Solvent extraction and stripping of silver ions in room-temperature ionic liquids containing calixarenes

Article abstract of DOI:10.1021/ac049549x

We found that a calix[4]arene-bearing pyridine is soluble in a typical room-temperature ionic liquid (RTIL), 1-alkyl-3-methylimidazolium hexafluorophosphate. Pyridinocalix[4]arene showed a high extraction ability and selectivity for silver ions. The extraction performance of the calix[4]arene was greatly enhanced by dissolution in RTILs compared to in chloroform. In a competitive extraction test using five different metal ions (Ag⁺, Cu²⁺, Zn²⁺, Co²⁺, Ni²⁺), only silver ions were transferred by the calix[4]arene from the aqueous feed phase into the RTILs, through a cation-exchange mechanism. The pyridinocalix[4]arene was found to form a stable 1:1 complex with silver ions, both by slope analysis and by Job's method. Since it is easy to strip silver ions from RTILs by controlling the aqueous-phase pH, the extraction performance of calix[4]arene in RTILs was maintained after five repeated uses.

Full text of DOI:10.1021/ac049549x

Anal. Chem. 2004, 76, 5039-5044
Solvent Extraction and Stripping of Silver Ions in
Room-Temperature Ionic Liquids Containing
Calixarenes
Kojiro Shimojo and Masahiro Goto*
Department of Applied Chemistry, Graduate School of Engineering, Kyushu University,
6-10-1 Hakozaki, Higashi-ku, Fukuoka, 812-8581, Japan
Calixarene dissolved in aliphatic diluents exhibited a high extrac-
tion efficiency for target molecules.
We found that a calix[4 ]arene-bearing pyridine is soluble
in a typical room-temperature ionic liquid (RTIL), 1 -alkyl-
3-methylimidazolium hexafluorophosphate. P yridinocalix-
[4 ]arene showed a high extraction ability and selectivity
for silver ions. The extraction performance of the calix[4 ]-
arene was greatly enhanced by dissolution in RTILs
compared to in chloroform. In a competitive extraction
Room-temperature ionic liquids (RTILs), which are composed
entirely of ions, are currently being investigated as alternative
reaction media to replace conventional organic solvents in a variety
of synthetic,5 catalytic,6 and electrochemical applications.7 RTILs
consisting of hydrophobic cations and anions are water immiscible,
thus appearing to be available for liquid-liquid extraction. The
unique properties of RTILs such as negligible volatility and
nonflammability are important advantages over conventional
organic diluents used in liquid-liquid extraction processes.
Several investigators have focused on the application of RTILs to
liquid-liquid extraction of metal ions. Dai et al.,8 for example,
first discovered that highly efficient extraction of strontium ions
can be achieved when dicyclohexano-18-crown-6 (DC18C6) is
combined with RTILs. Subsequently, Rogers et al.9 and Bartsch
et al.10 reported the extraction of various alkali metal ions with
crown ethers in RTILs. In a different extractant, our research
group11 and Visser and Rogers12 demonstrated that octyl(phenyl)-
N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) dis-
solved in RTILs enhances the extractability of lanthanides and
actinides in comparison to when it is dissolved in a conventional
organic solvent. In addition, the use of RTILs in liquid-liquid
extraction of organic substances was also investigated.13-15 Arm-
strong et al.14 reported the partition coefficients of a large variety
of organic compounds with different functionalities and mentioned
the moderately effective extraction of aromatic amino acids in the
presence of dibenzo-18-crown-6. Subsequently, Pletnev et al.15
test using five different metal ions (Ag+, Cu2 +, Zn2 +, Co2 +
,
Ni2 +), only silver ions were transferred by the calix[4 ]-
arene from the aqueous feed phase into the RTILs,
through a cation-exchange mechanism. The pyridinocalix-
[4 ]arene was found to form a stable 1 :1 complex with
silver ions, both by slope analysis and by Job’s method.
Since it is easy to strip silver ions from RTILs by control-
ling the aqueous-phase pH, the extraction performance
of calix[4 ]arene in RTILs was maintained after five re-
peated uses.
Liquid-liquid extraction is an effective analytical separation
method for various metal ions. The extractant has a crucial effect
on the separation and extraction efficiency. Selection of an
appropriate extractant often determines the success of an extrac-
tion process. To date, many extractants have been developed for
the extraction of various metal ions. In particular, a cyclic ligand
calixarene,1 which consists of phenol rings connected by meth-
ylene bridges, has attracted much attention because the cyclic
ligand can recognize the size of metal ions with its cavity.2 This
unique property often leads to an extremely high selectivity for a
target metal ion. However, calixarene has an inherent problem in
industrial applications: poor solubility in organic solvents other
than toxic chloric organic solvents. This shortcoming is a barrier
for the practical use of calixarene. In previous papers, we
succeeded in dissolving calixarene in aliphatic organic solvents
by the addition of reversed micellar solutions3 or alcohols.4
(5) (a) Welton, T. Chem. Rev. 1 9 9 9 , 99, 2071. (b) Seddon, K. R. J. Chem.
