18-Crown-6 and Dibenzo-18-crown-6 Assisted Extraction of Cesium from Water into Room Temperature Ionic Liquids and Its Correlation with Stability Constants for Cesium Complexes

Abstract

The pH-profiles of the extraction of Cs⁺ into four conventional (1-butyl-3- methylimidazolium hexafluorophosphate and bis[trifluoromethyl)sulphonyl]imides of 1- butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium, and 1-(2-ethylhexyl)-3-methylimidazolium) and two novel (trioctylmethylammonium salicylate and tetrahexyl-ammonium dihexylsulfosuccinate) room temperature ionic liquids have been determined both in the absence and in the presence of crown ether (18-crown-6 or dibenzo-18-crown-6). The pH-profiles of distribution ratio of crown ethers have been established in the same conditions. The relationship of cesium extraction efficiency both with the stability of its complexes with crown ethers and crown ethers’ distribution ratio has been clarified.

1. Introduction

Hydrophobic crown ethers are applied as extractants in the separation and recovery of ¹³⁷Cs and ⁹⁰Sr radioactive isotopes from high-level nuclear waste solutions, containing high concentrations of acids and salts, into molecular solvents [1,2,3,4,5]. Room-temperature ionic liquids (RTILs) are attracting increasing attention in solvent extraction processes due to important advantages over conventional organic diluents such as negligible vapor pressure, low flammability, moisture stability, unusual extraction properties and possibility to eliminate aqueous phase acidification [6,7,8,9,10,11,12,13]. It was demonstrated that extraction efficiency of RTIL can be modulated by using complexing agents, for example, crown ethers. Dai et al. [8] discovered that highly efficient extraction of strontium ions can be achieved when dicyclohexane-18-crown-6 is combined with RTILs. Rogers et al. [9] and Bartsch et al. [12] reported the extraction of various alkali metal ions with crown ethers in RTILs. Visser and Rogers demonstrated that octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide dissolved in RTILs enhances the extractability of lanthanides and actinides in comparison to conventional organic solvents [13]. The extraction of silver ions was found to be greatly enhanced by a combined application of RTIL and calix[4]arene compared to that of chloroform [14]. In addition, the task-specific RTILs with complexing functionality built in the RTIL cation have been reported [15,16]. Recently the efficiency of chelate extraction of 3d-cations with 8-sulfonamidoquinoline [17], Pu(IV) with carbamoylmethylphosphine oxide [18] and uranyl ion with tributylphosphate (TBP) [19] from aqueous phase into RTILs was reported. The higher selectivity of dibenzo-18-crown-6 to K⁺ over Na⁺ in N-octadecylisoquinolinium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate compared with that in molecular solvents suggests that RTIL provides a unique solvation environment for the complexation of crown ethers with the ions [20].

Besides the issues of cation, ligand and complex solubility in water and in RTIL, the relative stabilities of complex formation in both phases are of significant importance for extraction selectivity. In our recent communication [21] we have reported on the thermodynamics of complex formation of cesium ions with 18-crown-6 (18C6, L) in six hydrophobic RTILs (see Scheme 1): trioctylmethylammonium salicylate ([TOMA][Sal]), tetrahexylammonium dihexylsulfosuccinate ([THA][DHSS]), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]), 1-butyl-3- methylimidazolium bis[trifluoromethyl)sulphonyl]imide ([BMIM][N(Tf)₂]), 1-hexyl-3-methyl- imidazolium bis[trifluoromethyl)sulphonyl]imide ([HMIM][N(Tf)₂]), and 1-(2-ethylhexyl)-3-methyl- imidazolium bis[trifluoromethyl)sulphonyl]imide ([EtHMIM][N(Tf)₂]) by a ¹³³Cs-NMR technique. The present work aims to study the extraction capability of cesium by the above mentioned RTILs in the presence of 18C6 and dibenzo-18-crown-6 (DB18C6, L) and to clarify its relationship with the stability of complexes formed in aqueous and organic phases. Another objective is to study the impact of crown ether’s nature on the extraction efficiency. For this reason the two crown ethers with similar cavity, but different hydrophobicity (18C6 and DB18C6) have been chosen.

2. Results and Discussion

The results of the extraction experiments are presented in

Table 1. Distribution ratios of cesium (C_w⁰(Cs) = 1.5 × 10^-3 mol·dm^-3) between RTIL/aqueous phases in the absence of crown ether as a function of pH at 22 °C.

