Abstract
Transient electric birefringence (TEB) measurements on aqueous solutions of flexible polyelectrolytes (in pure water, with added salt and with oppositely charged surfactants) are presented. The results evidence the different polarization mechanism of free and condensed counterions. Free counterions polarize perpendicular to the main polymer axis and give a negative contribution to the birefringence; condensed counterions polarize parallel to the main polymer axis and give a positive contribution. We present a simple model that allows to explain the change in sign of the birefringence when polymer, salt and surfactant concentrations are varied, according to which polarization mechanism prevails. From the model we can estimate the fraction of free counterions obtaining values which are close to the values calculated with Manning theory. We demonstrate the interest of TEB technique in the study of polyelectrolyte/surfactant systems.
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