Technol. Biotechnol. 1 9 9 7 , 68, 351.
(6) (a) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2 0 0 0 , 39, 3773. (b)
Sheldon, R. Chem. Commun. 2 0 0 1 , 2399.
(7) Fuller, J.; Carlin, R. T.; Osteryoung, R. A. J. Electrochem. Soc. 1 9 9 7 , 144,
3881.
(8) Dai, S.; Ju, Y. H.; Barnes, C. E. J. Chem. Soc., Dalton Trans. 1 9 9 9 , 1201.
(9) Visser, A. E.; Swatloski, R. P.; Reichert, W. M.; Griffin, S. T.; Rogers, R. D.
Ind. Eng. Chem. Res. 2 0 0 0 , 39, 3596.
* Corresponding author: (e-mail) mgototcm@mbox.nc.kyushu-u.ac.jp; (phone/
fax) +81-92-642-3575.
(1) Gutsche, C. D. Acc. Chem. Res. 1 9 8 3 , 16, 161.
(10) Chun, S.; Dzyuba, S. V.; Bartsch, R. A. Anal. Chem. 2 0 0 1 , 73, 3737.
(11) Nakashima, K.; Kubota, F.; Maruyama, T.; Goto, M. Anal. Sci. 2 0 0 3 , 19,
1097.
(2) Ludwig, R. Fresenius J. Anal. Chem. 2 0 0 0 , 367, 103.
(3) (a) Kubota, F.; Shinohara, K.; Shimojo, K.; Oshima, T.; Goto, M.; Furusaki,
S.; Hano, T. Sep. Purif. Technol. 2 0 0 1 , 24, 93. (b) Goto, M.; Shinohara, K.;
Shimojo, K.; Oshima, T.; Kubota, F. J. Chem. Eng. Jpn. 2 0 0 2 , 35, 1012.
(4) (a) Shimojo, K.; Watanabe, J.; Oshima, T.; Goto, M. Solvent Extr. Res. Dev.,
Jpn. 2 0 0 3 , 10, 185. (b) Nakashima, K.; Oshima, T.; Goto, M. Solvent Extr.
Res. Dev., Jpn. 2 0 0 2 , 9, 69.
(12) Visser, A. E.; Rogers, R. D. J. Solid State Chem. 2 0 0 3 , 171, 109.
(13) Huddleston, J.; Willauer, G. H. D.; Swatloski, R. P.; Visser, A. E.; Rogers, R.
D. Chem. Commun. 1 9 9 8 , 1765.
(14) Carda-Broch, S.; Berthod, A.; Armstrong, D. W. Anal. Bioanal. Chem. 20 03 ,
375, 191.
(15) Smirnova, S. V.; Torocheshnikova, I. I.; Formanovsky, A. A.; Pletnev, I.
V.Anal. Bioanal. Chem. 2 0 0 4 , 378, 1369.
10.1021/ac049549x CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/10/2004
Analytical Chemistry, Vol. 76, No. 17, September 1, 2004 5039

Synthesis of P yridinocalix[4 ]arene and Its Monomer
Analogue. tert-Butylcalix[4]arene was prepared as described
elsewhere.17 Pyridinocalix[4]arene tBu[4]CH2Py was subsequently
reacted with 2-(chloromethyl)pyridine in a Williamson ether
synthesis, as described previously.18 Briefly, the synthetic proce-
dure was as follows: Under a nitrogen atmosphere, a slurry of
tert-butylcalix[4]arene (5.0 g, 7.6 mmol) and sodium hydride (6.9
g, 173 mmol, 60% in oil) in anhydrous DMF (50 mL) was stirred
for 2 h at room temperature. After the solution was cooled,
2-(chloromethyl)pyridine (25 g, 152 mmol, 20 mol equiv) was
carefully added and the reaction was stirred for 30 h at 40 °C.
