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Transport of alkali metal cations across liquid membranes by crown ether carboxylic acids
Journal of Membrane Science, 10 (1982) 35---47 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
35
TRANSPORT OF ALKALI METAL CATIONS ACROSS LIQUID MEMBRANES BY CROWN ETHER CARBOXYLIC ACIDS
JERZY
STRZELBICKI*
Department
and RICHARD A. BARTSCH**
of Chemistry,
Texas Tech University, Lubbock,
Texas 79409
(U.S.A.)
(Received July 18, 1981, accepted in revised form September 17, 1981)
Summary Competitive alkali metal transport from an alkaline aqueous source phase through a chloroform phase to an acidic aqueous receiving phase facilitated by nine crown ethers with pendant carboxylic acid groups has been investigated. Transport selectivity is controlled by the size of the polyether cavity of the carrier. Increasing the lipophilicity of the carrier, while maintaining a constant polyether cavity size, enhances the total transport rate but does not affect the selectivity. There is poor agreement between the results of competitive transport and the behavior anticipated on the basis of single cation transport studies.
Introduction The use of liquid surfactant membranes is a developing technique for the separation and concentration of metal ions from aqueous solutions [l-7]. The liquid surfactant membrane technique which involves emulsions and a complex aqueous phase-organic phaseaqueous phase system has some advantages and disadvantages when compared with widely-used solid membranes (diaphragms). The advantages are: (1) Large contact areas between aqueous and organic phases result in high transport rates through the organic layer (membrane); (2) The possibility for recovery and ye-use of the organic membrane components; and (3) The use of solid membranes which often exhibit rather poor physical properties may be avoided. The most important disadvantages of the liquid surfactant membrane technique are the extensive investigations required to establish the composition of the organic solution which forms the liquid membrane, the determination of the other operating parameters, and the careful maintenance of these parameters during the metal ion separation process. *Present address: Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Technical University of Wroclaw, 50-370 Wroclaw (Poland). * * To whom correspondence should be addressed.
0376-7388/82/0000-0000/$02.75
0 1982 Elsevier Scientific Publishing Company
36
A first step in developing a liquid surfactant membrane system often involves studies of the selectivity of the carrier which will be used to transport cations from the external aqueous phase across the organic membrane and into the internal aqueous phase of an emulsion droplet. Such investigations may involve two phase solvent extraction systems or, perhaps better, three phase (aqueous phase- organic phaseaqueous phase, but no emulsion) bulk liquid membrane systems to determine the complexing agent selectivity. We now report the results for a bulk liquid membrane model system in which alkali metal cations are transported from a source aqueous phase into a receiving aqueous phase through a chloroform phase which contains a crown ether carboxylic acid. Effects of structural modification of the complexing agent upon transport rates and selectivity are assessed for crown ether carboxylic acids 1-9. Experimental Apparatus Concentrations of alkali metal cations in the aqueous phases were determined with a Dionex Model 10 ion chromatograph. Organic complexing agent concentrations in the chloroform phases were measured with a Cary Model 17 ultraviolet-visible spectrophotometer. Measurements of pH were made with a Fisher Scientific Accumet Model 620 pH meter using a Fisher Scientific E-5A glass body combination electrode. During the transport experiment, constant pH levels were maintained with a Fisher Scientific Model 650 pH controller using a Corning Model 76050 semi-micro combination electrode to control a Sage Instrument Model 341A syringe pump. Phases were mixed using 200 r.p.m. constant speed motors (Model CA, Hurst Industries, Princeton, IN). Reagents sym-Dibenzo-13-crown-4-oxyacetic acid (l), symdibenzo-14-crown-4-oxyacetic acid (2)) sym dibenzo-16-crown-5-ouyacetic acid (3)) sym dibenzo-19crown-6-oxyacetic acid (4), 2-(symdibenzo-16-crown-5-oxy)-butanoic acid (5), 2-(symdibenzo-16-crown-5-oxy)-hexanoic acid (6), 2-(symdibenzo-16crown&oxy)-octanoic acid (7), and 2-(sym-dibenzo-16crown-5-oxy)decanoic acid (8) were synthesized by reactions of the corresponding hydroxy crown ethers [8] with NaH in THF followed by the addition of the appropriate IYbromocarboxylic acid [9]. The 5-(symdibenzo-16-crown-5-oxy)-pentanoic acid (9) was prepared by reaction of the sodium salt of sym-hydroxydibenzo16-crown-5 [8] with ethyl 5-bromovalerate (Aldrich, Milwaukee, WI) in THF and subsequent basic hydrolysis of the resulting crown ether carboxylic abid ester to 9. The LiCl, NaCl, RbCl, RbOH, CsCl, and CsOH were reagent grade and were obtained from Aldrich. Sources of other reagent grade inorganic chemicals include: MC & B (Norwood, OH), LiOH, NaOH; Baker (Phillipsburg, NJ), KCl; and Merck (Rahway, NJ), KOH. Demineralized water and purified chloroform were prepared as described previously [lo] .
37
5 fi 2 s
-C%,CH, -(C H&C H, -K H& H 3 AC H&C H 3
Procedure The liquid membrane transport experiments were conducted using the Utube cell depicted in Fig. 1. The organic phase (30 ml) was initially a 1.0 X 10m3N solution of the crown ether carboxylic acid in chloroform. The receiving phase (13 ml) was 0.1 N aqueous HCl. The source phase (16 ml) was a 0~25 N aqueous solution of the alkali metal chloride (for single cation transport) or of each alkali metal chloride (for competitive transport) for which the pH was adjusted to 9.0 by the addition of the appropriate alkali metal hydroxide(s). During the transport experiment, the pH of the source phase was kept at a constant value of 9.0 by adding an aqueous hydroxide solution which was 0.25 N in the alkali metal cation (single cation transport) or in each of the alkali metal cations (competitive transport) investigated. Contact areas of each aqueous phase-organic phase boundary were 8.04 cm2. The aqueous and organic phases were stirred at 200 r.p.m. The alkali metal cation transport was conducted for 48 h with 1.0 ml samples of the receiving phase being removed (usually every 12 hr) for measurement of the metal ion concentration. When a sample was withdrawn, 1.0 ml of 0.1 N HCl was added to the receiving phase to maintain its volume. The measured metal ion concentrations in the receiving phase were adjusted to compensate for these additions.
38
I +-----60mm-
Fig. 1. Liquid membrane transport cell (A = basic aqueous source phase, B = chloroform phase, C = acidic receiving phase, D = 200 r.p.m. glass stirrer, E = combination electrode, F = rubber septum, G = 21 gauge syringe needle for adding MCI-I).
