pubs.acs.org/Langmuir © 2009 American Chemical Society
Electrochemical and Spectroscopic Investigation of Counterions Exchange in Polyelectrolyte Brushes C. Combellas,† F. Kanoufi,† S. Sanjuan,‡ C. Slim,† and Y. Tran*,‡ †
Laboratoire Environnement et Chimie Analytique, ESPCI, CNRS UMR 712, 10 rue Vauquelin, 75231 Paris cedex 05, and ‡Laboratoire de Physico-chimie des Polym eres et des Milieux Dispers es, ESPCI, UPMC Univ Paris 6, CNRS UMR 7615, 10 rue Vauquelin, 75231 Paris cedex 05, France Received October 15, 2008. Revised Manuscript Received December 19, 2008
Scanning electrochemical microscopy (SECM) is employed to characterize the transport of redox-active probe ions through quenched polyelectrolyte brushes. The counterion exchange through polyelectrolyte brushes is also investigated by infrared spectroscopy in attenuated total reflection (FTIR-ATR), X-ray photolectron spectroscopy (XPS), and cyclic voltammetry (CV). The synthesis of poly(methacryloyloxy)ethyl trimethylammonium chloride (PMETAC) brushes is performed using surface-initiated atom transfer radical polymerization followed by in situ quaternization reaction. The chloride (Cl-) counterions of the positively charged polymer brush are exchanged by ferrocyanide (Fe(CN)46 ) and ferricyanide (Fe(CN)36 ) ions that are both detectable by spectroscopy and electrochemically active. A good agreement is found when comparing the results obtained by spectroscopic (FTIR-ATR and XPS) and electrochemical (SECM and CV) methods. The counterions exchange is completely reversible and reproducible. We show that (Fe(CN)46 ) and (Fe(CN)36 ) species form stable ion pairs with the quaternary ammonium groups of the polymer brush. The transport of iodide (I-) redox-active ions is also investigated. In all cases (ferrocyanide, ferricyanide, or iodide), we find that chloride counterions are partially replaced by electroactive ions. This partial exchange may be attributed to an osmotic effect, since the external salt concentration for the exchange is much lower than the counterion concentration inside the brush.
Introduction Polyelectrolyte brushes are made of polyelectrolyte chains that are densely attached by one end to a surface. They have attracted increasing attention for the development of sensitive-responsive actuators,1-6 owing to their ability to bear rapid and high deformation according to external stimuli. In the presence of an aqueous solvent, the structure and the properties of polyelectrolyte brushes are governed by electrostatic interactions between the charged chains and the counterions. They can change from a stretched conformation to a collapsed one when increasing the salt concentration or varying pH conditions. Different experimental methods have been explored to study the conformational transitions of polyelectrolyte brushes including ellipsometry,7-11 neutron reflectivity,12-14 atomic force microscopy (AFM),15 and quartz crystal microbalance with *Correspondence to Y. Tran (E-mail:
[email protected]). Phone number: Int 33 1 40 79 58 12 Fax number: Int 33 1 40 79 46 40. ::
(1) Advincula, R. C.; Brittain, W. J.; Caster, K. C. Ruhe, J. (Eds.) Polymer Brushes; Wiley-VCH Verlag: Weinheim, 2004. :: :: (2) Ruhe, J.; Ballauff, M.; Biesalski, M.; Dziezok, P.; Grohn, F.; Johannsmann, D.; Houbenov, N.; Hugenberg, N.; Konradi, R.; Minko, S.; Motornov, M.; Netz, R.; Schmidt, M.; Seidel, C.; Stamm, M.; Stephan, T.; Usov, D.; Zhang, H. Adv. Polym. Sci. 2004, 165, 79. (3) Luzinov, I.; Minko, S.; Tsukruk, V. V. Prog. Polym. Sci. 2004, 29, 635. (4) Minko, S. Polym. Rev. 2006, 46, 397. (5) Brittain, W. J.; Boyes, S. G.; Granville, A. M.; Baum, M.; Mirous, B. K.; Akgun, B.; Zhao, B.; Blickle, C.; Foster, M. D. Adv. Polym. Sci. 2006, 198, 125. (6) Brittain, W. J.; Minko, S. J. Polym. Sci., Part B: Polym. Chem. 2007, 45, 3505. :: (7) Biesalski, M.; Johannsmann, D.; Ruhe, J. J. Chem. Phys. 1999, 111, 7029. :: (8) Biesalski, M.; Johannsmann, D.; Ruhe, J. J. Chem. Phys. 2002, 117, 4988. :: (9) Biesalski, M.; Ruhe, J. Macromolecules 2002, 35, 499. :: (10) Biesalski, M.; Ruhe, J. Macromolecules 2004, 37, 2196. :: (11) Biesalski, M.; Johannsmann, D.; Ruhe, J. J. Chem. Phys. 2004, 120, 8807. (12) Tran, Y.; Auroy, P.; Lee, L. T. Macromolecules 1999, 32, 8952. (13) Sanjuan, S.; Perrin, P.; Pantoustier, N.; Tran, Y. Langmuir 2007, 23, 5769. (14) Sanjuan, S.; Tran, Y. Macromolecules 2008, 41, 8721. (15) Farhan, T.; Azzaroni, O.; Huck, W. T. S. Soft Matter 2005, 1, 66. (16) Azzaroni, O.; Sergio, M.; Farhan, T.; Brown, A. A.; Huck, W; T. S. Macromolecules 2005, 38, 10192.
