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Study of the stability of some supported liquid membranes
j o u r n a l of MEMBRANE SCIENCE ELSEVIER
Journal of Membrane Science 114 (1996) 73-80
Study of the stability of some supported liquid membranes C. Hill, J-F. Dozol *, H. Rouquette, S. Eymard, B. Tournois D.C.C. / D.E.S.D. / S.E.P. / S.E.A.T.N. Centre d' ~tudes de Cadarache, Commisariat h l' Energie Atomique ( C.E.A. ). 13108 Saint-Paul-lez-Durance. France
Received 20 February 1995; accepted 10 November 1995
Abstract In repeated transport experiments through organic supported liquid membranes (SLMs), the decrease of the cation permeability coefficient " P M " , as defined in Danesi's thermodynamic model, has been chosen to characterize membrane stability. A simple linear relation between log PM and the number of runs (i - 1) was derived from the model and verified on several different experimental cases of selective coupled co-transport of metallic cations. SLMs contained either organophosphorous compounds or macrocycles such as classical crown-ethers or calixcrowns. All cases have demonstrated the validity of the proposed relation in as much as the physical characteristics of the SLM remained constant. Unfortunately, by leaching the SLM continuously, the daily renewed contacting aqueous solutions dissolved both the organic diluent and the carrier, leading to SLM instability. Some deviations thus occurred after many runs of repeated transport experiments (i > 20 in some cases). Nevertheless the mathematical relation reported in this paper allowed the apparent partition constant of the carrier to be experimentally estimated, thus permitting the hydrophobicity and efficiency of different carriers to be compared. Keywords: Supported liquid membranes; SLM stability; Crown-ethers; Calixcrowns; CMPO
1. I n t r o d u c t i o n One of the chemical separation processes currently studied for decontamination of both high and medium level activity liquid waste produced by nuclear fuel reprocessing operations is the selective transport of metallic cations through supported liquid membranes (SLMs) containing specific carriers [1]. The major radionuclides to be selectively extracted from concentrated radioactive wastes are mainly f l / y emitters such as Sr and Cs and a emitters such as transuranium elements: Np, Pu, Am, Cm, etc. with
* Corresponding author.
long half lives [2,3]. Many published papers deal with the potential treatment of nuclear liquid waste by means of S L M s containing either linear organophosphorous extractants [4] or macrocycles such as crown-ethers [5] or calixcrown compounds [61. Neither coupled nor counter-transport have yet been applied to any industrial process due to the fact that the membranes are always damaged either by the partitioning of the carriers between the organic diluent and the contacting aqueous solutions [7] or by the leakage of water across the S L M s under high osmotic pressure [8,9]. Nevertheless, basic research performed on flat sheet S L M s and hollow fibers have proved their potential technological application
0376-7388/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved SSDI 0376-7388(95)00306-1
C. Hill et a l . / Journal of Membrane Science 114 (1996) 73-80
74
[10-12]. Among all research, Danesi's studies [13] have focused on the permeation of complexed metallic cations through SLMs and have proposed a simple mathematical model describing the transport mechanisms. Different approaches can be found in literature in order to determine SLM lifetime or stability: measuring permeability coefficients over extended periods of time in the presence and absence of osmotic pressure gradients; measuring physico-chemical properties of the organic phases (interracial tensions, viscosities, contact angles and water solubilities [8]); following the progressive wetting of SLMs by aqueous solutions [9] or even using a non-invasive technique such as impedance spectroscopy [ 14], etc. This paper reports a study of the lifetime of some SLMs containing a variety of complexing carriers potentially applicable to the treatment of nuclear liquid waste and proposes the simplified theoretical model of Danesi to characterize SLM stability.
2. Experimental 2.1. Device, materials and transport experiments description We used the thin flat sheet SLM device described by Stolwijk [15] (Fig. 1).
~,
The internal volumes of the cells varied from 45 to 50 cm 3 and the areas of the membranes from 15 to 16 cm 2 depending on the glass devices manufactured by the Prodilab and Verre and Science companies. The carriers we chose for the decontamination of simulating medium active liquid wastes were: Octyl phenyl N,N'-diisobutyl carbamoyl methylene phosphine oxide (CMPO, compound 1): an organo-phosphorous compound first described by Horwitz in the Truex separation process [16]. CMPO was purchased from XYMAX Inc. and used to transport :52europium. Di(tertiobutyl)cyclohexyl-1 8-crown-6 (dtBuDC18C6, compound 2) from Aldrich and decyl-benzo-21-crown-7 (10-B21 C7, compound 3) from the Chimie Plus laboratory. These two crown-ethers were respectively tested to transport 85strontium and 137cesium [17]. 1,3-Dioctyloxy-2,4-crown-6-calix[4]arene, compound 4, and 1,3-di(2-nitrophenyl)-octyloxy-2,4crown-6-calix[4]arene, compound 5, were synthesized by the teams of Ungaro [18] from the University of Parma and Reinhoudt from the University of Twente. • Doubly-crowned calix[4]arene compounds 6, 7 and 8 (respectively bis-crown-6-, bis-(1,2benzo)crown-6- and bis-(1,2-naphtho)crown-6calix[4]arene) were synthesized by Vicens and Asfari from the university of Strasbourg [19].
I~iii~
~ \2"'
: Polyme~4c suppoi't : (Dr~a+-aicphase 2; : Screw ~: Scre~,v SUl01oo~t E~: I n t e r ~ a l m a n e t
,7-"
--"
Liquicfl~embra~
: E x t e r n a l ma~g~net
7 : Thei~+nostatedjacket Fig. ]. Flat sheet supported liquid membrane device for transport experiments. Feed solution: [NaNO3] ° = 4 tool l- I/[HNO3] ° = l tool l- l (spiked with Is2Eu). SLM: CMPO at various concentrations in 1,2-NPOE; polymeric support: Ce]gard ® 2500 or Accurel ®. Stripping solution: [ s o d i u m citrate] ° = 0 . 2 5 m o l 1- t.
C. Hill et al./ Journal of Membrane Science 114 (1996) 73-80 Table 1 Physical properties of the microporous polymeric supports used in this study Membrane support
Thickness (/xm)
Porosity (vol %)
Average pore diameter (/xm)
Celgard ~ 2500 Accurel *
25 100
45 60
0.04 0.10
Both families of crown-6-calix[4]arene compounds 4 to 8 were used to transport 137cesium. The supported liquid membranes were prepared as follows. Circular pieces of polypropylene microporous polymers from Celgard* or Accurel* (physical properties are listed in Table 1) were soaked for 15 rain under reduced pressure with an organic solution of the carrier in 1,2-nitrophenyloctylether (1,2-NPOE): a hydrophobic diluent of high interfacial tension with water, a high boiling-point and lower surface tension than polypropylene [20]. 1,2-NPOE was synthesized at the Chimie Plus laboratory and used without further purification. All inorganic salts used to prepare the synthetic feed and stripping solutions (sodium nitrate, sodium citrate and nitric acid) were analytical-grade products from Prolabo or Aldrich. Radioactive 85strontium, 137cesium and 152europium were provided by the Amersham company. The transport of these radionuclides was followed by regular measurements of the radioactivity decrease in the feed solutions (initial activity concentrations = 2000 kBq 1-~) by ,/ spectrometry analysis, using a detection chain from Intertechnique, equipped with germanium detectors. The counting was always long enough to ensure a relative error in the activity measurements minor to 5%.
