RHEOLOGICAL BEHAVIOR OF WORMLIKE MICELLES - EFFECT OF SALT CONTENT

We study the effect of salt content on the rheological properties of wormlike micelles formed from hexadecyltrimethylammonium bromide (CTAB) in presence of potassium bromide (KBr) and of cetylpyridinium chlorate (CPClO3) in presence of sodium chlorate (ClO3Na). Upon increasing the salt concentration, at fixed surfactant concentration, we observe for both systems a maximum of the zero-shear viscosity eta0. For salt concentrations less than that corresponding to the maximum of eta0, the variation of eta0 with the surfactant concentration C can be described by a reptation model. Beyond the maximum, the scaling laws obtained for eta0(C) are characterized by values of the exponent much smaller than those predicted by the existing theoretical models. The results are qualitatively interpreted by a description based on a structural evolution upon increasing salt content from a system of entangled linear micelles to a multiconnected network. Measurements of the plateau modulus of CTAB solutions, as a function of KBr, give results supporting the above hypothesis.