ELECTROOPTIC EFFECTS OF AQUEOUS FD-VIRUS SUSPENSIONS AT VERY-LOW IONIC-STRENGTH

The orientation in external electric fields of rod-like fd-virus parti cles (length l = 895 nm, diameter d = 9 nm) in aqueous suspensions is examined by the electric birefringence method. In aqueous suspensions the negatively charged fd-particles are surrounded by a diffuse Debye cloud of counterions, which is characterized by the Debye-Huckel param eter kappa. A special experimental set-up is used to vary the ionic st rength of the suspension, i.e. the Debye-Huckel parameter, and therefo re the electrostatic interparticle interaction. The birefringence sign al DELTAn is measured as a function of the strength and frequency of t he applied electric field in suspensions of very low ionic strength (1 0(-6) M-10(-4) M). At low field strengths Kerr-behaviour is found. Fro m the dependence of the electric anisotropy DELTAalpha(el) on the Deby e-Huckel parameter kappa it is concluded that the orientation of the f d-particles is correlated to an induced dipole due to a deformation of the diffuse Debye cloud. Saturation electric birefringence values are far from that theoretically expected. This can be interpreted as a de struction of the diffuse Debye cloud at high electric fields. At low f ield strengths the frequency dispersion below 1 kHz of DELTAn of the e lectrostatically interacting fd-virus suspensions shows anomalous beha viour. This negative electro-optic effect is an evidence for the orien tation of the particle's long symmetry axis perpendicular to the appli ed electric field. The dispersion has a positive maximum at about 2 kH z. This maximum could be explained by different frequency dependencies of the electric polarizabilities parallel and perpendicular to the lo ng symmetry axis of the fd-rods.