Size-Selective Voltammetry: Modification of the Interface between Two

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Langmuir 2007, 23, 3455-3461

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Size-Selective Voltammetry: Modification of the Interface between Two Immiscible Electrolyte Solutions by Zeolite Y S. Senthilkumar,†,‡ Robert A. W. Dryfe,*,‡ and R. Saraswathi† School of Chemistry, Madurai Kamaraj UniVersity, Madurai 625 021, Tamil Nadu, India, and School of Chemistry, The UniVersity of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom ReceiVed September 8, 2006. In Final Form: December 15, 2006 Size- and charge-selective ion transfer across the zeolite-Y-modified interface between two immiscible electrolyte solutions (ZM-ITIES) is described. The zeolite-Y membrane is prepared from pressed disks by healing with tetraethyl orthosilicate (TEOS). Size- and charge-selective transfer of the tetraethylammonium cation, size-selective exclusion of tetrabutylammonium cation, and charge-selective exclusion of the tetrafluoroborate and perchlorate anions are demonstrated at the ZM-ITIES. The exclusion studies suggest that the membrane is coherent and contains a low density of pinholes, after healing with TEOS. Various factors affecting the ion transfer such as analyte concentration, supporting electrolyte concentration, and scan rate are investigated. The diffusion coefficient of tetraethylammonium ions within the zeolite-Y pores is found to be on the order of 10-8 cm2 s-1.

Introduction An important theme in electrochemical studies over the past 20 years has been the modification of electrode surfaces to impart selectivity, often based on analyte size and/or charge, to the electrochemical response.1,2 Zeolite materials were identified at an early stage as promising candidates for the modification of electrodes, due to their well-characterized framework structures, with subnanometer diameter pores. A large number of publications have appeared during the past two decades concerning zeolitemodified electrodes (ZMEs), which frequently consist of a zeolite-polymer composite or a zeolite-impregnated carbon paste, attached to an underlying conductor.3-7 This field has been reviewed subsequently.8,9 Recent studies of selective films on electrodes have used films of insoluble molecular squares to impart a size-based selectivity to the voltammetric response.10 In this laboratory, we have been interested in applying such selectivity to charge-transfer processes at the interface between two immiscible electrolyte solutions (ITIES). As well as the intrinsic interest in charge transfer at the ITIES, the strategy of modifiying the ITIES with an appropriate material is, in some respects, more straightforward than its solid electrode counterpart because one is relying on capillary forces to fill the interior of a porous material, rather than using more complex approaches (e.g., evaporation of the metal) to contact the porous material to a conducting substrate. A further advantage is that zeolite materials, like the ITIES, are ionic conductors, and thus the voltammetric signal resulting from charge (ion) transfer at the * Corresponding author. Tel.: +44(0)161-306-4522. Fax: +44(0)161275-4734. E-mail: [email protected]. † Madurai Kamaraj University. ‡ The University of Manchester. (1) Bard, A. J.; Faulkner, L. R. Electrochemical methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons Inc.: New York, 2001. (2) Zen, J. M.; Kumar, A. S.; Tsai, D. M. Electroanalysis 2003, 15, 1073. (3) Murray, C. G.; Nowak, R. J.; Rolison, D. R. J. Electroanal. Chem. 1984, 164, 205. (4) De Vismes, B.; Bedioui, F.; Devynck, J.; Bied-Charreton, C. J. Electroanal. Chem. 1985, 187, 197. (5) Baker, M. D.; McBrien, M.; Burgess, I. J. Phys. Chem. B 1998, 102, 2905. (6) Ganesan, V.; Ramaraj, R. Langmuir 1998, 14, 2497. (7) Ahlers, C. B.; Talbot, J. B. Electrochim. Acta 2000, 45, 3379. (8) Rolison, D. R. Chem. ReV. 1990, 90, 867. (9) Walcarius, A. Electroanalysis 1996, 8, 971. (10) Be´langer, S.; Hupp, J. T.; Stern, C. L.; Slone, R. V.; Watson, D. F.; Carrell, T. G. J. Am. Chem. Soc. 1999, 121, 557.