2.2. Permeability Danesi's model
coefficient
determination
in
85Strontium, 137cesium or ~52europium all diffuse through the SLMs because of the nitrate concentration gradients established between the feed solutions ([NaNO3] ° = 4 mol 1- I/[HNO3] ° = 1 mol 1- 1) and the stripping solutions (demineralized water or [sodium citrate] ° = 0.25 mol 1- 1). Making the assumptions extensively described in
75
Danesi's mathematical model of mass transfer through SLMs [13] (permeation process only limited and controlled by diffusion, total back-extraction of the complexed ion-pair at the "SLM/stripping solution" interface, etc.), the logarithmic plot of the decrease of C / C ° versus time allows the permeability coefficient PM of any transported radionuclide to be graphically determined. C represents the activity concentration of the radionuclide in the feed solution at time " t " , and C °, its initial activity concentration. The ion-pair extraction by neutral solvating agent S is usually described by the following equilibrium: -- Kex _
_
Mm+ + mA- + sS ~ MS, A,,
(1)
D M which directly depends on the free carrier concentration in the supported liquid membrane is expressed by relation (2):
[~]eq
riM-- [Mm+loq =
K'e×[ A- ]eq[S]~ q
(2)
where K'ex is the apparent extraction constant of equilibrium (1) taking into account all thermodynamic activity coefficients, [ A- ]eq the anion concentration in the aqueous feed solution at equilibrium, and [~]eq the free carrier concentration at the "feed solution/SLM" interface at equilibrium. In Danesi's simplified model, PM becomes proportional to the distribution coefficient D M of the permeating species, under proper hydrodynamic conditions at the "'feed solution/SLM" interface (high stirring rate): m
noK'ex[ A - ]eq PM =
d()
s
[S]eq
(3)
Making the following assumptions: [MS, A,,]eq <[S]eq ever since all radionuclides spiking the aqueous feed solutions were added at trace level in front of the carrier initial concentration; • no or only very poor extraction of the aqueous salting agent by the carrier; • [ A - ] e q ~ [ A-] 0 if nitrate concentration gradients are properly maintained throughout the experiments ([NO3] 0 = 5 mol 1- l in the feed solutions). Even in the least case, less than 10% of the whole
C. Hill et al./Journal of Membrane Science 114 (1996) 73-80
76
Pe.
(cm
h-')
10 Accurel :0,3M
1
*
.
+,
A
Celgard :0,3M
--*
C e l g a r d :0,1 M
C e l g a r d : 0• , 0 5 M
0,1
.
.
.
0
.
"
+"
-
A
.
.
5
•
•
(i- 1)
.
10
15
, 20
Fig. 2. Logarithmic variations of europium permeability coefficients versus the number of runs in repeated transport experiments with CMPO, compound 1 (s = 3). Feed solution: [NaNO3] ° = 4 rnol 1- L/[HNO3]° = 1 tool I- t (spiked with SSSr). SLM: compound 2: [S] = 0.5 tool 1- ~ in 1,2-NPOE; polymeric support: Celgard ® 2500. Stripping solution: deionized water.
initial nitrate concentration was transported over the period of measurement of the permeability coefficient as long as SLM remained stable; • and no modification of the membrane physicochemical characteristics (thickness, viscosity, etc.);
Ps~ (cm
the mass balance equation applied to the carrier gives for the ith transport experiment:
[~1,
[~10
[ S ] i ÷ l = (1 + R / K p )
- (1
+g/Kp.i+ l)
(4)
h -1)
1 i i !
II
-
-
•
m
•
•
!
( i - 1)
0,1
--
0
,
,
5
l0
-
~
15
,
20
Fig. 3. Logarithmic variation of strontium permeability coefficient versus the number of runs in repeated transport experiments with dtBuDCl8C6, compound 2 (s = l). Feed solution: [NaNO3] ° = 4 mol I- I/[HNO3] ° = 1 mol I- i (spiked with t37Cs). SLM: compounds 3: [5] = 0.5 mol I - i or compounds 4 and 5: [5] = 0.01 tool l - i in 1,2-NPOE. Stripping solution: deionized water. Polymeric support: Celgard ~ 2500.
77
C. Hill et al. / Journal of Membrane Science 114 (1996) 73-80
Pcs
( cm h
-1 )
10
•
•
•
•
•
Q
compound 4
:.
compound 5
compound 3
0,1
(i-1) 0
5
10
15
20
Fig, 4. Logarithmic variations of cesium permeability coefficients versus the number of runs in repeated transport experiments with 10-B21C7, compound 3, and 1,3-dialkoxy-2,4-crown-6-cali×[4]arenes, compounds 4 and 5 ( s = 1). Feed solution: [NaNO3] ° = 4 tool I- ]/[HNO3] ° = 1 tool 1 i (spiked with 137Cs). SLM: compound 3: [5] = 0.5 tool I- 1 or compounds 6, 7 and 8: [5] = 0.01 tool I ~ in 1,2-NPOE. Stripping solution: deionized water. Polymeric support: Celgard ~ 2500.
w h e r e [S]0 is the initial c a r r i e r c o n c e n t r a t i o n introd u c e d in the S L M at the first run, R
,and Kp the a p p a r e n t p a r t i t i o n c o n s t a n t o f the carrier b e t w e e n the S L M a n d b o t h a q u e o u s feed a n d stripp i n g solutions. In r e p e a t e d t r a n s p o r t e x p e r i m e n t s , w h e r e b o t h the a q u e o u s f e e d a n d s t r i p p i n g s o l u t i o n s are r e n e w e d
greed -t- Wstri p
VSLM
Pc, ( cm h-1 ) 10~ i
~ i '1"¢oo. *
1!
i
I
i
compound 8 *
•
6ompound7 c
:'m
compound 3
0,1
•
•
compound 6
6-1)
0,01
0
5
10
15
20
Fig. 5. Logarithmic variations of cesium permeability coefficients versus the number of runs in repeated transport experiments with 10-B21C7, compound 3, and bis-crown-6-calix[4]arenes, compounds 6, 7 and 8 (s = l).