After 48 hr, the chloroform phase was separated and the concentration of the complexing agent (both the acid and carboxylate forms) was determined by measuring the ultraviolet absorption at 273-274 nm. For l-5 and 9, neither the position nor intensity of the absorption maxima varied significantly when the crown ether carboxylic acid was converted into its carboxylic acid form [lo-121. However, for 6-8, the carboxylate absorption is somewhat stronger than that of the unionized complexing agent [12] . Since this ultraviolet absorption analytical method does not differentiate between the acid and carboxylate forms, the final concentration of complexing agent in the chloroform phase may appear to be slightly larger than the initial concentration of the crown ether carboxylic acid (Table 1). Results and discussion Representative results for the competitive transport of alkali metal cations are shown in Figs. 2 and 3 for crown ether carboxylic acids 3 and 8, respectively. Numerical results for all nine crown ether carboxylic acids are summarized in Table 1. Single ion transport of alkali metal cations was measured only for crown ether carboxylic acids 3 and 8 with the results being illustrated in Figs. 4 and 5 and tabulated in Table 2. Cation transport rates are averages of the rates calculated using pairs of consecutively measured points. The relatively large values of the standard deviations for such average transport rates require explanation. These arise from
39
90 80 70 60 50 LO 30 20 10 0 0
10
20 Time
Time[hrs)
30
40
5
thrs)
Fig. 2. Competitive transport of alkali metal cations across a chloroform crown ether carboxylic acid 3 (0 = Li, * = Na, 0 = K, o = Rb, v = Cs).
membrane
by
Fig. 3. Competitive transport of alkali metal cations across a chloroform crown ether carboxylic acid 8 (0 = Li, A = Na, o = K, 0 = Rb, v = Cs).
membrane
by
30
:: 2 20
10
0
-1
0 Timethrs)
10
20 30 Time(hrs)
LO
Eio
Fig. 4. Single ion transport of alkali metal cations across a chloroform ether carboxylic acid 3 (n = Li, a = Na, 0 = K, o = Rb, v = Cs).
membrane
by crown
Fig. 5. Single ion transport of alkali metal cations across a chloroform ether carboxylic acid 8 (0 = Li, A = Na, O= K, o = Rb, v = Cs).
membrane
by crown
f f + + f
(1.1 (1.0 (1.5 (1.2 (2.3
Li’ Na’ K’ Rb’ Cs+
7.98
x 1O-4
4
0.5) 0.5) 0.6) 0.4) 0.6)
0.4) 3.0) 2.2) 0.5) 0.2)
f f f + f
(1.1 (8.4 (9.1 (3.0 (1.4
Lit Na’ K+ Rb+ Cs’
5.86 x 1O-4
3
f 2.2) +Z1.8) f 2.9) f 0.4) + 3.0)
(8.9 (7.8 (4.4 (1.3 (5.6
Li’ Na+ K+ Rb’ Cs’
1.45 x 1o-4
2
3.7) 0.7) 0.6) 0.5) 4.8)
k f t f f
9.15 x 1o-5
1
(2.9 (2.5 (2.1 (1.2 (9.5
Li’ Na’ K’ Rb’ cs+
Final carrier concentration in organic phase (NY’
Carrier
x X x x x
x X x x x
x X x x X
X X x x x
10-u lo-’ lo-’ 1O-7 lo-*
lo-’ 10T7 lo-’ lo-* lo-*
1o-6 lo-’ lo-’ lo-’ lo-’
10m9 lo-’ lo-* lo-’ 1o-9
Average cation transport rate and standard deviation (mol/hr)
Competitive transport of alkali metal cations across a chloroform
TABLE 1
Na’ > K+ > Rb’ > Cs+ > Li’ 9 28 58 74
9.9 x lo-’
K+ > Rb+ > Na’ > Cs’ > Li’ 13 15 65 135
Li’ > Na’ > K+ > Rb’ > Cs+ 1.1 2.0 6.8 16
2.3 x lo-’
1.8 x 10“
Na+ > K’ > Rb’ > Cs’ > Li’ 1.2 2.1 2.6 8.5
__-
Transport selectivity order and selectivity coefficientsb
acid carriers
5.1 x lo-&
--
Total cation transport rate (mol/hr)
membrane with crown ether carboxylic
k 2.6) + 0.8) + 0.7) ?r 0.8) + 1.2) + + + f +
(6.4 (1.7 (1.3 (2.1 (1.2 (3.8 (1.9 (1.9 (5.0 (3.3 (9.0 (7.6 (2.6 (6.3
Li’ Na+ K’ Rb’ Cs’
Li’ Na’ K’ Rb’ Cs’
Na+ K+ Rb’ CS’
1.06 x 10-3c
6.67
lo-’
+ c + +
1.3) X lo-’ 3.6) x lo-* 1.4) x lo-” 2.5) x 1O-9
x lo-” X lObe x lo-’ x lo-’ x 10-s
Na+ > K+ > Rb’ > Cs’ > Li+ 12 57 70 250
Na’ > K+ > Rb’ > Cs‘ > Li+ 10 48 62 75
Na’ > K+ > Rb’ > Cs’ > Li’ 13 81 140 260
Na+ > K’ > Rb’ > Cs* > Li+ 10 39 51 59
Na+ > K’ > Rb’ > Cs+ 1.2 3.4 14
1.3 x 1o-6
1.3 x 1o-6
1.8 x 1o-6
2.2 x lo+
2.0 x lo-’
aInitial carrier concentration in the organic phase = 1.00 X lo-’ N. bSelectivity coefficients are listed below the cation chemical symbol and express the ratio between rates for the most rapidly transported cation and the indicated cation. ‘See text.
X
4.2) 0.4) 0.4) 3.8) 0.4)
x 1O-9 x 10mb x 1O-7 x lo-’ x 1o-8
1.0) X lo-’ 0.3) X 10m6 0.3) x lo-’ 0.4) x lo-’ 1.4) x 10-8
1.12 x 1o-3 c
f + + f f
(1.5 (1.1 (1.1 (2.3 (1.8
Li’ Na+ K+ Rb+ cs+
2.2) X 10m9 0.6) X 1Oe6 4.0) x lo-* 1.6) x 10:” 1.6) x 10-O
1.29 x lo-SC
+ + ? f f
(4.6 (1.2 (9.6 (2.0 (1.6
x 1o-3
Li’ Na’ K+ Rb’ Cs’
1.00
Li’ Na+ K’ Rb’ CS’
Li+ Na+ K+ Rb* cs4
3
8
1.00 1.00 1.00 1.00 1.00
2.73 9.70 8.50 2.85 2.75 x x x x x
1o-3 1o-3 1o-3 1o-3 1o-3
x 10-a x 10-4 x 10-4 X 1O-4 x 1O-4
Final carrier concentration in organic phase (N)a
-
(7.4 (5.4 (6.3 (3.9 (7.2
(2.7 (2.1 (2.4 (1.1 (9.0 x x x x x
x x x x x
--.
