Crown ether solvent extraction

Reddy, M. L. P. and Francis, T. (2001) Recent advances in the solvent extraction of mercury(II) with calixarenes and crown ethers, Solvent Extraction and Ion Exchange, 19(5), 839-863. 

The extraction becomes significant only in the presence of crown ether, which strongly indicates that crown ether has a critical role as a complexing reagent. The ratio of the amino acid to crown ether in extractable species for Trp, Leu, Gly, Ala is 1 1 and for Arg and Lys 1 2. At pH 2 the dicationic forms of Arg and Lys dominate. It means that the extraction in ILs proceeds as in traditional solvents, where each ammonium group interacts with one molecule of crown ether. 

The distribution ratios (D s) for crown-ether-based extraction processes using conventional solvents depend on two major factors (1) the thermodynamic driving force for cafion complexation by a crown efher and (2) the solvation of fhe cafion and counfer anion by the organic solvent [1,4] the former factor is usually thermodynamically favored (see Equation 10.3). Difficulties in increasing the solvent-extraction efficiency of conventional solvent-extraction systems using crown ethers as extractants lie in the... 

An extraction efficiency of RTILs over conventional solvents was definitely observed for a series of cations in presence of neutral complexing agents (Dietz, 2006 Murding Tang, 2010). At the same time, unlike for molecular solvents the Dm values for alkali and alkaline earth cations M in numerous RTIL/water systems in the presence of different crown ethers declined as HNQs concentration in aqueous phase increased, then the Dm dependence passed a minimum at c.a. 1 mol/ dmP concentration and started to increase as nitric acid concentration increased up to 3 mol/dm3 (EHetz, 2006 Egorov et al, 2010). Our group has demonstrated that the pH-dependence of crown-ether assisted extraction from water into RTIL phase correlates well with the relative distribution of a crown (Vendilo, et al, 2009), Fig. 1,2... 

Cesium isotopes can be recovered from fission products by digestion in nitric acid, and after filtration of waste the radioactive cesium phosphotungstate is precipitated using phosphotungstic acid. This technique can be used to prepare radioactive cesium metal or compounds. Various processes for removal of Cs isotopes from radioactive waste have been developed including solvent extraction using macrocycHc polyethers (62) or crown ethers (63) and coprecipitation with sodium tetraphenylboron (64). 

Various types of research are carried out on ITIESs nowadays. These studies are modeled on electrochemical techniques, theories, and systems. Studies of ion transfer across ITIESs are especially interesting and important because these are the only studies on ITIESs. Many complex ion transfers assisted by some chemical reactions have been studied, to say nothing of single ion transfers. In the world of nature, many types of ion transfer play important roles such as selective ion transfer through biological membranes. Therefore, there are quite a few studies that get ideas from those systems, while many interests from analytical applications motivate those too. Since the ion transfer at an ITIES is closely related with the fields of solvent extraction and ion-selective electrodes, these studies mainly deal with facilitated ion transfer by various kinds of ionophores. Since crown ethers as ionophores show interesting selectivity, a lot of derivatives are synthesized and their selectivities are evaluated in solvent extraction, ion-selective systems, etc. Of course electrochemical studies on ITIESs are also suitable for the systems of ion transfer facilitated by crown ethers and have thrown new light on the mechanisms of selectivity exhibited by crown ethers. 

The first examples of the application of phase-transfer catalysis (PTC) were described by Jarrousse in 1951 (1), but it was not until 1965 that Makosza developed many fundamental aspects of this technology (2,3). Starks characterized the mechanism and coined a name for it (4,5), whilst Brandstrom studied the use of stoichiometric amounts of quaternary ammonium salts in aprotic solvents, "ion-pair extraction" (6). In the meantime Pedersen and Lehn discovered crown-ethers (7-9) and cryptands (10,11), respectively. 

