Langmuir 2002, 18, 2313-2318
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Kinetics of IrCl62- Ion Transfer across the Water/ 1,2-Dichloroethane Interface and the Effect of a Phospholipid Monolayer Jie Zhang and Patrick R. Unwin* Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K. Received October 3, 2001. In Final Form: December 4, 2001 IrCl62- ion transfer (IT) across a native water/1,2-dichloroethane (DCE) interface has been investigated using microelectrode measurements at expanding droplets and scanning electrochemical microscopy (SECM). The interfacial potential drop was controlled by ClO4- ion in each phase, which also maintained charge neutrality during IrCl62- IT. IrCl62- transferred across the interface with an apparent transfer coefficient of 0.63 ( 0.05. SECM-double potential step chronoamperometry was used to study the effect of L-Rphosphatidylethanolamine, dilauroyl on the transfer of IrCl62- from water to the DCE phase. The phospholipid was found to significantly diminish the rate of IT, with the retardation effect dependent on the interfacial phospholipid concentration. Kinetic data obtained from these studies were successfully explained using a simple energy barrier model.
Introduction Immiscible liquid/liquid interfaces with adsorbed phospholipid monolayers represent a useful model for investigating fundamental physicochemical processes in biological (cellular) membranes. This configuration is particularly attractive because the interfacial potential drop is relatively well-defined and readily controlled.1,2 Compared to lipid bilayers,3 phospholipid monolayers are more readily formed1,2,4,5 and have high physical stability. Ion transfer (IT) between two immiscible liquid phases is currently attracting interest, with a view to understanding processes of fundamental,6 biological,7 and industrial8 importance. Moreover, IT across phospholipid monolayers at liquid/liquid interfaces has been advocated as a model system for studying the permeability of cell membranes.2 It has been shown that monolayers may affect the electrical potential near the interface, and thus the driving force for IT, and also introduce steric (blocking) effects on IT.1,2,9,10 * To whom correspondence should be addressed. E-mail address:
[email protected]. (1) Grandell, D.; Murtoma¨ki, L.; Sundholm, G. J. Electroanal. Chem. 1999, 469, 72 and references therein. (2) Kakiuchi, T. In Liquid-Liquid Interfaces: Theory and Methods; Volkov, A. G., Deamer, D. W., Eds.; CRC Press: Boca Raton, FL, 1996; p 317 and references therein. (3) Tien, H. T. Bilayer Lipid Membranes (BLM): Theory and Practice; Marcel Dekker: New York, 1974. (4) (a) Grandell, D.; Murtoma¨ki, L. Langmuir 1998, 14, 556. (b) Grandell, D.; Murtoma¨ki, L.; Kontturi, K.; Sundholm, G. J. Electroanal. Chem. 1999, 463, 242. (5) Liljeroth, P.; Ma¨lkia¨, A.; Cunnane, V. J.; Kontturi, A. K.; Kontturi, K. Langmuir 2000, 16, 6667. (6) For recent papers, see for example: (a) Kakiuchi, T.; Teranish, Y. Electrochem. Commun. 2001, 3, 168. (b) Ohde, H.; Uehara, A.; Yoshida, Y.; Maeda, K.; Kihara, S. J. Electroanal. Chem. 2001, 496, 110. (c) Terui, N.; Nakatani, K.; Kitamura, N. J. Electroanal. Chem. 2000, 494, 41. (d) Marcus, R. A. J. Chem. Phys. 2000, 113, 1618. (e) Sawada, S.; Osakai, T. Phys. Chem. Chem. Phys. 1999, 1, 4819. (f) Katano, H.; Senda, M. Bull. Chem. Soc. Jpn. 1999, 72, 2085. (g) Kontturi, K.; Manzanares, J. A.; Murtoma¨ki, L.; Schiffrin, D. J. J. Phys. Chem. B 1997, 101, 10801. (7) (a) For recent papers, see for example: (a) Ulmeanu, S.; Lee, H. J.; Fermin, D. J.; Girault, H. H.; Shao, Y. H. Electrochem. Commun. 2001, 3, 219. (b) Samec, Z.; Trojanek, A.; Langmaier, J.; Samcova, E.; Malek, J. Electroanalysis 2000, 12, 901. (c) Senda, M.; Kubota, Y.; Katano, H. Anal. Sci. 1997, 13, 285. (d) Arai, K.; Kusu, F.; Takamura, K. Anal. Sci. 1997, 13, 173. (8) (a) Freiser, H. Chem. Rev. 1988, 88, 611. (b) Danesi, P. R.; Chiarizia, R. CRC Crit. Rev. Anal.Chem. 1980, 10, 1. (c) Atherton, J. H. Res. Chem. Kinet. 1994, 2, 193.