+_2.3) * 2.7) f 2.4) + 0.8) + 2.4)
+ 1.4) * 0.8) f 0.6) * 0.5) +_1.3)
~~
10-8 lo-’ 1O-7 10“ 10-a
10‘” lo-’ lo-’ 1o-7 10-a
Average cation transport rate and standard deviation (mol/hr)
acid carriers
--._-
K’ > Na’ > Rb’ > Li’ > Cs’ 1.2 1.6 8.6 8.8
K’ > Na+ > Rb+ > Cs+ > Li’ 1.1 2.1 2.6 8.9
Apparent transport selectivity order and selectivity coefficientsb
membrane with crown ether carboxylic
‘Initial carrier concentration in organic phase = 1.00 X low3 N. bSelectivity coefficients are listed below the cation chemical symbol and express the ratio between rates for the most rapidly transported cation and the indicated cation.
.-_-
Alkali metal cation
Carrier
Single cation transport of alkali metal cations across a chloroform
TABLE 2
43
two sources. First, the cation concentrations in the receiving aqueous phase are very low (below 1OA N) which introduces uncertainty into the concentration measurements. This is particularly important for cations which show very low transport rates. However, a more significant second factor is the somewhat non-linear relationship between the amount of alkali metal cations transported into the receiving phase and the time (Figs. 2-5). This phenomenon is especially visible during the initial stages of the transport experiment. In some cases, linear behavior is observed after sufficient time has elapsed (Figs. 3 and 4). The non-linear behavior indicates that, in contrast with the reports of others for multidentate ligand facilitated cation transport across liquid membranes [ 13-161, equilibration does not occur instantaneously in our system. Variation of the crown ether portion of the cornplexing agent in competitive transport The first four crown ether carboxylic acids listed in Table 1 are all oxyacetic acid derivatives which differ from each other by variation of the poly&her cavity size and/or the number of oxygen donor atoms. Cavity sizes for l-4, as estimated from Corey-Pauling-Kortum (CPK) space-filling models, are cl-2 A, 1.2-l-5 4 2.0-2.4 A, and 3.0-3.5 A, respectively. Ionic diameters of the alkali metal cations are: L1*, 1.36 A; Na’, 1.96 A; IS+, 2.66 A; Rb+, 2.98 a ; and CS+, 3.30 a [17] . Based upon an optimal ratio for cation diameter/ cavity diameter of 0.9 [18], it would be predicted that Li’ should be selectively transported by 2, Na’ by 3, and K’ or Rb’ by 4. These predictions are 1Il @UX]]Mlt agreement with the transport rates recorded in Table 1, even though the Li’ selectivity of 2 is not very high (see “selectivity coefficients”, Table 1). It should be noted that this relationship between the size of the polyether cavity of the carrier and the dominant cation transported was totally unexpetted based upon the selectivity observed in solvent extractions of alkali metal cations from water into chloroform using l-4 [10,11] . In these mlvent extractions, the dominant chloroform phase metal was K for all four crown ether carboxylic acids. Thus, extraction selectivity was found to be almost independent of the polyether cavity size of the complexing agent. The contrasting selectivities in the present bulk liquid membrane transport experiments and the previous solvent extraction studies may be rationalized by considering the interactions of cations with the polyether and carboxylate portions of the complexing agents under different conditions. For the extraction process, there is sufficient time for the two phase SYStern to come to complete equilibrium. It is proposed that under such conditions cation interactions with the complexing agent are governed by thermodynamically stronger cation-carboxylate attractions. Compound 1 provides a reference point, since the polyether cavity of the complexing agent derived from 1 is too small to accomodate even the smallest alkali metal cation. Therefore, the selectivity for extraction of K noted for 1 suggests that Kcarboxylate interactions are stronger than those for the other alkali metal
44 Cations. Since the sele&ivity orders for extractions using 2-4 are very similar to that observed with 1, a dominant role of cation--carboxylate interactions in these complexing agents is also indicated. For active transport [ 191 of the alkali metal cations across liquid membranes, there is insufficient time for numerous exchange rE%iCtiOnSOf SOurCe aqueous phase Cations with the crown ether carboxylate before it migrates into the chloroform layer with subsequent release of the cation to the receiving aqueous phase. It is postulated that under such conditions cation interactions with the complexing agent are kinetically-controlled, cation-polyether attractions. Therefore, cations are selectively transported for which the relationship between the polyether cavity and cation sizes is appropriate. In agreement, 2-4 show markedly different selectivity orders than does 1 in the transport experiments. For the complexing agent derived from 1, cationpolyether interactions should be minimal. Comparison of the total alkali metal cation transport rates for crown ether carboxylic acids 14 is rendered difficult by their varying concentrations in the organic phase (as measured at the completion of the transport experiments, Table 1). However, if it is assumed that the transport rate is proportional to the complexing agent concentration in the chloroform phase [13] , a correction may be applied to the transport rate data and the relative efficiencies for transport are 4 > 3 > 2 > 1.The overall efficiency of alkali metal transport by 14 is small. In all cases the amount of cations transported in 48 hours is less than the amount of complexing agent present in the organic phase.
