- Email: [email protected]
Facilitated transport from ternary cation mixtures through water—chloroform—water membrane systems containing macrocyclic ligands
Journal of Membrane Science, 20 (1984) 273-284 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
273
FACILITATED TRANSPORT FROM TERNARY CATION MIXTURES THROUGH WATER-CHLOROFORM-WATER MEMBRANE SYSTEMS
CONTAININGMACROCYCLIC LIGANDS* REED M. IZA’IT, ROBERT M. HAWS, JOHN D. LAMB, DAVID V. DEARDEN, PHILIP R. BROWN, DON W. MCBRIDE, Jr. and JAMES J. CHRISTENSEN Departments of Chemistry from the Thermochemical (U.S.A.)
and Chemical Engineering and Contribution Institute, Brigham Young University, Provo,
(Received September 12,1983;
Number 331 UT 84602
accepted in revised form April 13, 1984)
Summary Cation fluxes were determined for various three-component, equimolar mixtures of alkali metal, alkaline earth, and Pb*+ cations in a H,O--CHCl,-H, 0 liquid membrane system incorporating macrocyclic polyethers as carriers. Carrier ligands studied were 18-crown-6, dicyclohexano-X3-crown-6, l,lO-diaza-18-crown-6, 21-crown-7, dibenzo-24crown-8, and cryptand [2.2.2]. Correlations were found between transport and relative cation:polyether cavity radii, the type of substituents present on the polyether ring, and the type and number of donor atoms present. All the ligands studied transported Pb2+ at higher rates than the other Ma+ in the mixtures. Transport behavior in these multi-cation systems can be predicted from M “+-polyether complex stability constant data in most cases.
Introduction
In recent years, there has been great interest in using the selective cationcomplexing ability of macrocyclic and macrobicyclic ligands for separating metals. With this application in mind, workers have incorporated macrocycles into liquid membrane systems and investigated the ability of these systems either to facilitate transport of individual metals across the organic membrane or to effect separations of metals from mixtures [l--5]. The water-chloroform-water liquid membrane system has been the one most commonly used [ 51. This system has been employed to collect data for single ion “unitary” transport and for two-cation “binary? systems involving Pb2+ [6], Sr2+ [ 71, Cs’ 171, Na’ [7], Ag’ [8], Cd’+ [9], Tl+ [lo], K’ [lo], and Hg2+ [ll] in mixtures with other alkali and alkaline-earth metal ions. A mathematical model describing cation transport in these membrane systems has been devel-
*Presented at the Western Regional ACS Meeting, Pasadena, CA, U.S.A., October 1983.
0376.7388/84/$03.00
o 1984 Elsevier Science Publishers B.V.
274
aped, which provides qualitative predictions of cation fluxes and selectivities [12] in unitary and binary systems. The present study extends the work on macrocycle-mediated cation transport to separations involving three-component, equimolar “ternary” mixtures of cations. This extension is important for two reasons. First, it tests the validity of using results from binary experiments to predict transport .selectivities in more complex multi-cation systems. This test is needed since in many cases cation fluxes in binary systems could not be predicted from unitary transport data alone [ 5-111. Second, ternary data provide a test of the ability of the mathematical model to predict transport results in these more complex systems. The macrocycles used in the study are shown in Fig. 1.
n = 1: 18-Crown-8 (18C6) n==2: 21-Crown-7 (21C7)
Dicyclohexano-1 B-Crown-6 (DC1 8C8)
Dibenzo-24-Crown-8 (DB24C8)
1, 10.Diaza-l8-Crown-6 (DAlSC6)
Cryptend (2.2.21
Fig. 1. Macrocycles used in study.
Experimental Liquid membrane experiments were performed in a specially thermostated room (25 f 1°C) using cells which were larger versions of those described previously [ 51. The cells consisted of a 6.0 mL membrane phase interfaced to both an 0.80 mL source phase (salt solution) and a 13.0 mL receiving phase (distilled deionized water). The membrane phase consisted of reagent grade chloroform (Fisher) which contained, initially, 1.0 mA4macrocycle and which was stirred at 120 rpm by a magnetic stirrer driven by a Hurst synchronous motor. After a period of 24 hr, a 7-8 mL sample of the receiving phase was withdrawn and the concentration of each cation was determined with an atomic absorption spectrophotometer (Perkin-Elmer model 603).
275
Three separate cells were used for each salt-macrocycle system to determine the reproducibility of the reported transport values. The standard deviation for each system was *15% or less. Control (no carrier) experiments were performed for the source phase salt solutions to determine cation leakage through the membrane. The amount of cation leakage varied with each cation but was always less than 0.8 X 10e8 mol/sec-m’. Reagent grade macrocyclic compounds were obtained from Parish Chemical Company (Orem, Utah) and were used without further purification. Reagent grade nitrate salts were obtained from the following sources: Rb+, Cs’ (Alfa); Pb2+, Ca2+ (Allied); Na+ (Mallinckrodt); K’ (J.T. Baker); Sr2+ (Fisher). All source phase solutions were 0.33 M in each cation, except for the ternary Rb’, Pb2’, M”+ solutions where a precipitate formed which was filtered before use. The precipitate was not identified. Values for cation radii are taken from Ref. [13], and those for macrocycle cavity radii are from Ref. [ 141. Results and discussion Macrocycle-mediated cation fluxes from source phases consisting of equimolar mixtures of three cation nitrates across CHCl, liquid membranes, and equilibrium constants, K, for 1: 1 Mn+-L interaction are presented in Table 1. Although unitary (one-cation) results cannot be expected to predict binary (two-cation) behavior [lo], binary results can in most cases be extended to the ternary systems since competition for a single ligand occurs in both binary and ternary systems. Past studies of unitary and binary systems showed correlations between cation fluxes and log K(CH3 OH) values [9-121. The various parameters and physical properties of both the ligand and the cation which affect complex stability have been reviewed [ 171. The dependence of transport and selectivity in binary systems on some of the more important parameters such as the ratio of the cation size to the macrocycle cavity size, macrocycle ring substituents, and the type of donor atoms in the macrocycle has been discussed also [e-11]. We now examine these parameters as they apply to ternary systems. Size effects
In unitary and binary studies, cations with a radius similar to that of the carrier macrocycle cavity generally have greater fluxes in a given membrane system than do those cations with radii different from that of the carrier cavity [5-111. This is not surprising since cations of the proper radius are able to interact more favorably with the ligand donor atoms than those which are too large or too small. Comparison of the ternary flux data for 18C6 and 21C7 shows that, in the absence of Pb2+, size is also important in these ternary systems. Of the univalent cations studied, K’, with a radius (1.38 A) most similar to that of the 18C6 cavity (1.34-1.43 A), has the highest flux value with this macrocycle. When divalent cations are also-present-in the
