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

Article abstract of DOI:10.3390/molecules14125001

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.

Full text of DOI:10.3390/molecules14125001

Molecules 2009, 14, 5001-5016; doi:10.3390/molecules14125001
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
1
8-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
1
2
2
2
Andrey G. Vendilo , Dmitry I. Djigailo , Svetlana V. Smirnova , Irina I. Torocheshnikova ,
1
,3
4
2,
Konstantin I. Popov , Vladimir G. Krasovsky and Igor V. Pletnev *
1
State Research Institute of Reagents and High Purity Substances (IREA), Bogorodskii val 3,
1
07258, Moscow, Russia; E-Mail: vendilo@ekos-1.ru (A.G.V.)
Department of Chemistry, M.V. Lomonosov Moscow State University, 1-3 Leninskiye gory,
19992, Moscow, Russia; E-Mails: djigailo@yandex.ru (D.I.D.), smirnova@analyt.chem.msu.ru
S.V.S.), dis@analyt.chem.msu.ru (I.I.T.)
2
1
(
3
4
Department of Physical and Colloid Chemistry, Moscow State University of Food Technologies,
Volokolamskoye Sh. 11, 125080, Moscow, Russia; E-Mail: ki-popov@mtu-net.ru (K.I.P.)
Zelinsky Institute of Organic Chemistry RAS, Leninsky pr. 47, 119991, Moscow, Russia;
E-Mail: miyusha@mail.ru (V.G.K.)
*
Author to whom correspondence should be addressed; E-Mail: pletnev@analyt.chem.msu.ru;
Tel.: +7-495-939-5464.
Received: 1 September 2009; in revised form: 24 November 2009 / Accepted: 27 November 2009 /
Published: 2 December 2009
+
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.

Molecules 2009, 14
Keywords: ionic liquids; crown ethers; cesium extraction; stability constants
5002
1
. Introduction
1
37
Hydrophobic crown ethers are applied as extractants in the separation and recovery of Cs and
Sr radioactive isotopes from high-level nuclear waste solutions, containing high concentrations of
9
0
acids and salts, into molecular solvents [1–5]. 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 [6–13]. 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][PF
methylimidazolium bis[trifluoromethyl)sulphonyl]imide ([BMIM][N(Tf) ]), 1-hexyl-3-methyl-
imidazolium bis[trifluoromethyl)sulphonyl]imide ([HMIM][N(Tf) ]), and 1-(2-ethylhexyl)-3-methyl-
6
]), 1-butyl-3-
2
2
1
33
imidazolium bis[trifluoromethyl)sulphonyl]imide ([EtHMIM][N(Tf)
2
]) by a
Cs-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.

Molecules 2009, 14
5003
Scheme 1. Structures of the RTILs studied.
+
N
N
+
+
N
N
N
N
O
O
F
F
O
O
S
O
O
F
F
F
F
]
F
F
F
F
F C S N S C F
F C S N
C
F C S N S
F
C
F
F
F
O
O
O
O
O
O
[
2
BMIM][N(Tf) ]
[
HMIM][N(Tf)
2
[
2
EtHMIM][N(Tf) ]
+
N
N
N
O
F
F
O
O
O
F P F
S
O
O
F F
O
[TOMA][Sal]
[
BMIM][PF
6
]
[THA][DHSS]
2
. Results and Discussion
The results of the extraction experiments are presented in Tables 1 to 3, where D refers to cesium
Cs
distribution ratio for the extraction from aqueous phase into RTIL in the absence of crown ethers,
1
8C6
DB18C6
D
Cs
as well as DCs
indicate the same in the presence of 18C6 and DB18C6, respectively,
Cs
Cs
while D18C6 and DDB18C6 are distribution ratios for 18C6 and DB18C6 in the presence of
cesium ions.
0
-3
-3
Table 1. Distribution ratios of cesium (C
w
(Cs) = 1.5 × 10 mol·dm ) between
RTIL/aqueous phases in the absence of crown ether as a function of pH at 22 °C.
RTIL
V
o
, mL
w
V , mL Aqueous pH logDCs
[
THA][DHSS]
0.5
5.0
5.94
1.21
1.11
0.84
0.69
0.76
0.74
0.10
-0.59
-0.82
-0.80
3
1
.36
.59
[TOMA][Sal]
0.5
0.5
5.0
5.0
5.98
4
3
1
.95
.01
.13
[BMIM][PF
6
]
5.90
3
1
.33
.25

