Electrowetting of Ionic Liquids: Contact Angle Saturation and

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J. Phys. Chem. C 2009, 113, 9321–9327

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Electrowetting of Ionic Liquids: Contact Angle Saturation and Irreversibility Jose´ Restolho,† Jose´ L. Mata,†,‡ and Benilde Saramago*,† Centro de Quı´mica Estrutural, Instituto Superior Te´cnico, T U Lisbon, AVenida RoVisco Pais, 1049-001 Lisboa, Portugal and Academia Militar, Pac¸o da Rainha, 29, 1150-244 Lisbon, Portugal ReceiVed: March 17, 2009; ReVised Manuscript ReceiVed: April 9, 2009

In this work, electrowetting of several room temperature ionic liquids having in common the cation or the anion, [EMIM][EtSO4], [EMPy][EtSO4], [EMIM][NTf2], [BMIM][BF4], [BMIM][TFA], and [OMIM][BF4], on poly(tetrafluoroethylene) (PTFE) was investigated. The typical behavior of aqueous salt solutions was found: symmetric parabolic curves of contact angle versus (positive and negative) applied voltage, at low voltages, and contact angle saturation after a threshold voltage. Furthermore, the contact angle did not recover its initial value when the voltage decreased, and this irreversibility was found even at low voltages. The dependence of the contact angle with the applied voltage is generally described by the Young-Lippmann equation before contact angle saturation. In contrast, the Langevin function as well as a modified form of the Young-Lippmann equation were found to fit the experimental electrowetting curves in the whole range of voltages. A correlation between the electrowetting behavior and the surface tension of the liquids was reported. Some physical effects that have been pointed out as possible causes for the contact angle saturation were investigated. The most plausible explanation for contact angle saturation in our systems seems to be the charge trapped across the solid/liquid interface. Introduction Electrowetting, defined as the phenomenon of contact angle decrease through the application of an external electrical potential to the solid/liquid interface, has long been known. However, in recent years, a renewed interest on this matter was motivated by the possibility of using electrowetting as a tool to manipulate minute quantities of liquids on a surface. In the review of Mugele et al.,1 commercial applications of electrowetting in the fields of lab-on-a-chip devices, microlenses, fiber optics, among others, are discussed. Typical liquids used in electrowetting are aqueous salt solutions which are usually considered as perfect conductors, while the solid substrates are electrodes covered with hydrophobic, low-energy polymers, such as poly(tetrafluoroethylene) (PTFE)2-4 or poly(ethylene terephthalate) (PET).2 The recent application of room-temperature ionic liquids (RTILs) as electrolytes in electrical and electrochemical processes and devices5 suggested the possibility of using these liquids as new electrowetting agents. Thus far, the number of publications on the electrowetting of ionic liquids is quite reduced. The first report by Millefiorini et al.6 describes the decrease in the contact angle of two imidazolium- and one pyrrolidinium-based ionic liquids on the surface of Teflon AF1600 by applying an external DC voltage. They found that, although the process seemed qualitatively similar to the electrowetting of aqueous electrolytes, several differences existed, namely, a much lower efficiency and an asymmetry in the saturation values of the contact angles obtained for positive and negative voltages. Dubois et al.7 studied electrowetting of commercial [BMIM][BF4], [BMIM][PF6], and a series of other ionic liquids synthesized in their laboratory with the objective of using them as fluidic actuators on the surface of a chip coated * Corresponding author. † Instituto Superior Te´cnico. ‡ Academia Militar.

with PTFE. The authors confirmed that the electrowetting of those ionic liquids is less efficient than aqueous salt solutions. Furthermore, they claimed that the contact angle hysteresis obtained when the potential is reduced after contact angle saturation has been achieved seems to depend on the nature of the anion. The values of the hysteresis varied between 2° and 3° for [TMBA][NTf2], [BMIM][NTf2], and [C10H20NO3][NTf2] and 8° for [BMIM][PF6]. Halka et al.8 performed electrowetting experiments under vacuum with [BMIM][PF6] and [HMIM][NTf2] on SiO2-Si and epoxide-Cu substrates and found slightly different results. The variation of the contact angle with the potential led to symmetrical curves, and a significant contact angle hysteresis of up to 15° was obtained when the applied voltage was reduced after saturation. The authors attributed the contact angle saturation to the decomposition of the ionic liquid. The variation of the contact angle, θ, with the applied external voltage, V, is described by the Young-Lippmann equation9

