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Macrocycle-mediated separation of Eu2+ from trivalent lanthanide cations in a modified thin-sheet-supported liquid membrane system
Journal
of Membrane Science, 50 (1990)
319-324
Elsevier Science Publishers B.V., Amsterdam
MACROCYCLE-MEDIATED TRIVALENT LANTHANIDE THIN-SHEET-SUPPORTED
-
319 Printed in The Netherlands
SEPARATION OF Et?’ FROM CATIONS IN A MODIFIED LIQUID MEMBRANE SYSTEM
C.Y. ZHU and R.M. IZATT*
Department of Chemistry, Brigham Young University, Provo, Utah 84602 (U.S.A.) (Received August 25,1989;
accepted in revised form November
61989)
Summary Macrocycle-mediated transport of Eu3+ and EL?+ from an aqueous solution through a modified thin-sheet-supported liquid membrane has been studied and compared with that of SF, Gd3+ and Nd3+. Transport of Et?+ was found to be more effective than that of trivalent lanthanide cations using bis (l-hydroxylheptyl)DC18C6 as the membrane carrier. The flux of EL?+ was similar to that of Sr2+ while those of Eu 3+, Gd3+ and Nd3+ were almost identical and much less than that of Sr’+.
Introduction
Selective transport of a particular species through a separation device often provides an effective means for recovering that species from a cation mixture. Carrier-mediated membrane transport has been studied for many years for this purpose and the crown ether compounds have proven to be excellent choices for metal cation separations because of their ability to complex particular cations selectively [ 11.These separations have been performed using alkali, alkaline earth, post transition and transition metal ions [ 21. The separation of trivalent lanthanide metal ions by these methods is expected to be difficult because these cations are similar to each other in cation size and aqueous chemistry [ 31. Earlier, it was found [ 41 that in single cation systems the DC18C6-mediated flux of Eu2+ in a H20-CHC13-H20 bulk liquid membrane system was large, being comparable to that of Sr2+. Under the same conditions, the flux of Eu3+ was negligible. These results suggested that in competitive systems it should be possible to separate Eu3+ selectively from other trivalent *To whom correspondence
0376-7388/90/$03.50
should be addressed.
0 1990 Elsevier Science Publishers
B.V.
320
lanthanide metal ions using a crown ether-mediated liquid membrane system first reducing Eu”+ to Eu2+ in the source phase and then
by
R = R
=
H:
dicyclohexano-18-crown-6
I-hydroxyheptyl:
(DC18C6)
bis(l-hydroxyheptyl)DCl8C6
transporting the Eu2+ selectively using an 18C6-type macrocycle as the carrier. The macrocycle 18C6 has been shown to have low affinity for the trivalent lanthanide ions in methanol solvent [ 51. This affinity is expected to be much lower in an aqueous solvent. In the present study, Eu”+ reduction has been accomplished electrically. The resulting Eu2+ has been separated from several trivalent lanthanide metal ions, and the fluxes of Et?+ and Sr”+ have been determined in competitive experiments using a modified thin sheet supported liquid membrane (TSSLM ) system. Experimental
The TSSLM system described earlier [6] was modified as shown in Fig. 1. In this system, a mercury pool located in the source phase was the cathode part of the electrolysis cell. A platinum coil and a calomel electrode were used as the anode and reference electrodes, respectively. The TSSLM was prepared by soaking a sheet of Celgard 2400 film in a 0.05 A4 carrier solution in phenylhexane. The effective membrane surface area was 20.5 cm2. Bis (l-hydroxylheptyl)DC18C6 was chosen as the carrier because of its appropriate cavity size for preferential complexing of Et?+ and its high hydrophobicity which minimized
Source
Phase
Receiving
Phase
Fig. 1. Schematic of modified TSSLM system: 1) platinum coil anode, 2) cathode, 3) calomel reference electrode, 4) mercury pool, 5) stirrers, 6) glass joint, 7) thin sheet supported liquid membrane, 8) sampling port.
