DOI: 10.1002/ijch.201300001

Highly Selective Li+ Ion Transport by PorousMolybdenum-Oxide Keplerate-Type NanocapsulesIntegrated in a Supported Liquid MembraneArnaud Gilles,[a] Simona Mihai,[a] Gihane Nasr,[a] Eugene Mahon,[a] Somenath Garai,[b] Achim M�ller,[b]

and Mihail Barboiu*[a]

Introduction

The transport of ions across cell membranes is a funda-mental biological process which depends on the uniqueproperties of hydrophilic domains of biomolecules, specif-ically on the functional complexity of the related mem-brane proteins.[1] Investigations of transport processesthrough model membrane systems, like those of theliquid type, can lead to information about basic propertiesof (novel) ion-receptor and related carrier systems.[2] Thefunctionality of cation transporting membrane proteins isrelated to the dynamic constitutional behaviors of theirbinding sites collectively contributing to the selectivecation transport.[1] To mimic related highly functional de-vices, one has to develop artificial constitutional dynamicmembrane systems that form effective patterns by relatedcollective self-assembly of carrier entities, leading to effi-cient cation translocation through the membrane barriers.Carrier molecules or ion-channel-type systems are usuallyused to facilitate the transport through lipophilic mem-brane barriers.[2] Recent publications showed, for in-stance, that nanosized systems, like Cu24(5-dodecyloxy-benzene-1,3-dicarboxylate)24, MOP-18,[3a] and pyrogallol-arene capsules[3b] can exhibit ion-channel activity in lipidbilayers.

In this context, we consider unique spherical porousanionic molybdenum-oxide based capsules.[4–6] Important-ly, their overall charges, as well as interiors, can be specif-ically, i.e. uniquely, tuned, allowing a controlled (step-wise) encapsulation of cations, while, for instance, theircrown-ether-type pores can regulate the ion-flux by clos-ing and opening them with plugs, in a supramolecular

fashion.[7,8] Moreover, earlier molecular dynamics simula-tions shed light on the process of embedding the nano-capsules�with their unprecedented molecular-scale filterproperties�into lipid bilayer membranes.[9]

Results and Discussion

Here we consider the porous, molybdenum-oxide basedanionic capsules 1a, 2a, and 3a of the compounds:(NH4)42[{(MoVI)MoVI

5O21(H2O)6}12{MoV2O4(CH3COO)}30]

· ca. [10CH3COONH4 +300H2O]� (NH4)42·1a· crystal ingredients �1[4b]

[MoVI72FeIII

30O252(CH3COO)12{Mo2O7(H2O)}2{H2Mo2O8-(H2O)}(H2O)91]· ca. 150H2O�2a·crystal ingredients�2[4c]

(NH4)72-n[{(H2O)81-n + (NH4)n}�{(MoVI)MoVI5O21(H2O)6}12-

{MoV2O4(SO4)}30]· ca. 200H2O � (NH4)72-n·3a

· crystal ingredients � 3[7,8]

The related lipophilic hybrid-type compounds,prepared with dimethyl-dioctadecylammonium-bromide

Abstract : Porous Keplerate-type molybdenum-oxide nano-capsules – encapsulated into cationic surfactants – act astransporting systems for alkali cations through supportedliquid membranes. The transport is based on the ability ofthe nanocapsules containing water molecules inside theircavities to attract and release the cations. This results in

specific nanoscaled translocation pathways, based on corre-sponding dynamic diffusional domains within the liquidbulk membrane phase, due to the self-assembly of the cap-sules. Li+ cations are preferentially extracted and transport-ed, thus allowing separation from Na+ and K+ cations,which are not transported to the receiving phase.