ITIES can be related to the associated transport of ions within the zeolite. Previously, we have shown that free-standing membranes of silicalite, a neutral framework zeolite, could be located at the water/1,2-dichloroethane interface.11-13 A sieving effect, based on the size of the ions transferred relative to the size of the silicalite pores, was observed. However, one complication was the low polarity of the silicalite interior, which gave rise to considerable distortions due to the Ohmic drop across the framework.13 Deposition of silicate materials, under potentiostatic control at the ITIES, has also been reported.14 A parallel to this modification of the ITIES with inorganic frameworks is the adsorption of phospholipids at the ITIES: electrochemical studies have been used to probe the structure of the adsorbates.15,16 The presence of the lipids has been exploited to slow electron transfer and thereby investigate the driving force dependence of electron-transfer rates at the ITIES.17 Voltammetric methods are readily suited to the measurement of transport parameters since a well-defined boundary condition is applied to an interface, and the resultant flux to that interface is measured. The measurement of transport parameters within microporous materials such as zeolites has been a topic of longstanding fundamental and technological interest. Barrer has developed analytical expressions for diffusion within microporous materials, with the diffusion coefficients related to the adsorption isotherm of the sorbate via expressions of the form:18

DX ) BXcX

d ln aX d ln aX ) BXcX d ln cX d ln θ

(1)

where D is the diffusion coefficient of adsorbate X, c is its concentration, a is the thermodynamic activity of X, θ is the (11) Dryfe, R. A. W.; Holmes, S. M. J. Electroanal. Chem. 2000, 483, 144. (12) Lillie, G. C.; Dryfe, R. A. W.; Holmes, S. M. Analyst 2001, 126, 1857. (13) Stephenson, M. J.; King, A. J.; Holmes, S. M.; Dryfe, R. A. W. J. Phys. Chem. B 2005, 109, 19377. (14) Marecˇek, V.; Ja¨nchenova´, H. J. Electroanal. Chem. 2003, 558, 119. (15) Kakiuchi, T.; Nakanishi, M.; Senda, M. Bull. Chem. Soc. Jpn. 1988, 61, 1845. (16) Samec, Z.; Troja´nek, A.; Kritl, P. Faraday Discuss. 2005, 129, 301. (17) Tsionsky, M.; Bard, A. J.; Mirkin, M. V. J. Am. Chem. Soc. 1997, 119, 10785. (18) Barrer, R. M. Molecular SieVe Zeolites - II; Adv. Chem. Ser. 102; American Chemical Society: Washington, DC, 1971.

10.1021/la0626353 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/06/2007

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fraction of available sites in the supercages, and B is the intrinsic mobility of X in the microporous host. Diffusion coefficients of gas-phase molecules adsorbed within zeolite frameworks can be predicted reasonably well using molecular dynamic methods.19 Experimentally, macroscopic diffusion coefficients of small molecule adsorbates can be measured using chromatographic pulse techniques.20 The corresponding microscopic parameters can be found using pulsedfield gradient NMR,21 or neutron scattering techniques.22 There is a smaller body of recent data reported for the transport of cations incorporated into zeolite frameworks. Early work exploited either conductivity measurements or self-diffusion of radioactive tracers to determine diffusion coefficients for framework cations.23,24 Reported values of the ionic self-diffusion coefficients vary depending on the extent of framework hydration.23 Recent reports of ionic diffusion parameters give values for certain systems, which are close to (or within 1 order of magnitude of) the value in bulk (aqueous) solution, while other values are several orders of magnitude lower than the bulk value. Examples include zeolite A, where the diffusion coefficient for Cs+ is reported to be similar to the bulk value;25 for Na+ the value is 1 or 2 orders of magnitude below the bulk value,26 whereas for Mg2+ a value ca. 6 orders of magnitude below bulk is reported.27 The preceding values have generally been determined via analysis of the contacting solution phase, often using unstable isotopes of the ions of interest. For such “bulk” chemical methods, a distinction must be made between selfdiffusion and exchange-diffusion processes. For zeolite Y, analysis of the solution content yielded a diffusion coefficient of 10-6 cm2 s-1 for Zn2+ within the zeolite interior,28 whereas a much lower value (10-8 cm2 s-1) was measured via an electrochemical method for Ag+.5 A more recent study of a ZME has given 1 × 10-9 cm2 s-1 as the diffusion coefficient of the tetraethylammonium cation within zeolite Y.29 However, a complication with attempts to measure transport parameters using conventional ZMEs is that the ion transfer process must be coupled to the redox (electron transfer) event, and thus the two processes must be deconvoluted. Moreover, given the composite nature of most ZMEs reported to date, factors relating to the mean crystallite size/coverage must be estimated to permit quantitative analysis of the voltammetric data. We have recently reported that ion exchange of suspensions of zeolite Y can be induced using electrochemical methods at the ITIES.30 Here, we describe the modification of the ITIES with free-standing membranes of zeolite Y, using the voltammetric response as a means to characterize the integrity and associated charge-/size-based selectivity of the membranes. We also attempt to use this setup to measure transport parameters, specifically the diffusion coefficient of the tetraethylammonium (TEA+) cation within the zeolite-Y framework. (19) Auerbach, S. M. Int. ReV. Phys. Chem. 2000, 19, 155. (20) Hufton, J. R.; Danner, R. P. AIChE J. 1993, 39, 962. (21) Heink, W.; Ka¨rger, J.; Pfeifer, H.; Datema, K. P.; Nowak, A. K. J. Chem. Soc., Faraday Trans. 1992, 88, 3505. (22) Jobic, H.; Be´e, M.; Pouget, S. J. Phys. Chem. B 2000, 104, 7130. (23) Breck, D. W. Zeolite Molecular SieVes; John Wiley & Sons: London, 1974. (24) Barrer, R. M.; Bartholomew, R. F.; Rees, L. V. C. J. Phys. Chem. Solids 1963, 24, 51. (25) Mon, J.; Deng, Y.; Flury, M.; Harsh, J. B. Microporous Mesoporous Mater. 2005, 86, 277. (26) Faux, D. A. J. Phys. Chem. B 1998, 102, 10658. (27) Coker, E. N.; Rees, L. V. C. Microporous Mesoporous Mater. 2005, 84, 171. (28) Dyer, A.; Townsend, R. P. J. Inorg. Nucl. Chem. 1973, 35, 3001. (29) Dome´nech, A. J. Phys. Chem. B 2004, 108, 20471. (30) Stephenson, M. J.; Holmes, S. M.; Dryfe, R. A. W. Angew. Chem., Int. Ed. 2005, 44, 3075.