Variation of the lipophilicity of the complexing agent in competitive
transport
Crown ether carboxylic acids 5-8 differ from 3 only by the attachment of alkyl groups to the a-position of the oxyacetic acid. Thus while holding the polyether and carboxylic acid portions of the crown ether carboxylic acid constant, the lipophilicity is systematically varied. As with 3, Na’ is selectively transported by 5-8. Selectivity coefficients (ratios of the rate for the most rapidly transported cation to that of another alkali metal cation) are quite similar for 3,5-S (Table 1). Therefore, the attachment of alkyl groups to the a! position of 3 has very little influence upon the selectivity (relative transport rates) of alkali metal cation transport. However, this structural modification does influence the total transport rate. Increasing the lipophilicity of the complexing agent enhances the rate of cation transport. In changing from 5 to 8 the total transport rate increases by approximately 70%. Since complexing agents derived from 5-8 remain completely in the organic phase, this increase does not arise from enhancement of the chloroform phase concentration of the complexing agent. It is noteworthy that for 8 the amount of cations transported into the receiving phase in 48 hours is 3.6 times higher than the amount of 8 in the system. The selectivity orders for solvent extractions using 5-S [ZO] are the same as those found in the present liquid membrane transport studies. This contrasts sharply with comparison between the results of solvent extractions and
45
liquid membrane transport of alkali metal cations using the parent crown ether carboxylic acid 3, For solvent extractions with 3, the selectivity order in the pH region of 6 -7 is K’ > Na’ > Rb’, Cs’ > Li’ and in the region 8-12 is Na’ > K’ > Rb’ = Cs’ > Li’. The selectivity order at higher pH is in agreement with the membrane transport rate. The reversal in Na’:K’ selectivity in the extraction studies for the two pH regions is attributed to a balance between the crown ether and carboxyl portions of the complexing agent [lo] . Under alkaline conditions, Na’ is selectively extracted, which suggests that the crown ether cavity is the dominant factor. However, under acidic conditions, K+ is extracted better than Na’ which indicates an enhanced importance of the carboxy1 portion of the complexing agent. If this reasoning is correct, the selectivity for Na at all pH values in extractions using 5-8 means that the carboxyl portion of the complexing agents is less important than in 3. For 3 and 5-8, pK values in water are 4.6, 5.3, 5.9, 7.2, and 7.4, respectively [9]. The diminished acidity caused by attaching lipophilic groups to 3 may be responsible for the lesser importance of the carboxyl portion of the complexing agents derived from 5-8. Variations of the relative positions of the polyether and carboxylated groups of the complexing agent in competitive transport Comparison of the transport study results for 9 with those for 5 and 6 (Table 1) reveals an important relationship between the relative positions of the polyether and carboxylate groups of the carrier. All three crown ether carboxylic acids share a common polyether ring structure and possess similar numbers of carbon atoms in the pendant carboxylic aeid groups. However, 5 and 6 are oxyacetic acid derivatives while 9 is a 6 -oxypentanoic acid derivative. Compared with 5 and 6, or even 3, the liquid membrane transport of alkali metal cations by 9 is very poor. Both the transport rate and selectivity suffer markedly. Thus, the proximity of the polyether and carboxylate portions of the complexing agent has important bearing upon the efficiency and selectivity of cation transport. Single cation transport To allow for the comparison of predictions for multi-ion transport behaviour which are extrapolated from single ion transport experiments with the results of competitive transport experiments, single ion transport of alkali metal cations across the liquid membrane by 3 and 8 was examined. Results are presented in Figs. 4 and 5 and in Table 2. Observed transport selectivities from competitive experiments (Table 1) show only slight similarity to the predictions based upon results from single ion transport experiments (Table 2). Others have noted similar discrepancies between single ion and competitive cation transport by neutral macrocyclic carriers [21] . In the single ion transport experiments for Li+ and Cs+ using 8, a noteworthy concentration of trace Na+ impurities was observed. Although the
46
concentration of Na’ impurity in the 0.25 N aqueous Li’ source phase was only 2 X lOA N, the transport rate of Na’ into the aqueous receiving phase [(8.1 f 4.1) X 10” mol/hr] was greater than that of Li’. Similarly for a 4 X 10V5 N impurity of Na’ in a 0.25 N aqueous Cs’ source phase, the transport rate of Na+ into the aqueous receiving phase [(1.8 k 0.4) X lo-’ mol/hr] surpassed that of Cs’. Acknowledgement This research was supported by the Department of Energy (Contract DE-ASOS-80ER-10604) and the Texas Tech University Center for Energy Research (postdoctoral fellowship to JS). References D.K. Schiffer, A. Hochhauser, D.F. Evans and E.L. Cussler, Concentrating solutes with membranes containing carriers, Nature, 250 (1974) 484. E.L. Cussler and D.F. Evans, How to design liquid membrane separations, Separ. Purif. Methods, 3 (1974) 399. A.M. Hochhauser and E.L. Cussler, Concentrating chromium with liquid surfactant membranes, AICHE Symposium Series, 152 (1975) 136. E.S. Matulevicius and N.N. Li, Facilitated transport through liquid membranes, Separ. Purif. Methods, 4 (1975) 73. J. Strzelbicki and W. Charewicz, Separation of copper by liquid surfactant membranes, J. Inorg. Nucl. Chem., 40 (1978) 1415. J. Strzelbicki and W. Charewicz, Separation of cobalt by liquid surfactant membranes, Separ. Sci. Techn., 13 (1978) 141. J. Strzelbicki and W. Charewicz, The liquid surfactant membrane separation of copper, cobalt and nickel from multicomponent aqueous solutions, Hydrometallurgy, 5 (1980) 243. G.S. Heo, R.A. Bartsch, L.L. Schlobohm and J.G. Lee, Preparation of hydroxy crown ethers by reactions of diphenols with epichlorohydrin, J. Org. Chem., 46 (1981) 3574. R.A. Bartsch, G.S. Heo, S.I. Kang, Y. Liu and J. Strzelbicki, Synthesis and acidity of crown ethers with pendant carboxylic acid groups, J. Org. Chem., in press. 10 J. Strzelbicki and R.A. Bartsch, Extraction of alkali metal cations from aqueous solutions by a crown ether carboxylic acid, Anal. Chem., 53 (1981) 40. 11 J. Strzelbicki, G.S. Heo and R.A. Bartsch, Solvent extraction of alkali metal cations from aqueous solutions by crown ether carboxylic acids, Separ. Sci. Technol., in press. 12 J. Strzelbicki and R.A. Bartsch, Extraction of alkaline earth cations from aqueous solutions by crown ether carboxylic acids, Anal. Chem., in press. 13 C.F. Reusch and E.L. Cussler, Selective membrane transport, AICHE J., 19 (1973) 736. 14 K.H. Wong, K. Yagi and J. Smid, Ion transport through liquid membranes facilitated by crown ethers and their polymers, J. Membrane Biol., 18 (1974) 379. 15 Y. Kobuke, K. Hanji, K. Horeguchi, 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. 16 J.J. Christensen, J.D. Lamb, S.R. Izatt, S.E. Starr, G.C. Weed,M.S. Astin, B.D. Stitt and R.M. Izatt, Effect of anion type on rate of facilitated transport of cations across liquid membranes via neutral macrocyclic carriers. J. Amer. Chem. Sot., 100 (1978) 3219.
47 17 W.E. Moore, D. Amman, R. Bissig, E. Pretsch and W. Simon, Cation selectivity of neutral macrocyclic and nonmacrocyclic complexing agents in membranes, in: R.M. Izatt and J.J. Christensen (Eds.), Progress in Macrocyclic Chemistry, Vol. 1, WileyInterscience, New York, 1979, Chap. 1, p. 9. 18 J.J. Christensen, D.J. Eatough and R.M. Izatt, The synthesis and ion binding of synthetic multidentate macrocyclic compounds, Chem. Rev., 74 (1974) 351. 19 R.A. Schwind, T.J. Gill&and and E.L. Cussler, Developing the commercial potential of macrocyclic molecules, in: R.M. Izatt and J.J. Christensen (Eds.), Synthetic Multidentate Macrocyclic Compounds, Academic Press, New York, 1978, Chap. 6, pp. 298-299. 20 J. Strzelbicki and R.A. Bartsch, Solvent extraction of alkali metal cations from aqueous solutions by highly lipophilic crown ether carboxylic acids, Anal. Chem., in press. 21 J,D, Lamb, R.M. Izatt, P.A. Robertson and J.J. Christensen, Highly selective membrane transport of Pb z+ from aqueous metal ion mixtures using macrocyclic carriers, J. Amer. Chem. Sot., 102 (1980) 2452.