Keywords: membranes · nanodevices · self-assembly · structure-property relationship · supramolecular materials

[a] A. Gilles, S. Mihai, G. Nasr, E. Mahon, M. BarboiuAdaptive Supramolecular Nanosystems GroupInstitut Europ�en des Membranes -ENSCM/UM2/CNRS 5635,IEM/UM II, CC 047, Place Eug�ne Bataillon, 34095, Montpellier,Cedex 5 (France)fax: +33-467-14-91-19e-mail: barboiu@iemm.univ-montp2.fr

[b] S. Garai, A. M�llerUniversit�t Bielefeld, Fakult�t f�r ChemiePostfach 10 0131, 33501 Bielefeld (Germany)

102 � 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Isr. J. Chem. 2013, 53, 102 – 107

Full Paper M. Barboiu, et al.

DODABr,[10] were used to get information about thecompetitive Li+, Na+ , and K+ ion transport throughliquid membranes, which contain {DODA40(NH4)21a},[10a,b]

{DODAx2a} (x~7),[10c] and {DODA50(NH4)223a} (thelatter prepared for the first time here) as cationic trans-porters.

The mentioned hybrids were synthesized according tothe previously published procedures by mixing aqueoussolutions of 1, 2, and 3 with solutions of DODABr inchloroform (see Experimental Section). These hybridsform at the water/CHCl3 interface and are extracted inthe organic phase, while their isolation takes place afterevaporation of the solvent. FTIR spectra show the char-acteristic vibrational bands of 1a, 2a, and 3a in the regionbelow 1000 cm�1, as well as those of the DODA cations(e.g. nCH2as =2850 cm�1, nCH2sym =2920 cm�1).[10] The vibra-tional bands of 1, 2, and 3 remain practically unshifted,but are sharper, as expected after their encapsulation. Im-portantly, the discrete species, like {DODA40(NH4)21a}, inthe membrane are probably in equilibrium with the as-semblies (Figure 1). (The equilibrium strongly dependson the concentration of {DODA40(NH4)21a}.) It was pre-

viously shown that the large self-assembled architectures(Figure 1) are expected to be energetically favorable insolution, based on the optimization of hydrophobic inter-actions between the DODA-type alkyl chains, as well aselectrostatic interactions between the anionic capsulesand cationic DODA head groups. Furthermore, it wasshown that cations like Li+ ions can enter the nanocap-sule cavities containing H2O molecules and show specificlocal interactions with the internal SO4

2� ligands (see e.g.ref. [4a]). Now it turns out that the capsules can facilitatecation transport and that the mentioned hybrid speciescan be used for selective membrane transport studies.

The goal of the present work was to study the lipophilichybrids {DODA40(NH4)21a}, {DODAx2a}, and {DODA50-(NH4)223a}, embedded in liquid membranes, as ion trans-porter systems, in order to evaluate their capacity for spe-cific alkali cation separations in solution (Figures 2, 3,

and 4). For this purpose, the hybrids were solubilized ino-nitrophenyl-n-octylether (NPOE), and supported liquidmembranes (SLMs) were prepared from these solutionsand hydrophobic Accurel

�

membranes with high porosity.The stagnant diffusional layer of the SLM is defined as

the thickness of the solid Accurel�

membrane film(60 mm), which separates the aqueous feed-type solutions(0.1 M LiCl, NaCl, and KCl) from the receiving-typecompartments (pure water). The concentrations of theLi+, Na+ , and K+ cations have been determined in thefeed and receiving phase at different time intervals (seeExperimental Section). The obtained profile concentra-tion, as a function of time (Figure 2), sheds light on theprocesses at the feed/membrane and membrane/receivingphase interfaces, as well as on the different transport be-haviors of the membrane systems employed.

Figure 1. A possible example for the self-association behavior ofsolutions of {DODA40(NH4)21a} (see also refs. [10a, b]). The presentone in CHCl3 in the membrane influences the cation transport (theposition of the NH4

+ ions is not known); for details see text. Thesituation for {DODAx2a} hybrids (x~7[10c]) should be completely dif-ferent because of the small number of surfactant molecules.

