The electric birefringence of polyelectrolytes. Maxwell-Wagner and electrokinetic relaxation

Frequency resolved electric birefringence experiments on dilute aqueous suspensions of charged rod-like fluorinated latex particles are reported. The electric charge of the particles was varied by adsorption of a ionic amphiphile. The frequency-dependence of the Kerr constant is found to change strongly with the particle charge and with the ionic strength of the suspension. The results are successfully interpreted by calculating the anisotropic electric polarizability of the particle surrounded by its counterion cloud through a modified Maxwell-Wagner model that is applicable only at high-frequency. The fit between theory and experiment is very good and provides a new and simple way to extract the particle charge from birefringence data. The electric charge of bare latex particles increases with ionic strength while surfactant-coated particles behave as constant charge entities.