Senthilkumar et al. Table 1. Properties of the Zeolite (Na)Y Used in This Study property

value

compositiona

typical unit cell density (g/cm3)b supercages per unit cellb aperture diameter (Å)a internal diameter of supercage (Å)b a

Na56[(AlO2)56(SiO2)136]‚250H2O 1.92 3.2 7.4 11.8

Reference 23. b Reference 37.

Experimental Section Chemicals. All of the aqueous solutions were prepared with water of a resistivity of 18.2 MΩ cm, obtained from a “PURELAB ultra” (Elga) system used in combination with a Millipore “RiOs3” prepurifier (Millipore Limited, Watford, UK). All organic solutions were prepared with 1,2-dichloroethane (DCE 99+%, Aldrich) as the solvent. The chemicals used in the aqueous phase were the chloride salts of lithium (99+%, Aldrich), tetraethylammonium (TEA+, g99%, Fluka), and tetrabutylammonium (TBA+, g97%, Fluka), the lithium salts of tetrafluoroborate (BF4-, 98%, Aldrich), and perchlorate (ClO4-, 95+%, Aldrich) and sodium chloride (99+%, Lancaster). Bis(triphenylphosporanylidene)ammonium tetrakis(4-chlorophenyl) borate (BTPPATPBCl4) was used as the organic supporting electrolyte throughout the experiments. BTPPATPBCl4 was prepared using bis(triphenylphosporanylidene)ammonium chloride (BTPPACl, g98%, Fluka) and the potassium salt of tetrakis(4-chlorophenyl) borate (TPBCl4-, g98%, Fluka) by metathesis using the procedure reported elsewhere.31 A final recrystallization step using toluene (>99%, BDH) was performed before air drying. The zeolite-Y membranes were prepared from commercially available molecular sieve catalyst supports of sodium Y zeolite (Aldrich), by healing free-standing pressed disks of the zeolite.32 The pressed disks were formed using a hydraulic press operating at approximately 10 tonnes of pressure, applied for 5 min using a stainless steel dye. The disk thickness and diameter were 0.75 and 20 mm, respectively. The disk was then healed using approximately 2 mL of tetraethyl ortho silicate (TEOS, 98%, Aldrich). The TEOS was placed in a flat bottom specimen tube (Samco, UK) over a pressed disk and then heated in a hot plate to drive off the ethanol from resulting TEOS decomposition, at approximately 170 °C, to form SiO2 in situ. After complete evaporation, healed disks were washed with acetone, followed by distilled water, to remove excess silica present at the surface of the disk, and then air-dried for further experiments. Table 1 lists the properties of the zeolite Y used in this study. Apparatus. The healed membranes were sealed to a homemade electrochemical cell using silicone rubber compound (RS Components, Corby, UK) as a sealant, which was allowed to cure overnight. The zeolite membrane was left in contact with the appropriate aqueous solution overnight, before being immersed in the organic phase for voltammetric experiments. The presaturation process allowed the aqueous solution to penetrate into the zeolite internal cavity. For TEA+ presaturation experiments, after the saturation period, the membrane was twice rinsed with distilled water to remove any excess TEA+ present at the surface of the membrane, and then used for further experiments. The homemade electrochemical cell used for voltammetric ion transfer studies is shown in Figure 1. The Ag/AgCl reference electrodes were prepared by the oxidation of 0.75 mm diameter silver wire (Advent Materials Ltd., Oxford, UK) immersed in a 2 M KCl solution. Platinum gauze (Advent Materials Ltd., Oxford, UK) was employed as counter electrodes. The counter electrode for the organic phase was constructed with the contacting wire coated with glass to prevent contact with the aqueous phase. (31) Fermı´n, D. J.; Duong, Z. F.; Ding, Z. F.; Brevet, P. F.; Girault, H. H. Phys. Chem. Chem. Phys. 1999, 1, 1461. (32) Senthilkumar, S.; King, A. J.; Holmes, S. M.; Dryfe, R. A. W.; Saraswathi, R. Electroanalysis 2006, 18, 2297.

