RHEOLOGICAL BEHAVIOR OF WORMLIKE MICELLES - EFFECT OF SALT CONTENT

We study the effect of salt content on the rheological properties of w ormlike micelles formed from hexadecyltrimethylammonium bromide (CTAB) in presence of potassium bromide (KBr) and of cetylpyridinium chlorat e (CPClO3) in presence of sodium chlorate (ClO3Na). Upon increasing th e salt concentration, at fixed surfactant concentration, we observe fo r both systems a maximum of the zero-shear viscosity eta0. For salt co ncentrations 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 f or eta0(C) are characterized by values of the exponent much smaller th an those predicted by the existing theoretical models. The results are qualitatively interpreted by a description based on a structural evol ution upon increasing salt content from a system of entangled linear m icelles to a multiconnected network. Measurements of the plateau modul us of CTAB solutions, as a function of KBr, give results supporting th e above hypothesis.