la
3 260 25
250 130 20
3 70 270
K’ Rb’ Cs’
Na’ K+ SP+
J
4.36 6.06 >5.5
6.06 5.32 4.79
4.36 6.06 4.79
Log K (CH,OH)b
do not include
18C6
which
Na’ K’ Cs’
Cation mixture
Systems
Pb”
15 20 840
280 130 20
7 210 30
DC18C6 J
12 4 26
e e e
e e e
DA18C6 J J
6 30 20
10 59 a7
3 21 77
1.73 4.22 1.77
4.22 4.86 5.01
1.73 4.22 5.01
Log K (CH,OH)b
from source phase@
210
Macrocycle (L)-mediated cation fluxes, &, (mol x 10-8/sec-m2) nitrates, and log K values for 1:l M”‘-L interaction
TABLE
7 3 e
f
e
2 1 10
J
of solutions
1.54 2.42 -
2.42 2.55 2.48
1.54 2.42 2.48
Log K (70% CH,OH)c
DB24C8
consisting
10 240 5
260 38 2
110 190 e
J
[2.2.2]
equimolar
cation
7.21 9.76 11.5
9.76 8.40 3.54
7.21 9.75 3.54
Log K (96% CH,OH)d
in three
160 30 e
2 8 420
84 35 280
K+ CY+ Caa+
Na’ CS+ Srz+
K+ Rb+ Sra+
6.06 5.32 >5.5
4.36 4.79 >5.5
6.06 4.79 3.46
6.06 3.86 >5.5
6.06 4.79 >5.5
70 20 420
-
210 30 2
33:
e
3;
e
1:
e
3 1 310
32:
210”
280 1 520
1
170
10 80 10
3 170 30
34 120 e
2:
35
20 140 20
4.22 4.86 1.77
1.73 5.01 1.77
4.22 5.01 _
4.22 1.77
4.22 5.01 1.77
1” e
1 13 e
1 7 e
3 e e
lo” e
2.42 2.55 _
1.54 2.48 -
2.42 2.48 -
_
2.42 2.48 -
220 42 4
20”
70
100 1 e
5”
270
4”
290
9.75 8.40 11.5
7.21 3.54 11.5
9.75 3.54 7.60
9.75 7.60 11.5
9.75 3.54 11.5
values across the CHCl,-receiving phase boundary can be calculated by multipIying the values in the table by 0.286, [14]. [15 1. [ 161. less than that of control [no carrier), 0.8 X 10sB mol/sec-m2.
87 2 420
K’ Caz+ Sr’+
*Flux bRef. CRef. dRef. eFlux
110 7 380
K+ Cs’ Sr”+
lb
Pb=+
Sr2+ Pb”
CS’
K’ CS’ Pb”
K’ Pb”
Nat
Cation mixture
Systems
which
4.33 2.51
6.5
10
35;
5 330
2.84 5.0 6.5
4.33 2.84 6.5
98
31;
2.76 4.33 6.5
Log K (70% CH,OH)C
Pb”
7 7 310
J
18C6
include
2;
7
28;
9.4
e
8:
77e
e
-
2.48 -
7 e e
59e
19
20
2.42 2.48 -
2 3 2
2 10 63
J
5.4 4.4 12.0
<2.0 8.0 12.0
5.4 <2.0 12.0
3.9 5.4 12.0
Log K (HzWd
in three
E2.2.2 ]
equimolar
2.42 _
1.54 2.42 _
-
of solutions
Log K (70% CH,OH)C 4 3 e
2 e
J
DB24C8
consisting
4.22 -
5.01 1.77 -
66 1” 10
24:
4.22 5.01 -
7 70 210
1
_ _
e 3.5 1000
1.73 4.22 -
3 9 240
1:
phasesa
(CH,OH)b
Log K
1 e 10
J
21c7
from source
_ -
DAlSC6
J
DClSC6
J
Macrocycle (L)-mediated cation fluxes, JM, (mol X 10-8/sec-m*) nitrates, and log K values for 1: 1 MntLL interaction
TABLE
cation
1 420
Rb’ Sr2+ Pb’+
aFlux bRef. CRef. dRef. *Flux
-
560
;
73;
1
49:
e
3 1 700
1:
e
19
e
1 a
_
-
28:
4.86 1.77 -
4.86 5.08 -
5.01 _. -
4.22 4.86 -
49
30 45 140
23;
63
e
1;
5 35 170
e
-
2 e e
2 4 e
11 e e
; e
values across the CHCl,-receiving phase boundary can be calculated by multiplying [14]. [ 151. [16]. less than that of control (no carrier), 0.8 x lo-’ mol/sec-m2.
-
35;
K’ Sr2+ Pba+
3.46 2.84 6.5
e
Rb’ CS’ Pb=+
3.46 5.0 6.5
2.84 2.51 6.5
e 280
4.33 3.46 6.5
4
42:
2
Ca2+ Pb”
CS+’
K’ Rb’ Pba+
6;
la
3 e 47
4;
3
5;
e
4 2 42
5.4 8.0 12.0
4.35 8.0 12.0
4.35 <2.0 12.0
< 2.0 4.4 12.0
5.4 4.35 12.0
the values in the table by 0.286.
_ -
2.55 _
2.55 2.43 -
2.48 _ -
2.42 2.55 _
280
mixture, size seems to be less important. For instance, Sr2+ (1.16 a) is smaller than either K+ or the 18C6 cavity, yet Sr2+ fluxes with 18C6 are much higher than are K’ fluxes with this carrier. Similar behavior involving K’ and Sr2’ was seen in the single and binary cation studies [ 5-111. With 21C7 (cavity radius = 1.7 A), the relative fluxes are different; Cs’ (1.70 W) fits better than the other univalent cations and has the highest flux in each combination where it is present, except for those where Pb2+ is also present. Rb’ (1.49 W) is second to Cs+ in similarity of size to the 21C7 cavity, and Rb’ fluxes are higher than are those for any other cation studied except Cs’ and Pb’+. Size comparisons fail to explain either the high fluxes obtained for Pb2’ with 18C6 and 21C7 or the selectivity of these ligands for Pb2+. However, binary cation studies using Pb2+ with either 18C6 or 21C7 also found that fluxes for other cations are low when Pb2+ is present, while Pb2’ flux remains appreciable [ 61. Log K(H20) data indicate that the Pb2+-18C6 complex is more stable than are those of alkali and alkaline earth cations [ 181, which is consistent with high Pb’+ fluxes and low M”’ fluxes in the 18C6 system: Pb2+ binds the carrier in the membrane and is transported, while little carrier remains available to bind and transport M”+. Substituent effects Investigation of single and binary cation systems with CHC13 membranes has shown that the presence of substituents on the macrocycle ring can markedly affect flux magnitudes and transport selectivities. These effects are assumed to be a result of some combination of changes in the water solubility, conformational flexibility, and electron distribution of the ligand upon addition of the substituent [ 5,6]. Similar effects are apparent in the results for the ternary systems studied. Comparison of the fluxes using 18C6 with those obtained using DC18C6 shows the influence of fusing cyclohexane rings to the 18C6 molecule. Fluxes with DC18C6 are generally either equal to or greater than those with 18C6. DC18C6 is much less soluble in water (0.036 M) than 18C6 (5 M) [19], so DC18C6 is expected to remain in the membrane -where it can function as a carrier - to a much greater extent than 18C6, which can be leached away. The cyclohexane rings also affect transport through their influence on the thermodynamic stabilities of some of the complexes. For example, Pb2+ fluxes are all significantly greater with DC18C6 than with 18C6, which agrees with log K(H20) data: 4.27 for 18C6, 4.95 for DC18C6 (181. No transport data are available for comparison of unsubstituted 24C8 with DB24C8. However, the flux values are generally small when DB24C8 is used as carrier ligand. Attachment of benzo groups to 18C6 has been found to lead to decreased fluxes in unitary and binary systems [ 5,6]. Donor atom effects Comparison of data for 18C6, DA18C6, and [2.2.2] shows the effect of changing the type and number of donor atoms. The selectivity of DA18C6
281
for Sr2+ has been noted in earlier binary studies [7], and is seen again in the ternary data: Sr” fluxes are higher than those for any other cation, except when Pb2+ is present. All of the fluxes except that of Na’ in one case (Na+, K+, Sr2+ group), that of Ca” in two cases (K+, Cs+, Ca2” and Cs+, Ca”, Pb2+ groups), and that of Sr’l’ in one case (K’, Rb’, Sr2+ group), are less with DAlSC6 than with 18C6, but the decrease in Sr2’ flux is generally less than that of the other cations. When Pb2+ is present, all the fluxes are small, including that of S?‘. This ability of Pb2+ to block the flow of other cations through the membrane in binary experiments has been noted and discussed
[6,201.