The solvent was evaporated under reduced pressure, and the
organic materials were extracted with chloroform. After the
organic layer was dried with anhydrous magnesium sulfate, the
solvent was removed in vacuo, and the residue was recrystallized
from methanol; white powder 6.2 g (80.3%), cone conformation:
1H NMR (250 MHz, CDCl3, TMS, 25 °C) (s, 36H, t-Bu) δ 3.06 (d,
4H, ArCHAAr), 4.41 (d, 4H, ArCHBAr), 4.99 (s, 8H, OCH2Py), 6.83
(s, 8H, ArH), 7.05 (t, 4H, 5-PyH), 7.27 (t, 4H, 4-PyH), 7.68 (d, 4H,
3-PyH), 8.48 (d, 4H, 6-PyH); MS, m/ e (% relative intensity) 1038
(MNa+, 20), 1016 (MH+, 100). Anal. Calcd for C68H76N4O4: C,
80.59; H, 7.56; N, 5.53. Found: C, 80.37; H, 7.57; N, 5.43.
Figure 1. Molecular structures and abbreviations of RTILs and
extractants.
reported that hydrophilic amino acids were quantitatively extracted
into RTILs containing DC18C6 even though in the presence of
crown ethers amino acids are typically not extracted into conven-
tional solvents. RTILs can provide an appropriate environment that
contributes to the performance of extractants and offer consider-
able potential as diluents in liquid-liquid extraction. The applica-
tion of RTILs to liquid-liquid extraction is, however, problematic
in that RTILs can dissolve only a few extractants. Thus, there has
been much interest in the development of RTIL-soluble extract-
ants.
Pyridine is miscible in RTILs, and it can also coordinate to
soft metal ions. In a previous study,16 we synthesized a calix[4]-
arene with pyridyl groups at the lower rim (tBu[4]CH2Py, Figure
1) and reported the solubilities of various calixarenes in RTILs as
well as their abilities to extract Ag+. Nonsubstituted tert-butyl or
tert-octylcalixarenes and their carboxylic acid derivatives were not
dissolved in 3-methylimidazolium hexafluorophosphate-based
RTILs ([Cnmim][PF6], n ) 4, 6, 8, Figure 1) at all; however, tBu[4]-
CH2Py was able to be dissolved in [Cnmim][PF6] (n ) 4, 6, 8),
and this compound showed an unprecedentedly high ability to
extract Ag+.
t
tert-Butylphenoxymethyl-2-pyridine Bu[1]CH2Py was also pre-
pared as a monomer analogue, by the same procedure. Thus,
2-(chloromethyl)pyridine was added to 4-tert-butylphenol. The final
product, a golden liquid (yield 98%), was purified by column
chromatography (SiO2) using a gradient of methanol in chloroform
as the eluent: 1H NMR (250 MHz, CDCl3, TMS, 25 °C) (s, 36H,
t-Bu) δ 5.19 (s, 8H, OCH2Py), 6.94 (s, 8H, ArHA), 7.20 (t, 4H,
5-PyH), 7.29 (s, 8H, ArHB), 7.54 (d, 4H, 3-PyH), 7.69 (t, 4H, 4-PyH),
8.58 (d, 4H, 6-PyH); MS, m/ e (% relative intensity) 244 (MH+,
100). Anal. Calcd for C16H19N1O1: C, 79.63; H, 7.94; N, 5.81.
Found: C, 79.91; H, 7.97; N, 5.73.
In the present study, we investigated the extraction perfor-
t
mance of Bu[4]CH2Py with respect to the target ion Ag+ and
RTIL Synthesis. Syntheses of 1-butyl-3-methylimidazolium
hexafluorophosphate [C4mim][PF6], 1-hexyl-3-methylimidazolium
hexafluorophosphate [C6mim][PF6], and 1-octyl-3-methylimidazo-
lium hexafluorophosphate [C8mim][PF6] were conducted as
described previously.13 The analogous chloride salts [Cnmim][Cl]
(n ) 4, 6, 8) were prepared by adding equal amounts of
1-methylimidazole and a primary alkyl chloride and reacting for
72 h at 70 °C. The resulting viscous liquids were washed with
ethyl acetate. After the last washing, the remaining ethyl acetate
was removed by heating to 90 °C under vacuum and freeze-drying.
The 1-alkyl-3-methylimidazolium hexafluorophosphate [Cnmim]-
[PF6] (n ) 4, 6, 8) was prepared by slowly adding hexafluoro-
phosphoric acid (1.3 mol equiv) to the corresponding chloride in
300 mL of water. After stirring for 12 h, the upper acidic aqueous
layer was decanted and the lower ionic liquid portion was washed
with water until the washings were no longer acidic. The
remaining water was removed by heating to 90 °C under vacuum
and freeze-drying.
discuss the mechanism of Ag+ extraction from an aqueous feed
t
solution into RTILs by using Bu[4]CH2Py. The extraction mech-
anism using ionic liquids was compared to that using chloroform.