RTIL	Vo, mL	Vw, mL	Aqueous pH	logDCs
[THA][DHSS]	0.5	5.0	5.94	1.21
			3.36	1.11
			1.59	0.84
[TOMA][Sal]	0.5	5.0	5.98	0.69
			4.95	0.76
			3.01	0.74
			1.13	0.10
[BMIM][PF6]	0.5	5.0	5.90	-0.59
			3.33	-0.82
			1.25	-0.80
[BMIM][N(Tf)2]	1.0	3.0	6.44	-0.67
			3.35	-1.24
			1.25	-1.31
[EtHMIM][N(Tf)2]	1.0	3.0	6.34	-0.81
			2.09	-0.62
[HMIM][N(Tf)2]	2.0	2.0	5.82	-1.24
			2.84	-1.36
			0.93	-2.21

,

Table 2. Distribution ratios of cesium [C_w⁰(Cs) = 1.5 × 10^-3 mol·dm^-3] and 18C6 [C_o⁰(18C6) = 1.5 × 10^-1 mol·dm^-3] in the presence of each other between RTIL/aqueous phases as a function of pH at 22 °C (V_o = 0.5 mL; V_w = 5.0 mL).

RTIL	Aqueous pH	logDCs18C6	logD18C6Cs
[THA][DHSS]	5.92	0.24	-0.12
	4.30	0.23	-0.16
	3.86	0.03	-0.41
	3.39	-0.38	-0.50
	2.48	0.57	-0.15
	1.60	0.75	-0.01
[TOMA][Sal]	5.84	0.83	0.48
	4.33	0.89	0.44
	3.81	0.90	0.36
	3.31	0.77	0.30
	2.22	0.36	0.21
	1.20	1.11	0.43
[BMIM][PF6]	5.84	-0.20	0.13
	4.33	-0.45	0.08
	3.81	-0.62	0.06
	3.31	-1.27	0.00
	2.22	-0.74	0.00
	1.20	-0.66	0.08
[BMIM][N(Tf)2]	6.88	1.56	0.77
	5.60	1.43	0.65
	3.56	-0.26	0.50
	2.26	-0.27	0.08
	1.27	-0.23	0.23
[EtHMIM][N(Tf)2]	6.50	0.56	-0.27
	2.15	0.30	-0.45
[HMIM][N(Tf)2]	5.60	0.82	0.25
	4.10	0.91	0.17
	2.96	0.61	0.06
	1.40	-0.22	0.08
	0.90	-0.33	0.27

and

Table 3. Distribution ratios of cesium and DB18C6 in the presence of each other between RTIL/aqueous phases as a function of pH at 22 °C (V_o = 0.5 mL; V_w = 5.0 mL).

RTIL	Cw0(Cs), mol·dm-3	Co0(18C6), mol·dm-3	Aqueous pH	logDCsDB18C6	logDDB18C6Cs
[THA][DHSS]	5.0·10-4	5.0·10-2	6.03	1.56	2.68
			2.85	1.27	2.41
			2.45	0.89	2.20
			1.89	0.74	2.15
			1.06	0.86	2.30
			0.14	1.02	2.68
[TOMA][Sal]	5.0·10-4	5.0·10-2	6.07	1.91	2.62
			3.54	1.86	2.32
			2.77	1.42	2.11
			2.11	1.12	2.05
			1.08	1.52	2.45
			0.13	1.96	3.05
[BMIM][N(Tf)2]	1.5·10-3	1.5·10-1	6.93	2.28	2.89
			5.09	2.06	2.71
			3.16	0.14	2.57
			1.92	-0.47	2.63
			0.38	-0.60	2.94

, where D_Cs refers to cesium distribution ratio for the extraction from aqueous phase into RTIL in the absence of crown ethers, D_Cs^18C6 as well as D_Cs^DB18C6 indicate the same in the presence of 18C6 and DB18C6, respectively, while D_18C6^Cs and D_DB18C6^Cs are distribution ratios for 18C6 and DB18C6 in the presence of cesium ions.

The RTILs studied demonstrate different extraction behavior towards cesium. [TOMA][Sal] and [THA][DHSS] appeared to be better solvents for this cation then water and provide positive logD_Cs values within the whole pH range, Table 1. This fact can be explained by the ability of salicylate and dihexylsulfosuccinate anions to interact with metal ion. Though such an interaction is not expected to be strong, worthy is mentioning that the bulk concentration of RTILs complexing anion is rather high: [Sal] = 1.68 and [DHSS] = 1.29 mol·dm^-3. In contrast, conventional-anion RTILs [BMIM][PF₆], [BMIM][N(Tf)₂], [HMIM][N(Tf)₂], and [EtHMIM][N(Tf)₂] reveal negative logD_Cs values, which are in a good agreement with those found for [BMIM][PF₆] earlier [12],

Table 4. Stability Constants of Cesium Complex Formation with 18C6 in RTIL and Extraction Efficiency from Water at 22 °C and pH 5 – 7^a.