Fluxes using cryptand [ 2.2.21 as carrier are significantly different from those with DA18C6. K’, which shows a small flux with DA18C6, has the highest flux using cryptand [2.2.2] in all of the combinations where it is present, except when Pb2+ is also present. Selectivity for Sr2+, as found with DA18C6, is not seen for [2.2.2] ; Sr2+ fluxes are small. Also, [2.2.2] is the only ligand studied for which Na’ had an appreciable flux, Previous work has shown that in many binary cases [2.2.2] is selective for Na+ over a second cation, but both K’ and Pb2+ had higher fluxes than Na’ in binary studies [7]. The same is true of the ternary results: when either K+ or Pb2’ is present, Na’ flux is low. Prediction of transport selectivities in many-cation systems The flow of cations through bulk liquid membranes can be thought of as a phase transfer catalytic process in which the macrocycle is the catalyst with the various metal ions competing for its active site, namely the ring cavity surrounded by its donor atoms. As a first approximation, the selectivity of this catalytic system for the various cations can be explained in terms of the relative stabilities of the cation-macrocycle complexes. The relevant log K values would, of course, be those valid in the membrane solvent. Unfortunately, log K(CHC1,) values have not been determined. Therefore, in order to test any model and formulate a correlation between cation flux and complex stability, we must rely on log K values valid in other solvents. For the systems reported in Table 1, a set of K values valid in a single solvent is not available. The log K values given in Table 1 are for H20-CH30H systems of various compositions. Limited comparisons of log K and cation flux for the ternary systems can be made using these data. It is seen in Table 1 that when three monovalent alkali cations are mixed, transport selectivities follow the log K selectivities. However, when monovalent and divalent cations are mixed the transport selectivity does not always follow the selectivity predicted from relative log K values. In the case of K+, Sr2+, and a third cation whose log K value is below that of either K’ or Sr”, log K values predict that for 18C6 and [2.2.2] Sr2+ should be selected over K+. For 18C6 this is the case, but for [2.2.2], K+ is selected over Sr2’. One possible explanation is that the relative magnitudes of the log K values valid in CHCIB are quite different from those valid in CH3OH. This is in agreement
282
with the observations of McBride et al. 1211, and deJong and Reinhoudt [22] who have pointed out that both the order and the magnitude of macrocycle selectivity are solvent-dependent. Admitting that this might be the case in a f4w isolated instances, examination of a larger data base [6-111 suggests that consideration’of log K data alone is not sufficient to predict transport selectivity from cation mixtures. Cation transport is dependent on the concentration of the cation-_ligand--anion complex in the organic phase (anions are included to preserve electroneutrality). In an equilibrium situation (or steady state with identical rate limiting diffusion coefficients for the various complexes) selectivity is determined by the relative concentrations of the various cation-ligand-anion complexes. In the transport model [ 121 and in its extension [10,21], the concentration of the complex near the source phase interface is related to the extraction coefficient, K kip, where K is the ion pair-macrocycle complexation constant in the organic phase and ki, is the water-organic liquid partition coefficient either for the ion pair (1: 1) or for the ion triplet (1: 2). In equimolar cation mixtures, the ratio of the concentrations of the complexes in the membrane is proportional to the ratio of the extraction coefficients, K kip, for the cations involved. Since selectivity is a function of two terms, a small log K value could be compensated by a large partition coefficient. The competitive aspect of selectivity, i.e., the competition by the various cations for a single ligand, is governed by the relative log K values of complexation, while the noncompetitive aspect is governed by the partition coefficients. Either of these terms can be the decisive factor in determining selectivity. In the case of a mixture of monovalent alkali cations, the partition coefficients can be considered approximately equal, thus selectivity is determined by the relative log K values. To a fist approximation, the same would be true in a mixture of divalent cations. However, in a mixture containing both monovalent and divalent cations, both K and ki, must be considered since it is their product which governs selectivity. An example of this is seen, for the cryptand [2.2.1], in mixtures of K’, Sr”, and a third cation with a log K value smaller than that of either K’ or Sr2+. In these cases, log K(CH,OH) for Sr2+ is nearly two orders of magnitude larger than log K(CH30H) for K’, yet K’ is transported selectively, This result can be rationalized as follows. McBride et al. [ 211 have pointed out that partition coefficients for ion triplets comprised of a divalent cation and two univalent anions should be much smaller than those for ion pairs comprised of univalent cations and anions. Reasons for this order of partition coefficients are twofold. First, the partitioning of the divalent cation itself is much smaller than that of the univalent cation (the free energy of transfer between two phases is proportional to .z~) and, second, two anions must partition for each divalent cation. (Although the ion triplet association constant for divalent cations is expected to be larger than the corresponding ion pair constant, the increase is not sufficient to overcome the decrease in single ion partitioning and the net result is a decrease in partitioning of ion triplets with respect to ion pairs when the anion is hydrophilic, such as
283