C. Hill et al./Journal of Membrane Science 114 (1996) 73-80
78
every day while the membrane remains the same as in the first run, the decrease of the permeability coefficient PM is deduced from relations (3) and (4) and can be explained simply by the partitioning of the carrier between the membrane and the aqueous solutions. P~) = DoKex [ A-]e~ [~]s
,t0
DoK'.[A-]~
[s];
do
(1 + e / K p ) 's
p(i) M (1 +
R/Kp) (i- l)~
= l o g Pit l-) --( M
log P~)
1)slog(1
+R/Kp)
(5)
P~) and --M p(l) being respectively the metallic cation permeability coefficients in the ith transport experiment and in the first run. The semi-logarithmic variation of PM versus the number of runs is then a way to characterise both the leaching of the SLM by the aqueous solutions (experimental determination of the carrier apparent partition constant Kp) and the membrane stability (slope of the straight line for given " s " and " R " ) .
3. Results and discussion The following diagrams (Figs. 2-5) show the experimental evolutions of the permeability coefficients PM versus ( i - 1) (the number of runs) for the different carriers tested in repeated transport experiments. As expected from the Danesi's simplified model, all reported experimental variations of log PM followed a linear decrease with ( i - 1) as long as the SLM characteristics remained adequate. Unfortunately, some deviations occurred after the last data points represented in Figs. 2-5, although some SLMs still remained apparently stable for a large number of runs (examples of compounds 4, 5, 7 and 8). The deviation of the experimental curves to linear depen-
dence of log PM with (i - 1), as defined in relation (5), presumably arises from changes in the SLM systems. These discrepancies can be explained by: • Specific chemical interactions between the organic diluent, the carrier and the microporous support (formerly supposed inert) probably induced by a radiolytic environment. • A change of the organic solution interfacial tension owing to the aggregation of surface active carriers, generating local emulsions at the "aqueous solutions/SLM" interfaces. These emulsions favour the water transport through the SLM responsible for its instability [8,21-23]. • A decrease of the SLM porosity because of complex precipitation inside the pores. • A change of the organic solution viscosity (due to a drastic decrease of the carrier concentration after partitioning) which alters the organic diffusion coefficient of the complex. • A decrease of membrane thickness due to the leaching of the SLM organic diluent. • An excessive extraction of the aqueous salts, which lowers the available carrier concentration in the SLM, etc. Fig. 2 shows both the influence of microporous support characteristics and of carrier concentration on the stability of the SLM. For the same concentration of CMPO, the decrease of PE~ was weaker in the case of Accurel* support than in the case of Celgard *, even though the permeation kinetics were initially higher in the latter case because of the smaller thickness of Celgard* support. The higher stability of the Accurel * SLM (smaller slope of chart "CMPO:0.3 M (Accurel)") could be explained by the internal physical characteristics of its support (manufacturing process, volumic porosity, thickness, etc., see Table 1). For a given microporous support, the carrier initial concentration in the SLM mainly affected the distribution coefficient of the extracted radionuclide and the organic diffusion coefficient of the complex: the three experimental linear variations are indeed quite parallel for [C---M-P-O]= 0.05 M, 0.1 M or 0.3 M. Fig. 4 demonstrates that crown-6-calix[4]arene compounds 4 and 5, which consist of a calixarene frame in the 1,3-alternate conformation (two opposite phenolic units being flipped upward and bridged with a polyethylene glycol chain [24-26], were more
C. Hill et a l . / Journal of Membrane Science 114 (1996) 73-80 Table 2 Apparent partition constants of compounds 3, 6, 7 and 8 between 1,2-NPOE and aqueous solutions. Experimental determination via linear regression of log Pcs versus (i - 1) Carriers Compound Compound Compound Compound
Apparent partition constants 3 6 7 8
128 100+_ 17 100 29 100 + 800 106 700 _+21 000 295 000 + 23 700
efficient to transport cesium than standard crownethers such as compound 3 [27]. Although decylbenzo-21-crown-7 was initially fifty times as concentrated as crown-6-calix [4] arene compounds 4 and 5, cesium permeation kinetics were much slower in the case of the crown ether because of a smaller organic diffusion coefficient of the complexed ion-pair and also because of a poorer selectivity towards cesium in the presence of sodium. Influence of carrier lipophilicity is shown in Fig. 5. Since cesium complex stoichiometry is identical for compounds 3 to 8 (s = 1 [6]), the slope of the linear decrease of log PM versus ( i - 1) gives an idea of the carrier lipophilicity. For a given value of R (phase volume ratio), the deeper the slope, the smaller the apparent partition constant Kp of the carrier. Danesi's simplified mathematical model is thus a practical tool for qualitatively determining and comparing carrier hydrophobicities. Table 2 sums up the experimentally determined apparent partition constants of carriers 3, 6, 7 and 8 between 1,2-NPOE and the aqueous solutions. As shown in Fig. 5, compound 6 rapidly leaked out of the Celgard ~ support (Pcs < 0.1 cm h- ~ after 15 runs) because of its low apparent partition constant, leading to SLM instability in less than 20 runs. Better stability and efficiency were observed with the more lipophilic benzo and naphtho derivatives of bis-crown-6-calix[4]arene (compounds 7 and 8). Introduction of aryl groups in the frame of the polyethylene glycol chains of the doubly-crowned calix[4]arenes not only increased the carrier preorganization, leading to higher extraction yields and selectivities [6], but also improved the cartier hydrophobicity (smaller slopes).
79
4. Conclusions Making some simplifying assumptions, Danesi's mathematical model of cation permeation through SLMs gave an explanation of the experimentally observed linear logarithmic decrease of permeability coefficients (PM) with repeated transport experiments (where both aqueous solutions were renewed every day while SLM remained the same as in the first run). Although Danesi's simplified mathematical model did not allow a full explanation of the variation of log PM on the whole system lifetime, a simple relation was developed to describe the leaching of the organic membrane solution at the beginning of the system lifetime (up to the 20th run in some particular cases) and demonstrated the deterrent influence of the carrier hydrophilicity on SLM stability. The higher the initial permeability coefficient p(0~ M and partition constant, the longer the stability of the SLM. Taking into account all possible parameters which are involved in the transport process and which contribute to the complexity of the pernleability coefficient PM, as defined by Danesi, would certainly have allowed an explanation of the observed deviations to linearity. This was unfortunately too difficult to model given the lack of the available experimental data.
Acknowledgements This work was partly financed by the European Community within the framework of the shared cost programme on radioactive waste management and storage ( 1990-1994).
References [1] J.F. Dozol, S. Eymard, R. Gambade, G La Rosa and J. Casas i Garcia, Decontamination des concentrats d'6vaporateur en c6sium, strontium et transuraniens, Rapport EUR 13887 FR (1992). [21 J. Lefevre, Dfchets radioactifs, Techniques del l'ingdnieurM6canique et chaleur, B8 II, B 3660-1 5_ B 3661-10 (1990). [3] Revue G6ndrale Nucl6aire, No. 5, September/October (1992).