Molecules 2009, 14
5004
Table 1. Cont.
RTIL
V
o
, mL
w
V , mL Aqueous pH logDCs
[
BMIM][N(Tf)
2
]
1.0
3.0
6.44
-0.67
-1.24
-1.31
-0.81
-0.62
-1.24
-1.36
-2.21
3
1
.35
.25
[
EtHMIM][N(Tf)
HMIM][N(Tf)
2
]
1.0
2.0
3.0
2.0
6.34
.09
5.82
2
[
2
]
2
0
.84
.93
0
-3
-3
Table 2. Distribution ratios of cesium [C
w
(Cs) = 1.5 × 10 mol·dm ] and 18C6
0
-1
-3
[
C (18C6) = 1.5 × 10 mol·dm ] in the presence of each other between RTIL/aqueous
o
o w
phases as a function of pH at 22 °C (V = 0.5 mL; V = 5.0 mL).
1
8C6
Cs
RTIL Aqueous pH logDCs
logD18C6
-0.12
-0.16
-0.41
-0.50
-0.15
-0.01
0.48
0.44
0.36
0.30
0.21
0.43
0.13
0.08
0.06
0.00
0.00
0.08
0.77
0.65
0.50
0.08
0.23
-0.27
-0.45
0.25
0.17
0.06
0.08
0.27
[
[
[
[
THA][DHSS]
TOMA][Sal]
5.92
0.24
0.23
0.03
4
3
3
2
1
.30
.86
.39
.48
.60
-0.38
0.57
0.75
0.83
0.89
0.90
0.77
0.36
1.11
-0.20
-0.45
-0.62
-1.27
-0.74
-0.66
1.56
1.43
-0.26
-0.27
-0.23
0.56
0.30
0.82
0.91
0.61
-0.22
-0.33
5.84
4
3
3
2
1
.33
.81
.31
.22
.20
BMIM][PF
6
]
5.84
4
3
3
2
1
.33
.81
.31
.22
.20
BMIM][N(Tf)
2
]
6.88
5
3
2
1
.60
.56
.26
.27
[
EtHMIM][N(Tf)
HMIM][N(Tf)
2
]
6.50
.15
5.60
2
[
2
]
4
2
1
0
.10
.96
.40
.90

Molecules 2009, 14
Table 3. Distribution ratios of cesium and DB18C6 in the presence of each other between
5005
RTIL/aqueous phases as a function of pH at 22 °C (V = 0.5 mL; V = 5.0 mL).
o
w
0
0
C
w
(Cs),
C
o
(18C6),
Aqueous
pH
DB18C6
Cs
RTIL
logDCs
logDDB18C6
-3
-3
mol·dm
mol·dm
-4
-4
-3
-2
[THA][DHSS]
[TOMA][Sal]
[BMIM][N(Tf)
5.0·10
5.0·10
1.5·10
5.0·10
5.0·10
1.5·10
6.03
1.56
2.68
2.41
2.20
2.15
2.30
2.68
2.62
2.32
2.11
2.05
2.45
3.05
2.89
2.71
2.57
2.63
2.94
2
2
1
1
0
.85
.45
.89
.06
.14
1.27
0.89
0.74
0.86
1.02
1.91
1.86
1.42
1.12
1.52
1.96
2.28
2.06
0.14
-0.47
-0.60
-2
6.07
3
2
2
1
0
.54
.77
.11
.08
.13
-1
2
]
6.93
5
3
1
0
.09
.16
.92
.38
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 logDCs
[
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:
-
3
[
Sal] = 1.68 and [DHSS] = 1.29 mol·dm . In contrast, conventional-anion RTILs [BMIM][PF
BMIM][N(Tf) ], [HMIM][N(Tf) ], and [EtHMIM][N(Tf) ] reveal negative logDCs values, which are
] earlier [12], Table 4. The cesium extraction
ability of all RTILs, with the exception of [EtHMIM][N(Tf) ], slightly diminishes as pH decreases.
6
],
[
2
2
2
in a good agreement with those found for [BMIM][PF
6
2
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] (logK ), Table 4, which
1
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 HNO
all RTILs, but at very low pH both logDCs
minimum at the curves logDCs = f(pH), in between pH 0 and pH 7 (Figure 1).
3
concentration in an aqueous phase decreases cesium’s recovery for
1
8C6
DB18C6
and logDCs
start to increase. Thus, there appears a