Figure 2. Profile concentration (�5 % error limit) versus time of (a)Li+ , Na+ , and K+ cations at the feed phase/membrane interfaceand of (b) Li+ cations in the feed, membrane, and receivingphases, based on the {DODA40(NH4)21a} present in the membrane.The flux values J, representing concentration of ions transportedper hour through feed/membrane or membrane/receiving interfa-ces, have been calculated from the slope of the initial nearly linearpart of the profiles (time 0–10 h), using the solution-diffusionmodel.[11]

Isr. J. Chem. 2013, 53, 102 – 107 � 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.ijc.wiley-vch.de 103

Selective Ion Transport by Nanocapsules

In this context we refer to the following points:

1. The ideal cation transport scenarios (shown in Fig-ure 3 a) according to the solution-diffusion mechanism(based on Fick�s law), assume that the driving force isdue to the cation concentration gradients. This shouldlead to an equal transport rate of all cations with anequilibrium state in which 50% of the initial concen-tration of the cations is present in each of the feed andreceiving phases.

2. In the presence of only a NPOE solution ofDODABr, the alkali cations are not extracted fromthe feed phase. 100% of the initial cation concentra-tions of Li+, Na+, and K+ (0.1 M, each) has beenmeasured in the feed phase at different time intervals(Figure 3 b).

3. The present SLM membrane containing the lipophilichybrid {DODA40(NH4)21a}, shows high selectivity forLi+ cations, which are exclusively transported, whilethe Na+ and K+ cations are only partially extractedinto the membrane, but are not present at all in the re-ceiving phase (Figure 3 c). The transport across the hy-drophobic liquid membrane phase should occur via

“ion-pairs”, i.e. of Li+, Na+, or K+ cations with theanionic nanocapsules, or of the Cl- anions with theDODA cations. The extraction/release performance ofthe ions of an alkali salt from the feed phase, throughthe membrane, to the receiving phase, in the case ofthe lipophilic hybrids {DODA40(NH4)21a}, can be eval-uated by comparison of alkali cation equilibrium con-centrations (measured after ~50 h) in the feed, mem-brane, and receiving phases, to their initial concentra-tions (0.1 M, each) (Figure 2a,b). The determinationof the profile concentration versus time in the receiv-ing phase and the calculated diffusion coefficient Di ofLi+ (Table 1)[11–14] show selective extraction/transportof Li+ (40% remaining in the feed phase, 20 % in themembrane, and 40% transferred into the receivingphase). Na+ and K+ cations, on the other hand remainmostly in the feed phase (85% and 92%, respectively)and are only slightly extracted to the membrane phase(15 % and 8 %, respectively), as shown in Figures 2aand 3 c.

The Mo9O9-type pores of 1a in the considered hybrid{DODA40(NH4)21a} (diameter of ca. 3 �[15])) are quitesimilar in size to those of the 27-crown-9. Therefore, hy-drated Li+ ions with a complete shell of four coordinatedwater molecules (diameter also around 3 �[16]) can pass.On the other hand, the larger hydrated Na+ and K+ re-quire partial dehydration before entering, which is en-thalpically unfavorable in the presence of internal acetateligands which have no receptor/attractor function. Theconsequence: Hydrated Na+ and K+ cations are posi-tioned outside the capsules.

For comparison: The self-diffusion coefficients of hy-drated Li+ , Na+, and K+ cations in water follow theseries DNa

+(H2O)n<DK

+(H2O)n ! DLi

+(H2O)n.

[16,17] This meansthat the transport of the Li+ cations through the capsulescontaining (disordered) H2O molecules could possibly�ina first order approximation�be influenced by the higherdiffusion rate of hydrated Li+ ions (see related remark inFigure 4). Importantly, for {DODA40(NH4)21a} there isa difference of one order of magnitude between the trans-port rates for Li+ cation uptake at the feed/membrane in-

Table 1. Diffusion coefficients of Li+ transported through the SLMmembranes (Na+ and K+ cations have not been detected in the re-ceiving phase).