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Figure 2. Cyclic voltammogram obtained for the size-selective exclusion of TBA+ (cell 1) across the ZM-ITIES at a scan rate of 0.025 V s-1.

Figure 1. Four-electrode electrochemical cell employed for the zeolite-Y membrane supported ITIES experiments. (RE1 and RE2 are the reference electrodes for the aqueous and organic phases, CE1 and CE2 are the counter electrodes for the aqueous and organic phases, 1 and 2 are the aqueous and organic phase electrolyte solutions, respectively, 3 is the reference solution for the organic phase, and 4 is the healed zeolite-Y membrane.) The ZM-ITIES was polarized using a four-electrode potentiostat (Autolab PGSTAT 20, Eco-Chemie, The Netherlands). Positive feedback compensation was applied via the potentiostat to the electrochemical cell to minimize the Ohmic (iR) drop across the ZM-ITIES, and through the solutions. Ohmic drop on the order of 1 kΩ was typically observed. The aluminous nature and associated appreciable ionic conductivity of zeolite Y meant most of the potential drop can be attributed to the organic phase. The electrochemical cell setup can be written as:

Figure 3. Cyclic voltammograms obtained for the charge-selective exclusion of anions across the ZM-ITIES: (A) BF4- (cell 2); (B) ClO4- (cell 2). In both cases, the scan rate was 0.02 V s-1. The bold and thin lines correspond to the presence and absence of the membrane, respectively. Time-dependent experiments were performed by pre-exchanging a zeolite-Y membrane with TEA+ (3 mM) solution for 24 h. The membrane was then rinsed twice with distilled water to remove any excess TEA+ present at the surface of the membrane, and then used for further experiments using cell 4. The healed zeolite-Y membrane was characterized by powder X-ray diffraction (XRD) (Bruker, aXS powder diffractometer, D8 Discover) and 29Si MAS NMR spectroscopy (UNITY Inova Spectrometer (300 MHz), Oxford Instruments). The MAS NMR experiments were performed at the EPSRC Solid-state NMR service (Department of Chemistry, University of Durham, UK). Scanning electron microscopy (SEM) was performed using a Quanta 200 Environmental SEM (FEI Corp., Hillsboro, OR). The sodium content of the zeolite, before and after ion exchange with TEA+, was determined by elemental analysis (Fisons Elemental analysis, Horizon, inductively coupled plasma-optical emission spectrometer (ICP-OES)).

Results and Discussion Structural Characterization of the Zeolite-Y Membrane. The basic need to characterize the healed zeolite-Y membrane is to check whether the zeolite framework is degraded during the healing process. The membrane is healed because the zeolite pressed disk consists of discrete crystals, which are only mechanically contacted, thus presenting inter-grain pathways to analyte transfer. Healing is applied to eliminate as many of these pathways as possible with TEOS functioning as a reactive silica source to bridge the inter-crystalline voids.32 X-ray diffraction, 29Si MAS NMR, and SEM studies were used to characterize the healed zeolite membranes and suggested that minimal degradation of the crystalline structure had occurred during the healing process and that at least the larger intercrystalline voids had been filled by the silica deposit.32,33