Molecules 2009, 14
5006
Figure 1. Distribution of cesium, 18C6, and DB18C6 vs. pH in [TOMA][Sal] based
systems.
CE
logD or logD
Cs
CE
4
Cs
1
8C6
DB18C6
| DB18C6
Cs
18C6
Cs
DB18CDB6
logD
18C6
C
l os Ng D
O
3
| 18C6
C
lo
s
g
N
D
O
3
1
l
8
o
C
g
6
D
C
s
C
s
3
2
1
0
pH
7.0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
A similar effect has been observed in [9] for cesium distribution in [BMIM][PF
,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],
8C6/[TOMA][Sal], and DB18C6/[TOMA][Sal]). Thus, unlike crown ether solutions in conventional
6
] in the presence of
4
1
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
+
-3
3 3
case of Cs (1 × 10 M) extraction from 2 M HNO aqueous solution into 0.1 M 18C6 in CHCl the
-3
value of DCs is extremely low (3 × 10 ; logDCs = -2.52). This value is lower than even the minimal
values of D obtained in the case of our RTILs-based systems. As a rule, the extraction efficiency of
Cs
the systems based on conventional solvents may be raised to the level of CE/RTIL-systems only by
1
8C6
addition of a bulk hydrophobic counter-ion (such as picrate) to the aqueous phase. So, the logDCs
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)
former are incomparably higher.
2
] and [TOMA][Sal], Table 4), while the hazardous properties of the
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][PF
6
],
[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].

Molecules 2009, 14
Table 4. Stability Constants of Cesium Complex Formation with 18C6 in RTIL and
5007
a
Extraction Efficiency from Water at 22 °C and pH 5 – 7 .
1
8C6
Cs
18C6
Cs
Solvent
logK
4.4
] 3.4
1
logK
1.13
1.16
1.29
2
logDCs
−1.24
−0.81
−0.67
logDCs
0.82
0.56
1.56
−0.20d
−0.52
logD18C6
0.25
−0.27
0.77
log[DCs
1.81
1.64
1.46
0.26
/(DCs
D
18C6 )]
[HMIM][N(Tf)
[EtHMIM][N(Tf)
[BMIM][N(Tf)
[BMIM][PF
2
]
2
2
]
3.4
2.3
6
]
−0.59
0.13
0.15
b
e
−
−
0.59
c
1.17
[
[
1
TOMA][Sal]
THA][DHSS]
1.45
0.77
0.69
1.21
−7.7
0.83
0.25
0.8
0.49
−0.38
−0.84
6.93
−0.12
f
f
g
h
i
,2-dichloroethane 7.98
2.58
0.03
a
−1
−1
Stability constants for RTILs are taken from [21] for 25 °C, K
1
= [Cs(18C6)][Cs] [18C6]
−
1
−1
b
c
d
K = [Cs(18C6) ][Cs(18C6)] [18C6]
;
Ref. [12];
Ref. [9], pH = 7;
Ref. [9],
2
2
0
-1
-3
e
0
-1
-3
C (18C6) = 1.0 × 10 mol·dm ; Ref. [9], C (18C6) = 1.0 × 10 mol·dm , blank experiment
o
o
f
g
h
i
(
without CsNO ); Ref. [23]; Ref. [2], picrate salt; Ref. [3], picrate salt; Ref. [1].
3
Generally, when an aqueous phase of cesium nitrate is in equilibrium with a RTIL organic phase,
L
containing crown ether L, the cesium distribution ratio DCs (for such RTILs as [TOMA][Sal],
6
[THA][DHSS] and [BMIM][PF ]), where only CsL species are formed is represented by a simple
equation (1):
L
RTIL
RTIL
w
w
D
Cs = ([Cs]
+ [CsL] )/([Cs] + [CsL] )
(1)
where the superscripts “RTIL” and “w” denote organic and aqueous phase, respectively. When the
t
t
total concentration [L] >> [Cs] and the stabilities of complexes in both phases are high enough
RTIL
w
RTIL
RTIL
w
w
(
logK
1
≥ 2; logK
1
≥ 2), then [Cs]
<< [CsL]
and [Cs] << [CsL] . In this case, the
RTIL
w
equilibrium concentrations of [Cs]
transformed into (2):
and [Cs] in (1) can be neglected. The equation (1) is therefore
L
RTIL
w
DCs = [CsL] /[CsL]
(2)
(3)
(4)
(5)
or:
L
RTIL
RTIL
RTIL
w
w
w
D
Cs = (K
1
1
[Cs] [L] )/(K [Cs] [L] ).
RTIL
w
RTIL
w
L
As far as [Cs] /[Cs] = DCs and [L] /[L] = D :
L
RTIL
w
D
Cs = K
1
Cs L 1
D D /K
or:
L
RTIL
w
logDCs = logK
1
L 1
+ logDCs + logD – logK .
L
RTIL
Thus, the plot of (logDCs – logDCs – logD
L
) versus logK
1
was expected to be linear with a slope
RTIL
w
of 1. In our case the initially stated requirement logK1
≥ 2; logK ≥ 2 does not take place for some
1
w
RTIL
RTILs and for water. Indeed, logK
1
= 0.98 [24] and for the RTILs the following values of logK
1
at
2
5 °C have been determined [21]: 1.45 (0.05) ([TOMA][Sal]); 0.77 (0.04) ([THA][DHSS]); 2.4 (0.2)
(
[BMIM][PF ]); 3.4 (0.5) ([BMIM][N(Tf) ]); 4.4 (0.1) ([HMIM][N(Tf) ]), and 3.4 (0.4)
6
2
2