A number of solvent extraction experiments have demonstrated that individual crown ethers in combination with a lipophilic sulfonic acid (such as didodecylnaphthalene sulfonic acid) are efficient, synergistic phase transfer agents for manganese(II) from aqueous solution into an organic phase." The X-ray structure of the manganese di-t-butylnaphthalenesul-fonate with cyclohexano-15-crown-5 ether as its toluene solvate has been reported." ... 

A benzo[18]crown-6 adduct (72) of Cgg (not shown) has been synthesized by the addition of the corresponding o-quinodimethane 71 in toluene [58]. The solubility of 72 in pro tic solvents such as MeOH strongly increases after the complexation of ions, as shown by extraction experiments. The combination of the crown ether and the fullerene moiety in 72 provides a highly amphiphilic character. This behavior allowed the preparation of Langmuir-Blodgett films of monolayers on mica of 72 and its complex. 

Dai, S., Ju, Y.H., and Barnes, C.E., Solvent extraction of strontium nitrate by a crown ether using room-temperature ionic liquids, /. Chem. Soc., Dalton Trans., 1201,1999. 

The distribution ratios obtained were compared to the corresponding distribution ratios between water and Toctanol. The authors also calculated solvent parameters of [C4Cilm][PFg] (Section 9.3). It was shown that phenolate-ion associates with [C4CiIm][PFg] more strongly than other ions. The authors also mention the possibility of extraction of amino acids into [C4CiIm][PPg] in the presence of crown ether dibenzo-18-crown-6, though at rather moderate efficiency. 

The most interesting point is that extraction of amino acids into IL occurs without addition of a counterion. Typically, for amino acid extraction into conventional solvents (including extraction with crown ether), a hydrophobic counterion is required. Moreover, in most cases, even the presence of such a counteranion does not provide an efficient recovery. 

High distribution ratios and almost quantitative recovery were observed for all amino acids. In contrast to conventional solvents, the extraction of the most hydrophilic amino acids such as Gly is quantitative. Recovery of Trp, Leu, Ala, Gly, Lys, and Arg is 96, 93, 92, 95, 94, and 92%, respectively. Also, amino acids, including highly hydrophilic, can be extracted with high efficiency from the mixture. For example, the recovery of Trp, Val, Gly from their equimolar mixture is equal to 99, 94, and 93%, respectively. Extraction was performed by adding 3 mL of IL with crown ether concentration 0.10 mol L with a 3 mL aqueous solution of amino acids (5 10 mol/L each 1.8 pH) and shaking for 15 min. 

From a thermodynamic perspective, the solvation of ionic species (see Equations 10.6 and 10.7), such as crown-ether complexes, NO3, and SO 4, in the ILs, should be much more favored thermodynamically than those of conventional solvent extractions (Equations 10.1 and 10.2). This is one of the key advantages of using ILs in separations involving ionic species. In this case, cationic crown-ether complexes and their counter anions are not expected to form ion pairs, but to be solvated separately by ionic species from the ILs. Therefore, the extraction process using crown ethers in ILs may not be an ion-pair extraction process. 

McDowell, W. J., Crown ethers as solvent extraction reagents Where do we stand, Sep. Sci. Technol, 23,1251-1268,1988. 

Blasius, E., Nilles, K.-H., The removal of cesium from medium-active waste solutions. 1. Evaluation of crown ethers and special crown-ether adducts in the solvent extraction of cesium, Radiochim Acta, 35,173-182,1984. 

Visser, A. E., Swatloski, R. P, Reichert, W. M., Griffin, S. T., Rogers, R. D., Traditional extractants in nontraditional solvents Groups 1 and 2 extraction by crown ethers in room-temperature ionic liquids, Ind. Eng. Chem. Res., 39,3596-3604,2000. 

Luo, H., Dai, S., Bonnesen, R V., Solvent extraction of Sr + and Cs+ based on room-temperature ionic liquids containing monoaza-substituted crown ethers. Anal. Chem., 76,2773-2779, 2004. 

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