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Electrochemical Characterization: Size and Charge Selectivity. Pinholes remaining within the membrane with a diameter of a few nanometers, or less, would be invisible to most electron microscopes. However, voltammetric methods can provide insight into the existence of such defects. The transfer of TBA+ (crystallographic radius, 4.94 Å),34 BF4- (crystallographic radius, 2.28 Å),35 and ClO4- (crystallographic radius, 2.92 Å)35 was attempted at the ZM-ITIES. Cyclic voltammograms recorded for the size (TBA+, cell 1)- and charge (BF4- and ClO4-, cell 2)-selective exclusion by the zeolite-Y membrane at the interface are shown in Figures 2 and 3. Cyclic voltammograms shown in Figure 2 indicate the sieving effect of zeolite Y through the size-selective exclusion of TBA+, where the diameter of TBA+ is greater than the channel openings. As expected, minimal currents over the background were observed, confirming the coherent nature of the zeolite-Y membrane. Given the background current observed, the upper limit on the TBA+ transfer current was estimated at 26 µA at a scan rate of 0.025 V s-1, corresponding to a maximum pinhole area of 5% assuming linear diffusion holds (vide infra). Cyclic voltammograms recorded for the chargeselective exclusion of BF4- and ClO4- are shown in Figure 3A and B, showing the corresponding sieving effect of the zeolite-Y membrane, even though the diameters of the anions are smaller than the channel diameter. Again, the upper limit on the anion transfer currents (given the non-Faradaic current observed) was estimated at 28 and 19 µA at a scan rate of 0.02 V s-1 for BF4and ClO4-, respectively. A linear diffusion analysis gives a maximum pinhole area of between 4% and 5% for the anionic analytes. From the above experiments, it is inferred that the healed zeolite-Y membranes are largely coherent and that the intercrystalline voids have been largely filled by the silica framework. Voltammetric Ion Transfer across the ZM-ITIES. Zeolite modification of the ITIES represents a useful route to size- and charge-selective ion transfer across the liquid/liquid (L/L) interface. The background potential window (x ) 0 mM and y ) 25 mM in cell 3), with and without the membrane, and voltammetric response obtained for the TEA+ transfer (x ) 0 and 1 mM; y ) 25 mM in cell 3) across the ZM-ITIES are given in Figure 4. The potential window available for the voltammetry at the L/L interface is determined by the standard transfer potentials of the supporting electrolytes in both the water and the DCE phases. From Figure 4A, it is clear that there is a considerable increase (ca. 300 mV) in the potential window in the presence of the zeolite-Y membrane, as has been observed previously with silicalite-1 modification of the ITIES.13 The extension of the positive limit of the potential window is consistent with the Gibbs energy of transfer for Li+ exceeding that of the organic phase anion (TPBCl4-). The increase at the negative end of the potential window implies that both chloride and the organic cation (BTPPA+) are excluded (on a charge and size basis, respectively) because silicalite modification of the ITIES has shown that both ions transfer at a similar potential.13 Because both ions are nominally excluded, the negative end of the potential window will be limited by the density of pinholes (allowing ion transfer) and/or any impurity cations present in the organic phase. This may explain the variability seen at the negative potential limit in Figure 3A and B. From Figure 4B, the transfer of TEA+ (bold line) across the ZM-ITIES can be observed. The crystallographic diameter reported36 for TEA+, (33) King, A. J.; Lillie, G. C.; Cheung, V. W. Y.; Holmes, S. M.; Dryfe, R. A. W. Analyst 2004, 129, 157. (34) Nightingale, E. R., Jr. J. Phys. Chem. 1959, 63, 1381. (35) Marcus, Y. Introduction to Liquid State Chemistry; John Wiley & Sons: London, 1977. (36) King, E. J. J. Phys. Chem. 1970, 74, 4590.

Senthilkumar et al.

Figure 4. Cyclic voltammogram for (A) the background response (x ) 0 mM and y ) 25 mM in cell 3) in the presence (thin line) and absence (bold line) of zeolite-Y membrane; and (B) TEA+ transfer (x ) 1 mM and y ) 25 mM in cell 3) and background (x ) 0 mM and y ) 25 mM in cell 3) in the presence of zeolite-Y membrane at the ITIES. The cyclic voltammograms were recorded at 0.05 V s-1.

Figure 5. Cyclic voltammetric responses observed for the transfer of TEA+ (x ) 3 mM and y ) 25 mM in cell 3) at various scan rates, in ascending order 0.01, 0.015, 0.02, 0.025, 0.03, 0.035, 0.04, 0.045, and 0.05 V s-1, across the ZM-ITIES.

of 6.2 Å, compares with the aperture diameter of 7.4 Å for the supercages of zeolite Y.23,33 Figure 5 shows cyclic voltammograms observed for TEA+ transfer (x ) 3 mM and y ) 25 mM in cell 3) across the ZMITIES at various scan rates in the range 0.01-0.05 V s-1. The peak current increases linearly with the square root of the scan rate; if the voltammetry is treated as Nernstian, analysis in terms of the Randles-Sevcˇik equation1 may be performed: where ip is

ip ) 0.4463

( ) n3F3 RT

1/2

AxDCoxV

(2)

the peak current, n is the charge number of the ion, A is the area

Size-SelectiVe Voltammetry

Figure 6. Linear sweep voltammetric responses observed for the transfer of TEA+ across the ZM-ITIES using cell 3 with y ) 25 mM and, in ascending order, x ) 0.2, 0.4, 0.6, 0.8, 1.0, 5.0, and 7.0 mM. The scan rate in each case was 0.05 V s-1.

