Anal. Chem. 2003, 75, 486-493
Hydrodynamic Study of Ion Transfer at the Liquid/ Liquid Interface: the Channel Flow Cell Simon S. Hill,† Robert A. W. Dryfe,*,† Edward P. L. Roberts,‡ Adrian C. Fisher,§ and Kamran Yunus§
Departments of Chemistry and Chemical Engineering, University of Manchester Institute of Science and Technology, P.O. Box 88, Sackville Street, Manchester M60 1QD, United Kingdom, and Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom
A hydrodynamic system based on the channel flow cell for voltammetric detection of ions at the liquid/liquid interface is reported. The current response for tetraethylammonium ion transfer across a membrane-supported liquid/liquid interface is shown to be consistent with existing theory for both the flow rate and voltage scan rate dependence of such processes, with no calibration factors or other adjustable parameters required. The analytical utility of such a device is discussed with specific regard to in situ measurements in flow systems. The measurement of solute transport across liquid/liquid interfaces is of relevance in various areas, including microextraction for subsequent chromatographic or mass spectrometric detection,1 membrane transport,2 and solvent extraction.3 Transmembrane ionic fluxes are also central to the function of biological cells.4 Voltammetric measurements offer a simple approach to the measurement of ionic fluxes across the liquid/liquid interface through direct polarization of the interface between two immiscible electrolyte solutions (ITIES). Under potentiostatic control, the flux of ions transferring across the ITIES as a function of the interfacial potential can be readily measured.5 Voltammetric measurement of ion transfer at the ITIES is of particular interest, since this method can be used to detect nonredox-active ions, such as alkali metal and alkaline earth cations.6 A natural extension of such voltammetric measurements, with specific regard to analytical applications, is the integration of the ITIES within flow systems, where the continuous monitoring of analyte concentration may be achieved.7 In such cases, the ion transfer current is measured as a function of flow rate, for which * To whom correspondence should be addressed. E-mail: robert.dryfe@ umist.ac.uk. † Department of Chemistry, University of Manchester Institute of Science and Technology. ‡ Department of Chemical Engineering, University of Manchester Institute of Science and Technology. § University of Bath. (1) Peng, S. X.; Branch, T. M.; King, S. L. Anal. Chem. 2001, 73, 708-714. (2) Shao, Y. H.; Mirkin, M. V. J. Phys. Chem. B 1998, 102, 9915-9921. (3) Hanna, G. J.; Noble, R. D. Chem. Rev. 1985, 85, 583-598. (4) Stintzi, A.; Barnes, C.; Xu L.; Raymond, K. N. Proc. Natl. Acad. Sci. 2000, 97, 10691-10696. (5) Girault, H. H. J.; Schiffrin, D. J. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15, pg 1. (6) Kakiuchi, T.; Senda, M. J. Electroanal. Chem. 1991, 300, 431-445. (7) Erbacher, C.; Bessoth, F. G.; Busch, M.; Verpoorte, E.; Manz, A. Mikrochim. Acta 1999, 131, 19-24.
486 Analytical Chemistry, Vol. 75, No. 3, February 1, 2003
at least one of the liquid phases is “mobile”. Various reports have described the construction of so-called hydrodynamic “ionode” devices. Two strategies have been presented to overcome the mechanical instability of the ITIES: The first route involves replacement of one liquid phase (often the organic phase) with a gel.8,9 The intrinsic drawback with this method is that the relationship between the physical and chemical properties of the gel and those of the original liquid phase are not clear; hence, the conclusions may not be of direct relevance to liquid/liquid systems. Moreover, the gel itself may be prone to swelling or chemical reaction over the lifetime of the experiment, complicating incorporation within a sealed unit, such as a flow cell. The second approach has involved the stabilization of the liquid/liquid interface with a membrane material.10,11 The intrinsic drawback here is that the membrane may not play a “passive” role in the interfacial transport process: mass-transfer through the membrane may be rate-determining; alternatively, adsorption processes on the membrane walls may also affect the overall response. To date, there have been few studies of hydrodynamic ion transfer in terms of known laminar flow regimes. Hundhammer et al adopted a walljet configuration in conjunction with a cellulose dialysis membrane, but the data was not fully interpreted in terms of wall-jet hydrodynamics.12 A possible reason for the empirical approach adopted was the considerable uncertainties with regard to role of the membrane; in particular, the cellulose membranes employed in the earliest studies are known to swell on contact with water, and their irregular pore size and pore