensity along the flow direction as observed in the rheo-SANS experiments also revealed the increase in MLV volume fraction in the shear thickening regime. The SANS data further showed a decrease in layer spacing with increasing shear which indicates that water is squeezed out of the vesicles.

Lamellar phases under shear: variation of the layer orientation across the couette gap

The shear induced orientation of a lyotropic lamellar phase in the system sodium dodecylsulfate (SDS)/decanol/water was studied. The distribution of the layer orientation across the gap of a couette type shear cell was investigated by means of small-angle X-ray scattering. As a function of shear rate, a transition from parallel to perpendicular layer orientation was observed. The measurements showed that the lamellar orientation depends on the position within the gap, with a parallel orientation preferred near the rotating outer wall of the shear cell. The inhomogeneous orientation distribution across the gap explains both the bimodal alignment reported earlier as well as the transition from parallel to perpendicular orientation over a broad range of shear rates in a shear controlled experiment. Therefore, quantitative studies on layer orientation need to consider the exact position within the gap where the beam has probed the sample.

Shear Induced Reorientations in a Defective Lyotropic Lamellar Phase

Surfactants are known to form lyotropic liquid crystalline phases in concentrated aqueous solution. Usually samples of macroscopic size are not homogeneously aligned but a polydomain structure is observed. Shear flow has a profound influence on structure and macroscopic alignment of liquid crystals and consequently the rheological properties of such materials are rather complex. Recently, samples with lamellar morphology attracted much attention in order to find unifying principles in different types of materials as e. g. aqueous surfactant solutions and block copolymer melts.