Selectivity of calixarene in RTILs was also evaluated by a
competitive extraction trial using transition metal ions. Further,
since we succeeded in recovering Ag+ that had been extracted
by the calixarene in the RTILs, we here report the stripping
t
behavior of Ag+ with Bu[4]CH2Py.
EXPERIMENTAL SECTION
Reagents. 4-tert-Butylphenol (98.0%) and 2-(chloromethyl)-
pyridine hydrochloride were obtained from Tokyo Kasei Kogyo
Co., Inc. (Tokyo, Japan). 1-Methylimidazole (99%) and hexafluo-
rophosphoric acid (60 wt % solution in water) were purchased
from Aldrich Chemical Co., Inc. (Milwaukee, WI). 1-Chlorobutane
(98%), 1-chlorohexane (97%), and 1-chlorooctane (95%) were
obtained from Wako Pure Chemical Industries (Osaka, Japan).
Silver nitrate (99.8%), copper nitrate 3-hydrate (99.5%), zinc nitrate
6-hydrate (99%), cobalt nitrate 6-hydrate (98%), nickel nitrate
6-hydrate (98%), and chloroform (HPLC grade) were purchased
from Kishida Chemical Co. Ltd. and used without further purifica-
tion. All other reagents were of commercially available analytical
grade and used as received.
1-Butyl-3-methylimidazolium hexafluorophosphate [C4mim]-
[PF6]: 68% yield; 1H NMR (250 MHz, acetone-d6, TMS, 25 °C) δ
0.94 (t, 3H), 1.37 (m, 2H), 1.92 (m, 2H), 4.02 (s, 3H), 4.32 (t, 2H),
(17) Gutsche, C. D.; Iqbal, M. Org. Synth. 1 9 9 0 , 68, 234.
(18) (a) Pappalardo, S.; Giunta, L.; Foti, M.; Ferguson, G.; Gallagher, J. F.; and
Kaitner, B. J. Org. Chem. 1 9 9 2 , 57, 2611. (b) Ohto, K.; Higuch, H.; Inoue,
K. Solvent Extr. Res. Dev., Jpn. 2 0 0 1 , 8, 37.
(16) Shimojo, K.; Goto, M. Chem. Lett. 2 0 0 4 , 33, 320.
5040 Analytical Chemistry, Vol. 76, No. 17, September 1, 2004

7.64 (d, 2H), 8.86 (s, 1H). Anal. Calcd for C8H15F6N2P1: C, 33.81;
H, 5.32; N, 9.86. Found: C, 33.81; H, 5.33; N, 9.87.
1-Hexyl-3-methylimidazolium hexafluorophosphate [C6mim]-
[PF6]: 72% yield; 1H NMR (250 MHz, acetone-d6, TMS, 25 °C) δ
0.87 (t, 3H), 1.33 (m, 6H), 1.90 (m, 2H), 4.02 (s, 3H), 4.33 (t, 2H),
7.65 (d, 2H), 8.89 (s, 1H). Anal. Calcd for C10H19F6N2P1: C, 38.46;
H, 6.13; N, 8.97. Found: C, 38.36; H, 6.11; N, 8.99.
1-Octyl-3-methylimidazolium hexafluorophosphate [C8mim]-
[PF6]: 74% yield; 1H NMR (250 MHz, acetone-d6, TMS, 25 °C) δ
0.84 (t, 3H), 1.27 (m, 10H), 1.90 (m, 2H), 4.00 (s, 3H), 4.28 (t,
2H), 7.67 (d, 2H), 8.82 (s, 1H). Anal. Calcd for C12H23F6N2P1: C,
42.35; H, 6.81; N, 8.23. Found: C, 42.39; H, 6.78; N, 8.24.
Liquid-Liquid Extraction of Metal Ions in RTILs or
Chloroform. An extracting phase was prepared by dissolving
tBu[n]CH2Py (n ) 1, 4) in [Cnmim][PF6] (n ) 4, 6, 8). For
comparison with the performance of RTILs, chloroform containing
the extractant was also prepared in the same manner. The organic
phases were mixed and shaken on a vortex mixer with an equal
volume of aqueous AgNO3 (0.1 mM), at 25 °C for 30 min, to attain
equilibrium. These mixtures were then centrifuged for 3 min to
promote phase separation. After each phase was separated, the
concentration of Ag+ in the aqueous phase was determined by
using atomic absorption spectroscopy (Shimazu AA-6700), which
then allowed calculation of the degree of extraction (E )
Figure 2. Extraction behavior of Ag+ in [C8mim][PF6]/aqueous
(closed symbols) or chloroform/aqueous (open symbols) systems.
