Impact of Monovalent Counter-ions on the Conformation of Flexible Polyelectrolytes Having Different Molecular Architectu
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Impact of Monovalent Counter-ions on the Conformation of Flexible Polyelectrolytes Having Different Molecular Architectures Alexandros Chremos1 and Jack F. Douglas1 1 Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, MD, 20899, U.S.A. ABSTRACT We explore the impact of monovalent counter-ions on the molecular conformation of highly charged flexible polyelectrolytes for a range of molecular topologies (linear chains, stars, and unknotted and trefoil rings) by molecular dynamics simulations that include an explicit solvent having short range interaction with the polyelectrolyte. In particular, we investigate how the counter-ions near the polyelectrolytes with variable mass influence the average molecular shape. We also characterize the interfacially “bound” counter-ions by calculating the timeaveraged number of interfacial counter-ions, as well as the degree to which the polyelectrolytes wrap around the counter-ions by calculating the number of contacts between the counter-ions and the polyelectrolyte. INTRODUCTION Polyelectrolytes are an important class of polymeric molecules that carry charged groups that release counter-ions to an extent that depends on their conformation and charge density when dissolved in polar solvents. Examples include sulfonated polystyrene and polyacrylic acid, as well as, many biological molecules such as DNA and proteins. Insights from the study of these polymers have potential significance in numerous applications, e.g., biomedical implant materials and encapsulating material pharmaceutical drug delivery systems [1, 2]. However, the modeling of synthetic and biological polyelectrolyte solutions is theoretically complicated due to the strong coupling between the counter-ion distribution of and polyelectrolyte conformation [3]. Theoretically, correlations between the counter-ions distribution and the polyelectrolyte are usually described based on the classical counter-ion condensation theory of Manning and subsequent revisions of this classic model of polyelectrolytes [4-7]. According to this theory, when the electrostatic interactions become comparable to thermal energy the counter-ions from their uniform distribution in the solution start to “condense” on the chain backbone, thus largely screening the backbone charge. In the original theory [4, 5], this instability takes place when ξ = λ lB > 1, (λ is the polyelectrolyte charge per length, and lB = e2 / (εr kBT) is the Bjerrum length and εr being the solvent dielectric constant). However, Manning theory models polyelectrolytes as infinitely long charged straight threads, while, real polyelectrolytes have a finite chain length and can be relatively flexible. The existence of a flexible backbone raises basic and theoretically unresolved questions about how the polyelectrolyte conformation affects the distribution of counter-ions distributed these polymers and about how the counter-ions, in turn, influence polymer conformation. Simulation studies [8-14] of flexible polyelectrolytes in so
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