distribution made interpretation of the data in terms of established mass-transport models difficult.12,13 Cellulose filter membranes were employed in a subsequent study of ion transfer across the membrane-stabilized interface by flow injection. Once more, uncertainties in the membrane geometry and structure complicated determination of the current dependence on flow rate.14,15 Girault and co-workers (8) Maracˇek, V.; Ja¨nchenova´, H.; Colombini, M. P.; Papoff, P. J. Electroanal. Chem. 1987, 217, 213-219. (9) Lee, H. J.; Girault, H. H. Anal. Chem. 1998, 70, 4280-4285. (10) Hundhammer, B.; Wilke, S. J. Electroanal. Chem. 1989, 266, 133-141. (11) Sawada, S.; Torii, H.; Osakai, T.; Kimoto, T. Anal. Chem. 1998, 70, 42864290. (12) Hundhammer, B.; Solomon, T.; Zerihun, T.; Abegaz, M.; Bekele, A.; Graichen, K. J. Electroanal. Chem. 1994, 371, 1-11. (13) Hundhammer, B.; Dhawan, S. K.; Bekele, A.; Seidlitz, H. J. J. Electroanal. Chem. 1987, 217, 253-259. (14) Wilke, S.; Franzke, H.; Mu ¨ ller, H. Anal. Chim. Acta 1992, 268, 285-292. (15) Wilke, S. Anal. Chim. Acta 1994, 295, 165-172. 10.1021/ac0259459 CCC: $25.00
© 2003 American Chemical Society Published on Web 01/03/2003
have used laser ablation of poly(ethylene terephthalate) (PET) films to fabricate well-defined microinterfaces between organic and aqueous electrolyte phases. The initial reports dealt with the steady-state current under quiescent conditions, analogous to a microdisk electrode, established at a single microinterface.16,17 Subsequent articles extended this work to the fabrication of regular arrays of microinterfaces, consisting of ∼100 array elements, for both quiescent and flowing solutions.18 A complication was the finite number of array elements, meaning that a significant number of the elements lay on the array’s perimeter, with different mass-transport regimes applying to the outlying elements.18 An alternative approach was pursued by Sawada and co-workers, who used a thin-layer flow cell with the stationary organic phase stabilized beneath a hydrophilic dialysis membrane to develop a protocol for the detection of lithium ions based on pulsed amperometry.11 Girault presented a similar approach to the detection of monovalent cations, although an organic gel was employed with the two phases separated by a micromachined PET film rather than a dialysis membrane.9 This latter approach was subsequently extended to the detection of ions by amperometry in a flow-injection system,19 whereas the former approach was extended to a thin-layer configuration allowing the complete conversion of the aqueous stream.20 In one of the above cases, it was shown that the aqueous stream flow rate affected the ion transfer current;9 however, interpretation in terms of established models of hydrodynamic voltammetry was not performed. There are two factors contributing to this: (i) the flow profile established in the aqueous stream has not been established, and (ii) the modification of the diffusion field resulting from the presence of the membrane was not known. The modification of solid electrode surfaces with various types of permeable overlayer materials has been widely reported.21,22 As well as using electroactive polymers with conductivity dependent on the polymer redox state,23 the transport of electroactive moieties through pores within insulating overlayers has been probed.24 The analysis of the transport to such modified electrodes is complex, since it can be difficult to differentiate between transport of electroactive material via diffusion through the polymer or via transport through the membrane pores.25 More recently, the advent of well-characterized electrode overlayers, such as self-assembled monolayers, and the advent of scanning probe microscopy techniques have enabled the correlation between overlayer structure and voltammetric response to be investigated.26,27 A further recent development has been the
application of “track-etched” polymer membranes, which consist of straight monodisperse pores with submicrometer diameters, to the modification of electrode surfaces.28 In particular, Martin and co-workers have reported the formation of microelectrode arrays by the electrodeposition of noble metals within the pores of track-etched membranes.29,30 Our group previously reported the modificaton of the metal/electrolyte interface with PET membranes and showed that, for the particular membranes employed in this text (vide infra), an overlapping mass-transport regime occurs, with the current being proportional to the entire membrane area,31 as has been predicted32,33 and observed with many other types of microelectrode array.29,30,34,35 Hydroydnamic experiments, such as rotating-disk voltammetry, have been widely employed to shed light on the mechanisms of charge transfer at modified solid electrodes.22,36 