Extracting phase: 0.5 mM tBu[4]CH2Py (circles) or 2 mM tBu[1]CH2-
Py (squares). Aqueous phase: 0.1 mM AgNO3.
order to compare its ability to extract Ag+ with that of the cyclic
ligand tBu[4]CH2Py. Both extractants are soluble in [Cnmim][PF6]
(n ) 4, 6, 8). The silver ion, which exhibits a high affinity for
ligands containing a soft donor, was employed as the target metal
ion. Figure 2 shows the extraction behavior of Ag+ in the systems
[Ag+]org.eq./ [Ag+]aq.ini.) and the distribution ratio (D ) [Ag+]org.eq.
/
t
with Bu[4]CH2Py or tBu[1]CH2Py in [C8mim][PF6] in comparison
[Ag+]aq.eq.). In the continuous variation method (Job’s method),
the total concentration of metal ion and extractant was kept
constant at 0.2 mM. In the competitive solvent extraction, solutions
containing various metal ions (AgNO3, Cu(NO3)2, Zn(NO3)2,
Ni(NO3)2, Co(NO3)2) were prepared at metal concentrations of
0.1 mM.
t
t
to the systems with Bu[4]CH2Py or Bu[1]CH2Py in chloroform.
Nitric acid was used as the source of counterions for the Ag+
transfer, because Ag+ forms a water-insoluble complex with
t
chloride. The concentration of Bu[1]CH2Py was 4-fold greater
than that of tBu[4]CH2Py, to provide the same number of
functional pyridyl groups. In the absence of these extractants, the
partitioning of Ag+ into [C8mim][PF6] or chloroform was con-
firmed to be negligibly small. The monomer analogue tBu[1]CH2-
Py did not extract Ag+ very well, whereas the cyclic ligand
tBu[4]CH2Py efficiently extracted Ag+ from the aqueous to the
organic phase, which indicates that the inclusion and chelating
effects are predominant factors in the exceptionally high Ag+
Solubility of RTILs in Water. A [Cnmim][PF6] (n ) 4, 6, 8)
was mixed and shaken on a vortex mixer with an equal volume
of deionized water at 25 °C for 30 min. These mixtures were then
centrifuged for 3 min to promote phase separation. After each
phase was separated, the aqueous phase was diluted with
deionized water by 1/ 50-fold for [C8mim][PF6] and 1/ 200-fold for
[C6mim][PF6] and [C4mim][PF6]. The absorbance of this solution
at 211 nm was measured by a UV-visible spectrophotometer
(Jasco U-best 570). The [Cnmim]+ concentrations in aqueous
phases were calculated with the Beer’s law plots formed for
[Cnmim][Cl] solutions.
t
extraction. The cavity size of Bu[4]CH2Py fits the Ag+ ion size
(ionic radius, 1.02 Å), which is also similar to that of Na+ (ionic
radius, 1.02 Å).
The extraction behavior of Ag+ with Bu[4]CH2Py in [C8mim]-
t
t
t
Stripping Test. Forward extraction of Ag+ (0.1 mM) with Bu-
[PF6] is quite different from that with Bu[4]CH2Py in chloroform.
In the case with chloroform, ∼90% of Ag+ was extracted by Bu-
t
[4]CH2Py (0.5 mM) was performed following the same procedure.
After equal volumes (15 mL) of the aqueous and RTIL solutions
were mixed and shaken on the vortex mixer, Ag+ was quantita-
tively transferred to the RTIL phase. The RTIL phase was divided
into 2.5-mL aliquots, and a fresh aqueous solution (2.5 mL)
containing nitric acid was added to each aliquot. Both phases were
mixed and shaken on the vortex mixer at 25 °C for 30 min. These
mixtures were then centrifuged for 3 min to promote phase
separation. The stripping solution was separated from the RTIL
phase and the Ag+ concentration was quantified in order to
[4]CH2Py at relatively high nitric acid concentrations (10-1-10-2
-
M; pH 1-2). The decrease in extractability at low NO3 concen-
trations could be attributed to the lack of counterions, and at the
high NO3- concentrations, the extraction efficiency was reduced
due to the protonation of pyridyl nitrogens in the cyclic ligand. In
contrast, in the ionic liquids, tBu[4]CH2Py exhibited high extrac-
-
tion affinity for Ag+ at a low NO3 concentration. In particular,
even when no HNO3 was present, Ag+ was efficiently transferred
into [C8mim][PF6]. These results suggest that metal complexation
in [C8mim][PF6] proceeds via a different mechanism from that in
a conventional organic solvent.
evaluate the stripping ratio () 100 × [Ag+]aq.eq./ [Ag+]org.ini.
