Water-Induced Restructuring of the Surface of a Deep Eutectic Solvent.

Rahul Gera,Huib J Bakker,Aditi Bhattacherjee,Carolyn J Moll

doi:10.1021/acs.jpclett.1c03907

Abstract

We study the molecular-scale structure of the surface of Reline, a DES made from urea and choline chloride, using heterodyne-detected vibrational sum frequency generation (HD-VSFG). Reline absorbs water when exposed to the ambient atmosphere, and following structure-specific changes at the Reline/air interface is crucial and difficult. For Reline (dry, 0 wt %, w/w, water) we observe vibrational signatures of both urea and choline ions at the surface. Upon increase of the water content, there is a gradual depletion of urea from the surface, an enhanced alignment, and an enrichment of the surface with choline cations, indicating surface speciation of ChCl. Above 40% w/w water content, choline cations abruptly deplete from the surface, as evidenced by the decrease of the vibrational signal of the −CH2– groups of choline and the rapid rise of a water signal. Above 60% w/w water content, the surface spectrum of aqueous Reline becomes indistinguishable from that of neat water.

Highlights

We study the molecular-scale structure of the surface of Reline, a deep eutectic solvents (DESs) made from urea and choline chloride, using heterodynedetected vibrational sum frequency generation (HD-VSFG)
I n the ongoing search for environment-friendly solvents, deep eutectic solvents (DESs) are emerging as a promising candidate to replace volatile organic solvents and possibly ionic liquids.[1−6] DESs are two-component mixtures, generally made up of an organic salt and a hydrogen bond donor in a specific molar ratio, with an eutectic melting point that is much lower than the melting points of the pure constituents.[3−5,7−10] DESs are nonflammable, have a low vapor pressure and high thermal stability, and can usually be produced at low costs
The unique physicochemical properties of these binary mixtures are attributed to the nature and strength of the intermolecular interactions, predominantly hydrogen bonds.[3−5,7−10] One of the most compelling arguments for using DESs is that their chemical composition allows them to be used as a green solvent.[3−18] As a result, DESs have already found applications in the fields of homogeneous bulk-phase chemistry related to organic chemistry reactions and mesoporous material synthesis,[19−21] electrochemistry,[22,23] industrial processes[18,24] such as metal ion sequestration,[25] and biotransformations.[26−28]

Summary

Introduction

We study the molecular-scale structure of the surface of Reline, a DES made from urea and choline chloride, using heterodynedetected vibrational sum frequency generation (HD-VSFG).

Results

Conclusion