Solvent-Modulated Specific Ion Effects: Poly(N-isopropylacrylamide) Brushes in Nonaqueous Electrolytes

Hayden Robertson; Isaac J Gresham; Andrew R J Nelson; Kasimir P Gregory; Edwin C Johnson; Joshua D Willott; Stuart W Prescott; Grant B Webber; Erica J Wanless

doi:10.1021/acs.langmuir.3c02596

Solvent-Modulated Specific Ion Effects: Poly(N-isopropylacrylamide) Brushes in Nonaqueous Electrolytes

Langmuir. 2024 Jan 9;40(1):335-347. doi: 10.1021/acs.langmuir.3c02596. Epub 2023 Dec 20.

Authors

Hayden Robertson¹, Isaac J Gresham², Andrew R J Nelson³, Kasimir P Gregory⁴, Edwin C Johnson⁵, Joshua D Willott¹, Stuart W Prescott⁶, Grant B Webber¹, Erica J Wanless¹

Affiliations

¹ College of Science, Engineering and Environment, University of Newcastle, Callaghan, New South Wales 2308, Australia.
² School of Chemistry, University of Sydney, Sydney 2052, Australia.
³ Australian Centre for Neutron Scattering, ANSTO, Locked Bag 2001, Kirrawee DC, New South Wales 2232, Australia.
⁴ Division of Biomedical Science and Biochemistry, Research School of Biology, The Australian National University, Canberra, Australian Capital Territory 0200, Australia.
⁵ Department of Chemistry, University of Sheffield, Dainton Building, Brook Hill, Sheffield S3 7HF, U.K.
⁶ School of Chemical Engineering, UNSW Sydney, Sydney, New South Wales 2052, Australia.

Abstract

Pertinent to cryopreservation as well as energy storage and batteries, nonaqueous electrolytes and their mixtures with water were investigated. In particular, specific ion-induced effects on the modulation of a poly(N-isopropylacrylamide) (PNIPAM) brush were investigated in various dimethyl sulfoxide (DMSO)-water solvent mixtures. Spectroscopic ellipsometry and neutron reflectometry were employed to probe changes in brush swelling and structure, respectively. In water-rich solvents (i.e., pure water and 6 mol % DMSO), PNIPAM undergoes a swollen to collapsed thermotransition with increasing temperature, whereby a forward Hofmeister series was noted; K⁺ and Li⁺ electrolytes composed of SCN^- and I^- salted-in (stabilized) PNIPAM chains, and electrolytes of Cl^- and Br^- salted-out (destabilized) the polymer. The cation was seen to play a lesser role than that of the anion, merely modulating the magnitude of the anion effect. In 70 mol % DMSO, a collapsed to swollen thermotransition was noted for PNIPAM. Here, concentration-dependent specific ion effects were observed; a forward series was observed in 0.2 mol % electrolytes, whereas increasing the electrolyte concentration to 0.9 mol % led to a series reversal. While no thermotransition was observed in pure DMSO, a solvent-induced specific ion series reversal was noted; SCN^- destabilized the brush and Cl^- stabilized the brush. Both series reversals are attributed to the delicate balance of interactions between the solvent, solute (ion), and substrate (brush). Namely, the stability of the solvent clusters was hypothesized to drive polymer solvation.