Urea + choline chloride

Related to ionic liquids are substances known as deep eutectic solvents or mixtures. A series of these materials based on choline chloride (HOCH2CH2NMe3Cl) and either zinc chloride or urea have been reported (Abbott et al., 2002 2003). The urea/choline chloride material has many of the advantages of more well-known ionic liquids (e.g. low volatility), but can be sourced from renewable feedstocks, is non-toxic and is readily biodegradable. However, it is not an inert solvent and this has been exploited in the functionalisation of the surface of cellulose fibres in cotton wool (Abbott et al, 2006). Undoubtedly, this could be extended to other cellulose-based materials, biopolymers, synthetic polymers and possibly even small molecules. 

Table 6.4 Solubility of various metal oxides in a 2 1 urea-choline chloride eutectic at 60 °C. ...

Yang H, Reddy RG (2014) Electrochemical deposition of zinc from zinc oxide in 2 1 urea/choline chloride ionic hquid. Electrochim Acta 147 513—519. doi 10.1016/j.electacta. 2014.09.137... 

Recently, a eutectic mixture of choline chloride and urea (commercially known as Reline) was used as a medium from which CdS, as well as CdSe and ZnS, thin films were electrodeposited for the first time [53]. Reline is a conductive room-temperature ionic liquid (RTIL) with a wide electrochemical window. The voltammetric behavior of the Reline-Cd(II)-sulfur system was investigated, while CdS thin films were deposited at constant potential and characterized by photocurrent and electrolyte electroabsorbance spectroscopies. 

Many of these problems may be overcome by using ionic liquids based on sugars [35] or deep eutectic mixtures. Deep eutectic mixtures such as that derived from choline chloride and urea (m. pt. 12°C [36]) or carboxylic acids [37] can be liquids and have very low vapour pressure. They have been successfully used as electrochemical solvents, but their use in catalysis remains little explored. Urea is a fertiliser and choline chloride (Vitamin B4) is a component of chicken feed so the mixture is environmentally acceptable. 

Abbott AP, Capper G, Davies DL et al (2003) Novel solvent properties of choline chloride/ urea mixtures. Chem Commun 1 70-71... 

Abbott, A.P.,G. Capper, D.L. Davies, R.K. Rasheed and V. Tambyrajah, Novel Solvent Properties of Choline Chloride/Urea Mixtures, Chemical Communications, 70-71 (2003). 

In this chapter some results on the electrodeposition of alloys from ionic liquids are summarized. Many fundamental studies have been performed in chloroaluminate first generation ionic liquids but the number of studies employing air- and water-stable ionic liquids rather than the chloroaluminates is increasing. Currently, new ionic liquids with better electrochemical properties are being developed. For example, Abbott et al. [47] have prepared a series of ionic liquids by mixing commercially available low-cost choline chloride and MCI2 (M = Zn, Sn) or urea and demonstrated that these ILs are good media for electrodeposition for pure metals (see Chapter 4.3). It can be expected that in the near future, the electrodeposition of alloys from ILs may become available for industrial applications. Furthermore, due to their variety, their wide electrochemical and thermal windows air- and water-stable ionic liquids have unprecedented prospects for electrodeposition. 

Processing of metal ores is a very energy-intensive process, and the use of RTILs in this area has therefore attracted a lot of attention. Metals have been selectively extracted from mixed metal oxides using choline chloride-urea DES (Table 6.4). The dissolved metals can be reclaimed using electrodeposition. 

In some cases, chemical functionalization of ILs is not necessary. Using a eutectic melt of choline chloride and urea, Abbott et al. dissolved several metal oxides. In particular, the eutectic melt dissolves Zn and P and leaves Fe and aluminosilicates behind [182], Abbott et al. have also reported on the behavior of eutectic mixtures of zinc chloride and urea or acetamide [183], Using FAB-MS it was possible to determine that the resulting ILs are made up of metal-containing anions. 

The simplest approach to obtain ILs from natural sources is surely the direct use of quaternary ammonium salts. Choline chloride, a salt melting at 302 °C, has been transformed recently by Davies and co-workers in a fluid at room temperature by mixing the organic salt with metal chlorides or other simple organic compounds (such as, urea or polyalcohols), which depress the melting point producing liquids with significant ionic character (Scheme 2). These liquids systems are known as eutectic mixtures. 

Moreover, recently DES of choline chloride/urea (or malonic acid) have been used as solvents in the synthesis of new coordination polymers, Zn(O30PCH2CO2)NI-E, of open-framework iron oxaphosphates and alumino-phospate zeolite analogues. Generally, the ionic medium not only acts as a solvent but also provides the template cations around which the inorganic frameworks order. 

The solubility of 17 common metal oxides in three ionic liquids based on choline chloride with urea, malonic acid, and ethylene glycol has been determined and compared with aqueous solutions of HCl and NaCl [20]. Details are presented in Table 5.1. The order of solubility was HCl > malonic acid > urea > NaCl > ethylene glycol. Higher solubility was observed with the more ionic oxides such as ZnO in... 