Molecules 2009, 14
5008
1
8C6
Cs
RTIL
(
[EtHMIM][N(Tf) ]). Nevertheless, the plot of (logDCs
18C6
– logD – logD ) versus logK1
for
2
Cs
[
TOMA][Sal], [THA][DHSS], and [BMIM][PF
6
] represents a perfectly strait line with a slope 0.73 and
2
R = 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 DCs for all RTILs in the range
0
w
-4
-2
-4
0
w
-3
C
(Cs) = 2 × 10 – 5 × 10 M. In all cases DCs remains nearly constant at 2·10 ≤ C
(Cs) ≤ 1.5·10
M (≤ 5·10 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) ] at pH = 6.88 (Table 2), which corresponds
-3
(
2
-4
to the residual free cesium concentration being equal to 2.9 × 10 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
[
+
-3
Cs -extraction from 1.5 × 10 M aqueous solution in the absence of crown ether in RTIL-phase).
1
8C6
Cs
RTIL
Figure 2. A plot of [logDCs
THA][DHSS] and [BMIM][PF
.6
− logD − logD
18C6
] vs. logK1
for [TOMA][Sal],
Cs
[
6
] at 22–25 °C.
0
18C6
Cs
[
logD
– logD – logD
18C6
]
Cs
Cs
0
.4
.2
.0
0.2
0.4
0.6 [THA][DHSS]
0.8
1.0
[
BMIM][PF6]
0
0
tgα = 0.73
-
[
TOMA][Sal]
-
-
-
RTIL
logK1
-
0.5
1.0
1.5
2.0
2.5
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 CsL complexes and the lack of such complexes in
2
2
water. However, all six RTILs fit equation (5) with a slope 0.89 and a correlation coefficient R = 0.98,
RTIL
RTIL
Figure 3. This can be explained by a domination of logK1
values over logK2
ones, Table 4.
RTIL
Another reason of low contribution of K
2
to the extraction is in the absence of enough sufficient
1
8C6
excess of crown re. cesium concentration. Anyway, the linear relationship of (logDCs
– logDCs –
Cs
RTIL
logD18C6 ) and logK
1
indicates clearly an importance of complex stability contribution for the
extraction of cesium into hydrophobic RTILs.
RTIL
Besides logK
1
L
, also a contribution of logD should be examined. Thus, along with cesium
extraction we have studied the water/RTIL distribution of crown ethers, Tables 2 and 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
Cs
sufficiently higher for all RTILs then that of 18C6, Tables 2 and 3. At the same time logDDB18C6
values are one to two log unites lower then those found for benzene, 1,2-dichloroethane, chloroform
and nitrobenzene [5].

