Competitive specific ion effects in mixed salt solutions on a thermoresponsive polymer brush

Hypothesis: Grafted poly(ethylene glycol) methyl ether methacrylate (POEGMA) copolymer brushes change conformation in response to temperature ('thermoresponse'). In the presence of different ions the thermoresponse of these coatings is dramatically altered. These effects are complex and poorly understood with no all-inclusive predictive theory of specific ion effects. As natural environments are composed of mixed electrolytes, it is imperative we understand the interplay of different ions for future applications. We hypothesise anion mixtures from the same end of the Hofmeister series (same-type anions) will exhibit non-additive and competitive behaviour. Experiments: The behaviour of POEGMA brushes, synthesised via surface-initiated ARGET-ATRP, in both single and mixed aqueous electrolyte solutions was characterised with ellipsometry and neutron reflectometry as a function of temperature. Findings: In mixed fluoride and chloride aqueous electrolytes (salting-out ions), or mixed thiocyanate and iodide aqueous electrolytes (salting-in ions), a non-monotonic concentration-dependent influence of the two anions on the thermoresponse of the brush was observed. A new term, delta, has been defined to quantitively describe synergistic or antagonistic behaviour. This study determined the specific ion effects imparted by salting-out ions are dependent on available solvent molecules, whereas the influence of salting-in ions is dependent on the interactions of the anions and polymer chains. (C) 2020 Elsevier Inc. All rights reserved.

Automated summary

This study investigates the thermoresponse of POEGMA brushes in the presence of different ions, revealing complex behaviors such as non-additive and competitive effects in mixtures of same-type anions. A new term, delta, is introduced to quantitatively describe the synergistic or antagonistic interactions between these anions. The specific ion effects of salting-out ions are dependent on solvent molecules, while those of salting-in ions are influenced by the interactions between anions and polymer chains.