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dissipation (QCM-D).16,17 From an electrochemical point of view, few groups have investigated ion exchange inside polyelectrolyte brushes with a thickness lower than 100 nm. Some examples of electrochemical studies are mentioned below. Schlenoff et al. have studied the exchange of 45Ca2+ cations in a styrene sulfonate gel.18 The exchange kinetics and the ion distribution at equilibrium were determined. Because of the low film thickness, the rate-determining step is mass transport in a liquid thin layer adjacent to the film. The counterion exchange using various cations shows that the kinetics is fast, so that even big and hydrophobic cations are quickly transferred inside the film. The authors have also studied by classical electrochemical techniques (rotating disk electrode and cyclic voltammetry) the exchange of electroactive species (ferrocyanide and ferricyanide) inside polyelectrolyte multilayers as a function of the film thickness, the ions charge and concentration, and the temperature.19,20 They show that the electroactive species flux is not linear with the concentration of the salt used to achieve the ion exchange, which allows the selection of ions according to their charge and favors transport of little charged ions. Choi et al. have characterized by cyclic voltammetry the stability, the electron exchange, and the swelling-collapse transition in polyectrolyte brushes conditioned by ferricyanide ions and exchanged by chloride ions.21 They have determined the effect of experimental parameters such as the ions charge and concentration and the brush thickness. They show that successive ion exchanges do not alter the brush properties and that ion exchange becomes more difficult as the brush thickness increases. This may result from the larger distance between the electrode and the redox (17) Moya, S.; Azzaroni, O.; Farhan, T.; Osborne, V. L.; Huck, W. T. S. Angew. Chem., Int. Ed. 2005, 44, 4578. (18) Graul, T. W.; Li, M.; Schlenoff, J. B. J. Phys. Chem. B 1999, 103, 2718. (19) Farhat, T. R.; Schlenoff, J. B. Langmuir 2001, 17, 1184. (20) Farhat, T. R.; Schlenoff, J. B. J. Am. Chem. Soc. 2003, 125, 4627. (21) Choi, E. Y.; Azzaroni, O.; Cheng, N.; Zhou, F.; Kelby, T.; Huck, W. T. S. Langmuir 2007, 23, 10389.
Published on Web 4/9/2009
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Scheme 1. Schematic illustration of counterion conditioning/salting-out process in the polymer brush. Conditioned and salted-out counterions are 3detected by FTIR-ATR (Fe(CN)46 and Fe(CN)6 ions) and XPS (all counterions). Only salted-out counterions are detected by CV and SECM.
centers and also from the brush hydrophobic character. The latter increases in the presence of ferricyanide ions due to the formation of stable and non-solvated ion pairs between the ferricyanide ions and the brush ions, which leads to the collapse. Such properties are sensitive to the hydrophobicity of the conditioning ions.22 More recently, the same group demonstrated from combined electrochemical and wettability experiments that polyelectrolyte brushes conditioned with ferricyanide ions are more hydrophobic than with ferrocyanide ions.23 These properties can be exploited to modulate the hydrophobicity of a polyelectrolyte brush using an electrochemical switch. Zhou et al. used cyclic voltammetry and impedance spectroscopy to study the dynamics of the conformation change of poly (methacryloyloxy)ethyl trimethylammonium chloride (PMETAC) chains grafted at a gold surface.24 Impedance spectroscopy is especially well adapted to study the swelling-collapse transition as a function of the conditioning ion concentration. Redox transport is fast for a swollen brush, while opposite behavior is observed for a collapsed brush; in the latter case, ion transport is not totally impeded, provided the brush is hydrated enough to ensure ion mobility. These results were confirmed by ellipsometry, AFM, and IR-ATR. The scanning electrochemical microscopy (SECM) has been already used to detect ion transport into polymeric membranes.25-33 The observation of ion fluxes extracted from membranes by SECM was rather dedicated to films deposited onto electrodes that were used to stimulate the ion extraction.25-31 To our knowledge, the SECM has never been used to quantify such ion exchanges inside thin films of ionic polymers immobilized on insulating surfaces. However, SECM should be well-adapted to characterize ionic exchanges inside polymer films due to (i) its selectivity and sensitivity (10-16 mol, that is 10-6 mol m-2) and (ii) its ability to follow rather quick kinetics (0.1 s) and to investigate charge or mass transfer mechanisms at many interfaces. Indeed, ion ejection is related to both free ion diffusion inside the film and charge transfer kinetics at the polymer/solution interface.