Molecules 2009, 14
Figure 3. A plot of [logDCs
5009
1
8C6
Cs
RTIL
− logD − logD
Cs
18C6
] vs. logK1
for [TOMA][Sal],
[
THA][DHSS], [BMIM][PF
6
], [BMIM][N(Tf)
2
], [HMIM][N(Tf)
2
], [EtHMIM][N(Tf) ] at
2
22–25 °C.
2
.5
18C6
logD
Cs
Cs
[
– logD – logD
Cs
]
18C6
[
HMIM][N(Tf)2]
EtHMIM][N(Tf)2]
BMIM][N(Tf)2]
2
.0
.5
.0
.5
.0
0.5
1.0
[
1
[
1
tgα = 0.89
0
[BMIM][PF6]
0
[
TOMA][Sal]
-
logK1RTIL
[THA][DHSS]
-
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
1
8C6
DB18C6
It should be noted that the variation of logDCs
and logDCs
with pH shows the same trend as
Cs
Cs
the crown ether distribution between aqueous phase and RTIL, logD18C6 and logDDB18C6 , Figure 1.
Cs
18C6
2
A linear relationship between logD18C6 and logDCs
(slope 0.91; R = 0.96) as well as between
Cs
DB18C6
2
logDDB18C6 and logDCs
(slope 1.24; R = 0.91) was found for TOMAS, Figure 4, indicating the
dominating role of relative crown ether solubility in water and RTIL.
CE
Cs
Figure 4. Plots of [logDCs – logDCs] vs. logDCE for TOMAS at 22 °C. (a) CE = 18C6;
b) CE = DB18C6.
(
18C6
Cs
0
.25
[
logD
- logDCs]
0
.20
.15
.10
.05
.00
0.05
0.10
0.15
0.20
pH 5.0
0
pH 6.0
0
tgα = 0.91
0
pH 3.0
0
-
-
pH 2.2
Cs
-
logD18C6
-
0.20
0.25
0.30
0.35
a)
0.40
0.45
0.50
(

Molecules 2009, 14
5010
Figure 4. Cont.
DB18C6
logD
Cs
2
.20
[
- logDCs]
pH 0.1
2
.00
.80
.60
.40
.20
.00
1
1
tgα = 1.24
1
pH 1.1
pH 6.0
1
pH 3.5
1
pH 2.1
0
.80
logDDB18C6Cs
pH 3.0
2.20
0
.60
2.00
2.40
2.60
2.80
3.00
3.20
(
b)
Analysis of the data presented in Table 4 for [BMIM][N(Tf)
2
], [HMIM][N(Tf)
2
], and
Cs
18C6
RTIL
[
EtHMIM][N(Tf)
2
] indicates that the contribution of logD18C6 into logDCs
prevails over logK
1
8C6
values. Indeed, the variation of logDCs
within this set of similar RTILs with the same anion
Cs
definitely follows the changes in logD18C6 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
RTIL
w
logK
1
– logK
1
) in Equation (5) becomes negligible. Moreover, this unfavorable for extraction
Cs
loss is excessively compensated by the fact that the values of logDDB18C6 are 2 to 3 log units higher
Cs
than logD18C6 values.
3
. Experimental
3
.1. General
2
[HMIM][N(Tf) ] (Solvent Innovation; puriss. 99%), Bromothymol Blue (Baum-Lux, Russia;
analytical reagent grade), KNO (analytical reagent grade), chloroform (Ekros, Russia), and absolute
3
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,
1
9
9% purity) was synthesized by Dr. V.M. Polosin and analyzed by H-NMR. NMR revealed no other
peaks than those corresponding to DB18C6. 18C6 (Aldrich, 99%) and DB18C6 have been melted at
0 and 180 °C, respectively, in a drying box for 3 h and then kept at 25 °C in a desiccator for 24 h to
7
remove water before use. NMR spectra were recorded on a 500 MHz Bruker DRX500 instrument is
the indicated solvents using TMS as reference.

