Attraction and ionic correlations between charged stiff polyelectrolytes.pdf

a r X i v : c o n d - m a t / 0 2 0 6 1 2 6 v 1 [ c o n d - m a t .s o f t ] 1 0 J u n 2 0 0 2 Attraction and ionic correlations between charged stiff polyelectrolytes Markus Deserno Department of Chemistry and Biochemistry, UCLA, USA Axel Arnold and Christian Holm Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany (Dated: June 10, 2002) We use Molecular Dynamics simulations to study attractive interactions and the underlying ionic correlations between parallel like-charged rods in the absence of additional salt. For a generic bulk system of rods we identify a reduction of short range repulsions as the origin of a negative osmotic coefficient. The counterions show signs of a weak three-dimensional order in the attractive regime only once the rod-imposed charge-inhomogeneities are divided out. We also treat the case of attraction between a single pair of rods for a few selected line charge densities and rod radii. Measurements of the individual contributions to the force between close rods are studied as a function of Bjerrum length. We find that even though the total force is always attractive at sufficiently high Bjerrum length, the electrostatic contribution can ultimately become repulsive. We also measure azimuthal and longitudinal correlation functions to answer the question how condensed ions are distributed with respect to each other and to the neighboring rod. For instance, we show that the prevalent image of mutually interlocked ions is qualitatively correct, even though modifications due to thermal fluctuations are usually strong. I. INTRODUCTION The interaction of polyelectrolytes or charged colloids in polar solvent depends sensitively on the structure of the electrical double layer surrounding the macroion. This double layer consists in the simplest case of small counterions of opposite charge. The linearized mean-field treatment of this layer lies at the heart of DLVO theory [1, 2], one of the most influential and s