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C. Hill et aL / Journal of Membrane Science 114 (1996) 73-80
[4] P.R. Danesi, R. Chiarizia, P. Richert and E.P. Horwitz, Separation of actinides and lanthanides from acidic nuclear wastes by supported liquid membranes, Solv. Extr. Ion Exch., 3(1/2) (1985) 111-147. [5] J.F. Dozol, J. Casas and A. Sastre, Stability of flat sheet supported liquid membranes in the transport of radionuclides from reprocessing concentrate solutions, J. Membrane Sci., 82 (1993) 237-246. [6] C. Hill, J.F. Dozol, V. Lamare, H. Rouquette, S. Eymard, B. Tournois, J. Vicens, Z. Asfari, C. Bressot, R. Ungaro and A. Casnati, Nuclear waste treatment by means of supported liquid membranes containing calixcrown compounds, J. Incl. Phenom. Mol. Rec. Chem., 19 (1995) 399-408. [7] T.B. Stolwijk, E.J.R. SudhiSlter and D.N. Reinhoudt, Effect of crown ether lipophilicity on the facilitated transport of guanidinium thiocyanate through an immobilized liquid membrane, J. Am. Chem. Soc., 111 (1989) 6321-6329. [8] P.R. Danesi, L. Reichley-Vinger and P.G. Rickert, Lifetime of supported liquid membranes: The influence of interfacial properties, chemical composition anti water transport on the long term stabilities of the membranes, J. Membrane Sci., 3 I (1987) 117-145. [9] H. Takeuchi, K. Takahashi and W. Goto, Some observations on the stability of supported liquid membranes, J. Membrane Sci., 34 (1987) 19-31; H. Takeuchi and N. Nakano, Progressive wetting of supported liquid membranes by aqueous solutions, J. Membrane Sci., 42 (1989) 183-188. [10] J.F. Dozol, H. Rouquette and B. Tournois, Properties required by extractants and diluents for the decontamination of liquid wastes using supported liquid membranes, 1993 Int. Conf. Nuclear Waste Manag. Environ Remediat., Praguia, Czech Republic, March, 1993. [11] W.F. Van Straaten-Nijenhuis, F. De Jong and D. Reinhoudt, Macrocyclic carriers in supported liquid membranes, Recl. Trav. Chim. Pays Bas, 112 (1993) 317-324. [12] P.R. Danesi, A simplified model for the coupled transport of metal ions through hollow fiber supported liquid membranes, J. Membrane Sci., 20 (1984) 231-248. [13] P.R. Danesi, Separation of metal species by a supported liquid membranes, Sep. Sci. Technol., 19(11/12) (1984/85) 857-894. [14] F.F. Zha, H.G.L. Coster and A.G. Fane, A study of stability of supported liquid membranes by impedance spectroscopy, J. Membrane Sci., 93 (1994) 255-271. [15] T.B. Stolwijk, E.J.R. Sudbblter and D.N. Reinhoudt, Crown ether mediated transport: A kinetic study of potassium perchlorate transport through a supported liquid membrane containing dibenzo 18 crown 6, J. Am. Chem. Soc., 109 (1987) 7042-7047. [16] E.P. Horwitz, D.G. Kalina, H. Diamond, G.F. Vandegrift and W. Shulz, A process for the extraction of the transuranic elements from nitric acid wastes utilizing modified Purex solvent, Solv. Extr. Ion Exch., 3(1) (1985) 75-109.
[17] J. Casas i Garcia, Decontamination selective du cesium et du strontium des concentrats d'6vaporation provenant des usines de retraitement de combustibles nucl6aires avec des Ethers couronne par transfen h travers des membranes liquides suppon6es, Thesis, University of Barcelona, 1991. [18] R. Ungaro, A. Casnati, F. Ugozzoli, A. Pochini, J.F. Dozol, C. Hill and H. Rouquette, 1,3-Dialkoxy caliz[4]arene crown 6 in 1,3-alternate conformation: cesium-selective ligands that exploit cation-arene interactions, Angew. Chem. Int. Ed. Engl., 33(14) (1994) 1506-1509. [19] Z. Asfari, S. Pappalardo and J. Vicens, New preorganized calix[4]arenes. Pan 1: A doubly crowned calix and a doublecalix crow n derived from 4,6,10,1 2,1 6,1 8 , 2 2 , 2 4 , 2 5 , 2 6 , 2 7 , 2 8 - d o d e c a m e t h y l 5,11,17,23-tetrahydroxycalix[4]arene, J. Incl. Phenom. Mol Recognit. Chem., 14(2) (1992) 189-192. [20] P.R. Brown, J.L. Hallman, L.W. Whaley, D.H. Desai, M.J. Pugia and R.A. Bartsch, Competitive proton-coupled, alkali metal cation transport across polymer-supported liquid membranes containing sym-decyl-dibenzo-16-crown-5-oxyacetic acid: variation of the alkyl 2 nitrophenyl ether membrane solvent, J. Membrane Sci., 56 (1991) 195-206. [21] A.M. Neplenbroek, D. Bargeman and C.A. Smolders, Mechanism of supported liquid membrane degradation: emulsion formation, J. Membrane Sci., 67 (1992) 133-148. [22] C. Fabiani, M. Merigiola, D. Scibona and A.M. Costagnola, Degradation of supported liquid membranes under an osmotic pressure gradient, J. Membrane Sci., 30 (1987) 97-104. [23] A.M. Neplenbroek, D. Bargeman and C.A. Smolders, Supported liquid membranes: instability effects, J. Membrane Sci., 67 (1992) 121--132. [24] E. Ghidini, F. Ugozzoli, R. Ungaro, S. Harkema, A. EI-Fadl and D.N. Reinhoudt, Complexation of alkali metal cations by conformationally rigid, stereoisomeric calix[4]arene crown ether: a quantitative evaluation of preorganization, J. Am. Chem. Soc., 112 (1990) 6979-6985. [25] W. Nijenhuis, E. Buitenhuis, F. De Jong, E. SudhSIter and D.N. Reinhoudt, Calix-crowns as selective potassium cation carriers in supported liquid membranes, J. Am. Chem. Soc., 113 (1991) 7963-7968. [26] P.J. Dijkstra, J. Brunink, K. Bugge, D.N. Reinhoudt, S. Harkema, R. Ungaro, F. Ugozzoli and E. Ghidini, Kinetically stable complexes of alkali cations with rigidified calix[4]arenes: synthesis, X-ray structures and complexation of calix crowns and calix spherands, J. Am. Chem. Soc., 111 (1989) 7567-7575. [27] W.J. McDowell and G.N. Case, Selective extraction of cesium from acidic nitrate solutions with didodecylnaphtalene sulfonic acid synergized with bis(tert-butyl benzo)-21crown-7, Anal. Chem., 64 (1992) 3013-3017.