Solvent	logK1	logK2	logDCs	logDCs18C6	logD18C6Cs	log[DCs18C6/(DCsD18C6Cs)]
[HMIM][N(Tf)2]	4.4	1.13	−1.24	0.82	0.25	1.81
[EtHMIM][N(Tf)2]	3.4	1.16	−0.81	0.56	−0.27	1.64
[BMIM][N(Tf)2]	3.4	1.29	−0.67	1.56	0.77	1.46
[BMIM][PF6]	2.3		−0.59 −0.59b −1.17c	−0.20 −0.52d	0.13 0.15e	0.26
[TOMA][Sal]	1.45		0.69	0.83	0.49	−0.38
[THA][DHSS]	0.77		1.21	0.25	−0.12	−0.84
1,2-dichloroethane	7.98f	2.58f	−7.7g	0.8h	0.03i	6.93

^a Stability constants for RTILs are taken from [21] for 25 °C, K₁ = [Cs(18C6)][Cs]⁻¹[18C6]⁻¹ K₂ = [Cs(18C6)₂][Cs(18C6)]⁻¹[18C6]⁻¹; ^b Ref. [12]; ^c Ref. [9], pH = 7; ^d Ref. [9], C_o⁰(18C6) = 1.0 × 10⁻¹ mol·dm^-3; ^e Ref. [9], C_o⁰(18C6) = 1.0 × 10⁻¹ mol·dm^-3, blank experiment (without CsNO₃); ^f Ref. [23]; ^g Ref. [2], picrate salt; ^h Ref. [3], picrate salt; ⁱ Ref. [1].

. The cesium extraction ability of all RTILs, with the exception of [EtHMIM][N(Tf)₂], slightly diminishes as pH decreases.

The situation becomes completely different when crown-ether molecules are introduced into RTIL/water biphasic systems. 18C6 significantly decreases cesium’s uptake by [THA][DHSS] but increases it for all other RTILs, Table 2. As far as we know, [THA][DHSS] is the first example of a negative influence of crown ether on a cation extraction into organic phase. Probably, the main reason is that crown-ether competes with a RTIL’s anion for cesium and, being better soluble in water, provides a partial transfer of the metal ion into an aqueous phase. This suggestion is indirectly supported by the low value of [Cs(18C6)]⁺ stability constant in [THA][DHSS] (logK₁), Table 4, which appears to be even lower than in water.

For DB18C6 an enhancement of cesium extraction is much more pronounced then for 18C6, Table 3. Notably, increasing HNO₃ concentration in an aqueous phase decreases cesium’s recovery for all RTILs, but at very low pH both logD_Cs^18C6 and logD_Cs^DB18C6 start to increase. Thus, there appears a minimum at the curves logD_Cs = f(pH), in between pH 0 and pH 7 (Figure 1).

A similar effect has been observed in [9] for cesium distribution in [BMIM][PF₆] in the presence of 4,4’-(5’)-di-(tert-butylcyclohexano)-18-crown-6. Notably, cesium recovery in strongly acidic media is usually lower than in neutral solutions (with the exception of systems 18C6/[THA][DHSS], 18C6/[TOMA][Sal], and DB18C6/[TOMA][Sal]). Thus, unlike crown ether solutions in conventional molecular solvents, such solutions in the studied RTILs do not need acidification to increase a recovery. Moreover, in most cases the addition of acid leads to the decrease of cesium recovery.

Worthy of mention is the fact that the extraction efficiency of solutions of CE in conventional solvents is often unsatisfactory even in strongly acidic media. According to Yakshin et al. [22], in the case of Cs⁺ (1 × 10^-3 M) extraction from 2 M HNO₃ aqueous solution into 0.1 M 18C6 in CHCl₃ the value of D_Cs is extremely low (3 × 10^-3; logD_Cs = -2.52). This value is lower than even the minimal values of D_Cs obtained in the case of our RTILs-based systems. As a rule, the extraction efficiency of the systems based on conventional solvents may be raised to the level of CE/RTIL-systems only by addition of a bulk hydrophobic counter-ion (such as picrate) to the aqueous phase. So, the logD_Cs^18C6 values for 1,2-dichloroethane in the presence of picrate in aqueous phase are of the same level that for some RTILs (e.g., [HMIM][N(Tf)₂] and [TOMA][Sal], Table 4), while the hazardous properties of the former are incomparably higher.

