The collapse of linear polyelectrolyte chains in a poor solvent: When does a collapsing polyelectrolyte collect its counter ions?

TL;DR

This study investigates the collapse behavior of polyelectrolyte chains in poor solvents, revealing that counter ions are collected during the initial stages of collapse even without counterion binding, using experimental and theoretical analysis.

Contribution

The paper introduces a method to quantify solvent quality for uncharged polyions and compares experimental collapse behavior with a new theory, highlighting counter ion collection during early collapse stages.

Findings

Polyion collects counter ions during initial collapse stages.

Solvent quality can be experimentally quantified for uncharged polyions.

Collapse occurs before counterion binding is expected based on Manning parameter.

Abstract

In order to better understand the collapse of polyions in poor solvent conditions the effective charge and the solvent quality of the hypothetically uncharged polymer backbone need to be known. In the present work this is achieved by utilizing poly-2-vinylpyridine quaternized to 4.3% with ethylbromide. Conductivity and light scattering measurements were utilized to study the polyion collapse in isorefractive solvent/non-solvent mixtures consisting of 1-propanol and 2-pentanone, respectively, at nearly constant dielectric constant. The solvent quality of the uncharged polyion could be quantified which, for the first time, allowed the experimental investigation of the effect of the electrostatic interaction prior and during polyion collapse, by comparing to a newly developed theory. Although the Manning parameter for the investigated system is as low as $l_B/l = 0.6$ ($l_B$ the Bjerrum length and $l$ the mean contour distance between two charges), i.e. no counterion binding should occur, a qualitative interpretation of the conductivity data revealed that the polyion chain already collects its counter ions when the dimensions start to shrink below the good solvent limit but are still well above the $\theta$-dimension.