Tuning the collapse transition of weakly charged polymers by ion-specific screening and adsorption

TL;DR

This study uses simulations and a mean-field model to explain how ion-specific effects influence the swelling and collapse of weakly charged polymers, revealing complex salt concentration-dependent behaviors aligned with experimental observations.

Contribution

It introduces a minimalistic simulation and a mean-field model to qualitatively and quantitatively explain ion-specific effects on polymer collapse and swelling.

Findings

Ion-specific screening and bridging cause collapse at low salt concentrations.

Re-entrant swelling occurs at high salt concentrations due to direct ion-polymer interactions.

An 'isospheric point' exists where ion-specific effects vanish, aligning with experimental Hofmeister series inversion.

Abstract

The experimentally observed swelling and collapse response of weakly charged polymers to the addition of specific salts displays quite convoluted behavior that is not easy to categorize. Here we use a minimalistic implicit solvent / explicit salt simulation model with a focus on ion-specific interactions between ions and a single weakly charged polyelectrolyte to qualitatively explain the observed effects. In particular, we demonstrate ion-specific screening and bridging effects cause collapse at low salt concentrations whereas the same strong ion-specific direct interactions drive re-entrant swelling at high concentrations. Consistently with experiments, a distinct salt concentration at which the salting-out power of anions inverts from the reverse to direct Hofmeister series is observed. At this, so called 'isospheric point', the ion-specific effects vanish. Furthermore, with additional simplifying assumptions, an ion-specific mean-field model is developed for the collapse transition which quantitatively agrees with the simulations. Our work demonstrates the sensitivity of the structural behavior of charged polymers to the addition of specific salt and shall be useful for further guidance of experiments.