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Aqueous solutions of rodlike fd virus particles (length l = 880 nm, diameter d = 9nm) are examined by the transient electric birefringence method. A special experimental setup is used, which allows a very sensitive variation of the screened Coulomb interaction between the particles. The birefringence signal is measured as a function of the virus particle concentration and the ionic strength of the solution. For low applied electric field strength it is demonstrated that anomalous birefringence is an effect of the steric and electrostatic interactions between the fd virus particles. It is shown that this behavior is correlated to a decrease of the rotational diffusion constant Dr. In the case of high ionic strength the deduced values of Dr are in agreement with the theory of Teraoka and Hayakawa1 up to a concentration of 8c* (c* = 1 particle/ length3 is the overlap concentration); at larger concentrations the values are significantly higher than predicted.

Electrophoretic mobility and conductivity data of aqueous suspensions of rodlike tobacco mosaic virus (TMV) (length 1 = 300 nm, diameter 2u = 18 nm) are presented in a wide range of particle concentration, below and above the overlap concentration c* = 1 particle/length3. The mobility is investigated in relation to the small ion concentration down to salt-free suspensions. At low small ion concentration a screened Coulomb interaction between the particles appears, i.e., the double layers surrounding the particles begin to overlap. This leads to a decrease of the mobility of TMV particles. Another effect which influences the particle mobility is the change in the dissociation degree of the weak surface groups on the particles which are responsible for the particle charge. It is investigated to what extent potential and particle charge can be determined by means of the standard theories.

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The electrical and optical properties of rodlike fd-virus particles (L = 880 nm, D = 9 nm) are investigated by magnetic and electric birefringence experiments and supplemented by electric field light-scattering measurements. In aqueous suspension the fd-virus particles are negatively charged and surrounded by diffuse counterion clouds. The examinations are performed at very low ionic strength (10"6 M). Under these conditions a liquid crystalline phase already occurs at fd-particle concentrations above 0.6 mg/mL. In magnetic fields the rodlike fd-virus is aligned parallel to the magnetic field due to its diamagnetic anisotropy. Almost complete particle alignment in magnetic fields is obtained for liquid crystalline samples, and the saturation value of the birefringence is determined. The application of electric fields results in a polarization of the counterion clouds and a statistical orientation of the dispersed particles. In low electric fields Kerr behavior is found, providing the electric anisotropy Aotet of a single rod. At higher electric fields the diffuse Debye cloud is partly stripped away and a complete particle orientation is hindered. Electric field light-scattering investigations yield quantitatively the decrease of the electric anisotropy Aoiei in high electric fields.