cos θ ) cos θ0 +

εε0 2 1 CV 2 ) cos θ0 + V 2γ 2γd

(1)

where θ0 is the contact angle at zero potential, γ is the surface tension of the liquid, C is the capacitance per unit area of the insulating polymer film under the drop, ε is the dielectric constant of the polymer, ε0 is the vacuum permittivity, and d is the thickness of the polymer film. The Young-Lippmann equation holds only for low voltages. As the voltage increases above a threshold value, the contact angle remains constant. The reasons for this contact angle saturation are still not clear. Recently, the Langevin function was used to model the contact angles of a series of ionic liquids measured on PTFE in the entire range of DC applied voltages.7 This equation, known to describe the orientation of electrical dipoles in the presence of an electric field,10 yields a linear dependence of cos θ with V2 at low voltages and a constant θ value when a threshold voltage is exceeded. Another possibility is the modified Young-Lippmann

10.1021/jp902393r CCC: $40.75  2009 American Chemical Society Published on Web 04/29/2009

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equation proposed by Verheijen and Prins.11 According to these authors, at high voltages, the applied voltage is reduced by the presence of trapped charges. We will discuss this hypothesis later on. In the present work the electrowetting behavior of a series of RTILs sharing the same anion or cation is investigated. The ionic liquids tested are the following: 1-ethyl-3-methylpyridinium ethyl sulfate, [EMPy][EtSO4], 1-ethyl-3-methylimidazolium ethyl sulfate, [EMIM][EtSO4], 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] imide, [EMIM][NTf2], 1-butyl-3-methylimidazolium trifluoroacetate, [BMIM][TFA], 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4], and 1-methyl-3-octylimidazolium tetrafluoroborate, [OMIM][BF4]. The experimental data are fitted to the Young-Lippmann equation, at low voltages, and to the Langevin function and the modified Young-Lippmann equation in the whole range of applied voltages. To check the consistency of the fittings to the Young-Lippmann equation, the film thickness is calculated and compared with the experimental value. The reversibility of the electrowetting process was tested by reducing the voltage after the contact angle plateau is reached. Attempts to correlate the efficiency of the reduction of the contact angle as well as the contact angle hysteresis observed when the process is reversed with the ionic liquid structure were made.

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Figure 1. Schematic of the experimental setup used to apply an external electric field to a sessile drop of an ionic liquid.

where drops of ionic liquids were made on the top of the substrate using a vertical syringe. The electrode dipped inside the liquid drop was a tungsten wire with 0.025 mm diameter (Goodfellow Cambridge Ltd.). Both electrodes were connected to a power supply (HP 712C, Hewlet-Packard), and a DC voltage was applied in increments of 25 V. The liquid drop was grounded during the experiments. For each liquid, at least three electrowetting curves were obtained. The reversibility was tested by decreasing the voltage in steps of 25 V from the maximum value applied to 0 V. The treatment of the images obtained was performed by the ADSA software formerly mentioned. Results

