In the quest for green solvents design: from hydrophilic to hydrophobic (deep) eutectic solvents.

Deep eutectic solvents (DESs) consist of a mixture of two or more solid components leading to a drop of the melting point of the mixture when compared to the starting materials. Until recently, only hydrophilic DESs were available, and despite their revolutionary role in the alternative solvents scenario, important issues in chemistry and chemical engineering, such as water related problems or the replacement of toxic volatile organic compounds, could not be tackled. Hydrophobic (Deep) - here in parenthesis due to the different depths of the eutectic´s melting points - Eutectic Solvents are a sub-class of DESs where both components are hydrophobic. The choice of low cost, naturally occurring compounds, with low toxicity and high biodegradability and straightforward preparation without purification steps, are among the key advantages that distinguish them as innovative solvents. Although research on hydrophobic DESs is scarce, since the first paper on the subject was only published in 2015, some interesting features and applications have been published and deserve to be reviewed and comparisons established. The current review is divided into two parts: first, a brief general introduction to DESs and the second part details on nomenclature using solid-liquid phase diagram analysis, chemical stability, thermophysical properties comparison and finally the most important emerging fields of application.