321
loss to the aqueous phase. Phenylhexane was chosen as the membrane solvent based on its high hydrophobicity and high boiling point which prevented drying out of the membrane [ 61. During each run, the source and receiving phases were each stirred at 200 rpm and both the cathode and anode cells were deaerated by softly blowing nitrogen gas into them. The cathode potential was controlled at -0.8 V (relative to the reference electrode) which is critical for reduction of Eu3+ in aqueous solution [ 31 and the cathode current density was about 1.0 mA/cm’. The supporting electrolyte in both cathode and anode cells was 0.5 M LiC104. The pH of the solution was maintained at about 7. The reduction efficiency was estimated to be over 90% according to the determination of the Eu2+ concentration in the samples from the cathode cell by oxidation-reduction titration with KMnO, as the titrant. In the transport experiments, the source phase was a dilute aqueous solution of mixed metal nitrates or chlorides with the concentration of each cation, except Li+, being 0.01 M and the receiving phase was deionized water. The receiving phase was sampled at appropriate time intervals and the concentrations of different metal cations in the samples were determined by ICP spectroscopy (Perkin-Elmer Plasma II Emission Spectrometer). All the experiments were run twice and the deviations from the averages were found to be less than 10%. Reagent grade chemicals were used in the experiments. Bis (l-hydroxylheptyl)DC18C6 was obtained from Parish Chemical Co., Orem, Utah and phenylhexane from Eastman Kodak. Eu(NO~)~ (MCB), Sr(N03)2 (Fisher), Nd ( NOB) 3 (Research Chemicals), GdC13 (Research Chemicals), and LiClO, (MCB ) were used without further purification. Results and discussion
The bis( 1-hydroxylheptyl)DC18CG-mediated transport of Eu3+ in the TSSLM system was compared with that of Sr2+, Gd3+ and Nd3+ in two competitive experiments. It was found that the flux of Eu3+ was about five times smaller than that of Sr2+ in experiment (I) (Table l), while those of Eu3+, Gd3+ and Nd3+ were about identical in experiment (II) (Table 1) . When Eu3+ was reduced electrically to Eu2+, the transport selectivity for Eu was improved markedly. The flux of Eu2+ was found to be even slightly larger than that of Sr2+ in experiment (III ) (Table 1)) and approximately five times larger than those of Gd3+ and Nd3+ in experiment (IV) (Table 1). Thus, Eu2+ was effectively separated from Gd3+ and Nd3+ in the TSSLM system. In Fig. 2(a)-(d), the results of experiments (I) - (IV) are illustrated by giving plots of percentage cation transport as a function of time. It should be noted that the system discussed above is not useful as a real separation process for two reasons. First, the flux using the TSSLM system is low since the surface area of the TSSLM is small. Second, there is a decrease of transport for all of the cations when the electric field is applied, as is seen
322 TABLE
1
Competitive EL?+, EL?+, Sr ‘+ , Nd’+ and Gd3+ fluxes” through TSSLM hydroxyheptyl)DC18CG Experiment
Cation
Flux (mol-sec-‘-m-2)
systems containing bis
(l-
X 10”
340 k 20
S?+
(Ilb UJb
Et?+
69+6
(II)” (II)” (II)”
Gd”+ Nd3+ EL?+
7ort5 76+5 68k4
(III)d (III)4
Sr2+
64+4
Eu*+
81+7
(IV)” (IV)’ (IV)e
Gd3+ Nd”+ Et?+
llkl 1852 84+7
“Calculated by obtaining the slope of a plot of receiving phase cation concentration (as a percentage of initial source phase cation concentration) vs. time and multiplying this quantity by the source phase initial concentration and the receiving phase volume and dividing by the membrane surface area. bExperiment (I ) : Competitive experiment involving Sr* + and ELI”+ , m the following system: source phase, 0.01 M Sr(NO,),, Eu(NO,),; membrane phase, 0.05 M macrocycle in phenylhexane on Celgard 2400 polypropylene support; receiving phase, water. “Experiment (II): Competitive experiment involving Et?+, Gd3+ and Nd3+, same conditions as footnote b. dExperiment
(III):
Competitive
experiment
involving
Sr2+ and Et?+,
same conditions
as foot-
note b except that an electric field is applied to reduce Eu”+ to EL?+. “Experiment (IV): Competitive experiment involving Et?+, Gd3+ and Nd”+, same condition footnote d.
as
by comparing the flux data between experiments (I) and (III) or (II) and (IV) (Table 1). This is because the source phase of the TSSLM system also acts as the cathode cell and the negative electric field produced by the cathode will not favor the transport of positively charged ions through the liquid membrane. The first limitation may be overcome by using a hollow-fiber supported liquid membrane system [7] where the surface area of the liquid membrane is increased thousands of times. The effect of the second limitation may be decreased by designing the system to compensate for the effect of the electric field on cation transport. The flux results can be understood by comparing the relative log K values for the interaction of 18C6 with the investigated cations. The log K( CH,OH) value for the interaction of 18C6 with Sr2+ ( > 5.5) is much larger than that with Eu”+ (1.84) [ 81. The relative magnitudes of these log K values are consistent with the relative fluxes of these two cations through the TSSLM. The
323 T
,
r
,T
(a)
B
0 ..-
r a n
Sr(IIl
5-
”
A -- Eu(IIIl
s P 0
4-
; (9%) Jr
0
10
20 30 Time(hr)
40
50
T
Cc)
0
, T r 0 ---
Sr(II)
A--Eu(II)
20 30 Timc(hr)
40
50
A --- E”(H)
Cd)
a ”
10
5.
