The influence of shear on lyotropic lamellar phases in the system sodium dodecyl sulfate (SDS)/decanol/water has been studied using small angle neutron and light scattering (SANS, SALS), birefringence and rheology. Eight different samples with a constant water content of 67.4%, but different surfactant-cosurfactant ratio were studied. Static SANS measurements showed that replacing of SDS with decanol leads to a transition from a defective lamellar phase, characterized first by a ribbon like structure and then by a pore like structure, to a classical lamellar phase. An orientation diagram was obtained from SANS, SALS and birefringence measurements under shear. For samples with low decanol content, shear flow leads to an alignment of lamellae but in addition to previous studies, we found two reorientations, from a parallel (at low shear rates) to a perpendicular alignment of the lamellae (with respect to the walls of the shear cell) and to a parallel alignment again at the highest shear rates available. At intermediate decanol content, a shear induced formation of multilamellar vesicles was observed in a certain shear rate region. Samples with classical lamellar structure at high decanol content exhibited no shear induced vesicle formation.

The shear-induced structure formation in lyotropic lamellar phases containing water-soluble polymers is investigated. The lyotropic phases consisted of sodium dodecyl sulfate/1-decanol/D2O and were mixed either with poly(n-isopropylacrylamide) (PNIPAM), hydroxyethyl starch (HES), poly(vinyl caprolactame) (PVCa), or poly(ethylenglycol)distearate (PEG-DS). Rheo-optical experiments (flow birefringence and small-angle light scattering, SALS) as well as small-angle neutron scattering (SANS) combined with a commercial rheometer were used to observe structural changes, e.g., layer reorientation or formation of multilamellar vesicles (liposomes). Equilibrium properties of the lamellar phases were investigated using quasi elastic light scattering (QELS) and static SANS, the latter was analyzed using a model proposed by Nallet et al. The polymer addition led to a viscosity increase but the flipping of aligned lamellae from parallel to perpendicular orientation was hardly affected by the polymers. The shear-induced formation of multilamellar vesicles (MLV), however, was strongly influenced by the macromolecules. The addition of small amounts of PNIPAM shifted the region where vesicles are formed to samples with higher decanol contents whereas HES, PVCa, and PEG-DS suppressed the MLV formation in all cases. Results from SANS and QELS indicate a possible correlation between the shear-induced vesicle formation and the viscoelastic properties of the surfactant bilayer.

Centrifugation can be used to remove the continuous aqueous phase of an oil-in-water emulsion. The cream that remains after most of the water has been removed has the structure of a biliquid foam; it can be redispersed in water. Examination of this cream through electron microscopy shows polyhedral oil cells separated by thin films. The thickness of these films has been measured through small-angle neutron scattering. The results yield a disjoining pressure isotherm, where the film thickness is solely determined by the pressure applied to extract water during centrifugation. For hexadecane-in-water biliquid foams, stabilized with sodium dodecyl sulfate (SDS), this isotherm has two states, the common black film (CBF; water thickness beyond 25 Å) and the Newton black film (NBF; (water thickness of 13 Å). At low pressures (1−50 atm), the films are in the CBF state, where the measured disjoining pressure matches the entropic pressure of the counterions, calculated from the Poisson−Boltzmann equation. At high pressures (20−300 atm), ionic correlations in the counterion layer reduce the disjoining pressure and the films jump discontinuously to the NBF. The thickness of the NBF is stabilized by hydration forces, which resist the dehydration of counterions and headgroups. The surface density of SDS molecules in these films has also been measured. As water is extracted, the concentration of counterions increases, and they screen the headgroups more efficiently; as a result, the surface density of SDS in the monolayers rises. In the NBF state, the monolayers are tightly packed, with an orientational order that exceeds that of the lamellar phase. This tighter packing of surfactant molecules may explain the surprisingly high metastability of biliquid foams when the films are in the NBF state.