A popular method for the formation of a monolayer at oil/water interfaces is based on the adsorption of phospholipid from the bulk organic phase.2 Early studies by Koryta and co-workers11 demonstrated that the transfer of Cs+ and tetramethylammonium (TMA+) ion, and the facilitated transfer of Na+ by dibenzo-18-crown-6, was retarded by lecithin monolayers at the water/nitrobenzene interface. Retardation effects were also observed by Girault and Schiffrin for tetraethylammonium (TEA+) IT across a lecithin monolayer at a water/1,2-dichloroethane (DCE) interface.12 Likewise, Cunnane et al.10 found that a monolayer of egg lecithin at the water/DCE interface inhibited the transfer of TEA+. These latter experimental results were analyzed using a simple model which considered that the Gibbs energy of activation for IT was equivalent to the work required to open a pore in the monolayer, with the size close to that of the transferring ion. In the absence of a monolayer, facilitated IT has been found to be rapid.13 Kakiuchi et al.14 measured the rate of TMA+ and TEA+ transfer across a phosphatidylcholine monolayer at the water/nitrobenzene interface, identifying that the IT rate decreased when the monolayer was in the liquid-condensed state, while in the liquid-expanded state the monolayer had no effect on the transfer rate of either ion. In contrast, Kontturi et al.15 found that the transfer of TEA+ across a phosphatidylcholine monolayer at a micropipet water/ DCE interface was slightly enhanced. A similar phenomenon was observed by Manzanares et al.9 for cation transfer across a distearoyl phophatidylcholine monolayer (9) Manzanares, J. A.; Allen, R. M.; Kontturi, K. J. Electroanal. Chem. 2000, 483, 188. (10) Cunnane, V. J.; Schiffrin, D. J.; Fleischmann, M.; Geblewicz, G.; Williams, D. E. J. Electroanal. Chem. 1988, 243, 455. (11) Koryta, J.; Hung, L. Q.; Hofmanova, A. Stud. Biophys. 1982, 25, 90. (12) Girault, H. H.; Schiffrin, D. J. In Charge and Field Effects in Biosystems; Allen, M. J., Usherwood, P. N. R., Eds.; Abacus Press: England, 1984; p 171. (13) (a) Shao, Y. H.; Mirkin, M. V. J. Am. Chem. Soc. 1997, 119, 8103. (b) Beattie, P. D.; Delay, A.; Girault, H. H. Electrochim. Acta 1995, 40, 2961. (14) Kakiuchi, T.; Kotani, M.; Noguchi, J.; Nakanishi, M.; Senda, M. J. Colloid Interface Sci. 1992, 149, 279. (15) Kontturi, A. K.; Kontturi, K.; Murtoma¨ki, L.; Quinn, B.; Cunnane, V. J. J. Electroanal. Chem. 1997, 424, 69.