x . . . Nd(II1)
S P 0
o--
Gd(III)
6) x
100
I
I
I
Time(hr)
Fig. 2. Plots of percentage cation transport (amount of cation transported as a percentage of its corresponding initial source phase concentration) as a function of time for (a) competitive transport of Sr2+ and EL?+, (b) competitive transport of Eu3+, Gd3+ and Nd3+, (c) competitive transport of S? and EL?+, and (d) competitive transport of Eu*+, Gd3+ and Nd3+.
log K(CH,OH) values for 18C6 interaction with Eu3+, Gd3+ and Nd3+ are 1.84, 1.32 and 2.44, respectively [5]. However, in aqueous solution, since the large dehydration energies of the trivalent lanthanide cations make the complexation of these cations by 18C6 unfavorable, the 1ogKvalues will be smaller. Therefore, the similar and small fluxes of the three trivalent lanthanide cations in experiment (II) were as expected. The log K value for 18C6-Eu2+ interaction has not been determined. However, the coordination chemistry of Eu2+ is expected to be similar to that of Sr2+ because of their similar cation radii (Eu2+, 1.31A; Sr2+, 1.27A) [ 91. These bivalent cations would be expected to form more stable complexes than Eu3T with 18C6 in aqueous solution since they fit the cavity of 18C6 (radius, 1.30 A) better than Eu3+ (radius, 1.09 A) does, and they have smaller dehydration energies than Eu3+. The fluxes of Eu2+, Sr2+ and Eu3+ compared to one another and to those of the other investigated cations were found to be in agreement with the above considerations.
324
Acknowledgements Appreciation for financial support of this research is expressed to the U.S. Department of Energy, Office of Basic Energy Sciences, Grant No. DE-FG0286ERl3463. Helpful discussions with Dr. Ronald L. Bruening and Ms. Anne M. Izatt are also acknowledged.
References 1
J.D. Lamb, R.M. Izatt and J.J. Christensen, Stability constants of cation-macrocycle complexes and their effect on facilitated membrane transport rates, in: R.M. Izatt and J.J. Christensen (Eds.), Progress in Macrocyclic Chemistry, Vol. 2, John Wiley & Sons, New York, NY,
2
1981, pp. 41-90. R.M. Izatt, G.A. Clark, J.S. Bradshaw, J.D. Lamb and J.J. Christensen,
3
transport of ions in liquid membrane systems, Sep. Purif. Methods, 15 (1986) 21-72. F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 5th edn., John Wiley & Sons,
4
5
6
7 8 9
Macrocycle-facilitated
New York, NY, 1988, pp. 955-979. P.R. Brown, R.M. Izatt, J.J. Christensen and J.D. Lamb, Transport ofEu’+ in a H,O-CHCl,H,O liquid membrane system containing the macrocyclic polyether 18.crown-6, J. Membrane Sci., 13 (1983) 85-88. R.M. Izatt, J.D. Lamb, J.J. Christensen
and B.L. Haymore,
Anomalous
stability sequence of
lanthanide(II1) chloride complexes with 18-crown-6 in methanol. Abrupt decrease to zero from Gd3+ to Tb3+, J. Am. Chem. Sot., 99 (1977) 8344-8346. J.D. Lamb, Y. Hirashima, R.L. Bruening, P.K. Tse, R.M. Izatt and J.J. Christensen, Characterization of a supported liquid membrane for macrocycle-mediated selective cation transport, J. Membrane Sci., 37 (1988) 13-26. R.M. Izatt, K.P. Roper, R.L. Bruening and J.D. Lamb, Macrocycle mediated cation transport using hollow fiber supported liquid membranes, J. Membrane Sci., 45 (1989) 73-84. R.M. Izatt, J.S. Bradshaw, S.A. Nielsen, J.D. Lamb, J.J. Christensen and D. Sen, Thermodynamic and kinetic data for cation-macrocycle interaction, Chem. Rev., 85 (1985) 271-339. R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr., Sect. A: Found. Crystallogr., 32 (1976) 751-767.