Journal of Membrane Science 114 (1996) 73-80
Study of the stability of some supported liquid membranes C. Hill, J-F. Dozol *, H. Rouquette, S. Eymard, B. Tournois D.C.C. / D.E.S.D. / S.E.P. / S.E.A.T.N. Centre d' ~tudes de Cadarache, Commisariat h l' Energie Atomique ( C.E.A. ). 13108 Saint-Paul-lez-Durance. France
Received 20 February 1995; accepted 10 November 1995
Abstract In repeated transport experiments through organic supported liquid membranes (SLMs), the decrease of the cation permeability coefficient " P M " , as defined in Danesi's thermodynamic model, has been chosen to characterize membrane stability. A simple linear relation between log PM and the number of runs (i - 1) was derived from the model and verified on several different experimental cases of selective coupled co-transport of metallic cations. SLMs contained either organophosphorous compounds or macrocycles such as classical crown-ethers or calixcrowns. All cases have demonstrated the validity of the proposed relation in as much as the physical characteristics of the SLM remained constant. Unfortunately, by leaching the SLM continuously, the daily renewed contacting aqueous solutions dissolved both the organic diluent and the carrier, leading to SLM instability. Some deviations thus occurred after many runs of repeated transport experiments (i > 20 in some cases). Nevertheless the mathematical relation reported in this paper allowed the apparent partition constant of the carrier to be experimentally estimated, thus permitting the hydrophobicity and efficiency of different carriers to be compared. Keywords: Supported liquid membranes; SLM stability; Crown-ethers; Calixcrowns; CMPO
1. I n t r o d u c t i o n One of the chemical separation processes currently studied for decontamination of both high and medium level activity liquid waste produced by nuclear fuel reprocessing operations is the selective transport of metallic cations through supported liquid membranes (SLMs) containing specific carriers [1]. The major radionuclides to be selectively extracted from concentrated radioactive wastes are mainly f l / y emitters such as Sr and Cs and a emitters such as transuranium elements: Np, Pu, Am, Cm, etc. with
* Corresponding author.
long half lives [2,3]. Many published papers deal with the potential treatment of nuclear liquid waste by means of S L M s containing either linear organophosphorous extractants [4] or macrocycles such as crown-ethers [5] or calixcrown compounds [61. Neither coupled nor counter-transport have yet been applied to any industrial process due to the fact that the membranes are always damaged either by the partitioning of the carriers between the organic diluent and the contacting aqueous solutions [7] or by the leakage of water across the S L M s under high osmotic pressure [8,9]. Nevertheless, basic research performed on flat sheet S L M s and hollow fibers have proved their potential technological application
0376-7388/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved SSDI 0376-7388(95)00306-1
C. Hill et a l . / Journal of Membrane Science 114 (1996) 73-80
74
[10-12]. Among all research, Danesi's studies [13] have focused on the permeation of complexed metallic cations through SLMs and have proposed a simple mathematical model describing the transport mechanisms. Different approaches can be found in literature in order to determine SLM lifetime or stability: measuring permeability coefficients over extended periods of time in the presence and absence of osmotic pressure gradients; measuring physico-chemical properties of the organic phases (interracial tensions, viscosities, contact angles and water solubilities [8]); following the progressive wetting of SLMs by aqueous solutions [9] or even using a non-invasive technique such as impedance spectroscopy [ 14], etc. This paper reports a study of the lifetime of some SLMs containing a variety of complexing carriers potentially applicable to the treatment of nuclear liquid waste and proposes the simplified theoretical model of Danesi to characterize SLM stability.
2. Experimental 2.1. Device, materials and transport experiments description We used the thin flat sheet SLM device described by Stolwijk [15] (Fig. 1).
~,
The internal volumes of the cells varied from 45 to 50 cm 3 and the areas of the membranes from 15 to 16 cm 2 depending on the glass devices manufactured by the Prodilab and Verre and Science companies. The carriers we chose for the decontamination of simulating medium active liquid wastes were: Octyl phenyl N,N'-diisobutyl carbamoyl methylene phosphine oxide (CMPO, compound 1): an organo-phosphorous compound first described by Horwitz in the Truex separation process [16]. CMPO was purchased from XYMAX Inc. and used to transport :52europium. Di(tertiobutyl)cyclohexyl-1 8-crown-6 (dtBuDC18C6, compound 2) from Aldrich and decyl-benzo-21-crown-7 (10-B21 C7, compound 3) from the Chimie Plus laboratory. These two crown-ethers were respectively tested to transport 85strontium and 137cesium [17]. 1,3-Dioctyloxy-2,4-crown-6-calix[4]arene, compound 4, and 1,3-di(2-nitrophenyl)-octyloxy-2,4crown-6-calix[4]arene, compound 5, were synthesized by the teams of Ungaro [18] from the University of Parma and Reinhoudt from the University of Twente. • Doubly-crowned calix[4]arene compounds 6, 7 and 8 (respectively bis-crown-6-, bis-(1,2benzo)crown-6- and bis-(1,2-naphtho)crown-6calix[4]arene) were synthesized by Vicens and Asfari from the university of Strasbourg [19].
I~iii~
~ \2"'
: Polyme~4c suppoi't : (Dr~a+-aicphase 2; : Screw ~: Scre~,v SUl01oo~t E~: I n t e r ~ a l m a n e t
,7-"
--"
Liquicfl~embra~
: E x t e r n a l ma~g~net
7 : Thei~+nostatedjacket Fig. ]. Flat sheet supported liquid membrane device for transport experiments. Feed solution: [NaNO3] ° = 4 tool l- I/[HNO3] ° = l tool l- l (spiked with Is2Eu). SLM: CMPO at various concentrations in 1,2-NPOE; polymeric support: Ce]gard ® 2500 or Accurel ®. Stripping solution: [ s o d i u m citrate] ° = 0 . 2 5 m o l 1- t.