Molecules 2009, 14
5011
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
−3
1
obtained with a density 0.943 g·cm ; T = (32.8 ± 0.4) °C, T = (14 ± 2) °C. Yield: 90%. H-NMR
m
f
(
CDCl ) 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);
3
1
3
6 3 2 3
C-NMR (DMSO-d ) 13.80 (C*CH ), 21.93 (C*H CH ), 21.25, 25.68, 28.27, 28.32, 31.04 (various
CH CH fragments); 47.41 (CH N); 60.50 (CH N); 115.35, 115.62, 120.73, 129.73, 130,88 (aromatic
2
2
3
2
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
−3
−3
0
.18% wt. (0.09 mol·dm ) for a solid product and 4.83% wt. (2.52 mol·dm ) for that equilibrated with
water RTIL.
Tetrahexylammonium dihexylsulfosuccinate was synthesized according to [26] as a transparent viscous
1
liquid (yield: 85%), analyzed by NMR, and then used without further purification. H-NMR (CDCl
3
)
1
3
0
.88 (18 H), 1.33 (36 H), 1.55 (12 H), 3.2 (12 H), 4.1 (3 H) ppm; C-NMR (DMSO-d
3.71 (CH ); 20.91, 21.76 (C*H CH ), 21.87, 24.82, 24.85, 25.34, 27,95, 27.99, 30.47, 30.75, 30.82
CH fragments); 34.01 (OOC*CH CH-); 57.63 (CH N); 63.89 (CH O); 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 AgNO test.
6
) 13.64, 13.69,
1
3
2
3
(
CH
2
2
2
2
2
3
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,
.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 (AgNO ). Then dichloromethane (400 mL) was added. The
2
(
3
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
1
reduced pressure (0.5 mm Hg) at 70 °C for 12 h. Yield 462 g (77.3%). H-NMR (300 MHz, DMSO-
d
6
): 0.91 (3H, t, NHCH
NCH CH CH CH ), 3.85 (3H, s, NCH
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,
3.81; H, 5.32; N, 9.86; F, 40.11; P, 10.90.
2
CH
2
CH
2
CH
3
), 1.27 (2H, m, NCH
2
CH
2
CH
2 3
CH ), 1.77 (2H, p,
2
2
2
3
3
), 4.16 (2H, t, NCH
2
CH CH
2
2
CH
3
), 7.72 (2H, m, C(5)H,
3

Molecules 2009, 14
5012
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 (AgNO ). The dichloromethane solution was mixed
3
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
1
1
62.6 g (77.5%). H-NMR (250 MHz, DMSO-d ): 0.89 (3H, t, NHCH CH CH CH ), 1.25 (2H, m,
6
2
2
2
3
NCH
2
2
CH
2
2
CH
2
2
CH
3
3
), 1.76 (2H, p, NCH
2
CH
2
CH
2
CH
3
), 3.84 (3H, s, NCH
3
), 4.15 (2H, t,
NCH
CH
CH
CH
), 7.72 (2H, m, C(5)H, C(4)H), 9.09 (1H, s, C(2)H); Analysis: found: C, 28.67; H,
3
1
.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,
5.26.
1
-(2-Ethylhexyl)-3-methylimidazolium bis[trifluoromethyl)sulphonyl]imide was kindly supplied by Dr.
1
V.E. Baulin (A.N. Frumkin Institute of Physical Chemistry and Electrochemistry of RAS). H-NMR
(
200 MHz, CDCl
H, CH N), 4.00 (d, J=7.0 Hz, 2H, CH
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:
3
) 0.85 (m, 6H, 2CH
2
CH
3 2 2 2
), 1.25 (m, 8H, 4CH ), 1.75 (m, 1H, CH CHCH ), 3.92 (s,
3
3
2
N), 7.25(s, 1Harom), 7.36 (s, 1Harom), 8.65 (s, 1Harom); Analysis:
1
3.49.
3
.3. Extraction
3
For a sample preparation the weighted amount of solid CsNO was dissolved in a distilled water to
-2
-3
obtain the initial solution of salt (1.5·10 mol·dm ), which was further used for the preparation of the
diluted samples. For adjusting pH values of aqueous phase, 0.01–3 M HNO
was used.
3
(analytical reagent grade)
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][PF ]; 1.0 mL for
6
-
3
-3
-
[
BMIM][N(Tf)
2
], [EtHMIM][N(Tf)
2
] and 2.0 mL for [HMIM][N(Tf)
2
]) to 1.5 × 10 mol·dm (2 × 10
4
-4
-3
-2
-2
-3
,
5 × 10 , 5 × 10 , 1 × 10 , and 5 × 10 mol·dm only at neutral pH, without addition of HNO
3
)
CsNO
3
aqueous solutions (5.0 mL for [TOMA][Sal], [THA][DHSS], and [BMIM][PF
6
]; 3.0 mL for
[BMIM][N(Tf)
2
], [EtHMIM][N(Tf) ] and 2.0 mL for [HMIM][N(Tf) ]). The biphasic systems were
2
2
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):
0
w
(
6)
w
R(Cs), % = [1 – C (Cs)/C
(Cs)]·100%,