of Membrane Science, 50 (1990)
319-324
Elsevier Science Publishers B.V., Amsterdam
MACROCYCLE-MEDIATED TRIVALENT LANTHANIDE THIN-SHEET-SUPPORTED
-
319 Printed in The Netherlands
SEPARATION OF Et?’ FROM CATIONS IN A MODIFIED LIQUID MEMBRANE SYSTEM
C.Y. ZHU and R.M. IZATT*
Department of Chemistry, Brigham Young University, Provo, Utah 84602 (U.S.A.) (Received August 25,1989;
accepted in revised form November
61989)
Summary Macrocycle-mediated transport of Eu3+ and EL?+ from an aqueous solution through a modified thin-sheet-supported liquid membrane has been studied and compared with that of SF, Gd3+ and Nd3+. Transport of Et?+ was found to be more effective than that of trivalent lanthanide cations using bis (l-hydroxylheptyl)DC18C6 as the membrane carrier. The flux of EL?+ was similar to that of Sr2+ while those of Eu 3+, Gd3+ and Nd3+ were almost identical and much less than that of Sr’+.
Introduction
Selective transport of a particular species through a separation device often provides an effective means for recovering that species from a cation mixture. Carrier-mediated membrane transport has been studied for many years for this purpose and the crown ether compounds have proven to be excellent choices for metal cation separations because of their ability to complex particular cations selectively [ 11.These separations have been performed using alkali, alkaline earth, post transition and transition metal ions [ 21. The separation of trivalent lanthanide metal ions by these methods is expected to be difficult because these cations are similar to each other in cation size and aqueous chemistry [ 31. Earlier, it was found [ 41 that in single cation systems the DC18C6-mediated flux of Eu2+ in a H20-CHC13-H20 bulk liquid membrane system was large, being comparable to that of Sr2+. Under the same conditions, the flux of Eu3+ was negligible. These results suggested that in competitive systems it should be possible to separate Eu3+ selectively from other trivalent *To whom correspondence
0376-7388/90/$03.50
should be addressed.
0 1990 Elsevier Science Publishers
B.V.
320
lanthanide metal ions using a crown ether-mediated liquid membrane system first reducing Eu”+ to Eu2+ in the source phase and then
by
R = R
=
H:
dicyclohexano-18-crown-6
I-hydroxyheptyl:
(DC18C6)
bis(l-hydroxyheptyl)DCl8C6
transporting the Eu2+ selectively using an 18C6-type macrocycle as the carrier. The macrocycle 18C6 has been shown to have low affinity for the trivalent lanthanide ions in methanol solvent [ 51. This affinity is expected to be much lower in an aqueous solvent. In the present study, Eu”+ reduction has been accomplished electrically. The resulting Eu2+ has been separated from several trivalent lanthanide metal ions, and the fluxes of Et?+ and Sr”+ have been determined in competitive experiments using a modified thin sheet supported liquid membrane (TSSLM ) system. Experimental
The TSSLM system described earlier [6] was modified as shown in Fig. 1. In this system, a mercury pool located in the source phase was the cathode part of the electrolysis cell. A platinum coil and a calomel electrode were used as the anode and reference electrodes, respectively. The TSSLM was prepared by soaking a sheet of Celgard 2400 film in a 0.05 A4 carrier solution in phenylhexane. The effective membrane surface area was 20.5 cm2. Bis (l-hydroxylheptyl)DC18C6 was chosen as the carrier because of its appropriate cavity size for preferential complexing of Et?+ and its high hydrophobicity which minimized
Source
Phase
Receiving
Phase
Fig. 1. Schematic of modified TSSLM system: 1) platinum coil anode, 2) cathode, 3) calomel reference electrode, 4) mercury pool, 5) stirrers, 6) glass joint, 7) thin sheet supported liquid membrane, 8) sampling port.
