Electro-optic effects of aqueous fd-virus suspensions at very low ionic strength

Abstract

The orientation 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 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 parameter κ. A special experimental set-up is used to vary the ionic strength of the suspension, i.e. the Debye-Huckel parameter, and therefore the electrostatic interparticle interaction. The birefringence signal Δn is measured as a function of the strength and frequency of the applied electric field in suspensions of very low ionic strength (10 -6 M-10 -4 M). At low field strengths Kerr-behaviour is found. From the dependence of the electric anisotropy Δα el on the Debye-Huckel 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 values 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 dispersion below 1 kHz of Δn of the electrostatically interacting fd-virus suspensions shows anomalous behaviour. This negative electro-optic effect is an evidence for the orientation of the particle's long symmetry axis perpendicular to the applied electric field. The dispersion has a positive maximum at about 2 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