35
TRANSPORT OF ALKALI METAL CATIONS ACROSS LIQUID MEMBRANES BY CROWN ETHER CARBOXYLIC ACIDS
JERZY
STRZELBICKI*
Department
and RICHARD A. BARTSCH**
of Chemistry,
Texas Tech University, Lubbock,
Texas 79409
(U.S.A.)
(Received July 18, 1981, accepted in revised form September 17, 1981)
Summary Competitive alkali metal transport from an alkaline aqueous source phase through a chloroform phase to an acidic aqueous receiving phase facilitated by nine crown ethers with pendant carboxylic acid groups has been investigated. Transport selectivity is controlled by the size of the polyether cavity of the carrier. Increasing the lipophilicity of the carrier, while maintaining a constant polyether cavity size, enhances the total transport rate but does not affect the selectivity. There is poor agreement between the results of competitive transport and the behavior anticipated on the basis of single cation transport studies.
Introduction The use of liquid surfactant membranes is a developing technique for the separation and concentration of metal ions from aqueous solutions [l-7]. The liquid surfactant membrane technique which involves emulsions and a complex aqueous phase-organic phaseaqueous phase system has some advantages and disadvantages when compared with widely-used solid membranes (diaphragms). The advantages are: (1) Large contact areas between aqueous and organic phases result in high transport rates through the organic layer (membrane); (2) The possibility for recovery and ye-use of the organic membrane components; and (3) The use of solid membranes which often exhibit rather poor physical properties may be avoided. The most important disadvantages of the liquid surfactant membrane technique are the extensive investigations required to establish the composition of the organic solution which forms the liquid membrane, the determination of the other operating parameters, and the careful maintenance of these parameters during the metal ion separation process. *Present address: Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Technical University of Wroclaw, 50-370 Wroclaw (Poland). * * To whom correspondence should be addressed.
0376-7388/82/0000-0000/$02.75
0 1982 Elsevier Scientific Publishing Company
36
A first step in developing a liquid surfactant membrane system often involves studies of the selectivity of the carrier which will be used to transport cations from the external aqueous phase across the organic membrane and into the internal aqueous phase of an emulsion droplet. Such investigations may involve two phase solvent extraction systems or, perhaps better, three phase (aqueous phase- organic phaseaqueous phase, but no emulsion) bulk liquid membrane systems to determine the complexing agent selectivity. We now report the results for a bulk liquid membrane model system in which alkali metal cations are transported from a source aqueous phase into a receiving aqueous phase through a chloroform phase which contains a crown ether carboxylic acid. Effects of structural modification of the complexing agent upon transport rates and selectivity are assessed for crown ether carboxylic acids 1-9. Experimental Apparatus Concentrations of alkali metal cations in the aqueous phases were determined with a Dionex Model 10 ion chromatograph. Organic complexing agent concentrations in the chloroform phases were measured with a Cary Model 17 ultraviolet-visible spectrophotometer. Measurements of pH were made with a Fisher Scientific Accumet Model 620 pH meter using a Fisher Scientific E-5A glass body combination electrode. During the transport experiment, constant pH levels were maintained with a Fisher Scientific Model 650 pH controller using a Corning Model 76050 semi-micro combination electrode to control a Sage Instrument Model 341A syringe pump. Phases were mixed using 200 r.p.m. constant speed motors (Model CA, Hurst Industries, Princeton, IN). Reagents sym-Dibenzo-13-crown-4-oxyacetic acid (l), symdibenzo-14-crown-4-oxyacetic acid (2)) sym dibenzo-16-crown-5-ouyacetic acid (3)) sym dibenzo-19crown-6-oxyacetic acid (4), 2-(symdibenzo-16-crown-5-oxy)-butanoic acid (5), 2-(symdibenzo-16-crown-5-oxy)-hexanoic acid (6), 2-(symdibenzo-16crown&oxy)-octanoic acid (7), and 2-(sym-dibenzo-16crown-5-oxy)decanoic acid (8) were synthesized by reactions of the corresponding hydroxy crown ethers [8] with NaH in THF followed by the addition of the appropriate IYbromocarboxylic acid [9]. The 5-(symdibenzo-16-crown-5-oxy)-pentanoic acid (9) was prepared by reaction of the sodium salt of sym-hydroxydibenzo16-crown-5 [8] with ethyl 5-bromovalerate (Aldrich, Milwaukee, WI) in THF and subsequent basic hydrolysis of the resulting crown ether carboxylic abid ester to 9. The LiCl, NaCl, RbCl, RbOH, CsCl, and CsOH were reagent grade and were obtained from Aldrich. Sources of other reagent grade inorganic chemicals include: MC & B (Norwood, OH), LiOH, NaOH; Baker (Phillipsburg, NJ), KCl; and Merck (Rahway, NJ), KOH. Demineralized water and purified chloroform were prepared as described previously [lo] .
37
5 fi 2 s
-C%,CH, -(C H&C H, -K H& H 3 AC H&C H 3
Procedure The liquid membrane transport experiments were conducted using the Utube cell depicted in Fig. 1. The organic phase (30 ml) was initially a 1.0 X 10m3N solution of the crown ether carboxylic acid in chloroform. The receiving phase (13 ml) was 0.1 N aqueous HCl. The source phase (16 ml) was a 0~25 N aqueous solution of the alkali metal chloride (for single cation transport) or of each alkali metal chloride (for competitive transport) for which the pH was adjusted to 9.0 by the addition of the appropriate alkali metal hydroxide(s). During the transport experiment, the pH of the source phase was kept at a constant value of 9.0 by adding an aqueous hydroxide solution which was 0.25 N in the alkali metal cation (single cation transport) or in each of the alkali metal cations (competitive transport) investigated. Contact areas of each aqueous phase-organic phase boundary were 8.04 cm2. The aqueous and organic phases were stirred at 200 r.p.m. The alkali metal cation transport was conducted for 48 h with 1.0 ml samples of the receiving phase being removed (usually every 12 hr) for measurement of the metal ion concentration. When a sample was withdrawn, 1.0 ml of 0.1 N HCl was added to the receiving phase to maintain its volume. The measured metal ion concentrations in the receiving phase were adjusted to compensate for these additions.
38
I +-----60mm-
Fig. 1. Liquid membrane transport cell (A = basic aqueous source phase, B = chloroform phase, C = acidic receiving phase, D = 200 r.p.m. glass stirrer, E = combination electrode, F = rubber septum, G = 21 gauge syringe needle for adding MCI-I).
