ABSTRACT
Deep eutectic solvents (DES) are a novel class of ionic liquid-based solvents, combining an organic salt and a hydrogen bond acceptor (HBA) at specific molar ratios. The resulting DES mixtures often have strongly depressed freezing points and feature properties well-known for ionic liquids as non-classical solvents. In this study, mixtures of 1-ethyl-3-methylimidazolium chloride ([EMIm][Cl]) and urea are investigated at different molar ratios mainly via electron paramagnetic resonance (EPR) spectroscopy on chemical environment-specific nitroxide-based spin probes, aided by differential scanning calorimetry (DSC) to obtain insights into the structure, dynamics, and molecular processes on the nanoscale. Molecular dynamics simulations, and Raman and pulse-field gradient (PFG) NMR spectroscopy are used to substantiate the insights in particular into the dynamic heterogeneities on the nanoscale. We find that indeed the mixing ratios leading to melting point extrema (two eutectic points, one dystectic point) show unusual EPR spectra indicating changes in the reorientational dynamics of the spin probes and their environmental polarity. By thorough EPR spectral analysis and simulation and in combination with data from the other methods, detailed assumptions on the nanostructure and dynamics in this DES can be made. It is shown that the macroscopic DES properties are governed by the nanoscale interface of IL-based nanoregions and urea-enriched regions. This nanointerface crucially depends on the chloride anion and its ability to form hydrogen bonds with urea which leads to distinctive structural changes.
ABSTRACT
The repair by co-antioxidants of the phenoxy radical of resveratrol, the famous health-preserving ingredient of red wine, is a key step of radical scavenging cascades in nature. To generate that radical, we employed 355 nm photoionization as a direct and selective access that reduces the chemical complexity and is equally applicable in organized phases; to monitor it, we used its hitherto unreported absorption in the red where no other species in our systems interfere. With this novel approach, we measured rate constants and H/D kinetic isotope effects for the repairs by ascorbate, trolox (a vitamin E analogue) and 4-aminophenol, and identified the mechanisms as one-step hydrogen abstractions. Cysteine and glutathione are unreactive. In micellar solution (SDS), the repair by ascorbate is much slower and involves only the hydrophilic phenoxy moieties protruding from the micelles. The new experimental strategy also led to a reevaluation of extinction coefficients, rate constants and mechanisms.
