From Hofmeister to hydrotrope: Effect of anion hydrocarbon chain length on a polymer brush

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

doi:10.1016/j.jcis.2022.12.114

From Hofmeister to hydrotrope: Effect of anion hydrocarbon chain length on a polymer brush

J Colloid Interface Sci. 2023 Mar 15:634:983-994. doi: 10.1016/j.jcis.2022.12.114. Epub 2022 Dec 23.

Affiliations

¹ College of Science, Engineering and Environment, University of Newcastle, Callaghan, NSW 2308, Australia.
² College of Science, Engineering and Environment, University of Newcastle, Callaghan, NSW 2308, Australia; Department of Materials Physics, Research School of Physics, Australian National University, Canberra, ACT 0200, Australia.
³ College of Science, Engineering and Environment, University of Newcastle, Callaghan, NSW 2308, Australia; Department of Chemistry, The University of Sheffield, Sheffield, UK.
⁴ School of Chemical Engineering, UNSW Sydney, Sydney, NSW 2052, Australia; School of Chemistry, University of Sydney, Sydney, NSW 2006, Australia.
⁵ Australian Centre for Neutron Scattering, ANSTO, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia.
⁶ Department of Materials Physics, Research School of Physics, Australian National University, Canberra, ACT 0200, Australia.
⁷ School of Chemical Engineering, UNSW Sydney, Sydney, NSW 2052, Australia.
⁸ College of Science, Engineering and Environment, University of Newcastle, Callaghan, NSW 2308, Australia. Electronic address: erica.wanless@newcastle.edu.au.

PMID: 36571860
DOI: 10.1016/j.jcis.2022.12.114

Abstract

Hypothesis: Specific ion effects govern myriad biological phenomena, including protein-ligand interactions and enzyme activity. Despite recent advances, detailed understanding of the role of ion hydrophobicity in specific ion effects, and the intersection with hydrotropic effects, remains elusive. Short chain fatty acid sodium salts are simple amphiphiles which play an integral role in our gastrointestinal health. We hypothesise that increasing a fatty acid's hydrophobicity will manifest stronger salting-out behaviour.

Experiments: Here we study the effect of these amphiphiles on an exemplar thermoresponsive polymer brush system, conserving the carboxylate anion identity while varying anion hydrophobicity via the carbon chain length. Ellipsometry and quartz crystal microbalance with dissipation monitoring were used to characterise the thermoresponse and viscoelasticity of the brush, respectively, whilst neutron reflectometry was used to reveal the internal structure of the brush. Diffusion-ordered nuclear magnetic resonance spectroscopy and computational investigations provide insight into polymer-ion interactions.

Findings: Surface sensitive techniques unveiled a non-monotonic trend in salting-out ability with increasing anion hydrophobicity, revealing the bundle-like morphology of the ion-collapsed system. An intersection between ion-specific and hydrotropic effects was observed both experimentally and computationally; trending from good anti-hydrotrope towards hydrotropic behaviour with increasing anion hydrophobicity, accompanying a change in hydrophobic hydration.

Keywords: Hofmeister; Hydrophobic effect; Hydrotrope; Polymer brush; Short-chain fatty acids; Specific ion effects; Surface chemistry; Thin films.

MeSH terms

Anions / chemistry
Hydrocarbons
Hydrophobic and Hydrophilic Interactions
Polymers* / chemistry
Sodium Chloride*

Substances

Polymers
Anions
Sodium Chloride
Hydrocarbons