BROWNIAN DYNAMICS OF THE POLARIZATION OF RODLIKE POLYELECTROLYTES - ANISOTROPY AND THE EFFECT OF HYDRODYNAMIC INTERACTIONS

The polarization of rodlike polyelectrolytes has been analyzed by Brow nian dynamics simulations, using a cube with a polymer in the center s urrounded by counterions and byions, Electrostatic interactions both i n the cube and beyond the cube were evaluated assuming periodic bounda ry conditions using Ewald summation. The effect of hydrodynamic intera ctions on the dynamics of the ions has been evaluated by modified Osee n interaction tensors. For polyions without added salt, the dipole ind uced in the presence of hydrodynamic interactions is smaller than in t heir absence for most of our simulated data, but under some conditions the effect of hydrodynamic interactions is rather small. For polyions with 40 and 60 residues, the dipole moment induced perpendicular to t he rod axis, mu(perpendicular to), is rather close to that induced par allel to the rod axis, mu(parallel to). The difference (mu(parallel to ) - mu(perpendicular to)) is positive under most conditions, but negat ive values of (mu(parallel to) - mu(perpendicular to)) were also found in some cases, All simulated dipole moments increase with the chain l ength, N;mu(perpendicular to) shows a linear increase with the chain l ength, whereas the increase of mu(parallel to) and (mu(parallel to) - mu(perpendicular to)) with N is more than linear, The dipole moment mu (parallel to) calculated for a polyion with 40 residues in the presenc e of hydrodynamic interactions increases with increasing concentration of added salt, whereas in the absence of hydrodynamic interactions mu (parallel to) is virtually independent of the salt concentration. This result indicates that the contribution of hydrodynamic interactions t o the dipole moment increases with increasing salt concentration. Elec tric fields induce not only dipole moments but also dissociation of io ns, which is reflected by an increase of the root mean square distance [s(c)(2)](1/2) of the counterions from the center of the cube. The in crease of [s(c)(2)](1/2) computed in the presence of hydrodynamic inte ractions is smaller than in their absence; this increase is also small er for perpendicular than for parallel orientation of the field vector and rod axis. The rise-time constants of the dipole moments are in th e time range of a few nanoseconds and do not show any detectable depen dence on the chain length. Because of the complexity of the results, w hich is due to the superposition of many different effects, it is not possible at the present state of the simulations to present simple equ ations or general rules fur the anisotropy of the polarizability, e.g. as a function of the field strength or of the chain length.