The imposition of flow on the solution phase with the intrinsic variation in mass transport assists in the identification of the transport process that acts as the rate-determining step. In addition, with regard to the modified liquid/liquid electrochemical systems discussed above, convective flow leads to a further advantage, since steady-state fluxes can be established at the ITIES with flow. Conventionally, voltammetry at the ITIES is performed under diffusion-only conditions, which gives rise only to steady-state currents for micrometer-scale interfaces.16,37 Measurements of transient current responses, particularly for trace analysis, are hampered by difficulties in resolving the Faradaic response of the analyte from the charging current. The difficulties in controlling the resistance of the organic phase in liquid/liquid experiments also lead to distortions in transient voltammetry; thus, steady-state measurements may offer improved analytical capabilities, such as lower limits of detection. Moreover, the rapid and controllable masstransport accessible with the hydrodynamic approach could provide information on the mechanism of charge-transfer processes at liquid/liquid interfaces, including the measurement of kinetics of interfacial extraction processes.3 Flow with a calculable laminar profile can readily be achieved in a channel configuration.38 Compton et al have developed the channel flow cell in which hydrodynamic voltammetry is performed at a metallic electrode embedded in one face of the channel. Analytical and numerical models allow the prediction of the analyte flux (electrode current) as a function of channel geometry and solution flow rate for various electrode reaction mechanisms.39 Kontturi and co-workers recently applied the
(16) Campbell, J. A.; Girault, H. H. J. Electroanal. Chem. 1989, 266, 465-469. (17) Osborne, M. D.; Shao, Y.; Pereira, C. M.; Girault, H. H. J. Electroanal. Chem. 1994, 364, 155-161. (18) Wilke, S.; Osborne, M. D.; Girault, H. H. J. Electroanal. Chem. 1997, 436, 53-64. (19) Lee, H. J.; Pereira, C. M.; Silva, A. F.; Girault, H. H. Anal. Chem. 2000, 72, 5562-5566. (20) Sawada, S.; Taguma, M.; Kimoto, T.; Hotta, H.; Osakai, T. Anal. Chem. 2002, 74, 1177-1181. (21) Murray, R. W. Acc. Chem. Res. 1980, 13, 135-142. (22) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001; Chapter 14. (23) Burgmayer, P.; Murray, R. W. J. Am. Chem. Soc. 1982, 104, 6139-6140. (24) Koochaki, Z.; Christie, I.; Vadgama, P. J. Membr. Sci. 1991, 57, 83-94. (25) Peerce, P. J.; Bard, A. J. J. Electroanal. Chem. 1980, 112, 97-115. (26) Sreenivas, G.; Ang, S. S.; Fritsch, I.; Brown, W. D.; Gerhardt, G. A.; Woodward, D. J. Anal. Chem. 1996, 68, 1858-1864. (27) Baker, W. S.; Crooks, R. M. J. Phys. Chem. B 1998, 102, 10041-10046.
(28) Hulteen, J. C.; Martin, C. R. J. Mater. Chem. 1997, 7, 1075-1087. (29) Penner, R. M.; Martin, C. R. Anal. Chem. 1987, 59, 2625-2630. (30) Cheng, I. F.; Whiteley, L. D.; Martin, C. R. Anal. Chem. 1989, 61, 762766. (31) Kralj, B.; Dryfe, R. A. W. Phys. Chem. Chem. Phys. 2001, 3, 3156-3164. (32) Gueshi, T.; Tokuda, K.; Matsuda, H. J. Electroanal. Chem. 1978, 89, 247255. (33) Shoup, D.; Szabo, A. J. Electroanal. Chem. 1984, 160, 19-26. (34) Sabatini, E.; Rubenstein, I.; Maoz, R.; Sagiv, J. J. Electroanal. Chem. 1987, 219, 365-371. (35) Cheng, I. F.; Martin, C. R. Anal. Chem. 1988, 60, 2163-2165. (36) Mallouk, T. E.; Cammarata, V.; Crayston, J. A.; Wrighton, M. S. J. Phys. Chem. 1986, 90, 2150-2156. (37) Stewart, A. A.; Taylor, G.; Girault, H. H.; McAleer, J. J. Electroanal. Chem. 1990, 296, 491-515. (38) Beek, W. J.; Muttzall, K. M. K.; van Heuven, J. W. Transport Phenomena, 2nd ed.; Wiley: New York, 1999; Chapter 2. (39) Cooper, J. A.; Compton, R. G. Electroanalysis 1998, 10, 141-155.
Analytical Chemistry, Vol. 75, No. 3, February 1, 2003
487
Figure 1. Diagram of the channel flow cell (cell 1) constructed for application to the ITIES. (A) Channel (aqueous) half of the flow cell: (1) compression block to apply pressure to the gasket, (2) glass channel cover plate, (3) aqueous electrodes, (4) aqueous inlet and outlet, (5) channel, (6) ITIES, and (7) Viton gasket. The coordinate scheme adopted is shown. (B) Schematic representation of the laminar flow profile in the mobile (aqueous) phase. (C) Stationary (organic) cell half: (1) PET membrane (supporting the interface when assembled with the channel), (2) Luggin capillary, (3) organic solution inlet (connected to a gastight syringe), (4) silver/silver tetraphenylborate reference electrode (sealed directly into the cell), and (5) platinum counter electrode (sealed directly into the cell). (D) Gravity-fed flow system: (1) constant head reservoir, (2) feed reservoir (for the constant head), (3) constant head overflow, (4) control capillaries, and (5) system waste outlet.
