Mixed Solids Form Low-freezing Liquids

When choline chloride and urea--both solids--are mixed together in a molar ratio of 1:2, a liquid mixture is formed that freezes at 12 C. As pure compounds, choline chloride ([HOCH₂CH₂N(CH₃)₃]⁺Cl^{) and urea [(NH₂)₂CO] melt at 302 C and 133 C, respectively.}

This curious behavior is not unique to choline chloride-urea mixtures. Indeed, researchers have described a family of liquids consisting of eutectic mixtures of choline chloride and either an organic hydrogen-bond donor or an inorganic metal salt that exhibit a significant drop in freezing point relative to that of their constituents. These so-called deep eutectics are proving potentially useful as solvents for organic reactions, for the recovery and extraction of metals, and for electroplating and electropolishing.

The choline chloride-urea eutectic fluid was reported by physical chemistry professor Andrew P. Abbott, senior lecturer David L. Davies, and coworkers at the University of Leicester, in England (Chem. Commun. 2003, 70). They showed that the fluid has unusual solvent properties. For example, aromatic acids, amino acids, several metal oxides, and inorganic salts such as silver chloride that are sparingly soluble in water are highly soluble in the eutectic fluid. The team later reported that choline chloride forms room-temperature eutectic fluids with other hydrogen-bond donors such as amides, alcohols, and carboxylic acids (J. Am. Chem. Soc. 2004, 126, 9142).

"The novelty of the mixtures is the magnitude of the freezing-point depression coupled with the environmental compatibility of the constituents," Abbott says. Choline chloride is used on the megaton scale as a chicken-feed additive, he notes. And urea is a common nitrogen-release fertilizer. The liquids made by mixing these two inexpensive chemicals "are insensitive to moisture, and their low volatility means that they have negligible volatile organic carbon emissions," Abbott says. "They are also nontoxic and even biodegradable. From a regulatory, health, and safety perspective, they have the advantage that their material safety data sheets are well-known.

"This approach to making environmentally compatible solvents is also applicable to a wide variety of quaternary ammonium and other organic salts and an even wider range of hydrogen-bond donors," he adds.

The fluid mixtures consist of the quaternary ammonium cation and a complex of the halide anion and the hydrogen bond donor. Their properties, such as nonflammability, conductivity, and polarity, are similar to those of room-temperature ionic liquids. "To distinguish eutectic liquids from ionic liquids, we adopted the term 'deep eutectic solvent' for them," Abbott says.

Eutectic mixtures of zinc chloride and choline chloride and other substituted quaternary ammonium salts have even larger depressions of freezing point--up to 270 C, according to the Leicester chemists. The mixtures have potential for large-scale applications such as zinc electroplating, batteries, and organic synthesis, they say. Last year, the team showed that low-melting-point eutectic mixtures can also be formed from tin(II), iron(III), and other metal halides and a variety of quaternary ammonium halides (Inorg. Chem. 2004, 43, 3447).

In 1998, Leicester University and Genacys--a wholly owned subsidiary of the Whyte Group, one of the largest privately owned chemical groups in the U.K.--set up the joint-venture company Scionix to develop and commercially exploit eutectic technology for industrial use.

"Scionix has developed several applications, including electropolishing, electroplating, catalyst recovery, biotransformation, surface cleaning, and synthesis," Genacys Director Khalid Shukri says. "The most commercially advanced is the electropolishing technology. We have developed a liquid that electropolishes stainless steel to a standard that is indistinguishable from the current technology that uses mixtures of harsh aqueous mineral acids such as phosphoric and sulfuric acids. Our technology has been scaled up from a 1-L beaker to a 1.25-ton tank."

The electropolishing project is being carried out in collaboration with one of Britain's largest electropolishing companies--Anopol, which is based in Birmingham, England.

"Electropolishing is a massive industry," Abbott remarks. "Electropolishing of stainless steel is an effective way of increasing corrosion resistance and therefore preventing rust. Electropolished components decrease wear and increase lubricity in engines and therefore reduce the risk of engine failure. The current acid-based technology is inherently inefficient, as only 10-20% of the energy supplied is used for electropolishing. In addition, the mineral acids are potentially harmful to work with and must be neutralized before disposal."

THE SCIONIX electropolishing technology employs a eutectic mixture of two relatively benign materials--choline chloride and ethylene glycol. In recent work, the Leicester chemists investigated the use of the mixture as an electropolishing electrolyte in an electrolytic cell containing a stainless steel alloy anode and a nickel cathode. They showed that the eutectic mixture reacts with the metal cations on the surface of the steel to form complexes with low saturation concentrations (Trans. Inst. Met. Finish. 2005, 82, 51).

"When the metal in the steel is electrochemically oxidized, it complexes with the hydrogen-bond donor--ethylene glycol--and an insoluble complex precipitates to the bottom of the electropolishing bath," Abbott explains. "The liquid can be filtered periodically and the metal complex collected."

The process enables the eutectic fluid and the metal to be recycled, thus preventing environmental emissions. Compared with the acid-based technology, the eutectic technology has improved current efficiency and evolves less hydrogen, according to Scionix.

The company is also developing eutectic mixtures of choline chloride and metal salts to plate chromium and other metals onto a wide range of metal substrates.

"Electroplating is the opposite of electropolishing," Abbott observes. "With electropolishing, a metal is removed, whereas electroplating deposits a metal.

