Weak polyelectrolyte complexation driven by associative charging

Abstract

Weak polyelectrolytes are relevant for a wide range of fields; in particular, they have been investigated as "smart" materials for chemical separations and drug delivery. The charges on weak polyelectrolytes are dynamic, causing polymer chains to adopt different equilibrium conformations even with relatively small changes to the surrounding environment. Currently, there exists no comprehensive picture of this behavior, particularly where polymer-polymer interactions have the potential to affect charging properties significantly. In this study, we elucidate the novel interplay between weak polyelectrolyte charging and complexation behavior through coupled molecular dynamics and Monte Carlo simulations. Specifically, we investigate a model of two equal-length and oppositely charging polymer chains in an implicit salt solution represented through Debye-Hückel interactions. The charging tendency of each chain, along with the salt concentration, is varied to determine the existence and extent of cooperativity in charging and complexation. Strong cooperation in the charging of these chains is observed at large Debye lengths, corresponding to low salt concentrations, while at lower Debye lengths (higher salt concentrations), the chains behave in apparent isolation. When the electrostatic coupling is long-ranged, we find that a highly charged chain strongly promotes the charging of its partner chain, even if the environment is unfavorable for an isolated version of that partner chain. Evidence of this phenomenon is supported by a drop in the potential energy of the system, which does not occur at the lower Debye lengths where both potential energies and charge fractions converge for all partner chain charging tendencies. The discovery of this cooperation will be helpful in developing "smart" drug delivery mechanisms by allowing for better predictions for the dissociation point of delivery complexes.

Small ion effects on self-coacervation phenomena in block polyampholytes

“…Overall, it is apparent that the inclusion of explicit counter-ions and small amounts of salt has only a weak effect on the thermodynamics and structure of near-symmetric block polyampholyte solutions, as seen in associative charging models in the weak association regime of polyelectrolyte complexation. 93,94 The small effects are largely predictable, such as renormalizing the electrostatic strength lB/b due to the additional charge screening by the small ions. The structures of the coacervate and supernatant are similarly weakly affected, even with dramatic increases in the levels of added small ions, in stark contrast to the large influence that sequence and molecular architecture have on the self-coacervation behavior and accessible chain conformations.…”

Effects of Non-Electrostatic Intermolecular Interactions on the Phase Behavior of pH-Sensitive Polyelectrolyte Complexes

“…Further, we hypothesize that cooperativity of electrostatic interactions between cationic and anionic chains can result in promoting the ionization of PAH chains. 66,67 We expect these observations to inspire theoretical and simulation investigations, that in turn will provide further insights into the phenomena.…”

Sequence-dependent self-coacervation in high charge-density polyampholytes

“…[60][61][62][63][64][65] Similar to the development of polyampholyte physics, molecular simulation has played a key role in developing a physical understanding; this is true both at the limit of dilute polyelectrolyte complexes between two chains, as well as for bulk phase separation. [80][81][82][83][84][85] Recent work on complex coacervation, inspired in part by the relevance to IDPs, has seen the emergence of a number of modeling approaches beyond the continuing efforts in using simu-lation and field theory. These have sought to further examine and account for the limitations present in many of the field theoretic approaches, and particularly the original Voorn-Overbeek theory.…”