C. Hill et al./ Journal of Membrane Science 114 (1996) 73-80 Table 1 Physical properties of the microporous polymeric supports used in this study Membrane support
Thickness (/xm)
Porosity (vol %)
Average pore diameter (/xm)
Celgard ~ 2500 Accurel *
25 100
45 60
0.04 0.10
Both families of crown-6-calix[4]arene compounds 4 to 8 were used to transport 137cesium. The supported liquid membranes were prepared as follows. Circular pieces of polypropylene microporous polymers from Celgard* or Accurel* (physical properties are listed in Table 1) were soaked for 15 rain under reduced pressure with an organic solution of the carrier in 1,2-nitrophenyloctylether (1,2-NPOE): a hydrophobic diluent of high interfacial tension with water, a high boiling-point and lower surface tension than polypropylene [20]. 1,2-NPOE was synthesized at the Chimie Plus laboratory and used without further purification. All inorganic salts used to prepare the synthetic feed and stripping solutions (sodium nitrate, sodium citrate and nitric acid) were analytical-grade products from Prolabo or Aldrich. Radioactive 85strontium, 137cesium and 152europium were provided by the Amersham company. The transport of these radionuclides was followed by regular measurements of the radioactivity decrease in the feed solutions (initial activity concentrations = 2000 kBq 1-~) by ,/ spectrometry analysis, using a detection chain from Intertechnique, equipped with germanium detectors. The counting was always long enough to ensure a relative error in the activity measurements minor to 5%.
2.2. Permeability Danesi's model
coefficient
determination
in
85Strontium, 137cesium or ~52europium all diffuse through the SLMs because of the nitrate concentration gradients established between the feed solutions ([NaNO3] ° = 4 mol 1- I/[HNO3] ° = 1 mol 1- 1) and the stripping solutions (demineralized water or [sodium citrate] ° = 0.25 mol 1- 1). Making the assumptions extensively described in
75
Danesi's mathematical model of mass transfer through SLMs [13] (permeation process only limited and controlled by diffusion, total back-extraction of the complexed ion-pair at the "SLM/stripping solution" interface, etc.), the logarithmic plot of the decrease of C / C ° versus time allows the permeability coefficient PM of any transported radionuclide to be graphically determined. C represents the activity concentration of the radionuclide in the feed solution at time " t " , and C °, its initial activity concentration. The ion-pair extraction by neutral solvating agent S is usually described by the following equilibrium: -- Kex _
_
Mm+ + mA- + sS ~ MS, A,,
(1)
D M which directly depends on the free carrier concentration in the supported liquid membrane is expressed by relation (2):
[~]eq
riM-- [Mm+loq =
K'e×[ A- ]eq[S]~ q
(2)
where K'ex is the apparent extraction constant of equilibrium (1) taking into account all thermodynamic activity coefficients, [ A- ]eq the anion concentration in the aqueous feed solution at equilibrium, and [~]eq the free carrier concentration at the "feed solution/SLM" interface at equilibrium. In Danesi's simplified model, PM becomes proportional to the distribution coefficient D M of the permeating species, under proper hydrodynamic conditions at the "'feed solution/SLM" interface (high stirring rate): m
noK'ex[ A - ]eq PM =
d()
s
[S]eq
(3)
Making the following assumptions: [MS, A,,]eq <[S]eq ever since all radionuclides spiking the aqueous feed solutions were added at trace level in front of the carrier initial concentration; • no or only very poor extraction of the aqueous salting agent by the carrier; • [ A - ] e q ~ [ A-] 0 if nitrate concentration gradients are properly maintained throughout the experiments ([NO3] 0 = 5 mol 1- l in the feed solutions). Even in the least case, less than 10% of the whole
C. Hill et al./Journal of Membrane Science 114 (1996) 73-80
76
Pe.
(cm
h-')
10 Accurel :0,3M
1
*
.
+,
A
Celgard :0,3M
--*
C e l g a r d :0,1 M
C e l g a r d : 0• , 0 5 M
0,1
.
.
.
0
.
"
+"
-
A
.
.
5
•
•
(i- 1)
.
10
15
, 20
Fig. 2. Logarithmic variations of europium permeability coefficients versus the number of runs in repeated transport experiments with CMPO, compound 1 (s = 3). Feed solution: [NaNO3] ° = 4 rnol 1- L/[HNO3]° = 1 tool I- t (spiked with SSSr). SLM: compound 2: [S] = 0.5 tool 1- ~ in 1,2-NPOE; polymeric support: Celgard ® 2500. Stripping solution: deionized water.
initial nitrate concentration was transported over the period of measurement of the permeability coefficient as long as SLM remained stable; • and no modification of the membrane physicochemical characteristics (thickness, viscosity, etc.);
Ps~ (cm
the mass balance equation applied to the carrier gives for the ith transport experiment:
[~1,
[~10
[ S ] i ÷ l = (1 + R / K p )
- (1
+g/Kp.i+ l)
(4)
h -1)
1 i i !
II
-
-
•
m
•
•
!
( i - 1)
0,1
--
0
,
,
5
l0
-
~
15
,
20
Fig. 3. Logarithmic variation of strontium permeability coefficient versus the number of runs in repeated transport experiments with dtBuDCl8C6, compound 2 (s = l). Feed solution: [NaNO3] ° = 4 mol I- I/[HNO3] ° = 1 mol I- i (spiked with t37Cs). SLM: compounds 3: [5] = 0.5 mol I - i or compounds 4 and 5: [5] = 0.01 tool l - i in 1,2-NPOE. Stripping solution: deionized water. Polymeric support: Celgard ~ 2500.
77
C. Hill et al. / Journal of Membrane Science 114 (1996) 73-80
Pcs
( cm h
-1 )
10
•
•
•
•
•
Q
compound 4
:.
compound 5
compound 3
0,1
(i-1) 0
5
10
15
20
Fig, 4. Logarithmic variations of cesium permeability coefficients versus the number of runs in repeated transport experiments with 10-B21C7, compound 3, and 1,3-dialkoxy-2,4-crown-6-cali×[4]arenes, compounds 4 and 5 ( s = 1). Feed solution: [NaNO3] ° = 4 tool I- ]/[HNO3] ° = 1 tool 1 i (spiked with 137Cs). SLM: compound 3: [5] = 0.5 tool I- 1 or compounds 6, 7 and 8: [5] = 0.01 tool I ~ in 1,2-NPOE. Stripping solution: deionized water. Polymeric support: Celgard ~ 2500.
w h e r e [S]0 is the initial c a r r i e r c o n c e n t r a t i o n introd u c e d in the S L M at the first run, R
,and Kp the a p p a r e n t p a r t i t i o n c o n s t a n t o f the carrier b e t w e e n the S L M a n d b o t h a q u e o u s feed a n d stripp i n g solutions. In r e p e a t e d t r a n s p o r t e x p e r i m e n t s , w h e r e b o t h the a q u e o u s f e e d a n d s t r i p p i n g s o l u t i o n s are r e n e w e d
greed -t- Wstri p
VSLM
Pc, ( cm h-1 ) 10~ i
~ i '1"¢oo. *
1!
i
I
i
compound 8 *
•
6ompound7 c
:'m
compound 3
0,1
•
•
compound 6
6-1)
0,01
0
5
10
15
20
Fig. 5. Logarithmic variations of cesium permeability coefficients versus the number of runs in repeated transport experiments with 10-B21C7, compound 3, and bis-crown-6-calix[4]arenes, compounds 6, 7 and 8 (s = l).