(22) Azzaroni, O.; Brown, A. A.; Huck, W. T. S. Adv. Mater. 2007, 19, 151. (23) Spruijt, E.; Choi, E.-Y.; Huck, W. T. S. Langmuir 2008, 24, 11253. (24) Zhou, F.; Hu, H.; Yu, B.; Osborne, V. L.; Huck, W. T. S. Anal. Chem. 2007, 79, 176. (25) Kwak, J.; Anson, F. C. Anal. Chem. 1992, 64, 250. (26) Lee, C.; Anson, F. C. Anal. Chem. 1992, 64, 528. (27) Arca, M.; Mirkin, M. V.; Bard, A. J. J. Phys. Chem. 1995, 99, 5040. (28) Troise Frank, M. H.; Denuault, G. J. Electroanal. Chem. 1994, 379, 399. (29) Kapui, I.; Gyurcsanyi, R. E.; Nagy, G.; Toth, K.; Arca, M.; Arca, E. J. Phys. Chem. B. 1998, 102, 9934. (30) Bertoncello, P.; Ciani, I.; Li, F.; Unwin, P. R. Langmuir 2006, 22, 10380. (31) Gyurcsanyi, R. E.; Pergel, E.; Nagy, R.; Kapui, I.; Lan, B. T. T.; Toth, K.; Bitter, I.; Lindner, E. Anal. Chem. 2001, 73, 2104. (32) Mandler, D.; Unwin, P. R. J. Phys. Chem. B 2003, 107, 407. (33) O’Mullane, A. P.; Macpherson, J. V.; Unwin, P. R.; Cervera-Montesinos, J.; Manzanares, J. A.; Frehill, F.; Vos, J. G. J. Phys. Chem. B 2004, 108, 7219.
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Here, we will investigate conditioning and salting-out processes in the same way as in our previous work.34 Conditioning will be performed by three electroactive anions (Fe(CN)46 , Fe(CN)36 , I ) and salting out by the non-electroactive Cl ion. These processes will be characterized by spectroscopic methods such as infrared spectroscopy in attenuated total reflection (FTIR-ATR) and X-ray photoelectron spectroscopy (XPS) and also by electrochemical techniques, scanning electrochemical microscopy (SECM) and cyclic voltammetry (CV). Each method has its own drawbacks and advantages. With FTIR-ATR, it will be possible to study the conditioning process, but the method is not suitable for the iodide ions. With SECM and CV, the behavior of the three ions can be studied but only the salting-out process may be detected. XPS can be used to investigate both processes with the three ions, but it probes only the upper 8-nm-thick layer of the film. These spectroscopic and electrochemical methods are properly complementary, as sketched in Scheme 1. Experimental Section Chemical Products. 2-(Dimethylamino)ethylmethacrylate (DMAEMA, Aldrich, 98%), triethylamine (Aldrich, 99.5%), and solvents such as dimethylformamide (SDS, 99%) and tetrahydrofuran (SDS, 95%) were passed through a column of activated basic alumina and degassed with high-purity nitrogen prior to use. Copper(I) bromide (CuBr) (Aldrich 98%) was purified as described in the literature. Dimethylchlorosilane (Roth Sochiel, 98%), chlorododecyldimethylsilane (Aldrich, 95%), 10-undecen-1-ol (Aldrich, 98%), 2-bromoisobutyryl bromide (Aldrich, 98%), 1,1,4,7,10,10-hexamethyltriethylene tetramine (HMTETA, Aldrich, 97%), ethyl-2-bromoisobutyrate (Aldrich, 98%), anhydrous tetrahydrofuran (Aldrich, 99.9%), anhydrous toluene (Aldrich, 99.8%), K4[Fe(CN)6] and K3[Fe (CN)6] (Aldrich, 99.99%), and NaI (Prolabo, pur sec) were used as received. All other chemical reagents were purchased from Aldrich (Saint-Quentin Fallavier, France) and used as received. Ultrapure water (Millipore, resistivity g18.2 MΩ.cm) was obtained in the laboratory. Substrates. Silicon (Si) wafers were purchased from ACM. (72 10 1.5 mm3) trapezoidal crystals (with an angle of 45) were required for FTIR-ATR measurements. For SECM experiments, small silicon wafers (∼1 1 cm2) were cut for use. Silicon substrates were cleaned by treatment with freshly prepared “piranha” solution (70:30 v/v concentrated H2SO4/30% aqueous H2O2) at 150 C for 30 min. The substrates were then rinsed with pure water (Millipore, resistivity g18.2 MΩ.cm), cleaned by ultrasound in water for 1 min and dried under nitrogen. Liquid cell for FTIR-ATR measurements containing zinc selenide (ZnSe) crystals were used for the calibration of ferrocyanide and ferricyanide aqueous solutions. The trapezoidal (34) Combellas, C.; Kanoufi, F.; Mazouzi, D.; Thiebault, A. Polymer 2003, 44, 19.
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Scheme 2. Synthetic strategy for the formation of quenched polyelectrolyte brushes via surface-initiated controlled radical polymerization. First, the initiator was covalently grafted on silicon substrates in a self-assembled monolayer. The polymer was then grown by atom transfer radical polymerization from the surface-attached initiator. The quaternization reaction, which was performed directly on the neutral polymer brush, allowed the formation of the quenched polyelectrolyte brush.