The crown ether influence on cesium extraction by RTILs is evidently explained by a formation of rather stable complexes. Previously, we found [21] that Cs⁺ forms stable complexes [Cs(18C6)]⁺ in [TOMA][Sal], [THA][DHSS], [BMIM][PF₆], [BMIM][N(Tf)₂], [HMIM][N(Tf)₂] and [EtHMIM][N(Tf)₂], and the stability constants have been measured. Besides, in [BMIM][N(Tf)₂], [HMIM][N(Tf)₂], and [EtHMIM][N(Tf)₂] the species [Cs(18C6)₂]⁺ are also formed [21]. In an aqueous phase only [Cs(18C6)]⁺ species are registered [24].

Generally, when an aqueous phase of cesium nitrate is in equilibrium with a RTIL organic phase, containing crown ether L, the cesium distribution ratio D_Cs^L (for such RTILs as [TOMA][Sal], [THA][DHSS] and [BMIM][PF₆]), where only CsL species are formed is represented by a simple equation (1):

D_Cs^L = ([Cs]^RTIL + [CsL]^RTIL)/([Cs]^w + [CsL]^w)

(1)

where the superscripts “RTIL” and “w” denote organic and aqueous phase, respectively. When the total concentration [L]^t >> [Cs]^t and the stabilities of complexes in both phases are high enough (logK₁^RTIL ≥ 2; logK₁^w ≥ 2), then [Cs]^RTIL << [CsL]^RTIL and [Cs]^w << [CsL]^w. In this case, the equilibrium concentrations of [Cs]^RTIL and [Cs]^w in (1) can be neglected. The equation (1) is therefore transformed into (2):

D_Cs^L = [CsL]^RTIL/[CsL]^w

(2)

or:

D_Cs^L = (K₁^RTIL[Cs]^RTIL[L]^RTIL)/(K₁^w[Cs]^w[L]^w).

(3)

As far as [Cs]^RTIL/[Cs]^w = D_Cs and [L]^RTIL/[L]^w = D_L:

D_Cs^L = K₁^RTIL D_CsD_L/K₁^w

(4)

or:

logD_Cs^L = logK₁^RTIL + logD_Cs + logD_L − logK₁^w.

(5)

Thus, the plot of (logD_Cs^L − logD_Cs − logD_L) versus logK₁^RTIL was expected to be linear with a slope of 1. In our case the initially stated requirement logK₁^RTIL ≥ 2; logK₁^w ≥ 2 does not take place for some RTILs and for water. Indeed, logK₁^w = 0.98 [24] and for the RTILs the following values of logK₁^RTIL at 25 °C have been determined [21]: 1.45 (0.05) ([TOMA][Sal]); 0.77 (0.04) ([THA][DHSS]); 2.4 (0.2) ([BMIM][PF₆]); 3.4 (0.5) ([BMIM][N(Tf)₂]); 4.4 (0.1) ([HMIM][N(Tf)₂]), and 3.4 (0.4) ([EtHMIM][N(Tf)₂]). Nevertheless, the plot of (logD_Cs^18C6 − logD_Cs − logD_18C6^Cs) versus logK₁^RTIL for [TOMA][Sal], [THA][DHSS], and [BMIM][PF₆] represents a perfectly strait line with a slope 0.73 and R² = 0.999, Figure 2, Table 4. In order to confirm the legitimacy of the derivation of the equation 4 we have determined the concentration profiles of D_Cs for all RTILs in the range C_w⁰(Cs) = 2 × 10⁻⁴ − 5 × 10⁻² M. In all cases D_Cs remains nearly constant at 2·10⁻⁴ ≤ C_w⁰(Cs) ≤ 1.5·10⁻³ M (≤ 5·10⁻³ M − for [TOMA][Sal] and [THA][DHSS]). At the same time the maximal cesium recovery (in the case of 18C6) is observed for 18C6/[BMIM][N(Tf)₂] at pH = 6.88 (Table 2), which corresponds to the residual free cesium concentration being equal to 2.9 × 10⁻⁴ M (i.e., even in this case the ratio [free Cs in the organic phase]/[free Cs in the aqueous phase] is the same that in the case of Cs⁺-extraction from 1.5 × 10⁻³ M aqueous solution in the absence of crown ether in RTIL-phase).