NO;.) Although log K for the Sr2+- [2.2.2] complexation is large, it is not large enough to compensate for the smaller partition coefficient of Sr*‘.‘The net result is that the extraction coefficient for K+ is larger than that of Sr*‘, leading to a selective transport of K’. Nearly all of the transport selectivity seen in Table 1 can be rationalized using the extraction coefficient relationship. The most notable exception is K+ and Sr*’ with a third cation in the presence of 21C7. In each case, we would expect K+ to be transported selectively over Sr*+. However, the two are transported at an approximately equal and low level in all cases. Whether this demonstrates the need to improve and refine’the model based on the extraction coefficient or simply demonstrates a failure of log K (CH,OH) data to predict behavior in CHC13, we cannot, at this time, ascertain. In mixtures containing Pb*’ (Table l), Pb*+ is always the best transported. Also, log K for formation of the Pb*+-ligand complex is always the largest in each set. Apparently, the log K value is sufficiently large in these cases to compensate for the presumed small partition coefficient of Pb*+ and to give Pb*’ the largest extraction coefficient. Acknowledgement We acknowledge the financial support of this study by U.S. Department of Energy Contract No. DE-AC02-78ER05016 and appreciate the work of Gypzy Lindh who carried out the atomic absorption analyses. References 1 2 3
4 5
6
7
8
C.F. Reusch and E.L. Cussler, Selective membrane transport, AIChE J., 19 (1973) 736. K.H. Wong, K. Yagi and J. Smid, Ion transport through liquid membranes facilitated by crown ethemand their polymers, J. Membrane Biol., 18 (1974) 379. Y. Kobuke, K. Hanji, K. Horiguchi, M. Asada, Y. Nakayama and J. Furukawa, Macrocyclic ligands composed of tetrahydrofuran for selective transport of monovalent cations through liquid membranes, J. Amer. Chem, Sot., 98 (1976) 7414. M. Kirch and J.-M. Lehn, Selective transport of alkali metal cations through a liquid membrane by macrobicyclic carriers, Angew. Chem., Int. Ed. Engl., 14 (1975) 555. J.D. Lamb, R.M. Izatt, D.G. Garrick, J.S. Bradshaw and J.J. Christensen, The influence of macrocyclic ligand structure on carrier-facilitated cation transport rates and selectivities through liquid membranes, J. Membrane Sci., 9 (1981) 83. J.D. Lamb, R.M. Izatt, P.A. Robertson and J.J. Christensen, Highly selective membrane transport of Pb*+ from aqueous metal ion mixtures using macrocyclic carriers, J. Amer. Chem. Sot., 102 (1980) 2454. J.D. Lamb, P.R. Brown, J.J. Christensen, J.S. Bradshaw, D.G. Garrick and R.M. Izatt, Cation transport at 25°C from binary Na+-M”+, Cs+-M”+, and Sr*+-Mn+ nitrate mixtures in a H, 0-CHCl,-H, 0 liquid membrane system containing a series of macrocyclic carriers, J. Membrane Sci., 13 (1983) 89. R.M. Izatt, D.V. Dearden, P.R. Brown, J.S. Bradshaw, J.D. Lamb and J.J. Christensen, 0 liquid memCation fluxes from binary Ag+-Mn+ mixtures in a H, 0-CHCI,-H, brane system containing a series of macrocyclic ligand carriers, J. Amer. Chem. Sot., 105 (1983) 1785.
284 9
10
11
12
13 14
15
16 17 18
19 20 21
22
R.M. Izatt, S.R. Izatt, D.W. McBride, Jr., J.S. Bradshaw and J.J. Christensen, Cation 0 liquid transport at 25°C from binary Cdl+-M”’ mixtures in a H, 0-CHCl,-H, membrane system containing a series of macrocyclic carriers, Isr. J. Chem., in press. G.A. Clark, D.W. McBride, Jr., J.J. Christensen, J.S. Bradshaw and R.M. Izatt, Cation transport at 25°C from binary TI+/Mn’ and K+/Mn+ nitrate mixtures in a H,O-CHCI,H, 0 liquid membrane system containing a series of macrocyclic polyether carriers, J. Membrane Sci., in press. i R.M. Izatt, M.B. Jones, D.W. McBride, Jr., J.S. Bradshaw and J.J. Christensen, Macrocycle-mediated cation transport at 25°C from binary Hg’+-M”’ mixtures in a H,O-CHCl,-H,O liquid membrane system, in preparation. J.D. Lamb, J.J. Christensen, J.L. Oscarson, B.L. Nielsen, B.W. Asay and R.M. Izatt, The relationship between complex stability constants and rates of cation transport through liduid membranes by macrocyclic carriers, J. Amer. Chem. Sot., 102 (1980) 6820. R.D. Shannon and C.T. Prewitt, Effective ionic radii in oxides and fluorides, Acta Crystallogr., B25 (1969) 925. J.D. Lamb, R.M. Izatt, C.S. Swain and J.J. Christensen, A systematic study of the effect of macrocycle ring size and donor atom type on the log K, AH, and TAS of reactions at 25°C in methanol of mono- and divalent cations with crown ethers, J. Amer. Chem. Sot., 102 (1980) 475. R.M. Izatt, R.E. Terry, D.P. Nelson, Y. Chan, D.J. Eatough, J.S. Bradshaw, L.D. Hansen and J.J. Christensen, Calorimetric titration study of the interaction of some uni- and bivalent cations with benzo-15-crown-5-, l8-crown-g, dibenzo-24-crown+, and dibenzo-27-crown-9 in methanol-water solvents at 25°C and fi = 0.1, J. Amer. Chem. Sot., 98 (1976) 7626. J.M. Lehn and J.P. Sauvage, [2] -Cryptates: Stability and selectivity of alkali and alkaline-earth macrobicyclic complexes, J. Amer. Chem. Sot., 97 (1975) 6700. J.D. Lamb, R.M. Izatt, J.J. Christensen and D.J. Eatough, in: G.A. Melson (Ed.), Coordination Chemistry of Macrocyclic Compounds, Plenum, New York, 1979. R.M. Izatt, R.E. Terry, B.L. Haymore, L.D. Hansen, N.K. Dalley, A.G. Avondet and J.J. Christensen, Calorimetric titration study of the interaction of several uni- and bivalent cations with 15crown-5, 18-crown-6 and two isomers of dicyclohexo-1% crown-6 in aqueous solution at 25°C and g = 0.1, J. Amer. Chem. Sot., 98 (1976) 7620 C.J. Pedersen, Cyclic polyethers and their complexes with metal salts, J. Amer. Chem. Sot., 89 (1967) 7017. J.D. Lamb, R.M. Izatt and J.J. Christensen, in: R.M. Izatt and J.J. Christensen (Eds.), Progress in Macrocyclic Chemistry, Vol. 2, Wiley-Interscience, New York, 1981. D.W. McBride, Jr., R.M. Izatt, J.D. Lamb and J.J. Christensen, in: J.L. Atwood, J.E.D. Davies and D.D. MacNicol (Eds.), Inclusion Compounds, Vol. 3, Academic Press, New York, 1984. F. deJong and D.N. Reinhoudt, Stability and reactivity of crown-ether complexes, Adv. Phys. Org. Chem., 17 (1980) 279.