After 48 hr, the chloroform phase was separated and the concentration of the complexing agent (both the acid and carboxylate forms) was determined by measuring the ultraviolet absorption at 273-274 nm. For l-5 and 9, neither the position nor intensity of the absorption maxima varied significantly when the crown ether carboxylic acid was converted into its carboxylic acid form [lo-121. However, for 6-8, the carboxylate absorption is somewhat stronger than that of the unionized complexing agent [12] . Since this ultraviolet absorption analytical method does not differentiate between the acid and carboxylate forms, the final concentration of complexing agent in the chloroform phase may appear to be slightly larger than the initial concentration of the crown ether carboxylic acid (Table 1). Results and discussion Representative results for the competitive transport of alkali metal cations are shown in Figs. 2 and 3 for crown ether carboxylic acids 3 and 8, respectively. Numerical results for all nine crown ether carboxylic acids are summarized in Table 1. Single ion transport of alkali metal cations was measured only for crown ether carboxylic acids 3 and 8 with the results being illustrated in Figs. 4 and 5 and tabulated in Table 2. Cation transport rates are averages of the rates calculated using pairs of consecutively measured points. The relatively large values of the standard deviations for such average transport rates require explanation. These arise from
39
90 80 70 60 50 LO 30 20 10 0 0
10
20 Time
Time[hrs)
30
40
5
thrs)
Fig. 2. Competitive transport of alkali metal cations across a chloroform crown ether carboxylic acid 3 (0 = Li, * = Na, 0 = K, o = Rb, v = Cs).
membrane
by
Fig. 3. Competitive transport of alkali metal cations across a chloroform crown ether carboxylic acid 8 (0 = Li, A = Na, o = K, 0 = Rb, v = Cs).
membrane
by
30
:: 2 20
10
0
-1
0 Timethrs)
10
20 30 Time(hrs)
LO
Eio
Fig. 4. Single ion transport of alkali metal cations across a chloroform ether carboxylic acid 3 (n = Li, a = Na, 0 = K, o = Rb, v = Cs).
membrane
by crown
Fig. 5. Single ion transport of alkali metal cations across a chloroform ether carboxylic acid 8 (0 = Li, A = Na, O= K, o = Rb, v = Cs).
membrane
by crown
f f + + f
(1.1 (1.0 (1.5 (1.2 (2.3
Li’ Na’ K’ Rb’ Cs+
7.98
x 1O-4
4
0.5) 0.5) 0.6) 0.4) 0.6)
0.4) 3.0) 2.2) 0.5) 0.2)
f f f + f
(1.1 (8.4 (9.1 (3.0 (1.4
Lit Na’ K+ Rb+ Cs’
5.86 x 1O-4
3
f 2.2) +Z1.8) f 2.9) f 0.4) + 3.0)
(8.9 (7.8 (4.4 (1.3 (5.6
Li’ Na+ K+ Rb’ Cs’
1.45 x 1o-4
2
3.7) 0.7) 0.6) 0.5) 4.8)
k f t f f
9.15 x 1o-5
1
(2.9 (2.5 (2.1 (1.2 (9.5
Li’ Na’ K’ Rb’ cs+
Final carrier concentration in organic phase (NY’
Carrier
x X x x x
x X x x x
x X x x X
X X x x x
10-u lo-’ lo-’ 1O-7 lo-*
lo-’ 10T7 lo-’ lo-* lo-*
1o-6 lo-’ lo-’ lo-’ lo-’
10m9 lo-’ lo-* lo-’ 1o-9
Average cation transport rate and standard deviation (mol/hr)
Competitive transport of alkali metal cations across a chloroform
TABLE 1
Na’ > K+ > Rb’ > Cs+ > Li’ 9 28 58 74
9.9 x lo-’
K+ > Rb+ > Na’ > Cs’ > Li’ 13 15 65 135
Li’ > Na’ > K+ > Rb’ > Cs+ 1.1 2.0 6.8 16
2.3 x lo-’
1.8 x 10“
Na+ > K’ > Rb’ > Cs’ > Li’ 1.2 2.1 2.6 8.5
__-
Transport selectivity order and selectivity coefficientsb
acid carriers
5.1 x lo-&
--
Total cation transport rate (mol/hr)
membrane with crown ether carboxylic
k 2.6) + 0.8) + 0.7) ?r 0.8) + 1.2) + + + f +
(6.4 (1.7 (1.3 (2.1 (1.2 (3.8 (1.9 (1.9 (5.0 (3.3 (9.0 (7.6 (2.6 (6.3
Li’ Na+ K’ Rb’ Cs’
Li’ Na’ K’ Rb’ Cs’
Na+ K+ Rb’ CS’
1.06 x 10-3c
6.67
lo-’
+ c + +
1.3) X lo-’ 3.6) x lo-* 1.4) x lo-” 2.5) x 1O-9
x lo-” X lObe x lo-’ x lo-’ x 10-s
Na+ > K+ > Rb’ > Cs’ > Li+ 12 57 70 250
Na’ > K+ > Rb’ > Cs‘ > Li+ 10 48 62 75
Na’ > K+ > Rb’ > Cs’ > Li’ 13 81 140 260
Na+ > K’ > Rb’ > Cs* > Li+ 10 39 51 59
Na+ > K’ > Rb’ > Cs+ 1.2 3.4 14
1.3 x 1o-6
1.3 x 1o-6
1.8 x 1o-6
2.2 x lo+
2.0 x lo-’
aInitial carrier concentration in the organic phase = 1.00 X lo-’ N. bSelectivity coefficients are listed below the cation chemical symbol and express the ratio between rates for the most rapidly transported cation and the indicated cation. ‘See text.
X
4.2) 0.4) 0.4) 3.8) 0.4)
x 1O-9 x 10mb x 1O-7 x lo-’ x 1o-8
1.0) X lo-’ 0.3) X 10m6 0.3) x lo-’ 0.4) x lo-’ 1.4) x 10-8
1.12 x 1o-3 c
f + + f f
(1.5 (1.1 (1.1 (2.3 (1.8
Li’ Na+ K+ Rb+ cs+
2.2) X 10m9 0.6) X 1Oe6 4.0) x lo-* 1.6) x 10:” 1.6) x 10-O
1.29 x lo-SC
+ + ? f f
(4.6 (1.2 (9.6 (2.0 (1.6
x 1o-3
Li’ Na’ K+ Rb’ Cs’
1.00
Li’ Na+ K’ Rb’ CS’
Li+ Na+ K+ Rb* cs4
3
8
1.00 1.00 1.00 1.00 1.00
2.73 9.70 8.50 2.85 2.75 x x x x x
1o-3 1o-3 1o-3 1o-3 1o-3
x 10-a x 10-4 x 10-4 X 1O-4 x 1O-4
Final carrier concentration in organic phase (N)a
-
(7.4 (5.4 (6.3 (3.9 (7.2
(2.7 (2.1 (2.4 (1.1 (9.0 x x x x x
x x x x x
--.