channel configuration to the gel-stabilized ITIES.40 In parallel, recent preliminary reports from our laboratories have described liquid/liquid measurements within a channel cell.41,42 The present report extends the latter work to the quantitative analysis of the ion transfer currents in the channel flow cell, using polyester tracketched membranes to stabilize the ITIES. The membrane employed at the ITIES has been investigated under stationary conditions and been shown to have a “passive” influence on the interfacial transfer of the ion investigated here. EXPERIMENTAL SECTION Apparatus The cell was constructed from two main parts (see Figure 1): the channel, containing the mobile aqueous phase, and a glass cell half containing the stationary phase (the organic electrolyte solution). The channel (Figure 1A) was milled into a PTFE block with dimensions of 40 (length), 7 (width) and 1.1 (40) Liljeroth, P.; Johans, C.; Kontturi, K.; Manzanares, J. A. J. Electroanal. Chem. 2000, 483, 37-46. (41) Fulian, Q.; Fisher, A. C.; Dryfe, R. A. W.; Roberts, E. P. L. J. Electroanal. Chem. 2000, 483, 197-200. (42) Yunus, K.; Marks, C. B.; Fisher, A. C.; Allsopp, D. W. E.; Ryan, T. J.; Dryfe, R. A. W.; Hill, S. S.; Roberts E. P. L.; Brennan C. M. Electrochem. Comm. 2002, 4, 579-583.
488
Analytical Chemistry, Vol. 75, No. 3, February 1, 2003
mm (depth). Flow in the channel was induced as shown schematically in Figure 1B. A 6-mm-diameter hole within the channel floor enabled the stationary cell half (Figure 1C) to be incorporated flush to the floor of the channel. A track-etched membrane, as employed by Martin and co-workers to modify the metal/ electrolyte interface,28-30 was used to stabilize the stationary organic phase against the mobile aqueous phase. The membrane was manufactured from PET (by Osmonics Inc., Livermore CA), the material used in the earlier laser ablation study by Girault and co-workers.16-18 PET is chemically resistant to the organic solvents required for liquid/liquid voltammetry43 and does not swell appreciably in such media. The membrane had a mean pore diameter of 100 nm, a high pore density (4 × 108 pores/cm2) and a thickness of 6 × 10-4 cm. The membrane was attached to the top of the stationary cell half using chemically resistant silicone sealant. The total area of exposed membrane, Amem, varied between 0.02 and 0.13 cm2. Exact dimensions of the exposed membrane were obtained via optical microscopy (Leica DM-IL inverted stereo microscope, Leica Microsystems, Wetzlar, Germany). The total pore area, Apore, can be obtained by multiplying (43) Poretics catalog; Osmonics Inc.: Livermore, CA, 1997; pp 32-35.
Figure 2. (A) Stationary liquid/liquid cell without membrane-supported interface. (B) Stationary liquid/liquid cell with the interface supported by PET membrane. Legend for both cells: (1) aqueous reference electrode, (2) aqueous counter electrode, (3) organic counter, (4) organic reference, and (5) PET membrane (cell B only).
Amem by the porosity of the membrane, (1 - θ). The manufacturers quote a value of 0.03 for (1 - θ).43 The top of the channel was sealed with a glass plate using a Viton rubber gasket to form the seal. An additional PTFE block was bolted on top to hold the channel together. The aqueous phase electrodes were placed on the inside of the glass cover plate: the reference electrode was placed upstream with respect to the ITIES, and the counter electrode was placed downstream to prevent any possible interference from electrolysis products formed at this electrode. The reference electrode was a 0.05-cmthick piece of silver foil (Advent) attached to the glass plate using the silicone sealant. The foil was then oxidized in a 3 M aqueous solution of lithium sulfate (Aldrich, 99+%) using a 9-V battery with a 1.5-kΩ resistor in series, forming a silver/silver sulfate electrode. The counter electrode was a 0.05-cm-thick piece of platinum foil (Advent) that spanned the width of the channel and had a length of 1 cm. The stationary cell half was employed to contain the organic phase and to support the membrane within the channel. This half of the cell was constructed from borosilicate glass with the electrodes sealed directly into the glass. The counter electrode was a 0.2-cm-diam platinum disk welded to a 0.05-cm-diam platinum wire (Advent), which was sealed into the main body of the cell. The reference electrode was a 0.075-cm-diam silver wire (Advent) sealed into a luggin capillary. The wire was then oxidized in a solution of 3 M sodium tetraphenylborate (Lancaster) in acetone (Prolabo) for 24 h in the dark, using a 9-V battery with a 5.1-kΩ resistor attached in series. The stationary cell half was attached to a 5-cm3 gastight syringe (Hamilton) using PTFE tubing. The combined cell was placed within a homemade Faraday cage and attached to an EG&G model 273 potentiostat, converted for 4-electrode use, which was controlled by a personal computer. The overall gravity-fed flow system is depicted schematically in Figure 1D. The head reservoir was designed to maintain a constant head level at the height of the overflow. The waste outlet allowed the overall system height to be adjusted, enabling volume flow rates in the range of 1.4 × 10-4 to 0.2 cm3 s-1 to be obtained. The three parallel control capillaries incorporated into the flow system allowed three different volume flow rates to be accessed at a given system height.