Membrane Constituents DLi+ · 1010 m2/s

{DODABr} 0[a]

{DODA40(NH4)21a} 1.62{DODAx2a}, (x~7) 1.75{DODA50(NH4)223a} 0[a]

[a] We noted no transport phenomena of ions through the mem-brane containing only the DODABr or {DODA50(NH4)223a} constitu-ents.

Figure 3. Concentrations of Li+ , Na+ and K+ cations at equilibriumstate in the feed, membrane (for thickness see text) and receivingphases, relative to the total initial concentration c0 = 0.1 M (foreach cation) with different membrane types: a) presuming idealpassive ion transport conditions; b) in the presence of onlyDODABr; and c) in the presence of {DODA40(NH4)21a}, acting as iontransporter.

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Full Paper M. Barboiu, et al.

terface (JLi+

,feed =4.7*10�3 mol/L/h) and their release atthe membrane/receiving interface (JLi

+,receiving =5.7 *

10�4 mol/L/h) (Figures 2 b and 3 c). This means that someof the Li+ cations are retained in the liquid membrane, asthe binding to the pore O atoms of the capsule 1a, witha charge of 42-, leads to an extreme lowering of the trans-port rate. The main contribution to the different transportrates and selectivities, in the case where the {DODA40-(NH4)21a} hybrid is present, is related to the different af-finity/uptake properties of the lipophilized nanoporouscapsules toward the Li+ , Na+, and K+ cations. The conse-quence is that only “Li” can pass through the capsules(Figure 4).

The observation has been confirmed by the permeabili-ty of Li+ cations through the hybrid {DODAx2a} (x~7)[10c] (note the relatively low negative capsule charge!) asthe Na+ and K+ cations cannot penetrate, because the{Mo3Fe3O6} pores exhibit an 18-crown-6-type crown-etherfunction, which causes the retention of these cationswithin the liquid membrane. In the Li+ cation transport,the diffusion-driven permeability through the membraneis a little bit more effective in case of the nearly neutralcapsule 2a compared to that of 1a (note: both have no re-ceptor ligands).

While the mentioned level of selectivity is influencedby different cation interactions with the capsule pores, itis evident, as mentioned before, that a second causeexists, namely possible internal ligand coordination, incase of a hybrid compound with sulphate ligands/recep-tors. For this reason, the “sulphate”-type hybrid

{DODA50(NH4)223a}, with known cation coordinatingproperties,[7,8,15] was tested. The experiments showed, cor-respondingly, that Li+ , Na+, and K+ cations are nolonger transported through the liquid membrane, as theyare mostly fixed, due to the SO4

2� ligand-type receptorproperties,[15] which is in contrast to the acetate-type cap-sule scenarios. As the high negative charge of the “sul-phate” cluster allows dehydration, all cations can enterthrough the pores, but cannot leave the capsule. (We donot consider here that uptake of cations lowers the capsu-le charge which changes the interaction with the DODAcations!) It was observed that the Li+ diffusion coeffi-cients (DLi+ (H2O) ~10�10 m2/s; Table 1) are six orders ofmagnitude lower than the diffusion coefficients of hydrat-ed Li+ cations in water (DLi+ =7.9*10�5 m2/s),[16,17] and ofthe same order of magnitude as the diffusion coefficientsdetermined by DOSY-NMR experiments for the “naked”nanocapsules 1a in DMSO solution (9.1*10�11 m2/s).[5]

Presuming that the dynamic diffusion of lipophilic self-as-sociated superstructures of the nanocapsules (Figure 1)with a larger hydrodynamic volume is smaller than diffu-sion of hydrated Li+ ions through the hydrated capsules,this behavior can be rationalized by considering thehybrid nanocapsules as “slower dynamic spectators” ofthe diffusion events of more mobile ionic partners likeLi+ cations, as previously observed with hybrid mem-brane systems.[14]