+_2.3) * 2.7) f 2.4) + 0.8) + 2.4)
+ 1.4) * 0.8) f 0.6) * 0.5) +_1.3)
~~
10-8 lo-’ 1O-7 10“ 10-a
10‘” lo-’ lo-’ 1o-7 10-a
Average cation transport rate and standard deviation (mol/hr)
acid carriers
--._-
K’ > Na’ > Rb’ > Li’ > Cs’ 1.2 1.6 8.6 8.8
K’ > Na+ > Rb+ > Cs+ > Li’ 1.1 2.1 2.6 8.9
Apparent transport selectivity order and selectivity coefficientsb
membrane with crown ether carboxylic
‘Initial carrier concentration in organic phase = 1.00 X low3 N. bSelectivity coefficients are listed below the cation chemical symbol and express the ratio between rates for the most rapidly transported cation and the indicated cation.
.-_-
Alkali metal cation
Carrier
Single cation transport of alkali metal cations across a chloroform
TABLE 2
43
two sources. First, the cation concentrations in the receiving aqueous phase are very low (below 1OA N) which introduces uncertainty into the concentration measurements. This is particularly important for cations which show very low transport rates. However, a more significant second factor is the somewhat non-linear relationship between the amount of alkali metal cations transported into the receiving phase and the time (Figs. 2-5). This phenomenon is especially visible during the initial stages of the transport experiment. In some cases, linear behavior is observed after sufficient time has elapsed (Figs. 3 and 4). The non-linear behavior indicates that, in contrast with the reports of others for multidentate ligand facilitated cation transport across liquid membranes [ 13-161, equilibration does not occur instantaneously in our system. Variation of the crown ether portion of the cornplexing agent in competitive transport The first four crown ether carboxylic acids listed in Table 1 are all oxyacetic acid derivatives which differ from each other by variation of the poly&her cavity size and/or the number of oxygen donor atoms. Cavity sizes for l-4, as estimated from Corey-Pauling-Kortum (CPK) space-filling models, are cl-2 A, 1.2-l-5 4 2.0-2.4 A, and 3.0-3.5 A, respectively. Ionic diameters of the alkali metal cations are: L1*, 1.36 A; Na’, 1.96 A; IS+, 2.66 A; Rb+, 2.98 a ; and CS+, 3.30 a [17] . Based upon an optimal ratio for cation diameter/ cavity diameter of 0.9 [18], it would be predicted that Li’ should be selectively transported by 2, Na’ by 3, and K’ or Rb’ by 4. These predictions are 1Il @UX]]Mlt agreement with the transport rates recorded in Table 1, even though the Li’ selectivity of 2 is not very high (see “selectivity coefficients”, Table 1). It should be noted that this relationship between the size of the polyether cavity of the carrier and the dominant cation transported was totally unexpetted based upon the selectivity observed in solvent extractions of alkali metal cations from water into chloroform using l-4 [10,11] . In these mlvent extractions, the dominant chloroform phase metal was K for all four crown ether carboxylic acids. Thus, extraction selectivity was found to be almost independent of the polyether cavity size of the complexing agent. The contrasting selectivities in the present bulk liquid membrane transport experiments and the previous solvent extraction studies may be rationalized by considering the interactions of cations with the polyether and carboxylate portions of the complexing agents under different conditions. For the extraction process, there is sufficient time for the two phase SYStern to come to complete equilibrium. It is proposed that under such conditions cation interactions with the complexing agent are governed by thermodynamically stronger cation-carboxylate attractions. Compound 1 provides a reference point, since the polyether cavity of the complexing agent derived from 1 is too small to accomodate even the smallest alkali metal cation. Therefore, the selectivity for extraction of K noted for 1 suggests that Kcarboxylate interactions are stronger than those for the other alkali metal
44 Cations. Since the sele&ivity orders for extractions using 2-4 are very similar to that observed with 1, a dominant role of cation--carboxylate interactions in these complexing agents is also indicated. For active transport [ 191 of the alkali metal cations across liquid membranes, there is insufficient time for numerous exchange rE%iCtiOnSOf SOurCe aqueous phase Cations with the crown ether carboxylate before it migrates into the chloroform layer with subsequent release of the cation to the receiving aqueous phase. It is postulated that under such conditions cation interactions with the complexing agent are kinetically-controlled, cation-polyether attractions. Therefore, cations are selectively transported for which the relationship between the polyether cavity and cation sizes is appropriate. In agreement, 2-4 show markedly different selectivity orders than does 1 in the transport experiments. For the complexing agent derived from 1, cationpolyether interactions should be minimal. Comparison of the total alkali metal cation transport rates for crown ether carboxylic acids 14 is rendered difficult by their varying concentrations in the organic phase (as measured at the completion of the transport experiments, Table 1). However, if it is assumed that the transport rate is proportional to the complexing agent concentration in the chloroform phase [13] , a correction may be applied to the transport rate data and the relative efficiencies for transport are 4 > 3 > 2 > 1.The overall efficiency of alkali metal transport by 14 is small. In all cases the amount of cations transported in 48 hours is less than the amount of complexing agent present in the organic phase.