273
FACILITATED TRANSPORT FROM TERNARY CATION MIXTURES THROUGH WATER-CHLOROFORM-WATER MEMBRANE SYSTEMS
CONTAININGMACROCYCLIC LIGANDS* REED M. IZA’IT, ROBERT M. HAWS, JOHN D. LAMB, DAVID V. DEARDEN, PHILIP R. BROWN, DON W. MCBRIDE, Jr. and JAMES J. CHRISTENSEN Departments of Chemistry from the Thermochemical (U.S.A.)
and Chemical Engineering and Contribution Institute, Brigham Young University, Provo,
(Received September 12,1983;
Number 331 UT 84602
accepted in revised form April 13, 1984)
Summary Cation fluxes were determined for various three-component, equimolar mixtures of alkali metal, alkaline earth, and Pb*+ cations in a H,O--CHCl,-H, 0 liquid membrane system incorporating macrocyclic polyethers as carriers. Carrier ligands studied were 18-crown-6, dicyclohexano-X3-crown-6, l,lO-diaza-18-crown-6, 21-crown-7, dibenzo-24crown-8, and cryptand [2.2.2]. Correlations were found between transport and relative cation:polyether cavity radii, the type of substituents present on the polyether ring, and the type and number of donor atoms present. All the ligands studied transported Pb2+ at higher rates than the other Ma+ in the mixtures. Transport behavior in these multi-cation systems can be predicted from M “+-polyether complex stability constant data in most cases.
Introduction
In recent years, there has been great interest in using the selective cationcomplexing ability of macrocyclic and macrobicyclic ligands for separating metals. With this application in mind, workers have incorporated macrocycles into liquid membrane systems and investigated the ability of these systems either to facilitate transport of individual metals across the organic membrane or to effect separations of metals from mixtures [l--5]. The water-chloroform-water liquid membrane system has been the one most commonly used [ 51. This system has been employed to collect data for single ion “unitary” transport and for two-cation “binary? systems involving Pb2+ [6], Sr2+ [ 71, Cs’ 171, Na’ [7], Ag’ [8], Cd’+ [9], Tl+ [lo], K’ [lo], and Hg2+ [ll] in mixtures with other alkali and alkaline-earth metal ions. A mathematical model describing cation transport in these membrane systems has been devel-
*Presented at the Western Regional ACS Meeting, Pasadena, CA, U.S.A., October 1983.
0376.7388/84/$03.00
o 1984 Elsevier Science Publishers B.V.
274
aped, which provides qualitative predictions of cation fluxes and selectivities [12] in unitary and binary systems. The present study extends the work on macrocycle-mediated cation transport to separations involving three-component, equimolar “ternary” mixtures of cations. This extension is important for two reasons. First, it tests the validity of using results from binary experiments to predict transport .selectivities in more complex multi-cation systems. This test is needed since in many cases cation fluxes in binary systems could not be predicted from unitary transport data alone [ 5-111. Second, ternary data provide a test of the ability of the mathematical model to predict transport results in these more complex systems. The macrocycles used in the study are shown in Fig. 1.
n = 1: 18-Crown-8 (18C6) n==2: 21-Crown-7 (21C7)
Dicyclohexano-1 B-Crown-6 (DC1 8C8)
Dibenzo-24-Crown-8 (DB24C8)
1, 10.Diaza-l8-Crown-6 (DAlSC6)
Cryptend (2.2.21
Fig. 1. Macrocycles used in study.
Experimental Liquid membrane experiments were performed in a specially thermostated room (25 f 1°C) using cells which were larger versions of those described previously [ 51. The cells consisted of a 6.0 mL membrane phase interfaced to both an 0.80 mL source phase (salt solution) and a 13.0 mL receiving phase (distilled deionized water). The membrane phase consisted of reagent grade chloroform (Fisher) which contained, initially, 1.0 mA4macrocycle and which was stirred at 120 rpm by a magnetic stirrer driven by a Hurst synchronous motor. After a period of 24 hr, a 7-8 mL sample of the receiving phase was withdrawn and the concentration of each cation was determined with an atomic absorption spectrophotometer (Perkin-Elmer model 603).
275
Three separate cells were used for each salt-macrocycle system to determine the reproducibility of the reported transport values. The standard deviation for each system was *15% or less. Control (no carrier) experiments were performed for the source phase salt solutions to determine cation leakage through the membrane. The amount of cation leakage varied with each cation but was always less than 0.8 X 10e8 mol/sec-m’. Reagent grade macrocyclic compounds were obtained from Parish Chemical Company (Orem, Utah) and were used without further purification. Reagent grade nitrate salts were obtained from the following sources: Rb+, Cs’ (Alfa); Pb2+, Ca2+ (Allied); Na+ (Mallinckrodt); K’ (J.T. Baker); Sr2+ (Fisher). All source phase solutions were 0.33 M in each cation, except for the ternary Rb’, Pb2’, M”+ solutions where a precipitate formed which was filtered before use. The precipitate was not identified. Values for cation radii are taken from Ref. [13], and those for macrocycle cavity radii are from Ref. [ 141. Results and discussion Macrocycle-mediated cation fluxes from source phases consisting of equimolar mixtures of three cation nitrates across CHCl, liquid membranes, and equilibrium constants, K, for 1: 1 Mn+-L interaction are presented in Table 1. Although unitary (one-cation) results cannot be expected to predict binary (two-cation) behavior [lo], binary results can in most cases be extended to the ternary systems since competition for a single ligand occurs in both binary and ternary systems. Past studies of unitary and binary systems showed correlations between cation fluxes and log K(CH3 OH) values [9-121. The various parameters and physical properties of both the ligand and the cation which affect complex stability have been reviewed [ 171. The dependence of transport and selectivity in binary systems on some of the more important parameters such as the ratio of the cation size to the macrocycle cavity size, macrocycle ring substituents, and the type of donor atoms in the macrocycle has been discussed also [e-11]. We now examine these parameters as they apply to ternary systems. Size effects
In unitary and binary studies, cations with a radius similar to that of the carrier macrocycle cavity generally have greater fluxes in a given membrane system than do those cations with radii different from that of the carrier cavity [5-111. This is not surprising since cations of the proper radius are able to interact more favorably with the ligand donor atoms than those which are too large or too small. Comparison of the ternary flux data for 18C6 and 21C7 shows that, in the absence of Pb2+, size is also important in these ternary systems. Of the univalent cations studied, K’, with a radius (1.38 A) most similar to that of the 18C6 cavity (1.34-1.43 A), has the highest flux value with this macrocycle. When divalent cations are also-present-in the
