Self-assembly of amphiphilic polyelectrolytes in trivalent salt solution

Abstract

Multivalent salt plays important roles in polyelectrolyte (PE) systems. Some special effects, such as ion mediated electrostatic correlation and reentrant condensation can be induced in the presence of multivalent salt. In this work, the self-assembly behaviors of diblock PEs in trivalent salt solutions are mainly investigated by molecular dynamics (MD) simulations, and partial results are qualitatively verified by experiments. The electrostatic correlation between PE chains and trivalent counterions is first enhanced with the increase of salt concentration, and then the large complexes of co-ions and trivalent counterions reduce the correlation at the excessive salt regime. Therefore, the electrostatic correlation mediated by trivalent ions shows a non-monotonic dependence on salt content. It should be noted that one-dimensional chain-like and two-dimensional planar network supra-micellar structures are obtained at different concentrations of PEs. It is the strong electrostatic correlation mediated by trivalent counterions that leads to the cross-linking of micelles, and homogeneous spherical micelles rather than anisotropic micelles are used as building blocks. Both of these supra-micellar structures predicted by simulations are qualitatively confirmed by experiments, and a micron sized long polymer chain and planar networks of inter-connected micelles are demonstrated in transmission electron microscopy (TEM) images. Reentrant behavior of the assembled micelles is demonstrated at a lower polymer concentration. The transitions from a dispersed micellar phase to a fully phase separated phase (where all the copolymer chains aggregate into one micelle to precipitate out of the solution) and to the re-dissolution of micelles are displayed with an increase in trivalent salt. The non-monotonic dependence of electrostatic correlation on the salt content is well displayed by the four states of the assembled micelles: (i) dispersed micelles, (ii) a single micelle comprising of all copolymer chains, (iii) micellar complex, and (iv) re-dispersed micelles. Our analysis may provide guidance for cross-linking of isotropic micelles into superstructures.