ELECTRIC-FIELD LIGHT-SCATTERING BY ROD-LIKE POLYELECTROLYTES IN AQUEOUS SUSPENSIONS

Static light scattering measurements are presented for rod-like fd-vir us particles (L = 880 nm, D = 9 nm) subjected to a pulsed alternating electric field in aqueous suspensions at very low ionic strength. In a queous suspensions the dispersed fd-particles are negatively charged a nd surrounded by a diffuse Debye counterion cloud. In an external elec tric field an induced dipole originating from a deformation of the dif fuse counterion cloud causes the alignment of the macromolecules. The anisotropic orientation distribution of the particles in the presence of the electric field results in a change of the angular distribution of the scattered light intensity with regard to the isotropic case. Th e steady-state electric field light scattering effect Delta l/l(0) is measured as a function of the electric field strength and its frequenc y at a fixed scattering angle. The determination of the anisotropy of the electric polarizability Delta alpha(el) of a fd-virus particle at higher electric held strengths, above the Kerr regime, shows a decreas e of Delta alpha(el) with increasing field. This is interpreted as a d estruction of the diffuse Debye cloud in high electric fields. The ori entational order parameter has been found to be as large as 0.93 indic ating an almost complete particle orientation along the external field at the highest fields. It is also shown that in the frequency regime below 1 kHz electrostatically interacting rods can align perpendicular to the external electric field, whereas at higher frequencies this an omalous behaviour disappears. From the scattered intensity the form-fa ctor and the static structure factor of interacting fd-virus particles have been determined. With increasing fields a substantial increase i n the peak height of the static structure factor is found. The data is in good agreement with Monte Carlo simulations using a simple interac tion model for the system. The orientation of the macromolecules in th e presence of an electric field is affected by the intermolecular elec trostatic repulsion.