Role of chain stiffness on the conformation of single polyelectrolytes in salt solutions

TL;DR

This study uses simulations to explore how chain stiffness influences the conformation of single polyelectrolytes in salt solutions, revealing distinct behaviors and structural transitions based on flexibility and salt concentration.

Contribution

It provides a comprehensive analysis of the role of chain stiffness on polyelectrolyte conformations in salt solutions, including a state diagram and insights into coil-globule transitions.

Findings

Flexible chains extend more at low salt concentrations.

Semiflexible chains form ordered globules like hairpins or toroids.

Chain stiffness affects the nature of coil-globule transitions.

Abstract

Conformation of single polyelectrolytes in tetravalent salt solutions is investigated under the framework of a coarse-grained model, using Langevin dynamics simulations. The chain size, studied by the radius of gyration, shows three different variational behaviors with salt concentration, depending on the chain stiffness. According to these behaviors, polyelectrolytes of fixed chain length are classified into three categories: flexible chain, semiflexible chain, and rigid chain. The worm-like chain model with persistence length predicted by the Odijk-Skolnick-Fixman theory is found to be able to qualitatively describe the end-to-end distance at low salt concentration. In a low-salt region, a flexible polyelectrolyte extends more significantly than a semiflexible chain, and in a high-salt region, regardless of chain stiffness, a chain attains a dimension comparable to that of its neutral polymer. The chain stiffness influences both the local and the global chain structures. A flexible chain exhibits a zigzagged local structure in the presence of salt ions and the condensed structure is a disordered, random globule. A semiflexible chain is locally smooth, and the condensed structure is orderly packed, taking a form such as hairpin or toroid. Moreover, the chain stiffness can also affect the nature of the coil-globule transition. The transition is occurred in a discrete manner for semiflexible chain, whereas in a continuous way for flexible chain. This discrete feature is happened not only at low salt concentration when a semiflexible chain is collapsed, but also at high salt concentration when the collapsed chain is reexpanded. At the end, the effects of chain stiffness and salt concentration on the conformation of single polyelectrolytes are summarized in a schematic state diagram.