)
RESULTS AND DISCUSSION
It is assumed that the properties of the extracting solvent affect
the ability of tBu[4]CH2Py to act as an extractant. The extraction
behavior of Ag+ with increasing Bu[4]CH2Py concentrations in
Extraction Equilibria of Silver Ions in RTILs or Chloro-
t
t
form. The monomer analogue Bu[1]CH2Py was synthesized in
Analytical Chemistry, Vol. 76, No. 17, September 1, 2004 5041

t
Figure 3. Degree of Ag+ extraction with increasing Bu[4]CH2Py
-
2-
Figure 4. Effect of the anionic species (NO3 (circles) or SO4
(squares)) on the extraction of Ag+ by tBu[4]CH2Py. Extracting
phase: 0.5 mM Bu[4]CH2Py in [C8mim][PF6] (closed symbols) or
chloroform (open symbols). Aqueous phase: 0.1 mM AgNO3.
concentration in [Cnmim][PF6] (n ) 4 (closed squares), n ) 6 (open
triangles), n ) 8 (closed circles)) and chloroform (open circles).
Aqueous phase: 0.1 mM AgNO3 in deionized water when using
[Cnmim][PF6] or 0.1 mM AgNO3 in 0.1 M HNO3 when using chloro-
form.
t
M
m+ + xLorg + mNO3- S MLx(NO3)m,org
(1)
various RTILs and in chloroform was investigated (Figure 3). This
trial was carried out under optimal conditions for the RTILs or
chloroform systems, based on the results presented in Figure 2:
The aqueous solution for the RTILs system was prepared by
dissolving 0.1 mM AgNO3 in deionized water, while in the
chloroform system, 0.1 mM AgNO3 in 0.1 M HNO3 was used as
the aqueous phase. The solubility of RTILs in deionized water
was measure to be 7.48 ( 0.07 ([C8mim][PF6]), 24.66 ( 0.23
([C6mim][PF6]), and 73.17 ( 0.10 mM ([C4mim][PF6]). In
[C4mim][PF6] and [C6mim][PF6], Ag+ distributed into the organic
phase to some extent without any extractant. The efficiency of
M
m+ + xLorg + mCnmim+ S MLx,orgm+ + mCnmim+ (2)
org
In the above equations, species present in the organic phase are
indicated by the subscript org.
In contrast, Dietz et al.21 argued that, as in conventional organic
solvents, Sr2+ transfer with a crown ether (DC18C8) in highly
hydrophobic RTILs proceeds through ion pair extraction. In the
present study, tBu[4]CH2Py dissolved in RTILs greatly enhanced
t
the extractability of Ag+ compared with that of Bu[4]CH2Py
dissolved in chloroform, which prompted us to investigate the
difference in the extraction mechanism between [C8mim][PF6]
and the chloroform system. Figure 4 shows the effect of the
t
Ag+ extraction by Bu[4]CH2Py increased slightly as the 1-alkyl
t
anionic species on the extraction of Ag+ by Bu[4]CH2Py. Nitric
group in the RTILs was elongated. The solubility of tBu[4]CH2Py
in [Cnmim][PF6] also increased as the 1-alkyl group in the RTILs
was elongated, because RTILs with a long alkyl chain result in
acid or sulfuric acid was employed as counterion. In chloroform,
the degree of extraction was reduced drastically when SO42- was
used as the anion. This result indicates that the anionic species
greatly affects Ag+ transfer by Bu[4]CH2Py. In fact, Bu[4]-
CH2Py in chloroform extracts Ag+ through ion pair extraction
accompanied by counterions. In [C8mim][PF6], the extraction
behavior of Ag+ with SO42- was similar to that with NO3-, which
demonstrates that the Ag+ transfer into [C8mim][PF6] does not
involve anion coextraction.
The influence of the aqueous-phase C8mim+ concentration on
Ag+ extraction by tBu[4]CH2Py is shown in Figure 5. The C8mim+
concentration in the aqueous phase was adjusted using [C8mim]-
[Cl], which is soluble in water. We carefully verified that the initial
concentration of Ag+ shows a steady value in all samples for a
long period. This result means that Ag+ does not form a precipitate
with Cl- under the present experimental conditions. The transfer
of Ag+ into chloroform was independent of the aqueous-phase
C8mim+ concentration. In contrast, in the system with [C8mim]-
[PF6], the degree of extraction decreased gradually with increases
in the aqueous-phase C8mim+ concentration. This means that
exchange of Ag+ for C8mim+ in the organic phase becomes
difficult when the concentration of C8mim+ in the aqueous phase
t
high hydrophobicity. Thus, it was suggested that Bu[4]CH2Py
t
t
in [C8mim][PF6] can form a more stable complex with Ag+ than
it can when in the other RTILs tested in this study.
