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Using molecular dynamics simulations we study the behavior of a dilute solution of strongly charged polyelectrolytes in poor solvents, where we take counterions explicitly into account. We focus on the chain conformational properties under conditions where chain-chain interactions can be neglected, but the counterion concentration remains finite. We investigate the conformations with regard to the parameters chain length, Coulomb interaction strength, and solvent quality, and explore in which regime the competition between short range hydrophobic interactions and long range Coulomb interactions leads to pearl-necklace like structures. We observe that large number and size fluctuations in the pearls and strings lead to only small direct signatures in experimental observables like the single chain form factor. Furthermore we do not observe the predicted first order collapse of the necklace into a globular structure when counterion condensation sets in. We will also show that the pearlnecklace regime is rather small for strongly charged polyelectrolytes at finite densities. Even small changes in the charge fraction of the chain can have a large impact on the conformation due to the delicate interplay between counterion distribution and chain conformation.

PACS. 61.25.Hq -Macromolecular and polymer solutions. PACS. 36.20.Ey -Polymer molecules -conformation. PACS. 87.15Aa -Theory and modeling; computer simulation.Abstract. -We present molecular dynamics simulations on strongly charged polyelectrolytes in poor solvent. The resulting pearl-necklace conformations are analyzed in detail. Fluctuations in the number of pearls and their sizes lead only to small signatures in the form factor and the force-extension relation, which is a severe obstacle for experimental observations. We find that the position of the first peak in the structure factor varies with the monomer density as ≈ ρ Polyelectrolytes (PEs) are polymers that carry ionizable groups that dissociate ions in aqueous solution. Technical applications range from viscosity modifiers, precipitation agents, superabsorbers to leak protectors [1]. In biochemistry and molecular biology they are of great importance because virtually all proteins, as well as DNA, are PEs.Many PEs contain a non-polar hydrocarbon backbone, for which water is a poor solvent. Therefore, in aqueous solution, there is a competition between the tendency to precipitate, the Coulomb interaction and the entropic degrees of freedom. This can lead to elongated strings of locally collapsed structures (pearls). Such necklace conformations have been predicted on the basis of scaling arguments in ref.[2] for a weakly charged single chain PE, and have been confirmed by simulations using the Debye-Hückel approximation [2,3] and with explicit counterions [4]. The formation of the necklace structure is due to the Rayleigh instability of a charged droplet, which leads to a split once a critical charge is reached. The size of the pearls is determined by the balance between electrostatic repulsion and surface tension. The distance between two pearls is governed by the balance of the electrostatic pearl-pearl repulsion and the surface tension. However, there is up to now no clear experimental proof for the existence of necklace chains [5,6]. Our previous papers [4] dealt with smaller system sizes and showed that necklaces exist also in the presence of counterions and exhibit a variety of conformational transitions as a function of density. The focus of the present paper is to analyze by extensive computer simulations in detail three possible experimental observables, namely the form factor, the structure factor and the force-extension relation, which can be probed by scattering and AFM techniques. Analyzing the fluctuations of necklace structures we find an extended coexistence regime between different necklace structures and broad distributions for the pearl sizes and the pearl-pearl distances that smear out the necklace signatures. In addition we find that the peak in the structure factor of the solution scales proportional to c EDP Sciences

We investigate end-effects in the ion distribution around strongly charged, flexible polyelectrolytes with a quenched charge distribution by molecular dynamics simulations of dilute polyelectrolyte solutions. We take the counterions explicitly into account and calculate the full Coulomb interaction via an Ewald summation method. We find that the free counterions of the solution are distributed in such a way that a fraction of the chain charges is effectively neutralized. This in turn leads to an effective charge distribution which is similar to those found for weakly charged titrating polyelectrolytes that have an annealed charge distribution. The delicate interplay between the electrostatic interactions, the chain conformation and the counterion distribution is studied in detail as a function of different system parameters such as the chain length N m , the charge fraction f , the charged particle density ρ, the ionic strength and the solvent quality. Comparisons are made with predictions from a scaling theory.