10.1021/la011511h CCC: $22.00 © 2002 American Chemical Society Published on Web 02/21/2002
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at the water/DCE interface. In a separate study,16 Kakiuchi et al. found that the liquid-condensed state of a phosphatidylethanolamine monolayer caused a decrease in the IT rate of TEA+ but slightly increased the IT rate of ClO4-. Chesniuk et al.17 studied the transfer of alkali and alkaline-earth cations across a dibehenoyl phosphatidylcholine monolayer. Both enhancement and blocking effects of the IT processes were observed, depending on the type of cation and its concentration. Following pioneering work by Brooks and Pethica on the design of Langmuir troughs for liquid/liquid interfaces,18 Grandell et al. developed a Langmuir trough to study IT across monolayers at the liquid/liquid interface.1,4 The transfer of propranolol and picrate across a distearoyl phosphatidylcholine monolayer at a water/DCE interface was selected as a model system and investigated using voltammetry under controlled surface pressure. It was found that the IT kinetics of both ions were too fast to be measured by the slow scan speed voltammetric method used. Liljeroth et al.5 formed phosphatidylcholine monolayers at an organic gel surface using a dipping method, but observed no effect of the monolayer on TEA+ transfer between the gel and water. Scanning electrochemical microscopy (SECM) is a proven technique for probing a myriad of interfacial processes,19 including interphase transfer across monolayers,20 bilayers,21 and cellular membranes.22 SECM has recently been used to study the kinetics of IT23 and molecular transfer24 across liquid/liquid interfaces. Although the effect of monolayers on electron transfer across liquid/liquid interfaces has received attention,25 there have not yet been any SECM investigations of the effect of monolayers on IT across liquid/liquid interfaces. The key advantage of SECM compared to other electrochemical techniques for this type of study is the high mass transfer rates available, which allows fast interfacial kinetics to be identified, free from bulk diffusional limitations. This is an important point in the light of recent work,6d which has indicated that many IT rates reported are close to the diffusional limit with the techniques employed. In this paper, the effect of L-R-phosphatidylethanolamine, dilauroyl (DLPE) on the transfer of IrCl62- from (16) Kakiuchi, T.; Kondo, T.; Kotani, M.; Senda, M. Langmuir 1992, 8, 169. (17) Chesniuk, S. G.; Dassie, S. A.; Yudi, L. M.; Baruzzi, A. M. Electrochim. Acta 1998, 43, 2175. (18) Brooks, J. H.; Pethica, B. A. Trans. Faraday Soc. 1964, 60, 208. (19) For recent reviews of SECM see for example: (a) Unwin, P. R. J. Chem. Soc., Faraday Trans. 1998, 94, 3183. (b) Barker, A. L.; Gonsalves, M.; Macpherson, J. V.; Slevin, C. J.; Unwin, P. R. Anal. Chim. Acta 1999, 385, 223. (c) Amemiya, S.; Ding, Z.; Zhou, J.; Bard, A. J. J. Electroanal. Chem. 2000, 483, 7. (d) Barker, A. L.; Slevin, C. J.; Unwin, P. R.; Zhang, J. In Liquid Interfaces in Chemical, Biological and Pharmaceutical Applications; Volkov, A. G., Ed.; Marcel Dekker: New York; p 283. (20) Slevin, C. J.; Ryley, S.; Walton, D. J.; Unwin, P. R. Langmuir 1998, 14, 5331. (21) (a) Amemiya, S.; Bard, A. J. Anal. Chem. 2000, 72, 4940. (b) Tsionsky, M.; Zhou, J. F.; Amemiya, S.; Fan, F. R.-F.; Bard, A. J.; Dryfe, R. A. W. Anal. Chem. 1999, 71, 4300. (22) (a) Liu, B.; Cheng, W.; Rotenberg, S. A.; Mirkin, M. V. J. Electroanal. Chem. 2001, 500, 590. (b) Yasukawa, T.; Kaya, T.; Matsue, T. Electroanalysis 2000, 12, 653. (c) Yasukawa, T.; Kondo, Y.; Uchida, T.; Matsue, T. Chem. Lett. 1998, 8, 767. (23) See for example: (a) Barker, A. L.; Unwin, P. R. J. Phys. Chem. B 2001, 105, 12019. (b) Selzer, Y.; Mandler, D. J. Phys. Chem. B 2000, 104, 4903. (c) Shao, Y. H.; Mirkin, M. V. J. Phys. Chem. B 1998, 102, 9915. (24) (a) Slevin, C. J.; Macpherson, J. V.; Unwin, P. R. J. Phys. Chem. B 1997, 101, 10851. (b) Barker, A. L.; Macpherson, J. V.; Slevin, C. J.; Unwin, P. R. J. Phys. Chem. B 1998, 102, 1586. (25) See for example: (a) Tsionsky, M.; Bard, A. J.; Mirkin, M. V. J. Am. Chem. Soc. 1997, 119, 10785. (b) Delville, M. H.; Tsionsky, M.; Bard, A. J. Langmuir 1998, 14, 2774. (c) Zhang, J.; Unwin, P. R. J. Electroanal. Chem. 2000, 494, 47.
