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

Hayden Robertson; Edwin C Johnson; Isaac J Gresham; Stuart W Prescott; Andrew Nelson; Erica J Wanless; Grant B Webber

doi:10.1016/j.jcis.2020.10.092

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

J Colloid Interface Sci. 2021 Mar 15:586:292-304. doi: 10.1016/j.jcis.2020.10.092. Epub 2020 Oct 26.

Authors

Hayden Robertson¹, Edwin C Johnson¹, Isaac J Gresham², Stuart W Prescott², Andrew Nelson³, Erica J Wanless¹, Grant B Webber⁴

Affiliations

¹ Priority Research Centre for Advanced Particle Processing and Transport, University of Newcastle, Callaghan, NSW 2308, Australia.
² School of Chemical Engineering, UNSW Sydney, NSW 2052, Australia.
³ ANSTO, Locked bag 2001, Kirrawee DC, NSW, 2232, Australia.
⁴ Priority Research Centre for Advanced Particle Processing and Transport, University of Newcastle, Callaghan, NSW 2308, Australia. Electronic address: grant.webber@newcastle.edu.au.

PMID: 33189318
DOI: 10.1016/j.jcis.2020.10.092

Abstract

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, δ, 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.

Keywords: Ellipsometry; Mixed electrolyte; Neutron reflectometry; Polymer brush; Specific ion effects; Thermoresponsive polymer.