The extraction ability of tBu[4]CH2Py was remarkably higher
in the RTILs than in chloroform. For example, Ag+ can be
quantitatively extracted into [C8mim][PF6] at a concentration of
t
only 0.2 mM Bu[4]CH2Py, which is only twice the concentration
of Ag+. In chloroform, however, a 10-fold higher concentration, 2
mM tBu[4]CH2Py, is required to obtain equivalent Ag+ extraction.
Determination of the Extraction Equilibrium Equation.
In recent studies of metal extraction in RTILs, it has been
demonstrated that the partitioning mechanism with crown ethers19
or CMPO20 changes from an ion pair extraction mechanism (eq
1) in conventional organic solvents to a cation-exchange extraction
mechanism (eq 2) in which the metal ion is exchanged for the
cationic constituent of the RTILs.
(19) (a) Dietz, M. L.; Dzielawa, J. A. Chem. Commun. 2 0 0 1 , 2124. (b) Jensen,
M. P.; Dzielawa, J. A.; Rickert, P.; Dietz, M. L. J. Am. Chem. Soc. 2 0 0 2 ,
124, 10664.
(20) Visser, A. E.; Jensen, M. P.; Laszak, I.; Nash, K. L.; Choppin, G. R.; Rogers,
R. D. Inorg. Chem. 2 0 0 3 , 42, 2197.
(21) Dietz, M. L.; Dzielawa, J. A.; Laszak, I.; Young, B. A.; Jensen, M. P. Green
Chem. 2 0 0 3 , 5, 682.
5042 Analytical Chemistry, Vol. 76, No. 17, September 1, 2004

Figure 5. Effect of aqueous-phase C8mim+ concentration on the
extraction of Ag+ by tBu[4]CH2Py. Extracting phase: 0.5 mM tBu[4]-
CH2Py in [C8mim][PF6] (closed symbols) or chloroform (open sym-
bols). Aqueous phase: 0.1 mM AgNO3.
t
Figure 7. Job’s plots of the complexation between Ag+ and Bu-
[4]CH2Py in [C8mim][PF6]. Total concentration of Ag+ and tBu[4]CH2-
Py was kept constant at 0.2 mM.
Figure 8. Competitive extraction of transition metal ions by tBu[4]-
t
Figure 6. Slope analysis of Ag+ extraction as a function of Bu[4]-
t
CH2Py in [C8mim][PF6]. Extracting phase: 0.5 mM Bu[4]CH2Py in
CH2Py concentration in [C8mim][PF6] (closed symbols) or chloroform
(open symbols). Aqueous phase: 0.1 mM AgNO3 in deionized water
when using [Cnmim][PF6] or 0.1 mM AgNO3 in 0.1 M HNO3 when
using chloroform.
[C8mim][PF6]. Aqueous phase: 0.1 mM AgNO3 (closed circles), 0.1
mM Cu(NO3)2 (open circles), 0.1 mM Zn(NO3)2 (closed squares), 0.1
mM Co(NO3)2 (open squares), and 0.1 mM Ni(NO3)2 (closed tri-
angles).
Ag+ + tBu[4]CH2Pyorg + C8mim+
S
org
is high, because metal ion transfer by tBu[4]CH2Py into [C8mim]-
[PF6] proceeds via a cation-exchange mechanism, as expressed
by eq 2.
Ag‚tBu[4]CH2Py,org+ + C8mim+ in [C8mim][PF6] (4)
To determine the fundamental stoichiometry of the Ag+-tBu[4]-
Competitive Extraction of Transition Metal Ions. The
unique property of calixarene is its high selectivity for a target
metal ion. To assess the selectivity of tBu[4]CH2Py, competitive
CH2Py complex, slope analysis was conducted as a function of
t
the equilibrium concentration of Bu[4]CH2Py in the organic
phase. As shown in Figure 6, linear relationships between log D
and log[tBu[4]CH2Py] with a slope of 1 were obtained for both
the RTIL and the chloroform systems. Furthermore, the stoichi-
ometry of the extraction complex was confirmed by Job’s method.