Zhang and Unwin
Figure 1. Schematic of MEMED for the study of IrCl62- transfer from water to DCE.
an aqueous solution to DCE is studied using SECMdouble potential step chronoamperometry (DPSC).24a This system was chosen since DLPE forms a condensed monolayer at a water/DCE interface.1,2 Excess ClO4- ion was used in both phases to control the interfacial potential drop and also maintain charge neutrality during the IrCl62- IT process. Both SECM-DPSC and microelectrode measurements at expanding droplets (MEMED)26 have been used to further investigate the kinetics of the IrCl62transfer process at a native water/DCE interface. Experimental Section Chemicals. All chemicals were used as received. From SigmaAldrich, these were potassium hexachloroiridate(III) (99%), NaClO4‚xH2O (A.R.), DLPE (98%), and DCE (HPLC grade). Other chemicals were 7,7,8,8-tetracyanoquinodimethane (TCNQ, 98%, Lancaster), NaCl (A.R., Fisons), sodium hexachloroiridate(IV) hexahydrate (99%, Strem), and tetra-n-hexylammonium perchlorate (THAP, crystalline, Alfa). All aqueous solutions were prepared from Milli-Q reagent water (Millipore Corp.). Apparatus and Procedures. All electrochemical measurements were made in a two-electrode arrangement using instrumentation described previously.19a,b A saturated calomel electrode (SCE) usually served as the reference electrode, and a glasscoated Pt disk ultramicroelectrode (UME) functioned as the working electrode tip. The UME had a diameter of 25 µm (RG ) 10) for SECM and 2 µm (RG ) 4) for MEMED. RG ) rs/a, where rs is the overall radius of the tip end (electrode plus insulating sheath) and a is the electrode radius. The construction of the electrodes was described previously.27 For MEMED studies, a DCE drop containing 0.1 M THAP, was grown into an aqueous receptor phase containing 0.25 mM IrCl62-, 0.1 M NaCl, and 0.01-0.25 M NaClO4, from a capillary with an internal diameter of about 200 µm. Amperometeric detection was used and the tip current for the local detection of IrCl62- in the receptor phase was recorded as a function of time, as the drop grew toward the tip. The small size of the tip used for these measurements ensured that the electrode was a noninvasive probe of the concentration boundary layer that developed adjacent to the droplet (Figure 1). A method for (26) For a review see: Slevin, C. J.; Unwin, P. R.; Zhang, J. In Liquid Interfaces in Chemical, Biological and Pharmaceutical Applications; Volkov, A. G., Ed.; Marcel Dekker: New York, 2001; p 325. (27) Macpherson, J. V.; Unwin, P. R. J. Phys. Chem. 1994, 98, 1704.
Kinetics of IrCl62- Transfer
Langmuir, Vol. 18, No. 6, 2002 2315 from a chloroform stock solution by evaporating chloroform and redissolving DLPE in DCE. The aqueous phase was introduced to the top of the DCE phase. Initially, time-dependent experiments of DLPE adsorption were carried out by measuring the blocking effect of the DLPE monolayer on electron transfer between electrogenerated Ru(CN)63- and decamethylferrocene as an indicator of the surface coverage, using conventional steadystate SECM approach curve measurements.25 DLPE adsorption achieved equilibrium after 40 min, in agreement with earlier studies by Tsionsky et al.25a This time scale for adsorption was also confirmed independently through surface tension measurements.31 The kinetics of IT across monolayers was measured after DLPE adsorption had reached equilibrium.
Results and Discussion
Figure 2. Schematic of SECM-DPSC measurements of IrCl62transfer across the water/DCE interface. obtaining the time-dependent droplet concentration profile, from the UME response, has been outlined fully elsewhere.28 The distance (and time) at which the tip contacted the liquid/liquid interface was taken as the point where the tip current suddenly changed on reaching the interface. To measure the rate constants for IT using SECM-DPSC, a flat interface was established between an aqueous (top) phase and a DCE (bottom) phase in a cell described previously.24,29 The working UME and reference electrode were placed in the aqueous phase, containing 5 mM IrCl63-, 0.2 M NaCl, and 10 mM NaClO4. The UME was stepped from a potential where there were no Faradaic processes, to a value where the