Materials and Methods The ionic liquids were purchased from Solchemar, with the exception of [EMIM][NTf2], which was supplied by Prof. Canongia Lopes (Instituto Superior Te´cnico), and have a purity of >98%. All ionic liquids were vacuum dried at 80 °C for, at least, 3 days, except [OMIM][BF4] and [EMPy][EtSO4], which were used as received. The water content, checked by Karl Fischer, is ∼690 ppm for [EMPy][EtSO4], ∼287 ppm for [EMIM][EtSO4], ∼ 122 ppm for [EMIM][NTf2], ∼1800 ppm for [BMIM][TFA], ∼384 ppm for [BMIM][BF4], and ∼344 ppm for [OMIM][BF4]. All liquids mentioned before were manipulated inside a glovebox filled with dried nitrogen. The densities, at 25 °C, were taken from the literature: 1.24 g · cm-3 for [EMIM][EtSO4],12 1.22 g · cm-3 for [EMPy][EtSO4],13 1.57 g · cm-3 for [EMIM][NTf2],14 1.20 g · cm-3 for [BMIM][BF4],15 1.22 g · cm-3 for [BMIM][TFA],14 and 1.10 g · cm-3 for [OMIM][BF4].16 The surface tension measurements were carried out by the pendant drop method inside a thermostatted ambient chamber (Rame´-Hart Inc., USA), using a video camera (jAi CV-A50) mounted on a Wild M3Z microscope to record the drop image. The video signal was transmitted to a frame grabber (Data Translation model DT3155), with the image acquisition and analysis performed on a computer, running the ADSA-P software (Axisymetric Drop Shape Analysis, Applied Surface Thermodynamics Research Associates, Toronto, Canada). The ambient chamber was dried with silica gel, and the surface tension measurements were made at 25 °C, allowing, at least 0.5 h for thermal stabilization. The experimental setup used for the electrowetting experiments is sketched in Figure 1. A poly(tetrafluoroethylene) (PTFE) film 10 µm thick (Goodfellow Cambridge Ltd.) was used as the dielectric, while the counter electrode was a polished brass cylinder of 23 mm diameter. Following the procedure suggested by Blake et al.,17 a very thin film of sodium chloride solution was introduced between the brass substrate and the PTFE film to avoid the presence of an air gap. The counter electrode coated with the PTFE film was loaded into a sessile drop apparatus (the same used for surface tension measurements)

The properties of the ionic liquids studied in this work are presented in Table 1. The influence of the radius of the drop, R, on the values of the contact angle, θ, was investigated for EMIM][NTf2]. Figure 2 shows the contact angle vs voltage for three values of the drop radius. The contact angle decreases when the drop radius increases and tends to a constant value for R > 1.5 mm. At zero voltage the reduction of the contact angle is more significant than at higher voltages, approaching the threshold voltage of saturation. This led us to always form drops with radii above 1.8 mm. The electrowetting curves are shown in Figure 3. The experimental results obtained in the voltage range - 100 e V e 100 V were fitted to the Young-Lippmann equation, and the resulting parabolic curves are represented as solid lines in the same figure. The two branches of the curves obtained with positive and negative potentials are mostly symmetric. The small shifts observed are within the experimental error. For positive voltages above 200 V and negative voltages below -200 V, the contact angles remain constant and saturation is said to be achieved. The irreversibility of the electrowetting process was demonstrated in all cases: the contact angles obtained by decreasing the voltage from the maximum value applied until 0 V kept a low, almost constant value. In the case of [OMIM][BF4], the irreversibility was investigated in each step. After each 25 V increase in the voltage, the contact angle was measured, then the voltage was decreased to the initial value, and the contact angle did not increase to the previous value. Discussion The electrowetting behavior of the six ionic liquids investigated in this work is similar to that observed for aqueous salt solutions. The decrease of the contact angle obtained before saturation was always larger than 25° and attained a maximum value of ∼48° for [OMIM][BF4]. This behavior contrasts with the results previously reported,6 which indicated electrowetting efficiency of ionic liquids as being 1 order of magnitude lower than that typical of aqueous salt solutions. For low values of the voltage (|V| e 100 V), the contact angles decrease with increasing voltage according to the Young-Lippmann equation

Electrowetting of Ionic Liquids

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TABLE 1: Properties of the Ionic Liquids: Structure, Conductivity, K, and Surface Tension, γ, at 25 °C

a

See ref 24. b See ref 25. c See ref 14. d See ref 26. e See ref 27. f See ref 16.

(eq 1). Fitting the data to this equation leads to the values of θ0, the contact angle at zero potential, and d, the thickness of the PTFE film, presented in Table 2. The thickness d was calculated using the values of the surface tension of the ionic liquids given in Table 1 and considering ε ) 2.1 for PTFE18 and ε0 ) 8.854 × 10-12 F · m-1. With the exception of [EMIM][NTf2], the calculated values for the film thickness are in good agreement with the value of 10 µm provided by the manufacturer. We should stress here that eq 1 is based on the assumption that the capacitance C of the drop/substrate system derives from a parallel-plate capacitor. In fact, there is a second

contribution associated to the parasitic fields between the surface of the drop and the ambient chamber walls, the stray capacitance, Cstr, which can be approximated4 to that of a sphere of radius, r, in vacuum, Cstr ) 4πε0r. The ratio of Cstr to that of the parallelplate capacitor is less than 1% in the worst case of our smallest drops. Therefore, the contribution of the stray capacitance was neglected. The electrowetting curves are symmetrical with respect to the zero voltage axis, and the values of the contact angle measured at zero potential, θ0, belong to the fitted parabolic curves. The values of the contact angles obtained at saturation

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X)

Figure 2. Contact angles, θ, vs applied voltage, V, for [EMIM][NTf2] using different drop radii: 1.31, 1.53, and 1.95 mm. The experimental values were obtained in single experiments.