As illustrated in Figure 7, the concentration of Ag+ in [C8mim]-
[PF6] reached a maximum value when the ratio of the tBu[4]CH2-
Py concentration to the total concentration was 0.5. These results
extraction of five different transition metal ions (Ag+, Cu2+, Zn2+
,
Co2+, Ni2+) by tBu[4]CH2Py in [C8mim][PF6] was conducted. As
summarized in Figure 8, Bu[4]CH2Py could separate only Ag+
t
from the aqueous phase containing the transition metal ions. Two
possible reasons for this selectivity could be that Ag+ fits the cavity
of tBu[4]CH2Py much better than the other metal ions tested in
this study (ionic radii: Ag+, 1.02 Å; Cu2+, 0.73 Å; Zn2+, 0.74 Å;
Co2+, 0.75 Å’ Ni2+, 0.69 Å), and is a monovalent cation, which is
favorable for exchange with C8mim+.
Stripping and Recycling Test. The back-transfer of metal ions
into a receiving phase is also important for separation and
concentration of a target metal ion. Recently, Wei et al.22 reported
that metal ions extracted with the acidic chelator dithizone in
t
suggest that one Bu[4]CH2Py is required to extract one Ag+,
indicating that a 1:1 complex is formed. On the basis of these
results, the extraction mechanisms in RTILs and in chloroform
were determined to be as follows:
Ag+ + tBu[4]CH2Pyorg + NO3- S
Ag‚tBu[4]CH2Py‚NO3,org in chloroform (3)
(22) Wei, G. T.; Yang, Z.; Chen, C. J. Anal. Chim. Acta 2 0 0 3 , 488, 183.
Analytical Chemistry, Vol. 76, No. 17, September 1, 2004 5043

conditions. In comparison to the results in chloroform, a 0.1 M
HNO3 solution ensures the recovery of 90% of the Ag+ in [C8mim]-
[PF6], whereas in chloroform, only 10% of the Ag+ is recovered
under the same conditions, and a 5 M HNO3 solution is required
to completely recover Ag+. In addition, it was found that Ag+
recovery could be achieved using a stripping solution containing
1 M thiourea. A recycling test for the RTILs phase was also carried
out, following the same procedure. A 0.5 M HNO3 solution was
employed as the receiving phase, and the RTIL phase was then
recycled. Even though five cycles of forward and back extraction
were carried out, the tBu[4]CH2Py-RTILs extraction system
maintained its high extraction ability (the average degree of
extraction was 99.5%). This extraction system was found to be
very reproducible.
Figure 9. Extraction and stripping profile of Ag+ in [C8mim][PF6].
CONCLUSION
Extraction (closed symbols); stripping (open symbols). Extracting
A solvent extraction system based on combining RTILs and
t
phase: 0.5 mM Bu[4]CH2Py in [C8mim][PF6]. Aqueous phase: 0.1
calixarene was investigated. Pyridinocalix[4]arene dissolved in
[C8mim][PF6] is able to extract silver ions much more effectively
than when it is dissolved in chloroform, and high selectivity for
silver ions from among five different transition metal ions (Ag+,
Cu2+, Zn2+, Co2+, Ni2+) was demonstrated. This compound
transfers silver ions into RTIL phases via a cation-exchange
mechanism and forms a stable 1:1 complex with silver ions.
Furthermore, recovery of silver ions from [C8mim][PF6] into a
receiving phase can be achieved under acidic conditions, which
are mild in comparison to those required for the chloroform
system. These results highlight the great potential of calixarenes
as extractants in RTILs systems. In the future, modification of
appropriate functional groups at the lower and/ or upper rims will
enable expansion of the frontiers of RTILs system applications.
mM AgNO3.
RTILs can be stripped into an acidic solution. However, since
neutral extractants such as crown ethers or CMPO are able to
extract metal ions under acidic conditions, researchers have
experienced difficulties in recovering cations extracted by neutral
extractants, despite the positive results for RTILs extraction
systems. Fortunately, the efficiency of Ag+ extraction by Bu[4]-
t
CH2Py in RTILs is reduced with increasing nitric acid concentra-
tion, even though tBu[4]CH2Py is a neutral extractant. The
stripping test for Ag+ extracted by tBu[4]CH2Py in [C8mim][PF6]
was performed using pH control of the receiving phase (Figure
9). The stripping efficiency of Ag+ was dependent on the
concentration of HNO3, and the profile of the stripping ratio was
the inverse of the profile for the degree of extraction. As a result,
Ag+ was efficiently transferred into the receiving phase at low-
Received for review March 23, 2004. Accepted June 6,
2004.
t
pH conditions because Bu[4]CH2Py was protonated under acidic
AC049549X
5044 Analytical Chemistry, Vol. 76, No. 17, September 1, 2004