In the case of [BMIM][N(Tf)₂], [HMIM][N(Tf)₂], [EtHMIM][N(Tf)₂] the equations (1), (2), (4) and (5) become more complicated due to formation of CsL₂ complexes and the lack of such complexes in water. However, all six RTILs fit equation (5) with a slope 0.89 and a correlation coefficient R² = 0.98, Figure 3. This can be explained by a domination of logK₁^RTIL values over logK₂^RTIL ones, Table 4. Another reason of low contribution of K₂^RTIL to the extraction is in the absence of enough sufficient excess of crown re. cesium concentration. Anyway, the linear relationship of (logD_Cs^18C6 − logD_Cs − logD_18C6^Cs) and logK₁^RTIL indicates clearly an importance of complex stability contribution for the extraction of cesium into hydrophobic RTILs.

Besides logK₁^RTIL, also a contribution of logD_L should be examined. Thus, along with cesium extraction we have studied the water/RTIL distribution of crown ethers, Table 2 and Table 3. 18C6 demonstrates similar affinity towards as the RTILs, so the conventional molecular solvents: 1,2-dichloroethane, chloroform and dichloromethane [5]. The detaining of DB18C6 in extracts is sufficiently higher for all RTILs then that of 18C6, Table 2 and Table 3. At the same time logD_DB18C6^Cs values are one to two log unites lower then those found for benzene, 1,2-dichloroethane, chloroform and nitrobenzene [5].

It should be noted that the variation of logD_Cs^18C6 and logD_Cs^DB18C6 with pH shows the same trend as the crown ether distribution between aqueous phase and RTIL, logD_18C6^Cs and logD_DB18C6^Cs, Figure 1. A linear relationship between logD_18C6^Cs and logD_Cs^18C6 (slope 0.91; R² = 0.96) as well as between logD_DB18C6^Cs and logD_Cs^DB18C6 (slope 1.24; R² = 0.91) was found for TOMAS, Figure 4, indicating the dominating role of relative crown ether solubility in water and RTIL.

Analysis of the data presented in Table 4 for [BMIM][N(Tf)₂], [HMIM][N(Tf)₂], and [EtHMIM][N(Tf)₂] indicates that the contribution of logD_18C6^Cs into logD_Cs^18C6 prevails over logK^RTIL values. Indeed, the variation of logD_Cs^18C6 within this set of similar RTILs with the same anion definitely follows the changes in logD_18C6^Cs values but not the stability constants variations.

A comparison of 18C6 and DB18C6 indicates a much higher extraction efficiency of the latter. The stability constants of cesium complexes with DB18C6 in the studied RTILs have not been measured yet, but for other than RTIL solvents they reveal 0.2 to 0.5 log units lower values than those for 18C6 [25]. However, bearing in mind that this is also valid for an aqueous solution, the difference in (logK₁^RTIL − logK₁^w) in Equation (5) becomes negligible. Moreover, this unfavorable for extraction loss is excessively compensated by the fact that the values of logD_DB18C6^Cs are 2 to 3 log units higher than logD_18C6^Cs values.

3. Experimental

3.1. General

[HMIM][N(Tf)₂] (Solvent Innovation; puriss. 99%), Bromothymol Blue (Baum-Lux, Russia; analytical reagent grade), KNO₃ (analytical reagent grade), chloroform (Ekros, Russia), and absolute ethanol (analytical reagent grade) were used as received. Cesium nitrate (Aldrich, 99.99%) was dried for 6 hours at 70 °C, and then kept 24 h in a desiccator before use. Dibenzo-18-crown-6 (DB18C6, 99% purity) was synthesized by Dr. V.M. Polosin and analyzed by ¹H-NMR. NMR revealed no other peaks than those corresponding to DB18C6. 18C6 (Aldrich, 99%) and DB18C6 have been melted at 70 and 180 °C, respectively, in a drying box for 3 h and then kept at 25 °C in a desiccator for 24 h to remove water before use. NMR spectra were recorded on a 500 MHz Bruker DRX500 instrument is the indicated solvents using TMS as reference.

