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

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 −CH 2 – 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.

Dissolution mechanism of cellulose in a benzyltriethylammonium/urea deep eutectic solvent (DES): DFT-quantum modeling, molecular dynamics and experimental investigation

“…The decrease in the eutectic point of the BTEAB/urea DES to around À15 1C may hinder the use of DES-H 2 O/DMSO as a solvent in organic, electrochemical, and other applications. [140][141][142][143][144]…”

Structure of Deep Eutectic Solvents (DESs): What We Know, What We Want to Know, and Why We Need to Know It

“…Many DESs contain water, and even when dried to remove water, a trace amount of water is an unavoidable impurity . There are a few reports of bulk DES structure as a function of water content and fewer reports of interfacial structure. ,,,,− …”

“…Experimental data show that interfacial structure, similar to bulk structure, is retained at high water contents. 62,66 At the gas/DES interface, Bakker et al observed a gradual depletion of urea as well as enrichment and alignment of choline cations at the surface with increasing water contents of up to 40 wt %. 66 At 60 wt % water, the interfacial structure of the DES is indistinguishable from that of pure water.…”

“…62,66 At the gas/DES interface, Bakker et al observed a gradual depletion of urea as well as enrichment and alignment of choline cations at the surface with increasing water contents of up to 40 wt %. 66 At 60 wt % water, the interfacial structure of the DES is indistinguishable from that of pure water. This phenomenon was also observed using atomic force microscopy force−distance curves at the solid/DES interface with 30−50 wt % water.…”

Interfacial Nanostructure and Hydrogen Bond Networks of Choline Chloride and Glycerol Mixtures Probed with X-ray and Vibrational Spectroscopies

“…53 Their results from vibrational SFG spectroscopy have indicated the surface of the mixture above 60 wt % H 2 O resembles the spectrum of pure water. 53 This observation agrees with our result from aerosol XPS spectra (Figure 4b). The XPS spectrum of 80 wt % H 2 O mixture (Figure 4a) is almost indistinguishable from the spectrum of pure glycerol (Figure S1) or glycerol−water mixtures, indicating minor surface presence of Ch + at higher water content.…”