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

Richard Chudoba,Joachim Dzubiella,Jan Heyda

doi:10.1039/c8sm01646a

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 beyond simple screening and shall be useful for further guidance of experiments.

Highlights

The Hofmeister effects on the polymeric structure and the phase behavior are very complex and polymer specific
Ureido copolymers are only partially miscible in water below their upper critical solution temperature (UCST), which can be modulated by addition of salts.[9]
Another example is the poly-N-isopropylacrylamide (PNIPAM) homopolymer or elastin-like polypeptides (ELPs), which both exhibit hydrophobic collapse transitions at their respective lower critical solution temperatures (LCST).[10,11,12]. These collapse transitions are analogous to cold-denaturation of proteins, in which ion-specific interactions (Hofmeister effects) play a prominent role

Summary

Introduction

The Hofmeister effects on the polymeric structure and the phase behavior are very complex and polymer specific. Ureido copolymers are only partially miscible in water below their upper critical solution temperature (UCST), which can be modulated by addition of salts.[9] Another example is the poly-N-isopropylacrylamide (PNIPAM) homopolymer or elastin-like polypeptides (ELPs), which both exhibit hydrophobic collapse transitions at their respective lower critical solution temperatures (LCST).[10,11,12] These collapse transitions are analogous to cold-denaturation of proteins, in which ion-specific interactions (Hofmeister effects) play a prominent role. The UCST is influenced by the copolymer composition and rises as the ureido content increases. Amine groups in the copolymer are protonated at pH B 7.5 making the polymer weakly positively charged.[9]

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Results

Conclusion