Deep eutectic solvent

Deep eutectic solvents are systems formed from a eutectic mixture of Lewis or Brønsted acids and bases which can contain a variety of anionic and/or cationic species. They are classified as types of ionic solvents with special properties. They incorporate one or more compound in a mixture form, to give a eutectic with a melting point much lower than either of the individual components. One of the most significant deep eutectic phenomenon was observed for a mixture of choline chloride and urea in a 1:2 mole ratio. The resulting mixture has a melting point of 12 °C (far less than the melting point of choline, 302 °C and urea, 133 °C), which makes it liquid at room temperature.

The first generation eutectic solvents were based on mixtures of quaternary ammonium salts with hydrogen bond donors such as amines and carboxylic acids. There are four types of eutectic solvents:

Type I Eutectics therefore also include the wide range of chlorometallate ionic liquids widely studied in the 1980s, such as the ever-popular imidazolium chloroaluminates which are based on mixtures of AlCl₃ + 1-Ethyl-3-methylimidazolium chloride. In addition to ionic liquids with discrete anions, the electrodeposition of a range of metals has been previously carried out in deep eutectic solvents (DESs). These are quaternary ammonium salts (e.g. choline chloride, ChCl), metal salts or metal salt hydrates and hydrogen bond donors (e.g. urea) and are commonly divided into four groups (Table 1), and have been particularly successful on a large scale for metal polishing and immersion silver deposition. While most ionic liquids and DESs include a quaternary ammonium ion as the cationic component, it has recently been shown that eutectics can also be formed between a metal salt (hydrate) and a simple amide or alcohol to form a metalcontaining solution composed of cations and anions via disproportionation processes e.g.

2AlCl₃ + urea ↔ [AlCl₂•urea]⁺ + [AlCl₄]⁻
These so-called Type 4 eutectics are useful as they produce cationic metal complexes, ensuring that the double layer close to the electrode surface has a high metal ion concentration.

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