10-undecen-1-ol with 2-bromo-isobutyryl bromide and the hydrosilylation with dimethylchlorosilane. The initiator was covalently grafted on silicon substrates by a self-assembling technique. The PDMAEMA brush was generated from the substrate by atom transfer radical polymerization of 2-(dimethylamino)ethylmethacrylate monomers. The free initiator ethyl-2bromoisobutyrate was added to the polymerization to provide an overall concentration of ester in the polymerization mixture, which controls the chain growth of both the surface-attached and bulk initiators.38-41 Polymer brushes were characterized with ellipsometry by measuring the dry thickness γ (nm). This thickness is associated with the amount of polymer per unit area. The surface concentration of monomers, Γe, was deduced from γ using eq 1: Γe ¼
crystals have a dimension of 75 10 5 mm3 with an angle of 45. ZnSe substrates were thoroughly rinsed with ethanol prior to use. Synthesis of Polymer Brushes. The synthesis of polymer brushes was reported in previous papers.13,35 The experimental procedure for the formation of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes and subsequent in situ conversion into poly(methacryloyloxy)ethyl trimethylammonium chloride (PMETAC) quaternized brushes is outlined in Scheme 2. The surface-attachable initiator was synthesized following the same strategy used by Matyjaszewski et al.36 and Husseman et al.37 Briefly, (11-2-(2-bromo-2-methyl)propionyloxy)undecyldimethylchlorosilane was obtained after the esterification of (35) Sanjuan, S.; Tran, Y. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 4305. (36) Matyjaszewski, K.; Miller, P. J.; Shukla, N.; Immaraporn, B.; Gelman, A.; Luokola, B. B.; Siclovan, T. M.; Kickelbick, G.; Vallant, T.; Hoffmann, H.; Pakula, T. Macromolecules 1999, 32, 8716. :: (37) Husseman, M.; Malmstrom, E. E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoit, D. G.; Hedrick, J. L.; Mansky, P.; Huang, A.; Russell, T.; Hawker, C. J. Macromolecules 1999, 32, 1424.
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γF M0
ð1Þ
where F is the density of the polymer (F = 1.318 g/cm3 for PDMAEMA) and M0 is the molecular weight of the monomer unit (M0 = 157.21 g/mol for DMAEMA unit). The polymer brushes we synthesized are from 32 to 73 nm thick. The corresponding surface concentrations vary from 2.7 10-4 mol/m2 to 6.1 10-4 mol/m2. The quaternization reaction of the PDMAEMA brush was performed with methyl iodide in ethanol to obtain a brush of positively charged PMETAC chains. A complete quaternization reaction without any chain degrafting was observed after 24 h using a temperature of 60 C. FTIR-ATR spectrometry was used to ensure that the quaternization reaction was complete. Procedure for Counterion Conditioning and Salting Out. For counterion conditioning, the polymer brush was dipped into ferrocyanide or ferricyanide aqueous solution at a concentration of 0.1 mol/L. NaI solution used for iodide ions conditioning was also at a concentration of 0.1 mol/L. After 40 min of conditioning, the silicon wafer was rinsed with pure water to remove electrolyte excess. Salting out was achieved by dipping the polymer brush into NaCl solutions at various concentrations for various times. For XPS, the NaCl concentration was fixed to 1 mol/L and the time was 200 s. The sample was dried under nitrogen prior to FTIR-ATR or XPS measurements. In addition, the thickness of the polymer brush was controlled after each step of drying to ensure that there was no chain degrafting. Instrumentation. Ellipsometric measurements were performed on a Sentech SE 400 apparatus. The light source was a He-Ne laser (λ = 632.8 nm), and the angle of incidence was set to 70. A multilayer model for a flat film was used for the calculation of the thickness of silica, initiator, and grafted polymer layers from the experimentally measured ellipsometric angles Ψ and Δ. The refractive indices n used for the calculations were 1.460 for the native silica layer, 1.508 for the initiator layer, and 1.517 for PDMAEMA. From the dry brush thickness γ (nm), we calculated the surface concentration of monomers as explained in the previous section. The ellipsometer was also required to control the thickness of the polymer brush during the conditioning/salting-out process. FTIR-ATR spectra were recorded on a Magna IR 550 (Nicolet) apparatus with a MCT detector cooled with liquid nitrogen. The spectra were recorded with a resolution of 2 cm-1. XPS spectra were recorded using a Thermo VG Scientific ESCALAB 210 system fitted with a monochromatic Al KR X-ray source (1486.6 eV). The X-ray beam of 6 mm 1 mm size was used at a power of 20 mA 15 kV. The spectra were acquired in the constant analyzer energy mode, with pass energy of 50 and 20 eV for, respectively, the survey and the narrow (38) Zhao, B.; Brittain, W. J. Macromolecules 2000, 33, 8813. (39) Ejaz, M.; Ohno, K.; Tsujii, Y.; Fukuda, T. Macromolecules 2000, 33, 2870. (40) Kong, X.; Kawai, T.; Abe, J.; Iyoda, T. Macromolecules 2001, 34, 1837. (41) Boyes, S. G.; Akgun, B.; Brittain, W. J.; Foster, M. D. Macromolecules 2003, 36, 9539.
