Evidence of Sticky Contacts between Wormlike Micelles in Viscoelastic Surfactant Solutions.

Many cationic surfactants form highly viscoelastic solutions at concentrations of only a few percent. These solutions contain wormlike micelles (WLM) which can be several micrometers long. The structural relaxation time, τs in these solutions can be as long as many seconds, and the zero-shear viscosity can be in the range of 106 Pas. Electric birefringence measurements on such solutions showed four different relaxation times with increasing concentration. The two shortest ones, τ1 and τ2 in the µs region were due to the alignment of small rodlike or ringlike micelles. The third one, τ3, was observed in the viscoelastic region in the ms region and finally, a fourth one, τ4, which was the same as the structural relaxation time, τs. In this article, it is shown that the τ3 process is due to the formation and opening of contacts between the WLM. The reason for the contacts is the attraction between the WLM which is due to the hydrophobic surfaces of the micelles. The contacts crosslink the WLM and thus form a three-dimensional network. This network is the reason for the high viscosity and viscoelastic properties of the samples. If the attraction between the WLM is reduced by adding glycerol to the solution, the viscosity of the solution breaks down. The same happens if the surface of the WLM is made hydrophilic by the addition of small amphiphilic molecules. The WLM are not destroyed by these procedures.

Controlled Exfoliation of Layered Silicate Heterostructures into Bilayers and Their Conversion into Giant Janus Platelets.

Ordered heterostructures of layered materials where interlayers with different reactivities strictly alternate in stacks offer predetermined slippage planes that provide a precise route for the preparation of bilayer materials. We use this route for the synthesis of a novel type of reinforced layered silicate bilayer that is 15 % stiffer than the corresponding monolayer. Furthermore, we will demonstrate that triggering cleavage of bilayers by osmotic swelling gives access to a generic toolbox for an asymmetrical modification of the two vis-à-vis standing basal planes of monolayers. Only two simple steps applying arbitrary commercial polycations are needed to obtain such Janus-type monolayers. The generic synthesis route will be applicable to many other layered compounds capable of osmotic swelling, rendering this approach interesting for a variety of materials and applications.

In-plane modulus of singular 2:1 clay lamellae applying a simple wrinkling technique.

Knowledge of the mechanical properties of singular clay lamellae is of crucial importance for the optimization of clay-polymer nanocomposites. On the basis of controlled stress release, singular 2:1 clay lamellae show regular wrinkles on a deformable substrate. A subsequent two-dimensional Fourier transformation gives an in-plane modulus of the clay lamella of approximately 150 GPa. Only readily-available topographical atomic force microscopy is required for analysis rendering that fast and facile procedure generally applicable for nanoplatelet characterization.