3.2. Synthesis and characterization of the RTILs

Trioctylmethylammonium salicylate. Aliquat® 336 (~0.2 mol) was mixed with 30% excess of sodium salicylate in chloroform (200 mL). The mixture was shaken for 4 h and then rinsed 20 times with a large amount of distilled water, then the solvent was evaporated and the liquid residue was heated up to 100 °C under reduced pressure for 5 h. After cooling to room temperature a white solid matter was obtained with a density 0.943 g·cm⁻³; T_m = (32.8 ± 0.4) °C, T_f = (14 ± 2) °C. Yield: 90%. ¹H-NMR (CDCl₃) 0.88 (9 H), 1.24 (30 H), 1.59 (6 H), 3.19 (3 H), 3.27 (6 H), 7.20 (1 H), 7.92 ppm (1 H); ¹³C-NMR (DMSO-d₆) 13.80 (C*CH₃), 21.93 (C*H₂CH₃), 21.25, 25.68, 28.27, 28.32, 31.04 (various CH₂CH₂ fragments); 47.41 (CH₃N); 60.50 (CH₂N); 115.35, 115.62, 120.73, 129.73, 130,88 (aromatic C); 163.17 (COH); 171.00 ppm (COO). Analysis: found: C, 76.06; H, 11.64; N: 2.62; calc.: C, 76.49; H, 11.89; N: 2.62 (taking into consideration 2:1 mol/mol relation between methyltrioctyl- and methyltridecylammonium cations in Aliquat® 336). After equilibration with water at ambient temperature the solid product transforms into a slightly yellowish viscous liquid with 0.942 g·cm⁻³ density and a freezing point below −18 °C. Water content measured by Karl Fisher titration constituted 0.18% wt. (0.09 mol·dm⁻³) for a solid product and 4.83% wt. (2.52 mol·dm⁻³) for that equilibrated with water RTIL.

Tetrahexylammonium dihexylsulfosuccinate was synthesized according to [26] as a transparent viscous liquid (yield: 85%), analyzed by NMR, and then used without further purification. ¹H-NMR (CDCl₃) 0.88 (18 H), 1.33 (36 H), 1.55 (12 H), 3.2 (12 H), 4.1 (3 H) ppm; 13C-NMR (DMSO-d₆) 13.64, 13.69, 13.71 (CH₃); 20.91, 21.76 (C*H₂CH₃), 21.87, 24.82, 24.85, 25.34, 27,95, 27.99, 30.47, 30.75, 30.82 (CH₂CH₂ fragments); 34.01 (OOC*CH₂CH-); 57.63 (CH₂N); 63.89 (CH₂O); 168.31, 170.97 COOR); Analysis: found: C, 66.67; H, 11.31; N: 1.97, S: 4.49; calc.: C, 66.71; H, 11.34; N: 1.94, S: 4.45. An absence of halogen-ions was proven by a AgNO₃ test.

1-Butyl-3-methylimidazolium hexafluorophosphate. A 1-L, one-necked, round-bottomed flask was equipped with a magnetic stirrer and charged with 1-butyl-3-methylimidazolium chloride (367.2 g, 2.10 mol, 1 equiv.) and potassium hexafluorophosphate (387.3 g, 2.10 mol, 1 equiv.) in distilled water (700 mL). The reaction mixture was stirred at room temperature for 2 h, affording a two-phase system. The organic phase was separated and washed with water (5 × 40 mL) until the aqueous fraction observed to be free of chloride (AgNO₃). Then dichloromethane (400 mL) was added. The dichloromethane solution was mixed with activated charcoal, stirred for 2 h, filtered and dried over anhydrous magnesium sulfate. After 1 h, the suspension was filtered and the volatile material was removed by rotary evaporation. The resulting colourless or light-yellow viscous liquid was dried under reduced pressure (0.5 mm Hg) at 70 °C for 12 h. Yield 462 g (77.3%). ¹H-NMR (300 MHz, DMSO-d₆): 0.91 (3H, t, NHCH₂CH₂CH₂CH₃), 1.27 (2H, m, NCH₂CH₂CH₂CH₃), 1.77 (2H, p, NCH₂CH₂CH₂CH₃), 3.85 (3H, s, NCH₃), 4.16 (2H, t, NCH₂CH₂CH₂CH₃), 7.72 (2H, m, C(5)H, C(4)H), 9.09 (1H, s, C(2)H); Analysis: found: C, 33.97; H, 5.37; N, 9.95; F, 39.91; P, 10.78; calc.: C, 33.81; H, 5.32; N, 9.86; F, 40.11; P, 10.90.