321
loss to the aqueous phase. Phenylhexane was chosen as the membrane solvent based on its high hydrophobicity and high boiling point which prevented drying out of the membrane [ 61. During each run, the source and receiving phases were each stirred at 200 rpm and both the cathode and anode cells were deaerated by softly blowing nitrogen gas into them. The cathode potential was controlled at -0.8 V (relative to the reference electrode) which is critical for reduction of Eu3+ in aqueous solution [ 31 and the cathode current density was about 1.0 mA/cm’. The supporting electrolyte in both cathode and anode cells was 0.5 M LiC104. The pH of the solution was maintained at about 7. The reduction efficiency was estimated to be over 90% according to the determination of the Eu2+ concentration in the samples from the cathode cell by oxidation-reduction titration with KMnO, as the titrant. In the transport experiments, the source phase was a dilute aqueous solution of mixed metal nitrates or chlorides with the concentration of each cation, except Li+, being 0.01 M and the receiving phase was deionized water. The receiving phase was sampled at appropriate time intervals and the concentrations of different metal cations in the samples were determined by ICP spectroscopy (Perkin-Elmer Plasma II Emission Spectrometer). All the experiments were run twice and the deviations from the averages were found to be less than 10%. Reagent grade chemicals were used in the experiments. Bis (l-hydroxylheptyl)DC18C6 was obtained from Parish Chemical Co., Orem, Utah and phenylhexane from Eastman Kodak. Eu(NO~)~ (MCB), Sr(N03)2 (Fisher), Nd ( NOB) 3 (Research Chemicals), GdC13 (Research Chemicals), and LiClO, (MCB ) were used without further purification. Results and discussion
The bis( 1-hydroxylheptyl)DC18CG-mediated transport of Eu3+ in the TSSLM system was compared with that of Sr2+, Gd3+ and Nd3+ in two competitive experiments. It was found that the flux of Eu3+ was about five times smaller than that of Sr2+ in experiment (I) (Table l), while those of Eu3+, Gd3+ and Nd3+ were about identical in experiment (II) (Table 1) . When Eu3+ was reduced electrically to Eu2+, the transport selectivity for Eu was improved markedly. The flux of Eu2+ was found to be even slightly larger than that of Sr2+ in experiment (III ) (Table 1)) and approximately five times larger than those of Gd3+ and Nd3+ in experiment (IV) (Table 1). Thus, Eu2+ was effectively separated from Gd3+ and Nd3+ in the TSSLM system. In Fig. 2(a)-(d), the results of experiments (I) - (IV) are illustrated by giving plots of percentage cation transport as a function of time. It should be noted that the system discussed above is not useful as a real separation process for two reasons. First, the flux using the TSSLM system is low since the surface area of the TSSLM is small. Second, there is a decrease of transport for all of the cations when the electric field is applied, as is seen
322 TABLE
1
Competitive EL?+, EL?+, Sr ‘+ , Nd’+ and Gd3+ fluxes” through TSSLM hydroxyheptyl)DC18CG Experiment
Cation
Flux (mol-sec-‘-m-2)
systems containing bis
(l-
X 10”
340 k 20
S?+
(Ilb UJb
Et?+
69+6
(II)” (II)” (II)”
Gd”+ Nd3+ EL?+
7ort5 76+5 68k4
(III)d (III)4
Sr2+
64+4
Eu*+
81+7
(IV)” (IV)’ (IV)e
Gd3+ Nd”+ Et?+
llkl 1852 84+7
“Calculated by obtaining the slope of a plot of receiving phase cation concentration (as a percentage of initial source phase cation concentration) vs. time and multiplying this quantity by the source phase initial concentration and the receiving phase volume and dividing by the membrane surface area. bExperiment (I ) : Competitive experiment involving Sr* + and ELI”+ , m the following system: source phase, 0.01 M Sr(NO,),, Eu(NO,),; membrane phase, 0.05 M macrocycle in phenylhexane on Celgard 2400 polypropylene support; receiving phase, water. “Experiment (II): Competitive experiment involving Et?+, Gd3+ and Nd3+, same conditions as footnote b. dExperiment
(III):
Competitive
experiment
involving
Sr2+ and Et?+,
same conditions
as foot-
note b except that an electric field is applied to reduce Eu”+ to EL?+. “Experiment (IV): Competitive experiment involving Et?+, Gd3+ and Nd”+, same condition footnote d.
as
by comparing the flux data between experiments (I) and (III) or (II) and (IV) (Table 1). This is because the source phase of the TSSLM system also acts as the cathode cell and the negative electric field produced by the cathode will not favor the transport of positively charged ions through the liquid membrane. The first limitation may be overcome by using a hollow-fiber supported liquid membrane system [7] where the surface area of the liquid membrane is increased thousands of times. The effect of the second limitation may be decreased by designing the system to compensate for the effect of the electric field on cation transport. The flux results can be understood by comparing the relative log K values for the interaction of 18C6 with the investigated cations. The log K( CH,OH) value for the interaction of 18C6 with Sr2+ ( > 5.5) is much larger than that with Eu”+ (1.84) [ 81. The relative magnitudes of these log K values are consistent with the relative fluxes of these two cations through the TSSLM. The
323 T
,
r
,T
(a)
B
0 ..-
r a n
Sr(IIl
5-
”
A -- Eu(IIIl
s P 0
4-
; (9%) Jr
0
10
20 30 Time(hr)
40
50
T
Cc)
0
, T r 0 ---
Sr(II)
A--Eu(II)
20 30 Timc(hr)
40
50
A --- E”(H)
Cd)
a ”
10
5.
