Swelling laws of growing and branching wormlike micelles
à 11h, en salle C. BROT
Wormlike micelles consist of long cylindrical supramolecular self-assemblies of amphiphilic molecules. Beside the main cylindrical curvature, wormlike micelle present heterogeneities in their curvature since they are necessarily terminated by spherical end cap and can present junctions that are locally flat. The concentrations of these defects of curvature determine the rheological properties and a better understanding calls for reliable determination of their value. The concentrations of end caps and junctions have been determined by SANS on model systems provided by gemini surfactants. The latter consist of several amphiphilic moieties tethered at the head group level through a variable length spacer. Cationic surfactants oligomers with short spacer are among the few systems, in which the transition from spherical to worm-like micelles (wlm) occurs at low concentration without requiring addition of salt or the presence of a hydrophobic counter-ion. They also provide binary systems where three fold junctions have been evidenced by cryo-TEM. Due to electrostatic correlation, small angle neutron scattering patterns of Gemini wormlike micelles are characterized by a maximum at finite wave vector q*. The volume fraction dependence of q* crosses over continuously through different scaling laws due to the morphological transitions. In the dilute regime, q* is determined by the number density of micelles and is analyzed in terms of aggregation number N. Its temperature dependence leads to an estimation of the excess energy associated to endcaps. In the semi-dilute regime, the length is no longer observable, but the junction density can be determined.
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