la
3 260 25
250 130 20
3 70 270
K’ Rb’ Cs’
Na’ K+ SP+
J
4.36 6.06 >5.5
6.06 5.32 4.79
4.36 6.06 4.79
Log K (CH,OH)b
do not include
18C6
which
Na’ K’ Cs’
Cation mixture
Systems
Pb”
15 20 840
280 130 20
7 210 30
DC18C6 J
12 4 26
e e e
e e e
DA18C6 J J
6 30 20
10 59 a7
3 21 77
1.73 4.22 1.77
4.22 4.86 5.01
1.73 4.22 5.01
Log K (CH,OH)b
from source phase@
210
Macrocycle (L)-mediated cation fluxes, &, (mol x 10-8/sec-m2) nitrates, and log K values for 1:l M”‘-L interaction
TABLE
7 3 e
f
e
2 1 10
J
of solutions
1.54 2.42 -
2.42 2.55 2.48
1.54 2.42 2.48
Log K (70% CH,OH)c
DB24C8
consisting
10 240 5
260 38 2
110 190 e
J
[2.2.2]
equimolar
cation
7.21 9.76 11.5
9.76 8.40 3.54
7.21 9.75 3.54
Log K (96% CH,OH)d
in three
160 30 e
2 8 420
84 35 280
K+ CY+ Caa+
Na’ CS+ Srz+
K+ Rb+ Sra+
6.06 5.32 >5.5
4.36 4.79 >5.5
6.06 4.79 3.46
6.06 3.86 >5.5
6.06 4.79 >5.5
70 20 420
-
210 30 2
33:
e
3;
e
1:
e
3 1 310
32:
210”
280 1 520
1
170
10 80 10
3 170 30
34 120 e
2:
35
20 140 20
4.22 4.86 1.77
1.73 5.01 1.77
4.22 5.01 _
4.22 1.77
4.22 5.01 1.77
1” e
1 13 e
1 7 e
3 e e
lo” e
2.42 2.55 _
1.54 2.48 -
2.42 2.48 -
_
2.42 2.48 -
220 42 4
20”
70
100 1 e
5”
270
4”
290
9.75 8.40 11.5
7.21 3.54 11.5
9.75 3.54 7.60
9.75 7.60 11.5
9.75 3.54 11.5
values across the CHCl,-receiving phase boundary can be calculated by multipIying the values in the table by 0.286, [14]. [15 1. [ 161. less than that of control [no carrier), 0.8 X 10sB mol/sec-m2.
87 2 420
K’ Caz+ Sr’+
*Flux bRef. CRef. dRef. eFlux
110 7 380
K+ Cs’ Sr”+
lb
Pb=+
Sr2+ Pb”
CS’
K’ CS’ Pb”
K’ Pb”
Nat
Cation mixture
Systems
which
4.33 2.51
6.5
10
35;
5 330
2.84 5.0 6.5
4.33 2.84 6.5
98
31;
2.76 4.33 6.5
Log K (70% CH,OH)C
Pb”
7 7 310
J
18C6
include
2;
7
28;
9.4
e
8:
77e
e
-
2.48 -
7 e e
59e
19
20
2.42 2.48 -
2 3 2
2 10 63
J
5.4 4.4 12.0
<2.0 8.0 12.0
5.4 <2.0 12.0
3.9 5.4 12.0
Log K (HzWd
in three
E2.2.2 ]
equimolar
2.42 _
1.54 2.42 _
-
of solutions
Log K (70% CH,OH)C 4 3 e
2 e
J
DB24C8
consisting
4.22 -
5.01 1.77 -
66 1” 10
24:
4.22 5.01 -
7 70 210
1
_ _
e 3.5 1000
1.73 4.22 -
3 9 240
1:
phasesa
(CH,OH)b
Log K
1 e 10
J
21c7
from source
_ -
DAlSC6
J
DClSC6
J
Macrocycle (L)-mediated cation fluxes, JM, (mol X 10-8/sec-m*) nitrates, and log K values for 1: 1 MntLL interaction
TABLE
cation
1 420
Rb’ Sr2+ Pb’+
aFlux bRef. CRef. dRef. *Flux
-
560
;
73;
1
49:
e
3 1 700
1:
e
19
e
1 a
_
-
28:
4.86 1.77 -
4.86 5.08 -
5.01 _. -
4.22 4.86 -
49
30 45 140
23;
63
e
1;
5 35 170
e
-
2 e e
2 4 e
11 e e
; e
values across the CHCl,-receiving phase boundary can be calculated by multiplying [14]. [ 151. [16]. less than that of control (no carrier), 0.8 x lo-’ mol/sec-m2.
-
35;
K’ Sr2+ Pba+
3.46 2.84 6.5
e
Rb’ CS’ Pb=+
3.46 5.0 6.5
2.84 2.51 6.5
e 280
4.33 3.46 6.5
4
42:
2
Ca2+ Pb”
CS+’
K’ Rb’ Pba+
6;
la
3 e 47
4;
3
5;
e
4 2 42
5.4 8.0 12.0
4.35 8.0 12.0
4.35 <2.0 12.0
< 2.0 4.4 12.0
5.4 4.35 12.0
the values in the table by 0.286.
_ -
2.55 _
2.55 2.43 -
2.48 _ -
2.42 2.55 _
280
mixture, size seems to be less important. For instance, Sr2+ (1.16 a) is smaller than either K+ or the 18C6 cavity, yet Sr2+ fluxes with 18C6 are much higher than are K’ fluxes with this carrier. Similar behavior involving K’ and Sr2’ was seen in the single and binary cation studies [ 5-111. With 21C7 (cavity radius = 1.7 A), the relative fluxes are different; Cs’ (1.70 W) fits better than the other univalent cations and has the highest flux in each combination where it is present, except for those where Pb2+ is also present. Rb’ (1.49 W) is second to Cs+ in similarity of size to the 21C7 cavity, and Rb’ fluxes are higher than are those for any other cation studied except Cs’ and Pb’+. Size comparisons fail to explain either the high fluxes obtained for Pb2’ with 18C6 and 21C7 or the selectivity of these ligands for Pb2+. However, binary cation studies using Pb2+ with either 18C6 or 21C7 also found that fluxes for other cations are low when Pb2+ is present, while Pb2’ flux remains appreciable [ 61. Log K(H20) data indicate that the Pb2+-18C6 complex is more stable than are those of alkali and alkaline earth cations [ 181, which is consistent with high Pb’+ fluxes and low M”’ fluxes in the 18C6 system: Pb2+ binds the carrier in the membrane and is transported, while little carrier remains available to bind and transport M”+. Substituent effects Investigation of single and binary cation systems with CHC13 membranes has shown that the presence of substituents on the macrocycle ring can markedly affect flux magnitudes and transport selectivities. These effects are assumed to be a result of some combination of changes in the water solubility, conformational flexibility, and electron distribution of the ligand upon addition of the substituent [ 5,6]. Similar effects are apparent in the results for the ternary systems studied. Comparison of the fluxes using 18C6 with those obtained using DC18C6 shows the influence of fusing cyclohexane rings to the 18C6 molecule. Fluxes with DC18C6 are generally either equal to or greater than those with 18C6. DC18C6 is much less soluble in water (0.036 M) than 18C6 (5 M) [19], so DC18C6 is expected to remain in the membrane -where it can function as a carrier - to a much greater extent than 18C6, which can be leached away. The cyclohexane rings also affect transport through their influence on the thermodynamic stabilities of some of the complexes. For example, Pb2+ fluxes are all significantly greater with DC18C6 than with 18C6, which agrees with log K(H20) data: 4.27 for 18C6, 4.95 for DC18C6 (181. No transport data are available for comparison of unsubstituted 24C8 with DB24C8. However, the flux values are generally small when DB24C8 is used as carrier ligand. Attachment of benzo groups to 18C6 has been found to lead to decreased fluxes in unitary and binary systems [ 5,6]. Donor atom effects Comparison of data for 18C6, DA18C6, and [2.2.2] shows the effect of changing the type and number of donor atoms. The selectivity of DA18C6
281
for Sr2+ has been noted in earlier binary studies [7], and is seen again in the ternary data: Sr” fluxes are higher than those for any other cation, except when Pb2+ is present. All of the fluxes except that of Na’ in one case (Na+, K+, Sr2+ group), that of Ca” in two cases (K+, Cs+, Ca2” and Cs+, Ca”, Pb2+ groups), and that of Sr’l’ in one case (K’, Rb’, Sr2+ group), are less with DAlSC6 than with 18C6, but the decrease in Sr2’ flux is generally less than that of the other cations. When Pb2+ is present, all the fluxes are small, including that of S?‘. This ability of Pb2+ to block the flow of other cations through the membrane in binary experiments has been noted and discussed
[6,201.