oxidation of IrCl63- was diffusion-controlled. The duration of this forward (generation) step was typically 0.01 s. Electrogenerated IrCl62- diffused to and transferred across the water/DCE interface. In the reverse (collection) step, the potential of the UME was switched to reduce IrCl62- to IrCl63- at a diffusion-controlled rate. These processes are shown schematically in Figure 2. As discussed elsewhere,24 the forward step transient provides information on the tipinterface separation (d), since the current is governed solely by the diffusion of IrCl63- to the UME in the aqueous phase. With a knowledge of the tip-interface separation, the reverse transient provides information on the kinetics of the IT process. Electroneutrality of both phases was maintained by the transfer of the common ClO4- ion, when the IT process of interest occurred at the liquid/liquid interface. The ratio of the bulk concentrations of ClO4- in the aqueous (w) and organic (o) phases, [ClO4-]w/[ClO4-]o, determined the interfacial potential drop, ∆w°φ, according to Nernst-Donnan equation30 0′ ∆w°φ ) ∆w°φClO - 4
RT [ClO4 ]w ln F [ClO4-]o
(1)
0′ where ∆w°φClO - is the formal transfer potential of ClO4 . 4 To investigate the effect of a monolayer on the IT kinetics, DLPE monolayers were formed by a simple equilibrium adsorption method.25 A DCE solution of DLPE (5-20 µM) was prepared
(28) (a) Slevin, C. J.; Unwin, P. R. Langmuir 1997, 13, 4799. (b) Slevin, C. J.; Unwin, P. R. Langmuir 1999, 15, 7361. (c) Zhang, J.; Slevin, C. J.; Unwin, P. R. Chem. Commun. 1999, 1501. (d) Zhang, J.; Unwin, P. R. Phys. Chem. Chem. Phys. 2000, 2, 1267. (e) Zhang, J.; Slevin, C. J.; Murtoma¨ki, L.; Kontturi, K.; Williams, D. E.; Unwin, P. R. Langmuir 2001, 17, 821. (29) Zhang, J.; Unwin, P. R. J. Phys. Chem. B 2000, 104, 2341. (30) Volkov, A. G.; Deamer, D. W.; Tanelian, D. L.; Markin, V. S. Liquid Interfaces in Chemistry and Biology; Wiley: New York, 1998.
Potential Dependence of the Kinetics of IrCl62Transfer across a Native Water/DCE Interface: MEMED Study. The nature of the MEMED experiments, where IrCl62- transferred to a droplet under sink conditions, meant that the process could be treated as driven irreversibly in the transfer direction from water to DCE. As discussed later, the transfer process is essentially irreversible in any case, which is attributed to the complexation of IrCl62- with THA+ in DCE. Under the conditions of the MEMED experiments, the rate constant for irreversible IT from water to DCE, kf, is expected to follow32
ln kf ) ln k0 -
RziF 0 (∆ °φ - ∆w°φIrCl 2-) 6 RT w
(2)
0 where ∆w°φIrCl 2- is the standard transfer potential of 6 2IrCl6 , k0 is the standard transfer rate constant, when 0 ∆w°φ ) ∆w°φIrCl 2-, zi is the charge number of the ion, and 6 R is the IT coefficient. Equations 1 and 2 can be combined to give
ln kf ) ln k0 + [ClO4-]w RziF 0 0′ (∆w°φIrCl (3) 2- - ∆w°φClO -) + Rzi ln 6 4 RT [ClO -] 4
o
The potential dependence of the IT kinetics was investigated by MEMED using the procedure outlined above. As defined in the Experimental Section, a relatively low concentration of IrCl62- (0.25 mM) was used to avoid solubility limitations of IrCl62- in the DCE phase, in the presence of 0.1 M THAP, particularly since MEMED operates on a fairly lengthy time scale (several seconds). Control experiments showed that the solubility limit was much less than 1 mM. Typical MEMED concentration profiles obtained with 0.1 M THAP in the DCE phase and a range of [ClO4-]w, are shown in Figure 3a, along with best fits obtained by numerical simulation. Full details of the treatment of mass transport for MEMED are given elsewhere28b,e and will not be repeated here. The following boundary condition at the droplet surface was used to extract kinetic values from these measurements
j ) kfc(i,w)
(4)
where j is the interfacial flux, kf is the rate constant for IrCl62- transfer from water to DCE, and c(i,w) is the concentration of IrCl62- close to the ITIES at the aqueous (31) Strutwolf, J.; Zhang, J.; Barker, A. L.; Unwin, P. R. Phys. Chem. Chem. Phys. 2001, 3, 5553. (32) Samec, Z. In Liquid-Liquid Interfaces: Theory and Methods; Volkov, A. G., Deamer, D. W., Eds.; CRC Press: Boca Raton, FL, 1996; p 155.