The stationary cells used for comparative purposes, with and without the PET track-etched membrane, are shown schematically in Figure 2. Chemicals The transfer of the tetraethylammonium (TEA) ion across the water/nitrobenzene (NB) interface was used as a model system in all experimental configurations.18 The aqueous supporting electrolyte, lithium sulfate (Aldrich, 99.99+%) and the TEA chloride (Fluka, electrochemical grade) were used as supplied, with water of resistivity of 18 MΩ cm that was prepared using a Milli-Q filtration system (Milli-pore, Watford, U.K.). The organic supporting electrolyte was made in-house, using a reported procedure,44 from sodium tetraphenylborate (Lancaster, 99%) and bis(triphenylphosphoranylidene)ammonium chloride (Lancaster, 97%). NB (Aldrich, 99.9%, ACS reagent grade) was used as the organic solvent for the recrystallized electrolyte, bis(triphenylphosphoranylidene)ammonium tetraphenylborate (BTPPA TPB). The resulting cell can be denoted as
Ag(s)|Ag2SO4(s)|0.05 M Li2SO4(aq), 1 × 10-3 M TEA Cl(aq)|| 0.02 M BTPPA TPB(NB)|AgTPB(s)|Ag(s)
where the double bar denotes the interface to be polarized. RESULTS AND DISCUSSION Membrane-Stabilized Interface Under Stationary Conditions As noted above, the uncertainties regarding the role of the membrane with respect to the interfacial mass-transport regime have hampered quantitative analyses of previous studies involving the ITIES under flowing conditions.9,11,12,14,15 The initial part of this study investigated the influence of the track-etched PET membrane with its well characterized geometry on TEA transfer using stationary solutions. The results obtained were compared against those acquired at a conventional (i.e., membrane-free) ITIES. The voltage scan rates (ν) used for these experiments were kept below 0.05 V s-1, because this was the maximum scan rate used subsequently in the hydrodynamic experiments. Peak-shaped voltammetry was observed, as expected for a diffusion-controlled (44) Shao, Y.; Stewart, A. A.; Girault, H. H. J. Chem. Soc., Faraday Trans 1991, 87, 2593-2597.
Analytical Chemistry, Vol. 75, No. 3, February 1, 2003
489
Figure 3. (A) Cyclic voltammetry as a function of ν (increasing from 0.01 V s-1 to 0.05 V s-1 by increments of 0.01 V s-1) for cell 2A. (B) Randles-Sevcˇ ik analysis of data from cell 2A (also increasing from 0.01 V s-1 to 0.05 V s-1 by increments of 0.01 V s-1). The solid and broken lines correspond to the forward (positive-going sweep) and reverse currents, respectively. (C) Cyclic voltammetry as a function of ν for cell 2B. (D) Randles-Sevcˇ ik analysis of data from cell 2B. The solid and broken lines correspond to the forward (positive-going sweep) and reverse currents, respectively.
ion transfer. The peak currents, Ip, were treated in terms of the Randles-Sevcˇik equation,45
Ip ) 0.4463
( ) z 3F 3 RT
1/2
AD1/2c0υ1/2
(1)
where z is the charge magnitude of the transferring ion (unity for TEA), A is the interfacial area, and c0 is the bulk concentration of TEA. The other symbols have their usual meanings. From Figure 3B, the TEA diffusion coefficient, D, was thus found to be 5.6 ( 0.5 × 10-6 cm2 s-1 at the experimental temperature (17 °C), which compares with literature values (at 25 °C) that range from 7.0 × 10-6 to 9.3 × 10-6 cm2 s-1.46,47 The two halves of the voltammogram were symmetrical because identical diffusion regimes are established in both electrolyte phases. The viscosity of NB is approximately twice that of water,48 leading to an (45) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001; p 231. (46) Taylor, G.; Girault, H. H. J. J. Electroanal. Chem. 1986, 208, 179-183. (47) Wandlowski, T.; Maracˇek, V.; Samec, Z. Electrochim. Acta 1990, 35, 11731175. (48) CRC Handbook of Chemistry and Physics; Lide, D. R., Frederikse, H. P. R., Eds.; CRC Press: Boca Raton, 1996; Chapter 6, p 247.