Fluxes using cryptand [ 2.2.21 as carrier are significantly different from those with DA18C6. K’, which shows a small flux with DA18C6, has the highest flux using cryptand [2.2.2] in all of the combinations where it is present, except when Pb2+ is also present. Selectivity for Sr2+, as found with DA18C6, is not seen for [2.2.2] ; Sr2+ fluxes are small. Also, [2.2.2] is the only ligand studied for which Na’ had an appreciable flux, Previous work has shown that in many binary cases [2.2.2] is selective for Na+ over a second cation, but both K’ and Pb2+ had higher fluxes than Na’ in binary studies [7]. The same is true of the ternary results: when either K+ or Pb2’ is present, Na’ flux is low. Prediction of transport selectivities in many-cation systems The flow of cations through bulk liquid membranes can be thought of as a phase transfer catalytic process in which the macrocycle is the catalyst with the various metal ions competing for its active site, namely the ring cavity surrounded by its donor atoms. As a first approximation, the selectivity of this catalytic system for the various cations can be explained in terms of the relative stabilities of the cation-macrocycle complexes. The relevant log K values would, of course, be those valid in the membrane solvent. Unfortunately, log K(CHC1,) values have not been determined. Therefore, in order to test any model and formulate a correlation between cation flux and complex stability, we must rely on log K values valid in other solvents. For the systems reported in Table 1, a set of K values valid in a single solvent is not available. The log K values given in Table 1 are for H20-CH30H systems of various compositions. Limited comparisons of log K and cation flux for the ternary systems can be made using these data. It is seen in Table 1 that when three monovalent alkali cations are mixed, transport selectivities follow the log K selectivities. However, when monovalent and divalent cations are mixed the transport selectivity does not always follow the selectivity predicted from relative log K values. In the case of K+, Sr2+, and a third cation whose log K value is below that of either K’ or Sr”, log K values predict that for 18C6 and [2.2.2] Sr2+ should be selected over K+. For 18C6 this is the case, but for [2.2.2], K+ is selected over Sr2’. One possible explanation is that the relative magnitudes of the log K values valid in CHCIB are quite different from those valid in CH3OH. This is in agreement
282
with the observations of McBride et al. 1211, and deJong and Reinhoudt [22] who have pointed out that both the order and the magnitude of macrocycle selectivity are solvent-dependent. Admitting that this might be the case in a f4w isolated instances, examination of a larger data base [6-111 suggests that consideration’of log K data alone is not sufficient to predict transport selectivity from cation mixtures. Cation transport is dependent on the concentration of the cation-_ligand--anion complex in the organic phase (anions are included to preserve electroneutrality). In an equilibrium situation (or steady state with identical rate limiting diffusion coefficients for the various complexes) selectivity is determined by the relative concentrations of the various cation-ligand-anion complexes. In the transport model [ 121 and in its extension [10,21], the concentration of the complex near the source phase interface is related to the extraction coefficient, K kip, where K is the ion pair-macrocycle complexation constant in the organic phase and ki, is the water-organic liquid partition coefficient either for the ion pair (1: 1) or for the ion triplet (1: 2). In equimolar cation mixtures, the ratio of the concentrations of the complexes in the membrane is proportional to the ratio of the extraction coefficients, K kip, for the cations involved. Since selectivity is a function of two terms, a small log K value could be compensated by a large partition coefficient. The competitive aspect of selectivity, i.e., the competition by the various cations for a single ligand, is governed by the relative log K values of complexation, while the noncompetitive aspect is governed by the partition coefficients. Either of these terms can be the decisive factor in determining selectivity. In the case of a mixture of monovalent alkali cations, the partition coefficients can be considered approximately equal, thus selectivity is determined by the relative log K values. To a fist approximation, the same would be true in a mixture of divalent cations. However, in a mixture containing both monovalent and divalent cations, both K and ki, must be considered since it is their product which governs selectivity. An example of this is seen, for the cryptand [2.2.1], in mixtures of K’, Sr”, and a third cation with a log K value smaller than that of either K’ or Sr2+. In these cases, log K(CH,OH) for Sr2+ is nearly two orders of magnitude larger than log K(CH30H) for K’, yet K’ is transported selectively, This result can be rationalized as follows. McBride et al. [ 211 have pointed out that partition coefficients for ion triplets comprised of a divalent cation and two univalent anions should be much smaller than those for ion pairs comprised of univalent cations and anions. Reasons for this order of partition coefficients are twofold. First, the partitioning of the divalent cation itself is much smaller than that of the univalent cation (the free energy of transfer between two phases is proportional to .z~) and, second, two anions must partition for each divalent cation. (Although the ion triplet association constant for divalent cations is expected to be larger than the corresponding ion pair constant, the increase is not sufficient to overcome the decrease in single ion partitioning and the net result is a decrease in partitioning of ion triplets with respect to ion pairs when the anion is hydrophilic, such as
283
