Deep
eutectic solvents (DESs) are lately expanding their use to
more demanding applications upon aqueous dilution thanks to the preservation
of the most appealing properties of the original DESs while overcoming
some of their most important drawbacks limiting their performance,
like viscosity. Both experimental and theoretical works have studied
this dilution regime, the so-called “water-in-DES” system,
at near-to stoichiometric amounts to the original DES. Herein, we
rather studied the high-dilution range of the “water-in-DES”
system looking for enhanced performance because of the interesting
properties (a further drop of viscosity) and cost (water is cheap)
that it offers. In particular, we found that, in the “water-in-DES”
system of a ternary DES composed of resorcinol, urea and choline chloride
(e.g., RUChClnW, where n represents mol of water
per mole of ternary DES), the tetrahedral structure of water
was distorted as a consequence of its incorporation, as an additional
hydrogen bond donor or hydrogen bond acceptor, into the hydrogen bond
complexes formed among the original DES components . DSC confirmed
the formation of a new eutectic, with a melting point below that of
its respective components, the original ternary DES and water. This
depression in the melting point was also observed in the same regime
of reline and malicine aqueous dilutions, thus suggesting the universality
of this simple procedure (i.e., water addition to reach the high-dilution
range of the “water-in-DES” system) to obtain deeper
eutectics eventually providing enhanced performances and lower cost.
Deep eutectic solvents (DESs) offer a suitable alternative to conventional solvents in terms of both performance and cost-effectiveness. Some DESs also offer certain green features, the greenness of which is notoriously enhanced when combined with water. Aqueous DES dilutions are therefore attracting great attention as a novel green medium for biotechnological processes, with the aqueous dilutions of reline - a DES composed of urea and choline chloride - being one of the most studied systems. Despite their macroscopic homogeneous appearance, both H NMR spectroscopic studies and molecular dynamics simulations have revealed the occurrence of certain dynamic heterogeneity at a microscopic molecular level. Ultrasonic measurements were also used with the aim of getting further insights but nonconclusive results were obtained. Herein, we have studied aqueous reline dilutions by Brillouin spectroscopy given its proved suitability for detecting local structure rearrangements in liquid mixtures of H-bonded co-solvents. Brillouin spectroscopy revealed the formation of a co-continuous structure resulting from local structure rearrangements and micro-segregation into aqueous and DES phases. Interestingly, there is agreement betweenH NMR and Brillouin spectroscopy when pointing to the DES content where microphase segregation and formation of co-continuous structures occurred.
Phosphorus-doped carbon–carbon nanotube hierarchical monoliths exhibiting energy densities of around 22.6 W h kg−1 at power densities of up to 10 kW kg−1 were capable of functioning as true three-dimensional electrodes in supercapacitor cells.
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