490
Analytical Chemistry, Vol. 75, No. 3, February 1, 2003
associated decrease in the TEA diffusion coefficient in this phase. The effect of nonequal diffusion coefficients is a small shift in the half-wave potential of the transfer peak.49 Experiments were performed at the membrane-supported ITIES under stationary conditions. In the range of ν used, the square-root variation in Ip with ν showed that the current response of the system was identical to that obtained in the absence of the membrane, which is in accord with our recent reports of ion transfer at the PET membrane-modified interface.50,51 This implies that an overlapping linear diffusion model describes transport to the membranesupported ITIES, and hence, treatment of the voltammetric data in terms of eq 1 using A equal to Amem, rather than Apore, yielded a value of D identical to the value derived from the unsupported ITIES (see Figure 3D). As noted in the Introduction, such behavior is well-known for metallic microelectrode arrays,32-35 whereas there are very few reports of overlapping diffusion behavior at the liquid/liquid interface. Previous theory predicts the onset of the overlapping diffusion regime from the dimensions of the electrode array, including (1-θ), and the time-scale of the volta(49) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001; p 229. (50) Dryfe, R. A. W.; Kralj, B. Electrochem. Comm. 1999, 1, 128-130. (51) Kralj, B.; Dryfe, R. A. W. Phys. Chem. Chem. Phys. 2001, 3, 5274-5282.
Figure 4. Comparison of voltammetric response of cells 2A and 2B. To account for differences in A, current density, J, is plotted as a function of potential difference for cell 2A (1) and cell 2B (2) under stationary conditions (ν of 0.02 V s-1). The peak current densities for TEA transfer from water to NB are highlighted by the arrows.
mmetric experiment, dictated by ν. A dimensionless parameter, Kλ1/2, dictates the transition from individual to overlapping diffusion fields. The parameter is defined as52
κλ1/2 )
1 0.6R0
xDRT Fν
(2)
where R0 is half the mean separation between the pores (obtained from the manufacturer’s quoted pore density of 4 × 108 cm-2). Substitution of the relevant parameters into eq 2 shows that for the ν values employed, Kλ1/2 is at least 19. Note that diffusion field overlap is believed to occur when the value of Kλ1/2 exceeds 10, indicating that the observed voltammetry is in accord with existing models of microelectrode arrays.52 The presence of the membrane causes an increase in the uncompensated resistance of the cell, Figure 4, as evidenced by the change in the baseline gradient. Whereas the voltammetry shown in Figure 3A is symmetric, inspection of Figure 3C demonstrates that there is some asymmetry with respect to the ion transfer process on either side of the liquid/liquid interface. This asymmetry can be attributed to the asymmetrical position of the ITIES with respect to the membrane pores.51 The PET surface is known to be hydrophobic and, therefore, preferentially wetted by the organic phase;18,53 hence, a less complete degree of diffusion field overlap will be established on the organic side of the ITIES, which is reflected in the lower sensitivity of the current to ν. The lack of symmetry concerning the mass-transport regimes in the aqueous and organic phases can be assessed from Figure 3B and D, which show that the ratio of forward (aqueous to organic) to reverse (organic to aqueous) peak currents is approximately unity for the membrane-free case but deviates increasingly at higher scan rates for the membrane case. At higher ν, there is less time for overlap between the adjacent diffusion fields to occur in the NB phase; (52) Amatore, C.; Save´ant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39-51. (53) Beriet, C.; Girault, H. H. J. Electroanal. Chem. 1998, 444, 219-229.
Figure 5. Voltammograms performed under flow conditions in the channel cell. In all cases, ν was 5 × 10-3 V s-1. Vf was (a) 3.43 × 10-3 cm3 s-1, (b) 1.53 × 10-2 cm3 s-1, and (c) 3.24 × 10-2 cm3 s-1.