Figure 7. Plot of the forward peak currents (water to DCE) against the square root of scan rate for the transfer of TEA+ at various concentrations across the ZM-ITIES.

of the interface, D is the diffusion coefficient of the transferred species, Co is its concentration, and V is the scan rate. For the lowest scan rates, we note that the peak potential difference observed for the transfer, ∆(∆φ), is very close to the expected value (59 mV) for diffusion-controlled transfer of a univalent ion. At the highest scan rate, however, the peak separation has increased to 100 mV. Effect of Concentration of TEA+. Linear sweep voltammograms (LSV) recorded for the transfer of TEA+ as a function of concentration are shown in Figure 6. The LSV data indicate that the transfer of TEA+ from the water-imbibed zeolite to the DCE phase increases with the concentration of TEA+. The forward peak currents (water/zeolite to DCE) plotted as a function of the square root of scan rate for the transfer of TEA+ at various concentrations across the ZM-ITIES are shown in Figure 7. The gradient values (ip/V1/2) obtained for the transfer of TEA+ as a function of concentration do not increase linearly across the concentration range, because a change in gradient is seen at higher concentrations (>1 mM). The forward peak currents observed for the transfer of TEA+ across the ZM-ITIES (cell 3) for a given scan rate as a function of TEA+ solution concentration are shown in Figure 8. The data points shown in the graph indicate that the experiments were duplicated to check the reproducibility of the results. The change in relation between the peak current at higher concentrations (above 1 mM) is again apparent in this presentation of the data. The gradient values obtained from the (37) Gemborys, H. A.; Shaw, B. R. J. Electroanal. Chem. 1986, 208, 95.

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Figure 8. Forward peak currents observed for the transfer of TEA+ across the ZM-ITIES (cell 3) as a function of concentration of TEA+ in solution in the range from 0.2 to 7.0 mM. Inset: Peak current observed at the lower concentrations of TEA+. The scan rate was 0.05 V s-1.

Figure 9. Peak current observed for the transfer of TEA+ ion across the TEA+ presaturated (3 mM for 24 h) zeolite-Y membrane at various time intervals (cell 4). Scan rate: 0.05 V s-1.

peak currents versus concentration plot (ip/Co) for the submillimolar concentrations of TEA+ also obey the following relation derived from the Randles-Sevcˇik equation,1

() () x ip Co

V1

ip Co

V2

)

V1 V2

(3)

For example, the ratio of the gradient values observed at scan rates of 0.05 and 0.02 V s-1, and of 0.03 and 0.02 V s-1 are 1.57 and 1.25, which are in good agreement with the square root of the ratio of the respective scan rates (1.58 and 1.22). Effect of Ionic Strength. The effect of varying supporting electrolyte (LiCl) concentration on the peak currents observed for transfer of TEA+ was also investigated (see Supporting Information). TEA+ and LiCl electrolyte were presaturated with the zeolite-Y membrane for 24 h. The optimum peak currents for both the forward and the reverse processes were observed at 15 mM LiCl in the aqueous phase, whereas at higher concentrations the peak currents decreased significantly. A similar trend was observed for both TEA+ concentrations investigated (1 and 3 mM). Diffusion Coefficient of TEA+ Ion across the ZM-ITIES. The diffusion coefficient of TEA+ within the zeolite-Y membrane can only be found if the concentration of TEA+ within the membrane is determined, because the adsorptive properties of

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Table 2. Elemental Analysis Results and Diffusion Coefficient of TEA+ at Various Concentrations in Zeolite Y concentration of TEA in bulk solution/(×10-3) mol cm-3

wt % of Na+ in NaY

wt % of TEA+ in NaY

concentration of TEA+ in NaY /(×10-5) mol cm-3

D/(×10-8) cm2 s-1 (water to DCE)

0.0 0.2 0.4 0.6 0.8 1.0 3.0 5.0 7.0

5.60 5.55 5.42 5.34 5.23 5.18 5.12 4.83 4.47

0.00 0.05 0.18 0.26 0.37 0.42 0.48 0.77 1.13

0.0 0.384 1.382 1.996 2.841 3.224 5.143 5.912 8.676

3.11 1.95 3.75 2.89 3.24 3.53 2.63 3.19

the framework mean it is likely to differ from the bulk solution value. The zeolite-Y membranes were presaturated with various concentrations of TEA+ for 24 h (with a constant LiCl concentration), and elemental analysis was subsequently carried out to find the percentage of Na+ remaining. The elemental analysis results obtained for the percentage weight of Na+ and, thereby, the diffusion coefficients found for TEA+ at various concentrations are shown in Table 2. The sodium percentage in the zeolite Y after exchange with 5 mL of 0.025 M aqueous LiCl solution was 5.6%. Although the previous literature indicates that exchange of Li+ is not a spontaneous process,23,38 some partial replacement of the native Na+ appears to occur under the conditions employed herein (the percentage content based on the nominal zeolite composition is 7.4%; the initial Na+ content actually found for the zeolite was 6.6%). As the concentration of TEA+ used for the presaturation increases, the sodium content was found to decrease further (Table 2), and it is assumed that this was due to the exchange of TEA+ into the zeolite-Y pores. The concentrations of TEA+ so determined within the zeolite-Y membrane are also shown in Table 2 and are seen to rise with the increase in the concentration of TEA+ in the bulk solution. The ion exchange process can be represented by the equation:23