Variation of the lipophilicity of the complexing agent in competitive
transport
Crown ether carboxylic acids 5-8 differ from 3 only by the attachment of alkyl groups to the a-position of the oxyacetic acid. Thus while holding the polyether and carboxylic acid portions of the crown ether carboxylic acid constant, the lipophilicity is systematically varied. As with 3, Na’ is selectively transported by 5-8. Selectivity coefficients (ratios of the rate for the most rapidly transported cation to that of another alkali metal cation) are quite similar for 3,5-S (Table 1). Therefore, the attachment of alkyl groups to the a! position of 3 has very little influence upon the selectivity (relative transport rates) of alkali metal cation transport. However, this structural modification does influence the total transport rate. Increasing the lipophilicity of the complexing agent enhances the rate of cation transport. In changing from 5 to 8 the total transport rate increases by approximately 70%. Since complexing agents derived from 5-8 remain completely in the organic phase, this increase does not arise from enhancement of the chloroform phase concentration of the complexing agent. It is noteworthy that for 8 the amount of cations transported into the receiving phase in 48 hours is 3.6 times higher than the amount of 8 in the system. The selectivity orders for solvent extractions using 5-S [ZO] are the same as those found in the present liquid membrane transport studies. This contrasts sharply with comparison between the results of solvent extractions and
45
liquid membrane transport of alkali metal cations using the parent crown ether carboxylic acid 3, For solvent extractions with 3, the selectivity order in the pH region of 6 -7 is K’ > Na’ > Rb’, Cs’ > Li’ and in the region 8-12 is Na’ > K’ > Rb’ = Cs’ > Li’. The selectivity order at higher pH is in agreement with the membrane transport rate. The reversal in Na’:K’ selectivity in the extraction studies for the two pH regions is attributed to a balance between the crown ether and carboxyl portions of the complexing agent [lo] . Under alkaline conditions, Na’ is selectively extracted, which suggests that the crown ether cavity is the dominant factor. However, under acidic conditions, K+ is extracted better than Na’ which indicates an enhanced importance of the carboxy1 portion of the complexing agent. If this reasoning is correct, the selectivity for Na at all pH values in extractions using 5-8 means that the carboxyl portion of the complexing agents is less important than in 3. For 3 and 5-8, pK values in water are 4.6, 5.3, 5.9, 7.2, and 7.4, respectively [9]. The diminished acidity caused by attaching lipophilic groups to 3 may be responsible for the lesser importance of the carboxyl portion of the complexing agents derived from 5-8. Variations of the relative positions of the polyether and carboxylated groups of the complexing agent in competitive transport Comparison of the transport study results for 9 with those for 5 and 6 (Table 1) reveals an important relationship between the relative positions of the polyether and carboxylate groups of the carrier. All three crown ether carboxylic acids share a common polyether ring structure and possess similar numbers of carbon atoms in the pendant carboxylic aeid groups. However, 5 and 6 are oxyacetic acid derivatives while 9 is a 6 -oxypentanoic acid derivative. Compared with 5 and 6, or even 3, the liquid membrane transport of alkali metal cations by 9 is very poor. Both the transport rate and selectivity suffer markedly. Thus, the proximity of the polyether and carboxylate portions of the complexing agent has important bearing upon the efficiency and selectivity of cation transport. Single cation transport To allow for the comparison of predictions for multi-ion transport behaviour which are extrapolated from single ion transport experiments with the results of competitive transport experiments, single ion transport of alkali metal cations across the liquid membrane by 3 and 8 was examined. Results are presented in Figs. 4 and 5 and in Table 2. Observed transport selectivities from competitive experiments (Table 1) show only slight similarity to the predictions based upon results from single ion transport experiments (Table 2). Others have noted similar discrepancies between single ion and competitive cation transport by neutral macrocyclic carriers [21] . In the single ion transport experiments for Li+ and Cs+ using 8, a noteworthy concentration of trace Na+ impurities was observed. Although the
46
concentration of Na’ impurity in the 0.25 N aqueous Li’ source phase was only 2 X lOA N, the transport rate of Na’ into the aqueous receiving phase [(8.1 f 4.1) X 10” mol/hr] was greater than that of Li’. Similarly for a 4 X 10V5 N impurity of Na’ in a 0.25 N aqueous Cs’ source phase, the transport rate of Na+ into the aqueous receiving phase [(1.8 k 0.4) X lo-’ mol/hr] surpassed that of Cs’. Acknowledgement This research was supported by the Department of Energy (Contract DE-ASOS-80ER-10604) and the Texas Tech University Center for Energy Research (postdoctoral fellowship to JS). References D.K. Schiffer, A. Hochhauser, D.F. Evans and E.L. Cussler, Concentrating solutes with membranes containing carriers, Nature, 250 (1974) 484. E.L. Cussler and D.F. Evans, How to design liquid membrane separations, Separ. Purif. Methods, 3 (1974) 399. A.M. Hochhauser and E.L. Cussler, Concentrating chromium with liquid surfactant membranes, AICHE Symposium Series, 152 (1975) 136. E.S. Matulevicius and N.N. Li, Facilitated transport through liquid membranes, Separ. Purif. Methods, 4 (1975) 73. J. Strzelbicki and W. Charewicz, Separation of copper by liquid surfactant membranes, J. Inorg. Nucl. Chem., 40 (1978) 1415. J. Strzelbicki and W. Charewicz, Separation of cobalt by liquid surfactant membranes, Separ. Sci. Techn., 13 (1978) 141. J. Strzelbicki and W. Charewicz, The liquid surfactant membrane separation of copper, cobalt and nickel from multicomponent aqueous solutions, Hydrometallurgy, 5 (1980) 243. G.S. Heo, R.A. Bartsch, L.L. Schlobohm and J.G. Lee, Preparation of hydroxy crown ethers by reactions of diphenols with epichlorohydrin, J. Org. Chem., 46 (1981) 3574. R.A. Bartsch, G.S. Heo, S.I. Kang, Y. Liu and J. Strzelbicki, Synthesis and acidity of crown ethers with pendant carboxylic acid groups, J. Org. Chem., in press. 10 J. Strzelbicki and R.A. Bartsch, Extraction of alkali metal cations from aqueous solutions by a crown ether carboxylic acid, Anal. Chem., 53 (1981) 40. 11 J. Strzelbicki, G.S. Heo and R.A. Bartsch, Solvent extraction of alkali metal cations from aqueous solutions by crown ether carboxylic acids, Separ. Sci. Technol., in press. 12 J. Strzelbicki and R.A. Bartsch, Extraction of alkaline earth cations from aqueous solutions by crown ether carboxylic acids, Anal. Chem., in press. 13 C.F. Reusch and E.L. Cussler, Selective membrane transport, AICHE J., 19 (1973) 736. 14 K.H. Wong, K. Yagi and J. Smid, Ion transport through liquid membranes facilitated by crown ethers and their polymers, J. Membrane Biol., 18 (1974) 379. 15 Y. Kobuke, K. Hanji, K. Horeguchi, 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. 16 J.J. Christensen, J.D. Lamb, S.R. Izatt, S.E. Starr, G.C. Weed,M.S. Astin, B.D. Stitt and R.M. Izatt, Effect of anion type on rate of facilitated transport of cations across liquid membranes via neutral macrocyclic carriers. J. Amer. Chem. Sot., 100 (1978) 3219.
47 17 W.E. Moore, D. Amman, R. Bissig, E. Pretsch and W. Simon, Cation selectivity of neutral macrocyclic and nonmacrocyclic complexing agents in membranes, in: R.M. Izatt and J.J. Christensen (Eds.), Progress in Macrocyclic Chemistry, Vol. 1, WileyInterscience, New York, 1979, Chap. 1, p. 9. 18 J.J. Christensen, D.J. Eatough and R.M. Izatt, The synthesis and ion binding of synthetic multidentate macrocyclic compounds, Chem. Rev., 74 (1974) 351. 19 R.A. Schwind, T.J. Gill&and and E.L. Cussler, Developing the commercial potential of macrocyclic molecules, in: R.M. Izatt and J.J. Christensen (Eds.), Synthetic Multidentate Macrocyclic Compounds, Academic Press, New York, 1978, Chap. 6, pp. 298-299. 20 J. Strzelbicki and R.A. Bartsch, Solvent extraction of alkali metal cations from aqueous solutions by highly lipophilic crown ether carboxylic acids, Anal. Chem., in press. 21 J,D, Lamb, R.M. Izatt, P.A. Robertson and J.J. Christensen, Highly selective membrane transport of Pb z+ from aqueous metal ion mixtures using macrocyclic carriers, J. Amer. Chem. Sot., 102 (1980) 2452.