Nanostructure of a deep eutectic solvent at solid interfaces

Aaron Elbourne; Nastaran Meftahi; Tamar L Greaves; Christopher F McConville; Gary Bryant; Saffron J Bryant; Andrew J Christofferson

doi:10.1016/j.jcis.2021.01.089

Nanostructure of a deep eutectic solvent at solid interfaces

J Colloid Interface Sci. 2021 Jun:591:38-51. doi: 10.1016/j.jcis.2021.01.089. Epub 2021 Feb 1.

Authors

Aaron Elbourne¹, Nastaran Meftahi², Tamar L Greaves³, Christopher F McConville⁴, Gary Bryant³, Saffron J Bryant⁵, Andrew J Christofferson⁶

Affiliations

¹ School of Science, RMIT University, Melbourne, VIC 3000, Australia. Electronic address: aaron.elbourne@rmit.edu.au.
² ARC Centre of Excellence in Exciton Science, School of Science, RMIT University, Melbourne, VIC 3001, Australia.
³ School of Science, RMIT University, Melbourne, VIC 3000, Australia.
⁴ School of Science, RMIT University, Melbourne, VIC 3000, Australia; Institute for Frontier Materials, Deakin University, Geelong, Victoria 3216, Australia.
⁵ School of Science, RMIT University, Melbourne, VIC 3000, Australia. Electronic address: saffron.bryant@rmit.edu.au.
⁶ School of Science, RMIT University, Melbourne, VIC 3000, Australia. Electronic address: andrew.christofferson@rmit.edu.au.

PMID: 33592524
DOI: 10.1016/j.jcis.2021.01.089

Abstract

Hypothesis: Deep eutectic solvents (DESs) are an attractive class of tunable solvents. However, their uptake for relevant applications has been limited due to a lack of detailed information on their structure-property relationships, both in the bulk and at interfaces. The lateral nanostructure of the DES-solid interfaces is likely to be more complex than previously reported and requires detailed, high-resolution investigation.

Experiments: We employ a combination of high-resolution amplitude-modulated atomic force microscopy and molecular dynamics simulations to elucidate the lateral nanostructure of a DES at the solid-liquid interface. Specifically, the lateral and near-surface nanostructure of the DES choline chloride:glycerol is probed at the mica and highly-ordered pyrolytic graphite interfaces.

Findings: The lateral nanostructure of the DES-solid interface is heterogeneous and well-ordered in both systems. At the mica interface, the DES is strongly ordered via polar interactions. The adsorbed layer has a distinct rhomboidal symmetry with a repeat spacing of ~0.9 nm comprising all DES species. At the highly ordered pyrolytic graphite interface, the adsorbed layer appears distinctly different, forming an apolor-driven row-like structure with a repeat spacing of ~0.6 nm, which largely excludes the chloride ion. The interfacial nanostructure results from a delicate balance of substrate templating, liquid-liquid interactions, species surface affinity, and packing constraints of cations, anions, and molecular components within the DES. For both systems, distinct near-surface nanostructural layering is observed, which becomes more pronounced close to the substrate. The surface nanostructures elucidated here significantly expand our understanding of DES interfacial behavior and will enhance the optimization of DES systems for surface-based applications.

Keywords: Atomic force microscopy; Deep eutectic solvents; HOPG; Interfacial nanostructure; Lateral nanostructure; Mica; Molecular dynamics simulations.