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regions. The Avantage software was used for data processing. Spectral calibration was determined by setting Ag3d5/2 peak at 368.3 eV and the fwhm was 0.95 eV. Electrochemical Procedures. Ultramicroelectrode (UME) tips were obtained according to literature.42 Platinum wires, used in the ultramicroelectrodes, were of 99.9% purity (Goodfellow, United Kingdom). The platinum microelectrode tip (metallic radius, a = 5 μm, ratio of glass to metallic radius, RG = 5), was assembled with a platinum counter electrode (radius = 125 μm) and an anodized Ag/AgCl reference wire (radius = 125 μm) as reported previously in order to perform electrochemical measurements in the confined domain defined by a 50 μL drop of solution.34 This electrode assembly was moved by a horizontal or vertical translation stage (speed: 0.5 μm/s) driven by an electrical microstep motor piloted by a computer (CMA motors driven by an ESP300, Newport). Potentials were imposed and currents measured by a potentiostat/galvanostat (CH660A, CH Instruments, USA). First, the microelectrode had to be positioned at a typical distance, d, of twice the microelectrode radius, d = 2a. For that, a 50 μL drop of a 10-3 M solution of the anion, A-, used for conditioning was introduced between the electrode assembly and the substrate surface. With I- or Fe(CN)46 as conditioning ions, the microelectrode was biased on the oxidation plateau of the anion (0.80 V/SCE), and with Fe(CN)36 , it was biased on the reduction plateau of the anion (0.20 V/SCE). Then, the positioning of the microelectrode tip was done by recording the microelectrode current, iT, as a function of the substratemicroelectrode distance, d.43 Since the polyelectrolyte layer was too thin to affect the insulating character of the substrate, during this first approach curve, the microelectrode current, iT, followed the approach curve of an insulating substrate in the feedback mode of the scanning electrochemical microscope. The tip was then positioned so that iT/iT,¥ ∼ 0.8, and iT, ¥ ¼ 4nFDa½A -
ð2Þ
where iT,¥ is the current at infinite distance, n, the number of exchanged electrons, F the Faraday constant (96 500 C), and D the A- diffusion coefficient. This corresponded to d ∼ 8 μm.42,44 The 10-3 mol/L solution of A- was then removed by suction, the substrate was rinsed with ultrapure water by several addition-suctions of approximately 50 μL drops of water. Then, a 50 μL drop of the 10-1 mol/L conditioning solution was introduced between the substrate and the electrode assembly and left in contact with the substrate for 40 min. A 10-1 mol/L concentration was not used directly during the tip positioning in order to avoid any electrode passivation. After several rinsing procedures of the substrate-electrodes with ultrapure water, the salting out of the conditioned substrate by a 20 μL drop of 0.1 and 1 mol/L NaCl was monitored on-line by SECM. For that, the tip was biased under the same conditions as for its positioning (see above), and the tip current was monitored as a function of time. CV Electrochemical Measurement. A global estimation of the amount of salted-out ions has also been performed. For that, after conditioning and rinsing, 200 μL of a 1 mol/L NaCl solution was deposited onto the substrate so that the whole surface was covered. After 200 s (same time as for IR experiments), the 200 μL volume was concentrated to 40 μL. The concentration of electroactive ions present in the solution was determined by cyclic voltammetry owing to relation 2. From the film surface area (between 1.0 and 1.3 cm2), it was then possible to deduce the surface concentration of the extracted electroactive ions. (42) Bard, A. J.; Fan, F.-R. F.; Mirkin, M. V. in Electroanalytical Chemistry, Bard A. J., Ed.; Marcel Dekker: New York, 1994; Vol 18, p 243. (43) Kanoufi, F.; Combellas, C.; Shanahan, M. E. R. Langmuir 2003, 19, 6711. (44) Scanning Electrochemical Microscopy, Bard, A. J.; Mirkin, M. V., Eds.; Marcel Dekker: New York, 2001.