Since 1980s, a large number of studies on the electrodeposition of metals have been reported. Historically, the water-sensitive chloroaluminate first-generation ILs are the most intensively studied. However, in future, the focus will rather be on air-and water-stable ionic liquids due to their variety and the less strict conditions under which they can be handled. Several review articles, books, and book chapters on the electrodeposition of metals and alloys from ILs have already been published [27-183], Ionic liquids can be classified as water/air sensitive (first-generation ionic liquids based on AIX3 (X=C1, Br)) and water/air stable (discrete anions based and eutectic solvents/ionic liquids such as ZnCl, urea, ethylene glycol, and choline chloride). A selection of published examples of the electrodeposition of metals and alloys from ionic liquids is listed in Table 5.3 the original work for each metal can be found in Refs. [23, 29-183], In this section, we mainly focus on the electrodeposition of active metals such as Al, Mg, and Ti in ionic liquids. 

Dale PJ, SamantiHeke AP, Shivagan DD (2007) Synthesis of cadmium and zinc semiconductor compounds from an ionic hquid containing choline chloride and urea. Thin Solid Films 515 5751-5760... 

DES = Biodegradable deep eutectic solvent generated from choline chloride and urea. 

Synthesis of aminooxazole derivatives via thermal and ultrasonic methods using deep eutectic solvent as the medium was recently reported by Singh et al. (2013). It was observed that the ultrasound-assisted method gave 90% yield in just 8 min as against 3.5 h required to achieve 69% yield by the thermal method. 2-Amino-4-aryloxazoles (80) were obtained via the reaction of phenacyl bromide derivatives (78) and amide derivatives (79) in a deep eutectic solvent (Scheme 8.25). The deep eutectic solvent used in this protocol was easily prepared from choline chloride (1 eq) and urea (2 eq) at 80°C by a previously reported method (Toukoniitty et al. 2006) with 100% atom economy. 

Golgovici F, Visan T (2012) Electrodeposition behaviour of cadmium telluride from choline chloride-urea ionic liquids. Chalcogenide Lett 9(4) 165-174... 

Deep Eutectics. Some years ago it was discovered that by mixing salts, e.g., choline chloride (melting point 302 °C), with urea (melting point 133 °C) in a molar ratio of 1 2, the melting point of the mixture drops down to 12 °C [7], These novel systems are somewhere in between Ionic Liquids and electrolyte solutions and have promising properties. 

E. Siddalingamurthy, K. M. Mahadevan, T. O. S. Kumar, Synth. Commun. 2013, 43, 3153—3162. Choline chloride/ urea ionic liquid eatalyzed a eonvenient one-pot synthesis of indole-3-propanamide derivatives. 

In another study, chitin nanofibers were prepared by using deep eutectic solvents (DESs), including mixtures of choline halide (chloride/bromide)-urea and choline chloride-thiourea. The produced chitin nanofibers were utilized for manufacturing calcium alginate bionanocomposite gel beads to improve elasticity and retard drug release [21]. 

Abbott et al. (2003) describe, as a typical example, mixtures of urea (m.p. 132.7°C) with choline chloride (m.p. 310°C), which exhibit a eutectic at ca. 64mol percent urea and 12°C. The fluidity (reciprocal of viscosity) and electrical conductivity of the eutectic mixture increase together with increasing temperature. The latter is similar to that of many ionic liquids, approximately 1 mS cm" at 30°C. They report solubilities at 50°C, in mol/1 LiCl >2.5 AgCl 0.66 benzoic acid 0.82 D-alanine 0.38 CuO 0.12. 

In addition to third-generation ILs, a new class of ionic solvent systems, called DES, has been developed. DES are typically formed by mixing a quaternary ammonium salt, such as choline chloride, and an uncharged hydrogen-bond donor, such as urea, carboxylic acids (e.g., oxalic or amino acids), or polyols (e.g., glycerol) [11,131]. Although DES are not entirely composed of ionic species, their physicochemical properties are very close to those of common ILs [132]. 

For the first DES that was formed between choline chloride (ChCl) and urea (U), there is a V-shaped relation between the freezing point and the ChCl/U ratio, producing a eutectic with the lowest T of -12°C at a molar ratio of 1 2 [5]. This indicates that for a molecule of choline chloride, two molecules of urea are required to form the eutectic with the most depressed freezing point. When carboxylic acids were used as the HBD in place of urea [19], one can find that a eutectic was formed at a salt/HBD... 

Figure 25.3 Impact of DES and its components on the growth of Hydra sinensis. The hydra survived after incubation for 7 h in aqueous solution containing the DES ChCl/U (1 1) but was disintegrated in the presence of choline chloride (ChCl), urea (U), and a mixture of ChCl and U at a molar ratio of 1 1 (ChCl+U).

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