Nanostructure of the deep eutectic solvent/platinum electrode interface as a function of potential and water content

The interfacial nanostructure of the three most widely-studied Deep Eutectic Solvents (DESs), choline chloride:urea (ChCl:Urea), choline chloride:ethylene glycol (ChCl:EG), and choline chloride:glycerol (ChCl:Gly) at a Pt(111) electrode has been studied as a function of applied potential and water content up to 50 wt%. Contact mode atomic force microscope (AFM) force–distance curves reveal that for all three DESs, addition of water increases the interfacial nanostructure up to ∼40 wt%, after which it decreases. This differs starkly from ionic liquids, where addition of small amounts of water rapidly decreases the interfacial nanostructure. For the pure DESs, only one interfacial layer is measured at OCP at 0.5 nm, which increases to 3 to 6 layers extending ∼5 nm from the surface at 40 or 50 wt% water. Application of a potential of ±0.25 V to the Pt electrode for the pure DESs increases the number of near surface layers to 3. However, when water is present the applied potential attenuates the steps in the force curve, which are replaced by a short-range exponential decay. This change was most pronounced for ChCl:EG with 30 wt% or 50 wt% water, so this system was probed using cyclic voltammetry, which confirms the interfacial nanostructure is akin to a salt solution

Brønsted acidity in deep eutectic solvents and ionic liquids.

Despite the importance of ionic liquids in a variety of fields, little is understood about the behaviour of protons in these media. The main difficulty arises due to the unknown activity of protons in non-aqueous solvents. This study presents acid dissociation constants for nine organic acids in deep eutectic solvents (DESs) using standard pH indicator solutes. The pKInvalue for bromophenol blue was found by titrating the DES with triflic acid. The experimental method was developed to understand the acid-base properties of deep eutectic solvents, and through this study it was found that the organic acids studied were slightly less dissociated in the DES than in water with pKavalues between 0.2 and 0.5 higher. pKInvalues were also determined for two ionic liquids, [Bmim][BF4] and [Emim][acetate]. The anion of the ionic liquid changes the pH of the solution by acting as a buffer. [Emim][acetate] was found to be more basic than water. It is also shown that water significantly affects the pH of ionic liquids. This is thought to arise because aqueous mixtures with ionic liquids form heterogeneous solutions and the proton partitions into the aqueous phase. This study also attempted to develop an electrochemical pH sensor. It was shown that a linear response of cell potential vs. ln aH+could be obtained but the slope for the correlation was less than that obtained in aqueous solutions. Finally it was shown that the liquid junction potential between two reference electrodes immersed in different DESs was dependent upon the pH difference between the liquids

Correction: Molecular and ionic diffusion in aqueous - deep eutectic solvent mixtures: probing inter-molecular interactions using PFG NMR.

Correction for 'Molecular and ionic diffusion in aqueous - deep eutectic solvent mixtures: probing inter-molecular interactions using PFG NMR' by Carmine D'Agostino et al., Phys. Chem. Chem. Phys., 2015, 17, 15297-15304

Thermodynamics of phase transfer for polar molecules from alkanes to deep eutectic solvents

Deep eutectic solvents (DESs) have been used for the purification of oils and the extraction of active ingredients from natural products but little is known about the mechanism of the extraction process. In this study a variety of molecular solutes are dissolved in alkanes and the thermodynamics of transfer into six DESs have been quantified. It is shown that the transfer of most solutes into the DES is endothermic and driven by entropy. The largest partition coefficients were demonstrated by the liquids with the lowest surface tensions and this is thought to arise because the enthalpy of hole formation controls the rate of solute transfer. Accordingly, it was shown that the size of the solute has an effect on the partition coefficient with smaller solutes partitioning preferably into the DES. As expected, solutes capable of strongly hydrogen bonding partitioned much better into the DES as the enthalpy of transfer was negative