hence, the reverse current in Figure 3C tends toward a recessed microelectrode response.54,55 The magnitude of the current is difficult to predict because the total charge passed is dictated by the charge passed on the forward part of the voltammogram. Hence, analysis of the voltammetry can only be performed by consideration of the coupled diffusion fields in the adjacent phases.55 Membrane-Stabilized Interface under Flowing Conditions. The PET membrane-stabilized ITIES was combined with the channel flow cell to induce laminar flow on the aqueous side of the ITIES. Mass transport can reach a steady state under these conditions, with convection along the length of the cell (x coordinate) balanced by diffusion of the ion in the coordinate normal to the ITIES (y coordinate; see Figure 1). Hence,
D
∂c ∂2c - vx ) 0 ∂x ∂y2
(3)
where vx is the velocity of the solution. Under laminar flow conditions, a parabolic flow profile is established with respect to the y coordinate, with the velocity reaching a maximum, v0, in the center of the channel,39
( )
vx ) v0 1 -
y2
h2
(4)
where h is the half-height of the channel. Figure 5 shows typical voltammograms observed for TEA transfer across the liquid/liquid interface at various volume flow rates. The sigmoidal forward sweep, representing the transfer of the TEA from the aqueous phase to the organic phase, is characteristic of a steady-state response, whereas the reverse sweep displays a diffusioncontrolled peak-shaped response. This asymmetry in the cyclic voltammetry is expected because of the different mass transport regimes controlling TEA supply on either side of the ITIES. Mass(54) Bond, A. M.; Luscombe, D.; Oldham, K. B.; Zoski, C. G. J. Electroanal. Chem. 1988, 249, 1-14. (55) Josserand, J.; Morandini, J.; Lee, H. J.; Ferrigno, R.; Girault, H. H. J. Electroanal. Chem. 1999, 468, 42-52.
Analytical Chemistry, Vol. 75, No. 3, February 1, 2003
491
Figure 6. Levich analysis: dependence of the steady-state limiting current on volume flow rate.
transport in the flowing aqueous phase is described by eq 3, whereas diffusion is the sole mode of mass-transport in the stationary organic phase. It has been shown previously that steadystate mass transport is established for a metallic channel electrode for experimental conditions similar to those employed here with ν less than 0.01 V s-1.56,57 The apparent steady-state current established for TEA transfer from the flowing aqueous phase to the stationary organic phase showed no sign of reducing over time (over the time-scale of an individual voltammogram, ∼2 min), which suggested that the buildup in concentration of the transferred ion on the organic side of the interface imposed no observable influence on the rate of transport to the ITIES. Instead, the rate of transfer appears to be controlled solely by the mass transport rate on the side of the ITIES where the ion originated (in this case, the aqueous phase), at least for the experimental conditions employed. As the voltammograms of Figure 5 indicate, control of the resultant steady-state ion transfer current, Ilim, could be achieved by adjusting the volume flow rate of the aqueous phase (Vf). A linear dependence of Ilim on the cube root of Vf was observed in accord with the Levich equation (eq 5), which describes the mass-transport limited current in a channel resulting from the solution of eqs 3 and 4,39
Ilim ) 0.925zFc0w
( ) xe2Vf D2
1/3
h2d
(5)
where d is the channel width, and xe and w, respectively, denote the length and width of the membrane-supported interface. The Levich plot is given as Figure 6. Combination of the least-squares gradient from Figure 6 with eq 4 gave D for TEA in aqueous solution of 5.7 ( 0.5 × 10-6 cm2 s-1, in close agreement with the value obtained under stationary conditions. The quantitative agreement between the observed voltammetry and that predicted by eq 5 indicates that the overlapping diffusion behavior is retained for the range of Vf employed experimentally. As with the membrane-supported stationary cell case, there is no simple analytical expression to describe the flow rate dependence of the (56) Aoki, K.; Tokuda, K.; Matsuda, H. J. Electroanal. Chem. 1986, 209, 247258. (57) Fisher, A. C.; Compton, R. G. J. Appl. Electrochem. 1992, 22, 38-42.
492
Analytical Chemistry, Vol. 75, No. 3, February 1, 2003
Figure 7. Transition from steady-state to peak-shaped voltammetry with increasing ν: (1) 0.010, (2) 0.013, (3) 0.014, (4) 0.015, (5) 0.020, and (6) 0.050 V s-1 under flow conditions (Vf ) 3.0 × 10-3 cm3 s-1).