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Results and Discussion Probing the Counterion Exchange by FTIR-ATR. The calibration of ferrocyanide and ferricyanide aqueous solutions was performed for FTIR-ATR measurements by using a liquid cell with ZnSe crystal, which is transparent in the range 400-4000 cm-1. The geometry of the ZnSe crystal was different from that of Si crystal, resulting in a disparity of the reflection number, which was 8 for ZnSe crystal and 49 for Si crystal. Thus, the absorbance value (determined at the suitable frequency) measured with ZnSe crystal was normalized to allow the comparison with the absorbance value measured with Si crystal. As the penetration depth (about 1 μm for the silicon-air interface) is much higher than the brush thickness (less than 100 nm), the attenuated total reflection infrared spectrum corresponds to the absorption of the whole brush. The FTIR-ATR spectra of ferrocyanide (a) and ferricyanide (b) aqueous solutions are shown in Figure 1. The spectrum of the ferrocyanide solution contained the expected peak at 2040 cm-1, which is attributed to a CN stretching vibration. For the spectrum of the ferricyanide ions, the absorption band is slightly shifted (the peak is at 2120 cm-1) and has a weaker intensity in comparison with the peak observed in the spectrum of Fe(CN)46 ions. This could be explained by the valence of the iron. The absorbance of each characteristic peak was measured for solutions at various concentrations. The calibration curve that represents the corrected absorbance of Fe(CN)46 (a) and Fe (CN)36 (b) versus the surface concentration is also displayed. The surface concentration was determined by normalizing the volume concentration to the penetration length of the infrared evanescent wave. The same procedure for counterion conditioning and salting out was achieved for spectroscopic and electrochemical measurements (see Experimental Section). The only distinction is that FTIR-ATR measurements were performed on dry samples. Figure 2 shows the evolution of the infrared spectra of the PMETAC brush after conditioning in ferrocyanide aqueous solution and dipping into pure water for various times, typically 1, 5, and 10 min. The spectra contained the peaks at 2040 cm-1 3and 2120 cm-1 associated with Fe(CN)46 and Fe(CN)6 coun3terions. The presence of Fe(CN)6 ions is due to the oxidation of during the process. The peak associated with Fe Fe(CN)46 3(CN)46 (respectively, Fe(CN)6 ) ions decreased (respectively, increased) with dipping time. After the salting out by a NaCl solution at a concentration of 1 mol/L for 5 min, the amount of 3Fe(CN)46 and Fe(CN)6 in the PMETAC brush became neg3ligible. The surface concentrations of Fe(CN)46 and Fe(CN)6 were determined at each step by using the calibration curves. The ratio of the concentration of counterions to that of monomers was also calculated. The values are displayed in Table 1. The 3overall ratio of Fe(CN)46 and Fe(CN)6 counterions was about 25% and remained unchanged during the dipping into pure water. In addition, the counterion exchange was completely reversible and reproducible, since the same results were obtained for the eight cycles of conditioning/dipping/salting out. 3The salting out of Fe(CN)46 (a) and Fe(CN)6 (b) counterions from the polymer brush was also investigated by FTIRATR. The ratio of the concentration of residual Fe(CN)46 (or Fe(CN)36 ) counterions inside the brush is represented as a function of time (Figure 3). The data are shown for various concentrations of external bathing NaCl solutions. Note that the conditioning of PMETAC brushes was performed by using 3either Fe(CN)46 or Fe(CN)6 aqueous solutions. About 60% of 4Fe(CN)6 counterions was salted out using the less concentrated DOI: 10.1021/la8034177
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Figure 1. FTIR-ATR spectra of K4[Fe(CN)6] (a) and K3[Fe(CN)6] (b) aqueous solutions at the concentration of 0.5 mol/L. Corrected 3absorbance of Fe(CN)46 (a) and Fe(CN)6 (b) as a function of the surface concentration. The calibration curve displays the corrected absorbance that takes into account the difference of geometry between Si and ZnSe trapezoidal crystals.
Figure 2. Evolution of the FTIR-ATR spectra of Fe(CN)46 and
Fe(CN)36 counterions after conditioning and dipping into pure water for various times. The brush investigated is 44 nm thick (the corresponding surface concentration of monomers is 3.7 10-4 mol/m2).
NaCl solution (0.1 mol/L) after 4 min. At long times (more than about 1 h), there was no more exchange between Fe(CN)46 and Cl- ions. It looks as if an osmotic equilibrium state was reached for this concentration after 1 h. On the other hand, the salting was rather complete after 4 min if the out of Fe(CN)46 concentration of NaCl solution was more than 0.3 mol/L. The exchange of Fe(CN)36 ions by Cl ions was more problematic. Actually, the ratio of salted-out Fe(CN)36 ions was less than 35% after 4 min for a NaCl concentration of 0.1 mol/L. The 5364
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osmotic equilibrium state seemed to be reached after 40 min with a maximum ratio of salted-out Fe(CN)36 ions equal to 40%. In addition, a complete salting out never occurred, although a 3 mol/L concentrated solution of NaCl was used. Actually, two major results can be highlighted. First, the salting out of electroactive ions using at least 1 M NaCl solutions is effective for less than 10 min; it is less effective at lower NaCl concentration. A 1 M concentration for NaCl was used for XPS, CV, and SECM measurements. Second, residual ferricyanide ions remain in the polycation brush, indicating that they are more tightly bound to the polycation than ferrocyanide ions. Probing the Counterion Exchange by Scanning Electrochemical Microscopy (SECM) and Cyclic Voltammetry (CV). Principle of the Local Electrochemical Measurement. The counterion exchange was monitored locally from the microelectrode tip held at a fixed (8 μm) distance from the substrate and held at the potential of detection of the expected ion. As soon as a drop of the salting out solution was put in contact with the substrate, the tip current rises toward a maximum in less than 10 s and then it decreases gently (Figure 4). This gives two pieces of information: (i) the time corresponding to the current maximum, which is characteristic of the saltingout anion kinetics and (ii) the total charge exchanged during salting out. The latter determination is not easy, since the residual current never reaches zero. However, a mathematical model was proposed to simulate the salting-out process.34 This model is based on the assumption that the electrode does not interfere with the salting-out process. Resolution of diffusion equations then gives the analytical equation for the concentration Langmuir 2009, 25(9), 5360–5370
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3Table 1. Surface Concentrations of Fe(CN)46 (Γferro) and Fe(CN)6 (Γferri) Ions during the Process of Conditioning, Dipping in Pure Water, and Salting Outa
Γferro,IR (mol/m2)
Γferri,IR (mol/m2)
Γferro, IR ð%Þ Γe
Γferri, IR ð%Þ Γe
Γferro, IR þ Γferri, IR ð%Þ Γe
3.54 10-5 17.3 9.6 26.9 conditioning 6.38 10-5 5.03 10-5 11.8 13.7 25.5 1 min dipping 4.36 10-5 -5 -5 5.77 10 8.3 15.7 24.0 5 min dipping 3.06 10 -5 -5 6.14 10 6.7 16.7 23.4 10 min dipping 2.47 10 1.12 10-5 0.7 3.0 3.7 salting out 1.95 10-6 a The ratio of the concentrations of ions to that of monomers, Γe, is also displayed. The brush investigated is 44 nm-thick (the corresponding surface concentration of monomers is 3.7 10-4 mol/m2).