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Figure 3. Effect of [ClO4-]w on the rate constant of IrCl62transfer from water to DCE. (a) Typical IrCl62- concentration profiles determined by MEMED, where [IrCl62-]* denotes the bulk aqueous concentration. From bottom to top, the first five solid experimental curves are for [ClO4-]w ) 0.01, 0.025, 0.05, 0.1, and 0.25 M. In each case, the aqueous phase also contained 0.25 mM IrCl62- and 0.1 M NaCl, while the DCE phase contained 0.1 M THAP. The top solid experimental curve is for the case where there was no electrolyte in the DCE phase (aqueous 0.01 M NaClO4, with the other conditions defined). From bottom to top, the dashed theoretical curves are for limiting diffusioncontrolled transfer and kf ) 0.03, 0.006, 0.0021, 0.0012, 0.0003, and 0 cm s-1. (b) Tafel plot of data from (a).
side. A semi-infinite boundary condition defining the recovery of the bulk aqueous concentration of IrCl62- in the receptor phase completed the description of the problem. It can be seen that as [ClO4-]w decreases, corresponding to an increase in the driving force for anion transfer from water to DCE, the concentration gradient for IrCl62steepens. No transfer was detected in the absence of THAP in the DCE droplets, indicating that a readily exchangeable partner anion (ClO4-) is needed in the DCE phase for the process to proceed. For [ClO4-]w ) 0.01 M, the rate constant of IrCl62- transfer was greater than 0.03 cm s-1 and could not readily be distinguished from the diffusion-controlled case. However, for higher [ClO4-]w the effect of finite kinetics is clearly evident in Figure 3a. A Tafel plot of the data from Figure 3a is shown in Figure 3b, in the form of eq 3. An IT coefficient, R ) 0.63 ( 0.05 was deduced, assuming that the charge number of the transferring ion was 2. Previous experimental results of the apparent charge-transfer coefficient for IT at potentials close to the standard transfer potential suggest a value of 0.5 ( 0.1.32 This analysis indicates that IrCl62- most likely transfers across the ITIES in its native form, rather than as an ion pair with THA+, which would reduce the charge on the transferring species. A follow-up chemical reaction in the DCE phase, with the formation of either an ion pair or the initial stages of precipitation,33 is likely, as explored in the next section.
Zhang and Unwin
Figure 4. SECM-DPSC measurements of IrCl62- transfer from water to DCE with 0.1 M THAP in the DCE phase. The aqueous phase contained 5 mM IrCl63-, 0.2 M NaCl, and 0.01 M NaClO4. (a) Forward step experimental transient (solid curve), and (from bottom to top) dashed theoretical curves for d ) 1.8, 1.9 (best fit, coincident with experiment), and 2.0 µm. (b) Reverse step experimental transient (i, solid curve). The corresponding coincident dashed theoretical curve (ii) is the best fit obtained to the irreversible model with kf ) 0.05 cm s-1. The dotted theoretical curves are the reverse step transients with no transfer (iii) and diffusion-controlled irreversible transfer (iv). The dash-dotted curve (v) represents the theoretical response for reversible diffusion-controlled transfer using the parameters outlined in the text.
Kinetics and Mechanism of IrCl62- Transfer across a Native Water/DCE Interface: SECM-DPSC Study. We have shown previously that a short forward step, i.e., a fast switching time, tswitch, in SECM-DPSC facilitates the study of rapid kinetics.24a Anticipating relatively fast kinetics based on the MEMED study, a short generation time (0.01 s) was applied in the following SECM-DPSC studies. Moreover, this limited the extent of the transfer process and thus any complications that might arise from the limited solubility of IrCl62- in the DCE phase, mentioned above. Typical SECM transient responses for the forward and reverse potential steps are shown in Figure 4. The tip response is presented as time-dependent current, i, normalized with respect to the steady-state current, i(∞), for the oxidation of IrCl63- with the probe in the bulk aqueous phase. To analyze the experimental results, a kinetic model is proposed based on the scheme in Figure 1, which results in a flux expression of the form
j ) kfc(i,w) - kbc(i,o)
(5)
where c(i,o) is the concentration of IrCl62- in the organic phase adjacent to the ITIES and kb is a first-order interfacial rate constant which can be understood by (33) So¨hnel, O.; Garside, J. Precipitation; Butterworth-Heinemann, Ltd.: Oxford, 1992.