reverse (organic to water) peak current, since the current is also dependent on the mass transport in the stationary organic phase and in the mobile aqueous phase, from where the transferring ion originated. The transition in the aqueous-phase mass-transport regime from a sigmoidal ion transfer voltammogram, reflecting steadystate mass transport, to a peak-shaped response, representing purely diffusive control, was investigated by varying ν at a constant value of Vf (3 × 10-3 cm3 s-1). The voltammograms exhibited a steady-state response for a ν of 0.01 V s-1. At higher ν, a peakshaped response became gradually more apparent (Figure 7), which evolved to the purely diffusive response expected on the basis of eq 1. Previous work has shown that the transition from the steady-state response of eq 5 to the peak-shaped response can be characterized by a normalized parameter, σ1/2, defined as56
σ1/2 ) D-1/6
x
( )
zFν hxe RT 2v0
1/3
(6)
The purely diffusive regime occurs for σ1/2 > 2.7, which corresponds to a σ of 0.015 V s-1, with the ITIES channel cell at the particular flow rate employed. Accordingly, the Ip values observed at 0.02 V s-1 and 0.05 V s-1 should follow the ν1/2 dependence predicted by eq 1. This is found to be the case. Finally, given the possible advantages of hydrodynamic voltammetry at the ITIES over its stationary counterpart with regard to sensitivity, a preliminary evaluation of the limits of detection of the three cells employed in this study was performed. The limit of detection for TEA, under the conditions employed in this article, was evaluated from the standard deviations of the intercepts of the Ilim or Ip plots as functions of c0 (see Supporting Information). The limits of detection obtained were 5 × 10-5 M for the flow cell, as compared to 3 × 10-4 M for the stationary cell with the unsupported ITIES (cell 2A) and 2 × 10-4 M for the stationary cell with the membrane-supported ITIES (cell 2B). It is notable that under these conditions, only a marginal improvement in sensitivity is observed for the membrane-supported cell, despite the advantages often claimed for microelectrode arrays.29,30,52 This point may be attributed to the greater difficulties in compensating
for cell resistance at the ITIES. Furthermore, the advantages suggested above with regard to the flow cell have been borne out: an order of magnitude improvement in the limit of detection has been observed. It should be noted that the conditions of these experiments have not been optimized. For example, use of higher Vf and smaller Amem should further improve the detection limit of the flow cell. CONCLUSION The modification of a liquid/liquid interface with the PET membrane gives a response analogous to that of the unmodified interface (Figure 3) as a result of the overlapping diffusion fields established at each of the microinterfaces, that is, the membrane pores, on the aqueous side of the interface. This result holds true at lower ν, where the diffusion fields for each of the microinterfaces are sufficiently large as to overlap with each neighboring field. The membranes employed have previously been demonstrated to play a “passive” role in TEA transfer;50,51 hence, mass transport is controlled by transfer across the boundary layers established on either side of the ITIES, rather than across the membrane itself. A noticeable phenomenon observed for all of the flow experiments performed was the apparent independence of Ilim, corresponding to aqueous to organic ion transport, from the concentration of the transferred ion on the organic side of the interface. The buildup of the transferred ion on the organic side of the ITIES with increasing Vf was confirmed by the (58) Ohkouchi, T.; Kakutani, T.; Osakai, T.; Senda, M. Rev. Polarogr. (Kyoto) 1985, 31, 123. (59) Kralj, B.; Dryfe, R. A. W. J. Phys. Chem. B 2002, 106, 6732-6739. (60) Wilke, N.; Iglesias, R. A.; Chesnuik, S. G.; Dassie, S. A.; Baruzzi, A. M. Bull. Chem. Soc. Jpn. 2002, 75, 235-240. (61) Harnisch, J. A.; Porter, M. D. Analyst 2001, 126, 1841-1849. (62) Ismagilov, R. F.; Ng, J. M. K.; Keins, P. J. A.; Whitesides, G. M. Anal. Chem. 2001, 73, 5207-5213.
increasing size of the diffusion-controlled reverse scan peaks at higher Vf. The cell developed here should be of utility for the study of chemical extraction processes across a liquid/liquid boundary. The distinct mass-transport regimes in both phases of the cell make this device analogous to the micropipet-supported ITIES pioneered by the groups of Senda58 and Girault,37,46 and the control over the mass-transport rates renders it similar to the more recent ITIES-based rotating-diffusion cell.59,60 In the latter case, the currents passed across the PET membrane-supported ITIES were also dependent on Amem, rather than Apore. We believe both the rotating-diffusion system and the channel-based device outlined here will be of considerable utility with regard to the mechanistic study of extraction/transfer processes at liquid/liquid interfaces as a result of the controllable steady-state mass-transport. Furthermore, the flow basis of the systems described here and enhanced sensitivity over stationary cells makes the channel device particularly useful for the on-line detection of ions, especially those that are nonredox-active. Extensions of the present on-line analytical system to liquid/liquid-based electrochemically modulated chromatography61 and to microfluidic systems are readily envisaged.62 ACKNOWLEDGMENT The financial support of the UK EPSRC (GR/M73590) is acknowledged. SUPPORTING INFORMATION AVAILABLE Figures showing current versus TEA concentration plots for A (cell 2A), B (cell 2B) and C (cell 1). This material is available free of charge via the Internet at http://pubs.acs.org. Received for review July 16, 2002. Accepted November 6, 2002. AC0259459
Analytical Chemistry, Vol. 75, No. 3, February 1, 2003
493
