Deep eutectic solvents (DESs) are rapidly emerging as environmentally benign and viable alternatives not only to common organic solvents but to relatively toxic ionic liquids as well. By judicious selection of the constituents and subsequent variation in their composition, the key physicochemical properties of the DESs can be easily tailored for a specific application. Impact of variation in constituents as well as their composition on important physical properties of natural DESs (NADESs) prepared using a common fatty acid n-decanoic acid (DA) and two naturally occurring monoterpenoids, thymol (Thy) and L(−)-menthol (Men), is assessed. Specifically, water miscibility, surface tension, density, and dynamic viscosity of 17 different DESs of three combinations of constituents, Men:DA, Thy:DA, and Thy:Men in four, four, and nine compositions, respectively, are measured. The water intake by the DESs under ambient conditions varies from 0.6 to 2.7 mol·kg–1 offering a means to gauge their hydrophobicity. Water miscibility is relatively lower for Men:DA DESs and higher for Thy:DA DESs. Surface tension values are within 28.37 mN·m–1 to 31.14 mN·m–1 under ambient conditions; they are higher for the DESs with the Thy constituent and vary systematically with DES composition. The densities of all the DESs are lower than that of water and decrease linearly with increasing temperature. While density does not vary with composition for Men:DA, it is found to increase linearly with an increase in Thy mole fraction for Thy:DA and Thy:Men DESs. The dynamic viscosity of the DESs are relatively lower as compared to the common tetralkylammonium salt- and metal salt-based DESs, and exhibit strong constituent and composition dependence. Rather than the Arrhenius-type expression, the temperature-dependence of the dynamic viscosity better conforms to the Vogel–Fulcher–Tammann (VFT) model with activation energy of the viscous flow varying systematically with the constituent and composition. Reported physical properties and their dependence on constituents/composition of the NADESs will enhance their utility and help establish them as novel alternate media in science and technology.

Deep eutectic solvents (DESs) have emerged as versatile and inexpensive solubilizing media with widely varying physicochemical properties. Establishing characteristics of a novel solvent milieu for polymer dissolution is an important...

Intrinsic fluorescence from l-tryptophan (l-Trp) is routinely used to obtain insight into the structural features and dynamics of proteins and enzymes. In contrast to aqueous enzymology, different parameters that control and influence the behavior of proteins and enzymes in nonaqueous media depend heavily on the solvent. Detailed analysis of the intrinsic fluorescence from l-Trp dissolved in two deep eutectic solvents (DESs), reline and glyceline, prepared by mixing salt choline chloride with H-bond donors urea and glycerol, respectively, in a 1:2 molar ratio within 298.15–358.15 K temperature range, is presented. Fluorescence emission maxima of l-Trp dissolved in DESs show bathochromic shift with increasing temperature. In comparison to water and several organic solvents, the fluorescence quantum yields of l-Trp in both DESs are significantly higher. While the rates of nonradiative decay in the two DESs are comparable and increase with increasing temperature, radiative decay rates are independent of temperature and are higher in glyceline than in reline, resulting in a higher fluorescence quantum yield of l-Trp in glyceline. Excited-state emission intensity decays of l-Trp fit best to a double exponential model irrespective of the identity of the DES and temperature. Average lifetime decreases with increasing temperature due to increased thermal deactivation; however, this decrease is much slower in DESs as compared to that in water. Both steady-state fluorescence anisotropy and rotational reorientation times for l-Trp are governed by the inherent complexity of the DESs as solubilizing milieu resulting in noncompliance to simple hydrodynamic treatment. Fluorescence quenching of l-Trp by acrylamide in reline is purely dynamic in nature. This is in contrast to the aqueous media where the decrease in fluorescence is a combined result of both dynamic and static quenching. The quenching within reline is fairly efficient considering the high viscosity of the medium. Significantly lower activation energy of the bimolecular quenching process as compared to the activation energy of the viscous flow indicates facilitation of the electron/charge transfer quenching of l-Trp by acrylamide within the ionic environment offered by reline. The effect of high viscosity is partly overcome by the strongly ionic environment of reline during the electron/charge transfer between l-Trp and acrylamide. The results highlight the structural complexity of these DESs especially within the cybotactic region of the probe, which is absent in common molecular solvents of similar high viscosity.

The pyrenyl (Py) groups in several dipyrenyl (or bispyrenyl) compounds, while do not dimerize in the ground-state, are known to intramolecularly aggregate exclusively in the excited-state to form excimers in...

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