Figure 3. Salting out of ferrocyanide (a) and ferricyanide (b) counterions. The ratio of the surface concentration of residual Fe(CN)46 or Fe (CN)36 counterions inside the brush is represented versus time for various concentrations of NaCl solutions used for the salting out. The brush investigated is 44 nm thick (the corresponding surface concentration of monomers is 3.7 10-4 mol/m2).
current is still given by iT, ¥ ¼ 4nFDaCðz, tÞ
Figure 4. Current profile during the salting out of Fe(CN)46 ions from the PMETAC brush with Γe = 5.3 10-4 mol/m2 (the corresponding brush thickness is 64 nm). The experimental curve (red) is compared to the simulated profile (dark with ΓCI = 3.5 10-5 mol/m2 and λ = 12 min-1).
profile of ions inside the solution KΓCl Cðz, tÞ ¼ pffiffiffiffiffiffiffi πD
Z 0
t
z2 exp - 4Dτ pffiffiffi exp½ -K ðt -τÞ dτ τ
ð3Þ
with C(z, t) the ion concentration in the solution at a distance z from the polymer film for a time, t; κ (in s-1) the rate of the limiting step of the ion extraction process from the polymer brush (ion diffusion into the brush and/or ion extraction at the polymer-solution interface); D the ion diffusion coefficient in the solution (D = 6 10-6 cm2 s-1); and ΓCI the surface concentration of salted-out ions. If the electrode does not interfere with the salting-out process, and if the tip is held sufficiently far from the substrate, the tip Langmuir 2009, 25(9), 5360–5370
ð4Þ
with z the tip-substrate separation distance. An example of the experimental and calculated curves is given in Figure 4. The presence of artifacts at short times is a consequence of the capacitive current; it is independent from any redox reaction of the electroactive ion. Moreover, based on the same mathematical model, the long-time limit of the time integration of the concentration profile gives rffiffiffiffiffiffiffi Z t t ð5Þ Cðz, tÞ dt f 2ΓCl lim tf¥ πD 0 Again, if the electrode does not infer with the salting-out process, the charge flowing through the tip during the saltingout process should be given at long times by rffiffiffiffiffiffi Z t Dt Cðz, tÞ f 8nFaΓCl ð6Þ qðtÞ ¼ 4nFDa π 0 Therefore, the total charge exchanged during salting out can then be extrapolated from the time-integration of the experimental current profile and compared with eq 6. Comparison with CV Measurements. The results obtained by both SECM and CV methods are reported as bars in Figure 5 3for Fe(CN)46 (a), Fe(CN)6 (b), and I (c) as conditioning ions (stippled corresponds to cyclic voltammetry and drawed to SECM). Salting out was performed with two NaCl concentrations (left, 1 mol/L; right, 0.1 mol/L). An average of 2 to 5 measurements was carried out on each brush sample, at the same place or at another place on the same sample. The percent of DOI: 10.1021/la8034177
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3Figure 5. Percent of exchange for different experiments of salting out of Fe(CN)46 (a), Fe(CN)6 (b), and I (c) by NaCl. The % of exchange
or the ratio of salted-out ions is obtained by cyclic voltammetry (stippled) and SECM (drawed), using a solution of NaCl at 1 mol/L (left) or 0.1 mol/L (right). The surface concentrations of monomer units measured by ellipsometry, Γe (104 mol/m2), are indicated above the bars.