Last year, Abbott, Davies, and coworkers reported that chromium can be efficiently electrodeposited onto stainless steel from a eutectic of choline chloride and hydrated chromium(III) chloride, CrCl₃6H₂O, mixed in the molar ratio of 1:2 (Trans. Inst. Met. Finish. 2004, 82, 14). They also found that the addition of lithium chloride to the mixture results in the deposition of black nanocrystalline chromium films that are corrosion resistant and crack free.

"Hard, wear-resistant, low-friction coatings are required for critical applications in civil and military aerospace, in the automotive and marine industries, and for industrial hydraulics," Abbott says. "For example, hard chromium coatings are essential for hydraulic rams in everything from aircraft landing gear to excavators."

Conventional chromium plating technology suffers from a major disadvantage: It employs hexavalent chromium, Cr(VI), which is highly toxic and carcinogenic. In addition, the reduction of Cr(VI) requires acid media, and the current efficiency of the process is low.

The eutectic mixture of choline chloride and CrCl₃6H₂O is significantly less toxic than Cr(VI) salts. Scionix is now using the eutectic mixture to develop a chromium electroplating process that has a current efficiency of more than 90%.

"This reduced power consumption reduces the cost and the overall environmental impact of the chromium-plating process," Shukri notes. "Hydrogen evolution is negligible, as we do not use aqueous solutions, and it is therefore possible to produce crack-free, highly corrosion resistant deposits. The process should allow thinner deposits to be formed, thus reducing overall material and power consumption even further."

Scionix has also shown that biocatalysis can be carried out in deep eutectic solvents such as its commercial product Reline, a mixture of urea and choline chloride that freezes at 12 C. The enzyme is added as a concentrated aqueous buffer to the mixture, and the substrate is then stirred in. After reaction, the products can be extracted with a variety of solvents that are immiscible with the deep eutectic solvent. The company has tested the system with -chymotrypsin, a proteolytic enzyme that hydrolyzes peptide bonds during the intestinal digestion of proteins.

Deep eutectic solvents are supercooled liquids with freezing points lower than their melting points, Shukri observes. The solvents can therefore be stored as solids. "We have found that -chymotrypsin can be dissolved in anhydrous Reline, and once frozen, the mixture can be returned to ambient temperature and stored," he notes. "We found that when the mixture was melted after a week, it retained most of its activity."

IN RELATED work, chemists at the University of Regensburg, in Germany, have been investigating the use of low-melting mixtures of carbohydrates, urea, and inorganic salts as solvents for organic reactions (Chem. Commun. 2005, 1170). They call the mixtures "sweet solutions."

"The solutions are mixtures of glucose, fructose, or sugar alcohols like sorbitol; urea; and inorganic salts, such as calcium chloride or ammonium chloride," explains organic chemistry professor Burkhard Knig, who leads the group. "The mixtures give stable, clear melts at 70 C. The decrease in melting temperature avoids any caramel formation, which is so well-known for sugars.

"Sweet solutions are cheap, nontoxic, and very green," he continues. "Their potential use as substitutes for organic solvents in chemical production could lead to more sustainable processes. The solvents are easily prepared by mixing readily available bulk compounds and heating the mixtures to the reaction temperature."

Knig estimates that the polarity of the mixtures is similar to that of dimethylformamide. "We have shown that the melts are suitable solvents for Diels-Alder reactions, homogeneously and heterogeneously catalyzed hydrogenations, and palladium-catalyzed couplings."

In a separate development at the University of St. Andrews, in Scotland, chemistry professor Russell E. Morris and coworkers have used ionic liquids and eutectic mixtures for the preparation of microporous crystalline zeolites (Nature 2004, 430, 1012). "Our work is based on the concept that using ionic liquids as both the solvent and the template for the synthesis of zeolite analogs removes the competition between template and solvent--usually water--in the synthesis," Morris says. "We have shown that the method can be used to prepare novel types of solids with chemistry that can be traced back to the type of ionic liquid or eutectic mixture used."

The group prepared, for example, an aluminophosphate zeolite by using a eutectic mixture of choline chloride and urea. Ammonium, formed by partial decomposition of the urea, acts to template the structure and balance the charge on the framework. The pores are small and therefore adsorb only the smallest molecules. Preliminary ion-exchange experiments indicate that the ammonium cations can be partly exchanged for metal cations such as Cu²⁺.

"The interesting and different chemistry of eutectic mixtures, plus the number of different types of ionic liquid that are available, point to many opportunities in materials synthesis, many of which we are currently pursuing," Morris says.

Abbott points to a number of other applications for deep eutectic solvents and metal-based eutectic ionic liquid mixtures. They are potentially useful, he says, for chromatography, dissolving peptides, and natural-product separations and extractions. They can also be used in catalysis, such as for Friedel-Crafts alkylations and acylations; as highly conducting electrolytes for battery technology and photovoltaics; and for other electrochemical applications, including the recovery and extraction of metals from ores, spent nuclear fuel, and slag.

In work yet to be published, the Leicester team determined the solubility of a variety of metal oxides in a 2:1 urea:choline chloride eutectic at 60 C. "Oxides such as ZnO, PbO₂, and Cu₂O exhibit appreciable solubility in the eutectic, whereas iron oxides and particularly Al₂O₃ are very poorly soluble," Davies notes. "The different solubilities and reduction potentials of the metal oxides provide a strategy for separating metal oxides and the selective extraction of metals from mixtures of metal oxides."

Abbott stresses that eutectic mixtures are easy and cheap to manufacture. "We are very excited about the possibilities of this new technology," he says. "The industrial interest in these new fluids has been extremely encouraging, and we hope that they will soon provide viable green alternatives for metal polishing, metal plating, and other technologies."