Explicit Ion Effects on the Charge and Conformation of Weak Polyelectrolytes.

Vikramjit Rathee,Hythem Sidky,Jonathan Whitmer,Benjamin Sikora

doi:10.3390/polym11010183

Abstract

The titration behavior of weak polyelectrolytes is of high importance, due to their uses in new technologies including nanofiltration and drug delivery applications. A comprehensive picture of polyelectrolyte titration under relevant conditions is currently lacking, due to the complexity of systems involved in the process. One must contend with the inherent structural and solvation properties of the polymer, the presence of counterions, and local chemical equilibria enforced by background salt concentration and solution acidity. Moreover, for these cases, the systems of interest have locally high concentrations of monomers, induced by polymer connectivity or confinement, and thus deviate from ideal titration behavior. This work furthers knowledge in this limit utilizing hybrid Monte Carlo–Molecular Dynamics simulations to investigate the influence of salt concentration, , pH, and counterion valence in determining the coil-to-globule transition of poorly solvated weak polyelectrolytes. We characterize this transition at a range of experimentally relevant salt concentrations and explicitly examine the role multivalent salts play in determining polyelectrolyte ionization behavior and conformations. These simulations serve as an essential starting point in understanding the complexation between weak polyelectrolytes and ion rejection of self-assembled copolymer membranes.

Highlights

Polyelectrolytes are the basis for a broad class of functional systems including self-assembled copolymer membranes, polymer-coated nanoparticles, polyelectrolyte microgels, and hydrogel networks [1,2,3,4,5,6,7,8,9,10,11]
While grand canonical Monte Carlo (GCMC) moves govern the exchange of ions with moves simulate the equilibrium dissociation and recombination of the monomer bead in another the bulk
We have presented a comprehensive picture of titration of weak polyelectrolytes taking into account the explicit pH and salt effects

Summary

Introduction

Polyelectrolytes are the basis for a broad class of functional systems including self-assembled copolymer membranes, polymer-coated nanoparticles, polyelectrolyte microgels, and hydrogel networks [1,2,3,4,5,6,7,8,9,10,11]. Though regarded as innately charged, polyelectrolytes frequently contain weakly acidic or basic moieties distributed along the polymer backbone This can be immensely useful, as monomer groups may protonate or deprotonate depending upon the local environmental conditions, enabling tunable “smart” materials [12,13,14]. Despite the prevalence of weakly acidic or basic polymers in various applications within the polyelectrolyte literature, there remains relatively little understanding of how different effects (including pH, salt concentration, topology, and solvation) balance in determining polyelectrolyte structure, morphology, and function. We build on an initial study which examined the titration of weak polyelectrolyte chains and stars using coupled reaction Monte Carlo and Molecular Dynamics in an implicit-salt screened-charge interaction limit [45], by explicitly accounting for the ionic species via grand-canonical Monte. Explicit ion effects should contribute prominently to the ion–polymer association and charging behavior, for multivalent species or in non-aqueous solvents where strong forces can lead to counterion adsorption onto the weak polyelectrolyte

Methods

METHODS

Results

A comparison of the swelling weakpolyelectrolyte polyelectrolyte chains

Conclusions