J. Phys. II France
Volume 4, Numéro 6, June 1994
Page(s) 1061 - 1074
DOI: 10.1051/jp2:1994184
J. Phys. II France 4 (1994) 1061-1074

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

H. Kramer1, C. Graf1, M. Hagenbüchle1, C. Johner1, C. Martin1, P. Schwind2 and R. Weber1

1  Fakultät für Physik, Universität Konstanz, 78434 Konstanz, Germany
2  Fakultät für Biologie, Universität Konstanz, 78434 Konstanz, Germany

(Received 5 November 1993, revised 7 January 1994, accepted 18 February 1994)

The orientation in external electric fields of rod-like fd-virus particles (length $\ell = 895$ 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-Hückel parameter $\kappa$. A special experimental set-up is used to vary the ionic strength of the suspension, i.e. the Debye-Hückel parameter, and therefore the electrostatic interparticle interaction. The birefringence signal $\Delta n$ 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 $\Delta \alpha_{\rm el}$ on the Debye-Hückel parameter $\kappa$ 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 $\Delta n$ 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.

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