C. Hill et al./Journal of Membrane Science 114 (1996) 73-80
78
every day while the membrane remains the same as in the first run, the decrease of the permeability coefficient PM is deduced from relations (3) and (4) and can be explained simply by the partitioning of the carrier between the membrane and the aqueous solutions. P~) = DoKex [ A-]e~ [~]s
,t0
DoK'.[A-]~
[s];
do
(1 + e / K p ) 's
p(i) M (1 +
R/Kp) (i- l)~
= l o g Pit l-) --( M
log P~)
1)slog(1
+R/Kp)
(5)
P~) and --M p(l) being respectively the metallic cation permeability coefficients in the ith transport experiment and in the first run. The semi-logarithmic variation of PM versus the number of runs is then a way to characterise both the leaching of the SLM by the aqueous solutions (experimental determination of the carrier apparent partition constant Kp) and the membrane stability (slope of the straight line for given " s " and " R " ) .
3. Results and discussion The following diagrams (Figs. 2-5) show the experimental evolutions of the permeability coefficients PM versus ( i - 1) (the number of runs) for the different carriers tested in repeated transport experiments. As expected from the Danesi's simplified model, all reported experimental variations of log PM followed a linear decrease with ( i - 1) as long as the SLM characteristics remained adequate. Unfortunately, some deviations occurred after the last data points represented in Figs. 2-5, although some SLMs still remained apparently stable for a large number of runs (examples of compounds 4, 5, 7 and 8). The deviation of the experimental curves to linear depen-
dence of log PM with (i - 1), as defined in relation (5), presumably arises from changes in the SLM systems. These discrepancies can be explained by: • Specific chemical interactions between the organic diluent, the carrier and the microporous support (formerly supposed inert) probably induced by a radiolytic environment. • A change of the organic solution interfacial tension owing to the aggregation of surface active carriers, generating local emulsions at the "aqueous solutions/SLM" interfaces. These emulsions favour the water transport through the SLM responsible for its instability [8,21-23]. • A decrease of the SLM porosity because of complex precipitation inside the pores. • A change of the organic solution viscosity (due to a drastic decrease of the carrier concentration after partitioning) which alters the organic diffusion coefficient of the complex. • A decrease of membrane thickness due to the leaching of the SLM organic diluent. • An excessive extraction of the aqueous salts, which lowers the available carrier concentration in the SLM, etc. Fig. 2 shows both the influence of microporous support characteristics and of carrier concentration on the stability of the SLM. For the same concentration of CMPO, the decrease of PE~ was weaker in the case of Accurel* support than in the case of Celgard *, even though the permeation kinetics were initially higher in the latter case because of the smaller thickness of Celgard* support. The higher stability of the Accurel * SLM (smaller slope of chart "CMPO:0.3 M (Accurel)") could be explained by the internal physical characteristics of its support (manufacturing process, volumic porosity, thickness, etc., see Table 1). For a given microporous support, the carrier initial concentration in the SLM mainly affected the distribution coefficient of the extracted radionuclide and the organic diffusion coefficient of the complex: the three experimental linear variations are indeed quite parallel for [C---M-P-O]= 0.05 M, 0.1 M or 0.3 M. Fig. 4 demonstrates that crown-6-calix[4]arene compounds 4 and 5, which consist of a calixarene frame in the 1,3-alternate conformation (two opposite phenolic units being flipped upward and bridged with a polyethylene glycol chain [24-26], were more
C. Hill et a l . / Journal of Membrane Science 114 (1996) 73-80 Table 2 Apparent partition constants of compounds 3, 6, 7 and 8 between 1,2-NPOE and aqueous solutions. Experimental determination via linear regression of log Pcs versus (i - 1) Carriers Compound Compound Compound Compound
Apparent partition constants 3 6 7 8
128 100+_ 17 100 29 100 + 800 106 700 _+21 000 295 000 + 23 700
efficient to transport cesium than standard crownethers such as compound 3 [27]. Although decylbenzo-21-crown-7 was initially fifty times as concentrated as crown-6-calix [4] arene compounds 4 and 5, cesium permeation kinetics were much slower in the case of the crown ether because of a smaller organic diffusion coefficient of the complexed ion-pair and also because of a poorer selectivity towards cesium in the presence of sodium. Influence of carrier lipophilicity is shown in Fig. 5. Since cesium complex stoichiometry is identical for compounds 3 to 8 (s = 1 [6]), the slope of the linear decrease of log PM versus ( i - 1) gives an idea of the carrier lipophilicity. For a given value of R (phase volume ratio), the deeper the slope, the smaller the apparent partition constant Kp of the carrier. Danesi's simplified mathematical model is thus a practical tool for qualitatively determining and comparing carrier hydrophobicities. Table 2 sums up the experimentally determined apparent partition constants of carriers 3, 6, 7 and 8 between 1,2-NPOE and the aqueous solutions. As shown in Fig. 5, compound 6 rapidly leaked out of the Celgard ~ support (Pcs < 0.1 cm h- ~ after 15 runs) because of its low apparent partition constant, leading to SLM instability in less than 20 runs. Better stability and efficiency were observed with the more lipophilic benzo and naphtho derivatives of bis-crown-6-calix[4]arene (compounds 7 and 8). Introduction of aryl groups in the frame of the polyethylene glycol chains of the doubly-crowned calix[4]arenes not only increased the carrier preorganization, leading to higher extraction yields and selectivities [6], but also improved the cartier hydrophobicity (smaller slopes).
79
4. Conclusions Making some simplifying assumptions, Danesi's mathematical model of cation permeation through SLMs gave an explanation of the experimentally observed linear logarithmic decrease of permeability coefficients (PM) with repeated transport experiments (where both aqueous solutions were renewed every day while SLM remained the same as in the first run). Although Danesi's simplified mathematical model did not allow a full explanation of the variation of log PM on the whole system lifetime, a simple relation was developed to describe the leaching of the organic membrane solution at the beginning of the system lifetime (up to the 20th run in some particular cases) and demonstrated the deterrent influence of the carrier hydrophilicity on SLM stability. The higher the initial permeability coefficient p(0~ M and partition constant, the longer the stability of the SLM. Taking into account all possible parameters which are involved in the transport process and which contribute to the complexity of the pernleability coefficient PM, as defined by Danesi, would certainly have allowed an explanation of the observed deviations to linearity. This was unfortunately too difficult to model given the lack of the available experimental data.
Acknowledgements This work was partly financed by the European Community within the framework of the shared cost programme on radioactive waste management and storage ( 1990-1994).