1-Butyl-3-methylimidazolium bis[trifluoromethyl)sulphonyl]imide. A 500-mL, one-necked, round- bottomed flask was equipped with a magnetic stirrer and charged with lithium bis(trifluoromethylsulfonyl)imide (143.693 g, 0.5 mol, 1 equiv.) and 1-butyl-3-methylimidazolium chloride (87.25 g, 0.5 mol, 1 equiv.) dissolved in distilled water (80 mL). The reaction mixture was stirred at room temperature for 2 h affording a two-phase system. Then dichloromethane (200 mL) was added. The lower organic layer was separated and washed with distilled water (5 × 20 mL) until the aqueous fraction observed to be free of chloride (AgNO₃). The dichloromethane solution was mixed with activated charcoal, stirred for 2 h, filtered and dried over anhydrous magnesium sulfate. After 1 h, the suspension was filtered and the volatile material was removed by rotary evaporation. The resulting colourless viscous liquid was dried under reduced pressure (0.5 mm Hg) at 70 °C for 12 h. Yield 162.6 g (77.5%). ¹H-NMR (250 MHz, DMSO-d₆): 0.89 (3H, t, NHCH₂CH₂CH₂CH₃), 1.25 (2H, m, NCH₂CH₂CH₂CH₃), 1.76 (2H, p, NCH₂CH₂CH₂CH₃), 3.84 (3H, s, NCH₃), 4.15 (2H, t, NCH₂CH₂CH₂CH₃), 7.72 (2H, m, C(5)H, C(4)H), 9.09 (1H, s, C(2)H); Analysis: found: C, 28.67; H, 3.63; N, 9.94; F, 27.30; S, 15.23; O, 15.23; calc.: C, 28.64; H, 3.61; N, 10.02; F, 27.18; S, 15.29; O, 15.26.

1-(2-Ethylhexyl)-3-methylimidazolium bis[trifluoromethyl)sulphonyl]imide was kindly supplied by Dr. V.E. Baulin (A.N. Frumkin Institute of Physical Chemistry and Electrochemistry of RAS). ¹H-NMR (200 MHz, CDCl₃) 0.85 (m, 6H, 2CH₂CH₃), 1.25 (m, 8H, 4CH₂), 1.75 (m, 1H, CH₂CHCH₂), 3.92 (s, 3H, CH₃N), 4.00 (d, J=7.0 Hz, 2H, CH₂N), 7.25(s, 1H_arom), 7.36 (s, 1H_arom), 8.65 (s, 1H_arom); Analysis: found: C, 35.40; H, 4.85; F, 24.00; N: 8.81; S: 13.52; calc.: C, 35.36; H, 4.88; F, 23.97; N: 8.84, S: 13.49.

3.3. Extraction

For a sample preparation the weighted amount of solid CsNO₃ was dissolved in a distilled water to obtain the initial solution of salt (1.5·10^-2 mol·dm^-3), which was further used for the preparation of the diluted samples. For adjusting pH values of aqueous phase, 0.01–3 M HNO₃ (analytical reagent grade) was used.

Cesium extraction process in the absence of crown ethers was performed by adding of water- saturated RTIL samples (0.5 mL for [TOMA][Sal], [THA][DHSS], and [BMIM][PF₆]; 1.0 mL for [BMIM][N(Tf)₂], [EtHMIM][N(Tf)₂] and 2.0 mL for [HMIM][N(Tf)₂]) to 1.5 × 10^-3 mol·dm^-3 (2 × 10^-4, 5 × 10^-4, 5 × 10^-3, 1 × 10^-2, and 5 × 10^-2 mol·dm^-3 only at neutral pH, without addition of HNO₃) CsNO₃ aqueous solutions (5.0 mL for [TOMA][Sal], [THA][DHSS], and [BMIM][PF₆]; 3.0 mL for [BMIM][N(Tf)₂], [EtHMIM][N(Tf)₂] and 2.0 mL for [HMIM][N(Tf)₂]). The biphasic systems were shaken for 2 h (adequate time for equilibrium state establishment for all RTILs studied at room temperature, 22 °C). After centrifugation for 15 min aqueous and organic phases were separated. An aqueous phase was analyzed for equilibrium pH (pH-meter pH-410; combined glass microelectrode ESLK-13.7, Aquilon, Russia) and cesium content (flame photometry, FPA-2; Zagorsk Optical and Mechanical Plant, Russia). The recovery (R(Cs), %) and the distribution constant (D(Cs)) of cesium were found according to eqns. (6) and (7):

R(Cs), % = [1 – C_w(Cs)/C_w⁰(Cs)]·100%,

(6)

D(Cs) = C_o(Cs)/C_w(Cs) = [V_w/V_o][R(Cs), %]/[100 – R(Cs), %],

(7)

where C_w⁰(Cs) and C_w(Cs) are the initial and equilibrium concentrations of Cs⁺ in an aqueous phase, respectively, V_w и V_o correspond to the total volumes of aqueous and organic phase, respectively (mL). The ratio V_w:V_o was 10 ([TOMA][Sal], [THA][DHSS], and [BMIM][N(Tf)₂]), 3 ([BMIM][PF₆], [EtHMIM][N(Tf)₂]) and 1 ([HMIM][N(Tf)₂]). To compensate the matrix effects the calibration standards were prepared by adding standard solutions of Cs⁺ to the aqueous solutions pre-equilibrated with the corresponding RTILs. Otherwise the error increased sufficiently.