Molecules 2009, 14
5013
D(Cs) = C (Cs)/C (Cs) = [V /V ][R(Cs), %]/[100 – R(Cs), %],
(7)
o
w
w
o
0
w
+
where C
(Cs) and C (Cs) are the initial and equilibrium concentrations of Cs in an aqueous phase,
w
respectively, V
w
и V correspond to the total volumes of aqueous and organic phase, respectively
o
(
[
mL). The ratio V :V was 10 ([TOMA][Sal], [THA][DHSS], and [BMIM][N(Tf) ]), 3 ([BMIM][PF ],
w o 2 6
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,
-1
-3
C(18С6) was 1.5 × 10 mol·dm for all RTILs studied. The same concentration was used for
0
DB18C6 in [BMIM][N(Tf) ]. Due to the solubility problems, the content of DB18C6 in [TOMA][Sal]
2
-2
-3
0
and [THA][DHSS] was lower (C(DB18С6) = 5 × 10 mol·dm ). 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) :C(Cs ) = 100 and ν(CE) :ν(Cs ) = 10 for all RTILs studied, the
0
0
0
0
+
initial concentration of Cs in aqueous phases contacting with [TOMA][Sal] and [THA][DHSS] was
+
-4
-3
0 w o
adjusted to C(Cs ) = 5 × 10 mol·dm . The volume ratio V :V = 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
2
DB18C6/[BMIM][N(Tf) ] were dissolved in an absolute ethanol taken in vol/vol ratio 1000 (for
DB18C6/[BMIM][N(Tf) ]) and 300 (for DB18C6/[TOMA][Sal] and DB18C6/[THA][DHSS]). Then
2
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 λmax = 274 nm (εmax = 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):
0
R(Cs), % = [C
o
(Cs)/C
o
(Cs)]·100%,
(8)
0
o o
where C (CE) and C (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 KNO were dissolved in an aliquot taken from an aqueous phase (to adjust
3
-3
-2
1
.2 × 10 mol·dm concentration of both of them). Then the extraction of the complex
+
–
3
[K(18С6) ·BTB ] into CHCl 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 (λmax = 400 ÷ 450 nm, εmax = 1,500 ÷ 2,000; the chloroform extract obtained in

Molecules 2009, 14
5014
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
0
that C (CE) = C (CE) – 10·C (CE), where C (CE) denotes an equilibrium concentration of crown
o
o
w
w
ether in aqueous phase.
. Conclusions
The data presented reveals that several ionic liquids are suitable for extraction of cesium in the
4
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.
Acknowledgements
The authors are grateful to V.E. Baulin who kindly provided [EtHMIM][N(Tf) ] ionic liquid. Our
2
thanks are also due to Russian Foundation for Basic Research (projects no. 07-08-00246 and no.
0
8-03-91319).
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2009 by the authors; licensee Molecular Diversity Preservation International, Basel, Switzerland.
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