Kinetics of IrCl62- Transfer
Langmuir, Vol. 18, No. 6, 2002 2317
referring to Figure 1. The form of the interfacial boundary condition for SECM-DPSC studies can be obtained by considering a steady state for IrCl62- in the DCE phase, yielding
c(i,o) )
kfc(i,w)
(6)
kb + k1[THA+]o
where k1 can be understood by referring to Figure 1. It follows from eqs 5 and 6 that
(
j ) kfc(i,w) 1 -
kb kb + k1[THA+]o
)
(7)
As mentioned above, the transfer process is promoted by THA+, and so it is reasonable to consider k1[THA+]o . kb, under the experimental conditions, for the transfer of the hydrophilic IrCl62- ion. This leads to
j ) kfc(i,w)
(8)
which implies that SECM-DPSC data may be treated in terms of irreversible transfer, and the kinetics derived should be consistent with MEMED for conditions where the two techniques overlap. For the particular case in Figure 4a, the forward transientswhich depends only on IrCl63- diffusion in the SECM geometrysallows the tip/ interface separation to be determined as d ) 1.9 µm. The reverse transient clearly shows that IrCl62- transfers across the interface, as the current response is lower than that predicted for an inert interface. On the other hand, the current is not as small as that predicted for a diffusioncontrolled process, and a rate constant, kf ) 0.05 cm s-1, is obtained from the best fit of the data to an SECM model for irreversible transfer.24a Although the aqueous phase contained 5 mM IrCl63-/2- compared to 10 mM ClO4-, the extent of IrCl62- transfer was relatively small due to kinetic effects and the short generation time. This is important since both ClO4- and IrCl62- could influence the local potential in these experiments, but the effect of IrCl62will be negligible. If IrCl62- transfer was reversible, the following boundary condition would apply at the ITIES:24b
(
j ) kfc(i,w) - kbc(i,o) ) kf c(i,w) -
)
c(i,o) Kp
(9)
where Kp is the partition coefficient of IrCl62- between water and DCE. This case was also simulated and shown to be inapplicable, as discussed below. For the reversible transfer model, a diffusion coefficient of 6.0 × 10-6 cm2 s-1 was assumed for IrCl62- in the DCE phase. This was estimated from the Stokes-Einstein equation,34 given the viscosity (at 25 °C) of DCE (0.779 g m-1 s-1) and water (0.890 g m-1 s-1),35 and the diffusion coefficient of IrCl62- in the aqueous phase (6.8 × 10-6 cm2 s-1).36 Even using a KP value of 0.1, which is certainly an overestimate, given the limited solubility of IrCl62- in DCE, the results in Figure 4b showed that the reversible model did not apply, even for the maximum limit of diffusioncontrolled transfer. The reversible model suggests that most of the electrogenerated IrCl62- in the aqueous phase would be detected in the reverse collection step, which (34) Atkins, P. W. Physical Chemistry, 5th ed.; Oxford University Press: Oxford, 1994. (35) Handbook of Chemistry and Physics, 80th ed.; CRC Press: Boca Raton, FL, 1999. (36) Birkin, P.; Silva-Martinez, S. Anal. Chem. 1997, 69, 2055.
Figure 5. Effect of DLPE on IrCl62- transfer from water to DCE with 0.1 M THAP in the DCE phase using SECM-DPSC. The aqueous phase contained 5 mM IrCl63-, 0.2 M NaCl, and 0.01 M NaClO4. From top to bottom, the solid experimental curves are typical reverse step transients obtained at welldefined tip-interface separations in the range 1.9-2.2 µm with cL ) 20.0, 15.0, 10.0, 7.5, 5.0, and 0 µM. The corresponding dashed theoretical curves (top to bottom) are for kf ) 0.01, 0.015, 0.02, 0.03, 0.04, and 0.05 cm s-1.