For cesium extraction study in the presence of crown ethers, the precise amounts of 18C6 or DB18C6 were dissolved in the RTILs saturated by a distilled water. Initial concentration of 18C6, C(18C6)₀ was 1.5 × 10^-1 mol·dm^-3 for all RTILs studied. The same concentration was used for DB18C6 in [BMIM][N(Tf)₂]. Due to the solubility problems, the content of DB18C6 in [TOMA][Sal] and [THA][DHSS] was lower (C(DB18C6)₀ = 5 × 10^-2 mol·dm^-3). This concentration was obtained after heating of these RTILs with the amounts of DB18C6 at 80–90 °C. Thus, these solutions are likely to be oversaturated, although no precipitation after cooling to room temperature was observed.

To provide an equal ratio C(CE)₀:C(Cs⁺)₀ = 100 and ν(CE)₀:ν(Cs⁺)₀ = 10 for all RTILs studied, the initial concentration of Cs⁺ in aqueous phases contacting with [TOMA][Sal] and [THA][DHSS] was adjusted to C(Cs⁺)₀ = 5 × 10^-4 mol·dm^-3. The volume ratio V_w:V_o = 10 was used for all 9 extraction systems studied (0.5 mL crown ether solution in RTIL and 5.0 mL of cesium nitrate aqueous solution with various pH). The aqueous solutions pre-equilibrated with the corresponding CE/RTIL systems were used for preparation of the calibration standards. All other experimental conditions were the same as for experiments run without crown ether.

Distribution of DB18C6 was studied in a parallel way within the same experiments on cesium extraction. The samples taken from phases of DB18C6/[TOMA][Sal], DB18C6/[THA][DHSS], and DB18C6/[BMIM][N(Tf)₂] were dissolved in an absolute ethanol taken in vol/vol ratio 1000 (for DB18C6/[BMIM][N(Tf)₂]) and 300 (for DB18C6/[TOMA][Sal] and DB18C6/[THA][DHSS]). Then the resulting solutions were analyzed spectrophotometrically (U-2900 UV-Vis Recording Spectrophotometer, Hitachi, Japan; 1 cm quartz cells) for DB18C6 content using crown ether absorption band λ_max = 274 nm (ε_max = 5,000 ÷ 5,500) and the reference solutions of the corresponding RTILs in ethanol with the same dilution as for DB18C6/RTILs. Detaining (R(CE), %) for DB18C6 has been calculated using equation (8):

R(Cs), % = [C_o(Cs)/C_o⁰(Cs)]·100%,

(8)

where C_o⁰(CE) and C_o(CE) denote initial and equilibrium concentrations of crown ether in organic phase (extract), respectively; while distribution constant (D(CE)) − using equation (7).

Distribution of 18C6 was controlled by the method described elsewhere [27]. Bromothymol Blue (H-BTB) and KNO₃ were dissolved in an aliquot taken from an aqueous phase (to adjust 1.2 × 10^-2 mol·dm^-3 concentration of both of them). Then the extraction of the complex [K(18C6)+·BTB–] into CHCl₃ was performed with 2 h contact time and phases volume ratio 1:1; pH of an aqueous phase was kept constant (pH = 6.9) by phosphate buffer for all solutions other then neutral. Then the system was centrifuged for 15 min, an organic phase was separated and analyzed spectrophotometrically (λ_max = 400 ÷ 450 nm, ε_max = 1,500 ÷ 2,000; the chloroform extract obtained in the blank experiment was used as the reference solution). Detaining (R(CE), %) and distribution ratio (D(CE)) were calculated by the eqns., completely analogous to (1) and (2), taking into consideration that C_o(CE) = C_o⁰(CE) − 10·C_w(CE), where C_w(CE) denotes an equilibrium concentration of crown ether in aqueous phase.

4. Conclusions

The data presented reveals that several ionic liquids are suitable for extraction of cesium in the presence of crown ethers. The liquids which show a higher retention of a crown ether and a higher stability of the complexes demonstrate better capabilities to extract Cs⁺. The former factor dominates evidently over the latter one. Among the crowns the ligand of a choice is the one that demonstrates higher hydrophobicity, and, therefore, a higher detaining in extract, e.g., DB18C6.