x . . . Nd(II1)
S P 0
o--
Gd(III)
6) x
100
I
I
I
Time(hr)
Fig. 2. Plots of percentage cation transport (amount of cation transported as a percentage of its corresponding initial source phase concentration) as a function of time for (a) competitive transport of Sr2+ and EL?+, (b) competitive transport of Eu3+, Gd3+ and Nd3+, (c) competitive transport of S? and EL?+, and (d) competitive transport of Eu*+, Gd3+ and Nd3+.
log K(CH,OH) values for 18C6 interaction with Eu3+, Gd3+ and Nd3+ are 1.84, 1.32 and 2.44, respectively [5]. However, in aqueous solution, since the large dehydration energies of the trivalent lanthanide cations make the complexation of these cations by 18C6 unfavorable, the 1ogKvalues will be smaller. Therefore, the similar and small fluxes of the three trivalent lanthanide cations in experiment (II) were as expected. The log K value for 18C6-Eu2+ interaction has not been determined. However, the coordination chemistry of Eu2+ is expected to be similar to that of Sr2+ because of their similar cation radii (Eu2+, 1.31A; Sr2+, 1.27A) [ 91. These bivalent cations would be expected to form more stable complexes than Eu3T with 18C6 in aqueous solution since they fit the cavity of 18C6 (radius, 1.30 A) better than Eu3+ (radius, 1.09 A) does, and they have smaller dehydration energies than Eu3+. The fluxes of Eu2+, Sr2+ and Eu3+ compared to one another and to those of the other investigated cations were found to be in agreement with the above considerations.
324
Acknowledgements Appreciation for financial support of this research is expressed to the U.S. Department of Energy, Office of Basic Energy Sciences, Grant No. DE-FG0286ERl3463. Helpful discussions with Dr. Ronald L. Bruening and Ms. Anne M. Izatt are also acknowledged.
References 1
J.D. Lamb, R.M. Izatt and J.J. Christensen, Stability constants of cation-macrocycle complexes and their effect on facilitated membrane transport rates, in: R.M. Izatt and J.J. Christensen (Eds.), Progress in Macrocyclic Chemistry, Vol. 2, John Wiley & Sons, New York, NY,
2
1981, pp. 41-90. R.M. Izatt, G.A. Clark, J.S. Bradshaw, J.D. Lamb and J.J. Christensen,
3
transport of ions in liquid membrane systems, Sep. Purif. Methods, 15 (1986) 21-72. F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 5th edn., John Wiley & Sons,
4
5
6
7 8 9
Macrocycle-facilitated
New York, NY, 1988, pp. 955-979. P.R. Brown, R.M. Izatt, J.J. Christensen and J.D. Lamb, Transport ofEu’+ in a H,O-CHCl,H,O liquid membrane system containing the macrocyclic polyether 18.crown-6, J. Membrane Sci., 13 (1983) 85-88. R.M. Izatt, J.D. Lamb, J.J. Christensen
and B.L. Haymore,
Anomalous
stability sequence of
lanthanide(II1) chloride complexes with 18-crown-6 in methanol. Abrupt decrease to zero from Gd3+ to Tb3+, J. Am. Chem. Sot., 99 (1977) 8344-8346. J.D. Lamb, Y. Hirashima, R.L. Bruening, P.K. Tse, R.M. Izatt and J.J. Christensen, Characterization of a supported liquid membrane for macrocycle-mediated selective cation transport, J. Membrane Sci., 37 (1988) 13-26. R.M. Izatt, K.P. Roper, R.L. Bruening and J.D. Lamb, Macrocycle mediated cation transport using hollow fiber supported liquid membranes, J. Membrane Sci., 45 (1989) 73-84. R.M. Izatt, J.S. Bradshaw, S.A. Nielsen, J.D. Lamb, J.J. Christensen and D. Sen, Thermodynamic and kinetic data for cation-macrocycle interaction, Chem. Rev., 85 (1985) 271-339. R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr., Sect. A: Found. Crystallogr., 32 (1976) 751-767.