NO;.) Although log K for the Sr2+- [2.2.2] complexation is large, it is not large enough to compensate for the smaller partition coefficient of Sr*‘.‘The net result is that the extraction coefficient for K+ is larger than that of Sr*‘, leading to a selective transport of K’. Nearly all of the transport selectivity seen in Table 1 can be rationalized using the extraction coefficient relationship. The most notable exception is K+ and Sr*’ with a third cation in the presence of 21C7. In each case, we would expect K+ to be transported selectively over Sr*+. However, the two are transported at an approximately equal and low level in all cases. Whether this demonstrates the need to improve and refine’the model based on the extraction coefficient or simply demonstrates a failure of log K (CH,OH) data to predict behavior in CHC13, we cannot, at this time, ascertain. In mixtures containing Pb*’ (Table l), Pb*+ is always the best transported. Also, log K for formation of the Pb*+-ligand complex is always the largest in each set. Apparently, the log K value is sufficiently large in these cases to compensate for the presumed small partition coefficient of Pb*+ and to give Pb*’ the largest extraction coefficient. Acknowledgement We acknowledge the financial support of this study by U.S. Department of Energy Contract No. DE-AC02-78ER05016 and appreciate the work of Gypzy Lindh who carried out the atomic absorption analyses. References 1 2 3
4 5
6
7
8
C.F. Reusch and E.L. Cussler, Selective membrane transport, AIChE J., 19 (1973) 736. K.H. Wong, K. Yagi and J. Smid, Ion transport through liquid membranes facilitated by crown ethemand their polymers, J. Membrane Biol., 18 (1974) 379. Y. Kobuke, K. Hanji, K. Horiguchi, M. Asada, Y. Nakayama and J. Furukawa, Macrocyclic ligands composed of tetrahydrofuran for selective transport of monovalent cations through liquid membranes, J. Amer. Chem, Sot., 98 (1976) 7414. M. Kirch and J.-M. Lehn, Selective transport of alkali metal cations through a liquid membrane by macrobicyclic carriers, Angew. Chem., Int. Ed. Engl., 14 (1975) 555. J.D. Lamb, R.M. Izatt, D.G. Garrick, J.S. Bradshaw and J.J. Christensen, The influence of macrocyclic ligand structure on carrier-facilitated cation transport rates and selectivities through liquid membranes, J. Membrane Sci., 9 (1981) 83. J.D. Lamb, R.M. Izatt, P.A. Robertson and J.J. Christensen, Highly selective membrane transport of Pb*+ from aqueous metal ion mixtures using macrocyclic carriers, J. Amer. Chem. Sot., 102 (1980) 2454. J.D. Lamb, P.R. Brown, J.J. Christensen, J.S. Bradshaw, D.G. Garrick and R.M. Izatt, Cation transport at 25°C from binary Na+-M”+, Cs+-M”+, and Sr*+-Mn+ nitrate mixtures in a H, 0-CHCl,-H, 0 liquid membrane system containing a series of macrocyclic carriers, J. Membrane Sci., 13 (1983) 89. R.M. Izatt, D.V. Dearden, P.R. Brown, J.S. Bradshaw, J.D. Lamb and J.J. Christensen, 0 liquid memCation fluxes from binary Ag+-Mn+ mixtures in a H, 0-CHCI,-H, brane system containing a series of macrocyclic ligand carriers, J. Amer. Chem. Sot., 105 (1983) 1785.
284 9
10
11
12
13 14
15
16 17 18
19 20 21
22
R.M. Izatt, S.R. Izatt, D.W. McBride, Jr., J.S. Bradshaw and J.J. Christensen, Cation 0 liquid transport at 25°C from binary Cdl+-M”’ mixtures in a H, 0-CHCl,-H, membrane system containing a series of macrocyclic carriers, Isr. J. Chem., in press. G.A. Clark, D.W. McBride, Jr., J.J. Christensen, J.S. Bradshaw and R.M. Izatt, Cation transport at 25°C from binary TI+/Mn’ and K+/Mn+ nitrate mixtures in a H,O-CHCI,H, 0 liquid membrane system containing a series of macrocyclic polyether carriers, J. Membrane Sci., in press. i R.M. Izatt, M.B. Jones, D.W. McBride, Jr., J.S. Bradshaw and J.J. Christensen, Macrocycle-mediated cation transport at 25°C from binary Hg’+-M”’ mixtures in a H,O-CHCl,-H,O liquid membrane system, in preparation. J.D. Lamb, J.J. Christensen, J.L. Oscarson, B.L. Nielsen, B.W. Asay and R.M. Izatt, The relationship between complex stability constants and rates of cation transport through liduid membranes by macrocyclic carriers, J. Amer. Chem. Sot., 102 (1980) 6820. R.D. Shannon and C.T. Prewitt, Effective ionic radii in oxides and fluorides, Acta Crystallogr., B25 (1969) 925. J.D. Lamb, R.M. Izatt, C.S. Swain and J.J. Christensen, A systematic study of the effect of macrocycle ring size and donor atom type on the log K, AH, and TAS of reactions at 25°C in methanol of mono- and divalent cations with crown ethers, J. Amer. Chem. Sot., 102 (1980) 475. R.M. Izatt, R.E. Terry, D.P. Nelson, Y. Chan, D.J. Eatough, J.S. Bradshaw, L.D. Hansen and J.J. Christensen, Calorimetric titration study of the interaction of some uni- and bivalent cations with benzo-15-crown-5-, l8-crown-g, dibenzo-24-crown+, and dibenzo-27-crown-9 in methanol-water solvents at 25°C and fi = 0.1, J. Amer. Chem. Sot., 98 (1976) 7626. J.M. Lehn and J.P. Sauvage, [2] -Cryptates: Stability and selectivity of alkali and alkaline-earth macrobicyclic complexes, J. Amer. Chem. Sot., 97 (1975) 6700. J.D. Lamb, R.M. Izatt, J.J. Christensen and D.J. Eatough, in: G.A. Melson (Ed.), Coordination Chemistry of Macrocyclic Compounds, Plenum, New York, 1979. R.M. Izatt, R.E. Terry, B.L. Haymore, L.D. Hansen, N.K. Dalley, A.G. Avondet and J.J. Christensen, Calorimetric titration study of the interaction of several uni- and bivalent cations with 15crown-5, 18-crown-6 and two isomers of dicyclohexo-1% crown-6 in aqueous solution at 25°C and g = 0.1, J. Amer. Chem. Sot., 98 (1976) 7620 C.J. Pedersen, Cyclic polyethers and their complexes with metal salts, J. Amer. Chem. Sot., 89 (1967) 7017. J.D. Lamb, R.M. Izatt and J.J. Christensen, in: R.M. Izatt and J.J. Christensen (Eds.), Progress in Macrocyclic Chemistry, Vol. 2, Wiley-Interscience, New York, 1981. D.W. McBride, Jr., R.M. Izatt, J.D. Lamb and J.J. Christensen, in: J.L. Atwood, J.E.D. Davies and D.D. MacNicol (Eds.), Inclusion Compounds, Vol. 3, Academic Press, New York, 1984. F. deJong and D.N. Reinhoudt, Stability and reactivity of crown-ether complexes, Adv. Phys. Org. Chem., 17 (1980) 279.