Na+(z) + TEA+(s) h Na+(s) + TEA+(z)

(4)

where z and s are the zeolite and solution phases, respectively. The effective diffusion coefficient of TEA+ in the zeolite-Y membrane was calculated using the Randles-Sevcˇik equation1 (eq 2). The gradient (ip/V1/2) values were found at different concentrations (Figure 7), with A, as the area of the membrane, and Co, the concentration of TEA+ in the zeolite pores, determined from the elemental analysis. The diffusion coefficients so determined for the various concentrations of TEA+, and shown in Table 2, are fairly constant. The concentrations determined are mean values over the entire membrane, which do not allow for local heterogeneity. The voltammetric data reflect the local concentration of TEA+ on the face of the membrane closest to the organic phase. Inhomogeneities in concentration may therefore lower the local concentration in the latter case, which would cause the D values found to be underestimates. Time-Dependent Experiments. Finally, the time-scale of the exchange process was investigated by recording the TEA+ transfer current as a function of time. Equilibration of TEA+ present within the zeolite-Y supercages was carried out across the ZMITIES using the NaCl solution (cell 4) at various time intervals. The forward and reverse peak currents observed over time are shown in Figure 9. The peak currents are almost constant over the 6 h period of observation, indicating that the TEA+ remains largely within the zeolite pores over this time period.

Discussion The transfer current observed, following equilibration of the zeolite-Y sample with the aqueous TEA+ solution, indicates that (38) Keane, M. A. Micoporous Mater. 1994, 3, 93.

partial replacement of the framework Na+ with TEA+ takes place. Although Barrer originally reported that TEA+ is too large to exchange into the faujasite framework structure,39 the subsequent consensus40,41 has contravened Barrer’s findings. The exchange of TEA+ occurs, albeit slowly, due to the ionic diameter, which is such that the ion is restricted to the supercages of the structure: exchange with the sodium present in the sodalite cage is precluded because the oxygen 6 ring of the sodalite cages has a smaller diameter23 of only 2.2 Å. The low rate of exchange is supported by the voltammetric data presented here (Figure 9), which shows that minimal back-exchange of the TEA+ with Na+ occurs over a 6 h period. In fact, levels of TEA+ exchange somewhat higher than those reported here have been quoted previously,42 using more concentrated solutions (ca. 1 M with respect to TEA+). More recently, TEA+ has been used as the electrolyte cation in an investigation of the electrochemical reduction of a manganese complex encapsulated within zeolite Y.29 By contrast, the voltammetric results obtained here (Figure 2) demonstrate that the larger TBA+ ion is excluded from the zeolite framework because of its size and that the anions investigated are also excluded by the charge of the zeolite-Y framework. The restricted uptake of TEA+, due to its exclusion from the sodalite cages, is also manifested in the data of Figure 8, where a nonlinear dependence of ion transfer peak current on bulk solution concentration is seen. During the presaturation period, the aqueous solution contains both TEA+ and supporting electrolyte (LiCl): in principle, both cations can undergo exchange with the sodium initially present in the supercage. Although Li+ incorporation is unfavored with respect to Na+,38 the solution conditions mean that some Li+ uptake (replacement of ca. 20% of the initial Na+ ions, see Table 2) into the zeolite framework does occur. For the TEA+ exchanged samples, the Li+ content was not explicitly determined; however, if the latter is assumed to be constant, then diffusion coefficients in the range from 1.9 × 10-8 to 3.8 × 10-8 cm2 s-1 are found for TEA+ within the zeolite framework. These values are calculated from eq 2 with the assumption that the diffusion fields within the individual zeolite pores overlap completely within the structure, due to its three-dimensional porosity. This analysis assumes the voltammetry is reversible, whereas an increase in peak separation (see above) with scan rate was observed. This could be interpreted in terms of a quasi-reversible component to the ion transfer, but the complex geometry of the interface means we have not attempted to explore this further. If diffusion field overlap does not occur, the apparent area for ion transfer would increase by a factor of (1 - θ), defined as the ratio of the combined pore surface area located at the interface to the total (39) Barrer, R. M. Proc. Chem. Soc. 1958, 99. (40) Kerr, G. T. Zeolites 1983, 3, 295. (41) Baker, M. D.; Senaratne, C.; Zhang, J. J. Chem. Soc., Faraday Trans. 1992, 88, 3187. (42) Coutant, M. A.; Le, T.; Castagnoda, N.; Dutta, P. K. J. Phys. Chem. B 2000, 104, 10783.