was not observed experimentally. For slower kinetics or a lower Kp value, the model predicted even higher currents than for the case shown. Thus, an irreversible process, with a follow-up reaction involving IrCl62- and THA+ in DCE describes IT. The fact that the IT rate measured by SECM was comparable to MEMED, under similar conditions, further confirmed that the irreversible transfer model was applicable. Effect of a DLPE Monolayer on the Kinetics of IrCl62- Transfer. Of the conditions examined, IrCl62transfer kinetics were fastest in the system with 0.01 M NaClO4 in the aqueous phase. This was considered to be most suitable for further investigations of the effect of a DLPE monolayer on the kinetics of IrCl62- transfer, since any diminution in rate constant would be manifested most sensitively in the SECM-DPSC measurements.24a Moreover, the most negative relative potential in the aqueous phase, obtained in this case (eq 1) also favored the adsorption of zwitterionic phospholipids.1 IT kinetics were investigated at a close tip-interface separation, measured precisely in the range 1.9-2.2 µm, by the SECM-DPSC procedure illustrated earlier. Information on the tip-interface separation was obtained from forward step transients, similar to that shown in Figure 4a, with an accuracy of ca. (5%. Typical transient results for the corresponding reverse steps, with a generation (switching) time of 0.01 s in the forward step, are shown in Figure 5. These data clearly show an increase in the collector current as DLPE concentration in the DCE phase increases from 5 to 20 µM. This behavior can be interpreted as a decrease in the rate of IrCl62- transfer, as indicated by the theoretical curves to which the experimental data have been fitted. The diminution in rate constant may be attributed to the effect of DLPE adsorption, which increases with bulk phospholipid concentration over this range.31 The inhibitory effect was analyzed using a simple energy barrier model,10 in which the Gibbs energy of IT (∆GTq) is equated to the work required to open a pore of radius, r, against the surface pressure, Π, of the adsorbed monolayer.10
∆GTq ) πr2Π From absolute rate theory
(10)
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Langmuir, Vol. 18, No. 6, 2002
Zhang and Unwin
kf ) Z exp(-∆GTq/kBT)
(11)
where kB is the Boltzmann constant, T is absolute temperature, and Z is a pre-exponential constant. The Gibbs energy of adsorption at an electrolyte interface is given by37
Γmax )
( ) ( ∂Π ∂µi
)
T
)
∂Π RT(∂ ln cL)
(12)
T
where Γmax is the maximum surface coverage of the surfactant and cL is the bulk concentration of lipid. Integrating eq 12 yields
Π ) constant + RTΓmax ln cL
(13)
Combining eqs 10, 11, and 13 results in
ln kf ) constant′ - (πr2NΓmax ) ln cL
(14)
where N is Avogadro’s number. Equation 14 predicts a linear dependence of ln kf on ln cL with a slope proportional to Γmax and r. Results of the analysis of the data in Figure 5 in terms of eq 14 are presented in Figure 6. These data clearly analyze well in terms of the simple model. By use of Γmax ) 3 × 10-10 mol cm-2 from surface tension measurements,31 r ) 4.8 ( 0.3 Å is obtained. This compares to a crystallographic radius of IrCl62- of 3.3 Å.38 The higher value deduced from these experiments may imply that IrCl62transfers with a hydration shell, as found for the transfer of ClO4- from aqueous solution to nitrobenzene.39 In principle, the presence of zwitterionic phospholipid at the interface could also affect the interfacial potential drop and thus the kinetics of IT.9,40 However, we have observed significant steric effects of zwitterionic phos(37) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980; p 491. (38) Slowinski, K.; Slowinska, K. U.; Majda, M. J. Phys. Chem. B 1999, 103, 8544. (39) Osakai, T.; Ogata, A.; Ebina, K. J. Phys. Chem. B 1997, 101, 8341. (40) Strutwolf, J.; Manzanares, J. A.; Williams, D. E. Electrochem. Commun. 1999, 1, 139.
Figure 6. ln kf-ln cL curve of the data in Figure 5.
pholipids and other amphiphiles on the transfer of small neutral molecules at the liquid/liquid31 and water/air interface,41 for which potential effects would be unimportant. With this knowledge, and based on the analysis above, the effect of DLPE on IT kinetics can be satisfactorily explained in terms of a steric effect. Conclusions The transfer of IrCl62- across the water/DCE interface has been investigated using both MEMED and SECMDPSC. An apparent transfer coefficient of 0.63 ( 0.05 has been obtained from MEMED studies under sink transfer conditions. Further investigations by SECM-DPSC have shown that the IT process is irreversible, attributed to ion-pair formation between IrCl62- and THA+ in the DCE phase. An adsorbed monolayer of DLPE at the water/ DCE interface has been shown to diminish the rate of IT. The blocking effect of the monolayer was successfully analyzed using a simple energy barrier model,10 yielding a pore size of 4.8 ( 0.3 Å. Acknowledgment. We thank the EPSRC (GR/ M90948) for support of this work. J.Z. thanks the ORS scheme, the University of Warwick, and Avecia for scholarships. Helpful discussions with Dr. John Atherton (Avecia, Huddersfield) are much appreciated. LA011511H (41) (a) Zhang, J.; Unwin, P. R. Langmuir, in press. (b) Zhang, J.; Unwin, P. R. Submitted for publication.
