Manipulating Polyelectrolyte Complexes in Salt Solutions: a Computer simulation study

Abstract

In this thesis, a series of Langevin dynamics simulations are performed to investigate the complexation of polyelectrolytes (PE) in multivalent salt solutions and the manipulation of those PE complexes by electric fields. Our study provides the following results. Firstly, the addition of multivalent salt can induce two consequent conformational (or phase) transitions of PE: a condensation of PE, followed by a decondensation. The chain stiffness affects the transition-order of both condensation and decondensation. It also determines whether or not the PE structure can be ordered. It modifies the ion distribution around the chain at the same time because the distribution profiles usually reflect either local or global structure of the PE chain. Secondly, a PE chain, which forms PE complex with condensed ions, can be unfolded in a direct-current (DC) electric field once the field strength exceeds a critical value E*. The value of E* dependence on Cs and the dependence is non-monotonic. This is because the difficulties for polarized a PE complex at different Cs are different. The difficulty is attributed to both the binding strength of condensed ions and the amount of them. E* also depends on the chain length N and shows a power-law-like relation E* ∼ N-θ. The exponent θ decreases with increasing salt valence Zs. The dependences of E_ on Cs, Zs and N provide a solid foundation to separate PE chains by length in free solutions. Finally, a collapsed PE chain can be unfolded by an alternating-current (AC) electric field when the field strength exceeds the DC threshold E* and the frequency is below a critical value. The critical frequency corresponds to the inverse charge relaxation/dissociation time of condensed multivalent counterions at the interface of the collapsed PE. The relaxation time is coincident with the DC chain fluctuation time. This suggests that, in an AC electric field, the dissociation of condensed polyvalent counterion on the collapsed PE interface controls the PE dipole formation and unfolding dynamics.