Size-SelectiVe Voltammetry

Langmuir, Vol. 23, No. 6, 2007 3461

surface area of zeolite located at the interface.43 This quantity has not been reported directly in the literature for zeolite Y, but the void fraction (κ) of this material has been reported as 0.48,23 thus:

(1 - θ) ) κ2/3

(5)

suggesting that (1 - θ) is approximately 0.6 for this material. Combination of eq 5 with the area term in eq 2, assuming Nernstian voltammetry, would increase the D values tabulated for TEA+ by a factor of 2.7. In either case, the values calculated are 2 orders of magnitude lower than the diffusion coefficient measured for TEA+ in bulk aqueous solution.44,45 The peak currents observed for the transfer of TEA+ across the ZM-ITIES are, however, affected by the LiCl concentration of the aqueous phase in contact with the zeolite membrane (see Supporting Information). The fall in the TEA+ flux seen at higher Li+ concentrations (>0.015 M) is indicative of replacement of TEA+ with Li+ within the zeolite framework. The rise in the TEA+ transfer current seen at lower Li+ concentrations has not been investigated in detail, but it may reflect an increase in the transport rate of TEA+ as the Li+ content of the sample increases (with Li+ replacing Na+). We note, however, that a diffusion coefficient a further order of magnitude lower than the value reported here was reported, using an indirect voltammetric approach, for TEA+ within zeolite Y.29

Conclusion Voltammetric studies have been described, using zeolite-Y membranes prepared from commercially available zeolite samples by healing, where TEOS is used as a silica source. The membranes were used to modify the ITIES and found to impose size- and charge-selective ion transfer across the resultant ZM-ITIES. TBA+, BF4-, and ClO4- were excluded from zeolite-Y membrane because of either molecular size (TBA+) or charge (BF4- and ClO4-). The voltammetric response for the transfer of TEA+ across the ZM-ITIES occurs through the (43) Amatore, C.; Save´ant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39. (44) Yuan, Y.; Wang, L.; Amemiya, S. Anal. Chem. 2004, 76, 5570. (45) Kralj, B.; Dryfe, R. A. W. Phys. Chem. Chem. Phys. 2001, 3, 5274.

supercages of the zeolite Y because of its molecular size. The peak currents observed for the transfer of TEA+ across the ZMITIES were directly proportional to the concentrations of TEA+, and the square root of the voltage scan rate, for concentrations in the range of 0.2-1.0 mM. The diffusion coefficients determined for intra-zeolite transport of TEA+ were of the order of 10-8 cm2 s-1, but were found to be sensitive to the ionic composition of the zeolite, although they were relatively insensitive to TEA+ concentration within the concentration range studied. Modification of the ITIES, as opposed to solid electrode surfaces, with zeolite materials presents an interesting alternative route to impose size- and charge-based selectivity on the electrode response and, by extension, presents a route to the measurement of intra-zeolite transport parameters. The use of the ITIES means that the studies are not restricted to redox-active ions, as would be the case with solid electrodes. The use of continuous freestanding membranes circumvents some of the problems associated with conventional ZMEs. Finally, the use of zeolites with appreciable ionic conductivity, such as zeolite Y, eliminates the problems with high membrane resistance observed previously with silicalite frameworks.13 This approach is promising as a route to determine the properties of zeolite membranes, and as a relatively simple means to determine transport parameters within such membranes. A final comment is that no account of microscopic factors such as pore geometry or surface roughness is included here. The work presented has not, therefore, considered the fractal geometry of the zeolite pore structure (reported as 2.5 for zeolite Y on the basis of sorption data46). An important future development would be the correlation of this type of experimental work with molecular dynamic simulations of diffusion processes within microporous solids.19 Acknowledgment. S.S. thanks the Commonwealth Scholarship Commission in the United Kingdom for the Split-Site Doctoral Scholarship (reference: INCN-2005-11), held at the University of Manchester, U.K. Supporting Information Available: Plots of peak currents for TEA+ transfer as a function of LiCl concentration of the aqueous electrolyte solution in contact with the zeolite. This material is available free of charge via the Internet at http://pubs.acs.org. LA0626353 (46) Huang, S. J.; Yu, Y. C.; Lee, T. Y.; Lu, T. S. Physica A 1999, 274, 419.