Electro-optic effects of electrostatically interacting rod-like polyelectrolytes

Abstract

Orientation behavior in external electric fields of rod-like fd-virus particles (length l=895 nm, diameter d=9 nm) in aqueous suspensions is examined by the electric birefringence mthod and static light-scattering measurements. In aqueous suspensions the negatively charged fd-particles are surrounded by a diffuse Debye cloud of counterions, which is characterized by the Debye-Hückel parameter κ. A special experimental setup is used to vary the ionic strength of the suspension, i.e., the Debye-Hückel parameter κ and, therefore, the electrostatic interparticle interaction. The birefringence signal Δn and the relative change of the scattered light intensity ΔI/I 0 is measured as a function of the strength and frequency of the applied electric field in suspensions of very low ionic strength (10−6–10−4 M). At low field strengths Kerr-behavior is found. From the dependence of the electric anisotropy Δα el on the Debye-Hückel parameter κ it is concluded that the orientation of the fd-particles is correlated to an induced dipole due to a deformation of the diffuse Debye cloud. Saturation electric birefringence and electric light-scattering data are far from that theoretically expected. This can be interpreted as a destruction of the diffuse Debye cloud at high electric fields. At low field strengths the frequency dispersions below 1 kHz of Δn and ΔI/I 0 of the electrostatically interacting gd-virus suspensions show anomalous behavior. This negative electro-optic effect is an evidence for the orientation of the particle long symmetry axis perpendicular to the applied electric field. The dispersion has a positive maximum at about 3 kHz. This maximum could be explained by different frequency dependencies of the electric polarizabilities parallel and perpendicular to the long symmetry axis of the fd-rods.Key wordsElectric birefringenceelectric field light scatteringfd-viruselectric polarizability