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C. Hill et aL / Journal of Membrane Science 114 (1996) 73-80
[4] P.R. Danesi, R. Chiarizia, P. Richert and E.P. Horwitz, Separation of actinides and lanthanides from acidic nuclear wastes by supported liquid membranes, Solv. Extr. Ion Exch., 3(1/2) (1985) 111-147. [5] J.F. Dozol, J. Casas and A. Sastre, Stability of flat sheet supported liquid membranes in the transport of radionuclides from reprocessing concentrate solutions, J. Membrane Sci., 82 (1993) 237-246. [6] C. Hill, J.F. Dozol, V. Lamare, H. Rouquette, S. Eymard, B. Tournois, J. Vicens, Z. Asfari, C. Bressot, R. Ungaro and A. Casnati, Nuclear waste treatment by means of supported liquid membranes containing calixcrown compounds, J. Incl. Phenom. Mol. Rec. Chem., 19 (1995) 399-408. [7] T.B. Stolwijk, E.J.R. SudhiSlter and D.N. Reinhoudt, Effect of crown ether lipophilicity on the facilitated transport of guanidinium thiocyanate through an immobilized liquid membrane, J. Am. Chem. Soc., 111 (1989) 6321-6329. [8] P.R. Danesi, L. Reichley-Vinger and P.G. Rickert, Lifetime of supported liquid membranes: The influence of interfacial properties, chemical composition anti water transport on the long term stabilities of the membranes, J. Membrane Sci., 3 I (1987) 117-145. [9] H. Takeuchi, K. Takahashi and W. Goto, Some observations on the stability of supported liquid membranes, J. Membrane Sci., 34 (1987) 19-31; H. Takeuchi and N. Nakano, Progressive wetting of supported liquid membranes by aqueous solutions, J. Membrane Sci., 42 (1989) 183-188. [10] J.F. Dozol, H. Rouquette and B. Tournois, Properties required by extractants and diluents for the decontamination of liquid wastes using supported liquid membranes, 1993 Int. Conf. Nuclear Waste Manag. Environ Remediat., Praguia, Czech Republic, March, 1993. [11] W.F. Van Straaten-Nijenhuis, F. De Jong and D. Reinhoudt, Macrocyclic carriers in supported liquid membranes, Recl. Trav. Chim. Pays Bas, 112 (1993) 317-324. [12] P.R. Danesi, A simplified model for the coupled transport of metal ions through hollow fiber supported liquid membranes, J. Membrane Sci., 20 (1984) 231-248. [13] P.R. Danesi, Separation of metal species by a supported liquid membranes, Sep. Sci. Technol., 19(11/12) (1984/85) 857-894. [14] F.F. Zha, H.G.L. Coster and A.G. Fane, A study of stability of supported liquid membranes by impedance spectroscopy, J. Membrane Sci., 93 (1994) 255-271. [15] T.B. Stolwijk, E.J.R. Sudbblter and D.N. Reinhoudt, Crown ether mediated transport: A kinetic study of potassium perchlorate transport through a supported liquid membrane containing dibenzo 18 crown 6, J. Am. Chem. Soc., 109 (1987) 7042-7047. [16] E.P. Horwitz, D.G. Kalina, H. Diamond, G.F. Vandegrift and W. Shulz, A process for the extraction of the transuranic elements from nitric acid wastes utilizing modified Purex solvent, Solv. Extr. Ion Exch., 3(1) (1985) 75-109.
[17] J. Casas i Garcia, Decontamination selective du cesium et du strontium des concentrats d'6vaporation provenant des usines de retraitement de combustibles nucl6aires avec des Ethers couronne par transfen h travers des membranes liquides suppon6es, Thesis, University of Barcelona, 1991. [18] R. Ungaro, A. Casnati, F. Ugozzoli, A. Pochini, J.F. Dozol, C. Hill and H. Rouquette, 1,3-Dialkoxy caliz[4]arene crown 6 in 1,3-alternate conformation: cesium-selective ligands that exploit cation-arene interactions, Angew. Chem. Int. Ed. Engl., 33(14) (1994) 1506-1509. [19] Z. Asfari, S. Pappalardo and J. Vicens, New preorganized calix[4]arenes. Pan 1: A doubly crowned calix and a doublecalix crow n derived from 4,6,10,1 2,1 6,1 8 , 2 2 , 2 4 , 2 5 , 2 6 , 2 7 , 2 8 - d o d e c a m e t h y l 5,11,17,23-tetrahydroxycalix[4]arene, J. Incl. Phenom. Mol Recognit. Chem., 14(2) (1992) 189-192. [20] P.R. Brown, J.L. Hallman, L.W. Whaley, D.H. Desai, M.J. Pugia and R.A. Bartsch, Competitive proton-coupled, alkali metal cation transport across polymer-supported liquid membranes containing sym-decyl-dibenzo-16-crown-5-oxyacetic acid: variation of the alkyl 2 nitrophenyl ether membrane solvent, J. Membrane Sci., 56 (1991) 195-206. [21] A.M. Neplenbroek, D. Bargeman and C.A. Smolders, Mechanism of supported liquid membrane degradation: emulsion formation, J. Membrane Sci., 67 (1992) 133-148. [22] C. Fabiani, M. Merigiola, D. Scibona and A.M. Costagnola, Degradation of supported liquid membranes under an osmotic pressure gradient, J. Membrane Sci., 30 (1987) 97-104. [23] A.M. Neplenbroek, D. Bargeman and C.A. Smolders, Supported liquid membranes: instability effects, J. Membrane Sci., 67 (1992) 121--132. [24] E. Ghidini, F. Ugozzoli, R. Ungaro, S. Harkema, A. EI-Fadl and D.N. Reinhoudt, Complexation of alkali metal cations by conformationally rigid, stereoisomeric calix[4]arene crown ether: a quantitative evaluation of preorganization, J. Am. Chem. Soc., 112 (1990) 6979-6985. [25] W. Nijenhuis, E. Buitenhuis, F. De Jong, E. SudhSIter and D.N. Reinhoudt, Calix-crowns as selective potassium cation carriers in supported liquid membranes, J. Am. Chem. Soc., 113 (1991) 7963-7968. [26] P.J. Dijkstra, J. Brunink, K. Bugge, D.N. Reinhoudt, S. Harkema, R. Ungaro, F. Ugozzoli and E. Ghidini, Kinetically stable complexes of alkali cations with rigidified calix[4]arenes: synthesis, X-ray structures and complexation of calix crowns and calix spherands, J. Am. Chem. Soc., 111 (1989) 7567-7575. [27] W.J. McDowell and G.N. Case, Selective extraction of cesium from acidic nitrate solutions with didodecylnaphtalene sulfonic acid synergized with bis(tert-butyl benzo)-21crown-7, Anal. Chem., 64 (1992) 3013-3017.