exchange or the ratio of salted-out ions was deduced from the charge percentages by taking into account the anion charge. The different values of the surface concentration of monomer units, Γe, are indicated above the bars. Γe is deduced from the film thickness measured by ellipsometry, γ, using eq 1. The experimental results show that the ratio of salted-out ions is always much lower than 100%. The values obtained by CV are generally higher (less than 20%) than those obtained by SECM, indicating that eq 4 is likely approximate. However, because of the high dispersity of the results, only trends can be ascertained. Comparison of the results for the extraction by 1 mol/L and 0.1 mol/L NaCl solutions shows that salting out is favored when the NaCl concentration increases, as observed from the IR measurements and as expected from thermodynamics for any ion exchange. The results obtained with 0.1 mol/L NaCl concentration will not be discussed more deeply because of the small number of measurements; however, the trends observed are still consistent with the IR measurements. The comparison of the ratio of salted-out ions for the different anions regardless of the measurement method (CV or SECM) gives comparable 3results for Fe(CN)46 and Fe(CN)6 (∼12 ( 5%) and higher 5366
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for I- (∼35 ( 5%). The ion percentages show that the efficiency of the conditioning followed by the salting-out process is highest for the iodide ion. Finally, the SECM also allowed, as shown in Figure 4, the observation of the salting-out kinetics when it is performed at high Cl- concentration. The kinetics seems to depend on the polymer brush structure, the exchanged ion, and the history of the measurements. However, the salting-out process by a 1 M NaCl solution can be characterized by an apparent first-order extraction rate ranging between 12 and 50 min-1. These values confirm that the extraction kinetics of the ions from the polymer brushes is mainly developed during the minute and that it cannot be observed on the IR time scale. FTIR data are additionally consistent with the SECM suggestion of a fast kinetic process (0.3 M) is higher than for less dense ones (0.1 M).21 SECM also revealed some interesting kinetic features of the extraction process, as the process could be understood as kinetically limited by an apparent first-order rate ranging from 12 to 60 min-1 for brushes of thicknesses, γ, ranging from 63 to 32 nm. It could be assumed that the kinetic limitation observed is mainly related to ion transport into the whole brush and that this transport is characterized by an apparent diffusion coefficient into the brush, Db. The apparent first-order rate estimated from SECM should then be Db/γ2, giving access to Db ranging between 6 10-12 and 2 10-11 cm2/s. These values are comparable to other experimentally available data on the diffusion of electroactive ions within thin polymer membranes.30,46,47 It is remarkable that this value is smaller but comparable to that reported for the transport of ferricyanide/ ferrocyanide in thinner and less dense PMETAC brushes.23 Our smaller value is not surprising, owing to Donnan exclusion but also to steric reasons, when considering transport through more dense polyelectrolyte brushes. Finally, on the PMETAC brushes of 14 and 24 nm thicknesses, the amount of electroactive ions estimated by cyclic voltammetry deviates from the value measured by UV-visible absorption. This difference accounted for an increasing difficulty of the electrontransfer reaction with increasing thickness of the brushes. It has already been observed from electrochemical impedance spectroscopy investigation of charge transport into thicker PMETAC brushes (100 nm) by the same group.24 Such transport difficulties could be attributed to possible brush collapse due either to charge screening or to the use of hydrophobic anions.23 This scheme is consistent with our own observation of an inhomogeneous exchange of counterions along the chains in the direction perpendicular to the surface using PMETAC brushes whose thicknesses are on the same order of magnitude.
Conclusion The transport of redox-active probe ions through quenched polyelectrolyte brushes was investigated using spectroscopic (FTIR-ATR and XPS) and electrochemical (SECM and CV) methods. The chloride (Cl-) counterions of the (46) Bertoncello, P.; Ciani, I.; Marenduzzo, D.; Unwin, P. R. J. Phys. Chem. C 2007, 111, 294. (47) Ikeda, T.; Schmehl, R.; Denisevich, P.; Willman, K.; Murray, R. W. J. Am. Chem. Soc. 1982, 104, 2683.
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poly(methacryloyloxy)ethyl trimethylammonium (PMETAC) brush were exchanged by ferrocyanide (Fe(CN)46 ) and ferricyanide (Fe(CN)36 ) multivalent ions and also by iodide (I-) monovalent ions. The ratio of conditioned and salted-out ions could be determined by spectroscopic methods, while the electrochemical methods could only probe salted-out ions. A good agreement was found for Fe(CN)46 ions when comparing the results obtained by FTIR-ATR, XPS, SECM, and CV. We also observed that only a fraction of chloride counterions of the polymer brush were replaced by electroactive ions. This might be due to a strong osmotic pressure inside the brush, since the internal concentration of Cl- counterions in the brush is higher than the concentration of conditioned and salted-out ions. These
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results show that scanning electrochemical microscopy (SECM) is a powerful tool to quantify ion exchanges inside thin films of ionic polymers such as polyelectrolyte brushes on any kind of surface (insulator or conductor). It would be interesting to extend this investigation to polyampholyte brushes. This would help in understanding the swelling-collapse transition of polyampholeyte chains in full detail via the exchanges of the polymer counterions. Acknowledgment. XPS spectra were recorded in the Commissariat a l’Energie Atomique (Gif, France) in the “Chimie des Surfaces et Interfaces”. J. Charlier and P. Jegou are gratefully acknowledged for the XPS measurements.
Langmuir 2009, 25(9), 5360–5370