Summary and Conclusion

The present results show highly selective transport of Li+

over Na+ and K+ cations through supported liquid mem-branes, in which special porous nanocapsules embeddedin surfactants occur in dynamic constitutional aggregates,used as specific nanoscaled translocation pathways forcations. The constitutional interactions among thesenanocapsules lead to the generation of dynamic nano-phases within the membrane phase, making it possible toimprove the cation-type diffusion within such percolatedconductive nanodomains. The controlled generation of“diffusional”-type nanoscaled channels within a mem-brane structure is important for the design of new mem-brane systems. It is worth mentioning that the ability ofthe porous nanocapsules to uptake/release cations isbased on the diffusion of the ions within the cavities,which are filled with water molecules. Perspectives forthe future include the development of these new method-ologies towards dynamic constitutional systems. This mayprovide new insights into basic features for the design ofnew materials mimicking biological ion channels, with ap-plications in nanoscaled separations, sensoring, and stor-age-delivery devices. Finally the results obtained extendthe application of Constitutional Dynamic Chemistry[18] tomaterials science.[19]

Figure 4. Top: Scanning electron microscope (SEM) images of thecross-section of the membrane films (a) before and (b) after fillingthe membrane phase with {DODA40(NH4)21a} hybrids, showing thatall membrane macropores are filled with the lipophilic nanocap-sules. Bottom: A detailed model of the processes in the SLM, inwhich the lipophilic nanocapsules under study contain internal“water.” Li+ cations (yellow spheres) are selectively transportedover Na+ (blue spheres) and K+ (red spheres) cations, by a com-bined diffusion/uptake-release driven transport mechanism.

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Selective Ion Transport by Nanocapsules

Experimental Section

Materials and methods: All reagents were obtained fromAldrich and used without further purification. All organicsolutions were routinely dried by using sodium sulphate(Na2SO4). The molybdenum-oxide based capsules 1a,[4b]

2a,[4c] and 3a[6] and the lipophilic porous hybrids{DODA40(NH4)21a}[10a,b] and {DODAx2a}[10c] were synthe-sized according to the previously published procedures,while {DODA50(NH4)223a} was synthesized for the firsttime (correct elemental analysis and IR spectrum).

Membrane transport procedure: Membrane transportexperiments were performed with magnetic stirring, ina conventional bi-compartmental Teflon device,[11] atroom temperature, in which the membrane (S=5.32 cm2)separated an aqueous feed phase of 50 mL of 10�1 MLiCl+10�1 M NaCl+10�1 M KCl solution from the re-ceiving phase of 50 mL of deionized water. The supportedmembrane was prepared by introducing either 10�3 M sol-utions of the hybrids {DODA40(NH4)21a}, {DODAx2a},and {DODA50(NH4)223a} or a 10�2 M solution of DODAin 2-nitrophenyloctyl-ether (NPOE) into the thin hydro-phobic Accurel

�

films of high porosity. Aliquots (0.01 ml)of both aqueous solutions were withdrawn at appropriateintervals. The Li+ , Na+, and K+ concentrations in aque-ous feed and receiving phases were determined by atomicabsorption spectrometry (�5% error limit). Their con-centration in the membrane phase was determined by thedifference between the initial concentration and the sumof the total concentration in the aqueous phases. The fluxvalues were calculated from the concentration versustime profiles in the feed or receiving phases.

Acknowledgements

This work was conducted as part of a DYNANO, PITN-GA-2011-289033 (www.dynano.eu) and ANR-10-BLAN-717-2. We thank Adinela Cazacu and Dr. Ana MariaTodea for the preliminary experiments. A. M. thanks theDeutsche Forschungsgemeinschaft and the Fonds derChemischen Industrie for long term support.

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Received: January 6, 2013Accepted: January 15, 2013

Published online: February 18, 2013

Isr. J. Chem. 2013, 53, 102 – 107 � 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.ijc.wiley-vch.de 107

Selective Ion Transport by Nanocapsules