TABLE 2: Contact Angle at Zero Voltage, θ0, and Thickness of the Insulating Polymer Layer, d, Resulting from the Fitting of the Young-Lippmann Equation to the Raw Data Obtained with Low Voltages (-100 V e V e 100 V) IL

θ0/deg

d/µm

[EMPy][EtSO4] [EMIM][EtSO4] [EMIM][NTf2] [BMIM][TFA] [BMIM][BF4] [OMIM][BF4]

91.9 91.8 67.7 77.7 93.4 80.4

12.0 10.8 15.4 11.2 11.3 10.8

with negative and positive voltages are similar. This behavior agrees with that observed with aqueous solutions.4 However, for ionic liquids there is no unanimity among the different authors. Halka and Freyland8 reported a symmetric parabolic dependence of the contact angles of a series of ionic liquids on the applied voltage over a reduced range. In contrast, Millefiorini et al.6 and Laskoski and Snow19 found, in some cases, an asymmetry in the saturation of the contact angles and a discontinuity in the electrowetting curve at zero voltage. The former study was performed under high-vacuum conditions on dielectric substrates, while the latter ones were done in air using PTFE substrates. Although our experimental conditions were similar to those of Millefiorini et al.6 and Laskoski and Snow,19 the results obtained are closer to those of Halka and Freyland.8 This observation rules out the justification advanced by the latter authors, who suggested that the observed differences were derived from the use of different substrates (SiO2-Si and epoxide dielectric films instead of PTFE). The saturation of the contact angle when a critical voltage is attained is a well-known phenomenon in the electrowetting processes. At this point the dependence of the contact angle on the applied voltage does not follow the Young-Lippmann equation. A mathematical function which describes the orientation of electrical dipoles (or magnetic moments) in the presence of an electric field (or a magnetic field), called Langevin function, seems to be particularly adequate to describe the evolution of the contact angles as the voltage increases including the saturation region. This function is defined as follows

L(X) ) coth(3X) where

1 3X

(2)

CV 2 2γ(cos θs - cos θ)

(3)

θs is the contact angle at saturation and the remaining variables were defined above. The curves obtained from the fittings of the experimental data (θ vs V) to the Langevin function are presented in Figure 4. The six ionic liquids yield curves cos θ vs V2 grouped in two sets. Each set has in common the value of the liquid surface tension (see Table 1). Ionic liquids with higher surface tension (46-48 mJ · m-2) lead to higher contact angles both at zero potential and at saturation. Curves shifted to lower contact angles correspond to ionic liquids with lower surface tension (33-35 mJ · m-2). The curves obtained with [EMIM][EtSO4] and [EMPy][EtSO4] are almost coincident, in agreement with the similarity between the two cations (see Table 1). Many physical effects have been pointed out as causes for the contact angle saturation.1 Most probably a unique explanation cannot be provided, and the importance of each effect depends on the specific experimental conditions of each experiment. We analyze now the adequacy of the various hypotheses to our experimental results. Vallet et al.2 attributed the contact angle saturation to instabilities of the three-phase contact line at high voltages. They observed the ejection of little droplets at the edge of the liquid drops when the applied voltage exceeded a threshold value. In our work, no instabilities of the triple line were observed. Saphiro et al.20 found a correlation between the saturation behavior and the electric conductivity of the liquid and claimed that “a reasonable amount of liquid resistance will cause contact angle saturation”. They introduced the solid/liquid resistivity ratio defined as

A0 ) FS h/FL R0

(4)

where FS and FL are, respectively, the solid and the liquid resistivities, h is the thickness of the insulator, and R0 is the nominal radius of a spherical drop with the same volume as the liquid drop. For a perfectly conducting liquid, A0 f ∞ and no contact angle saturation would occur. However, even a small FL may lead to a finite A0, assuming that h , R0. This is not the case of our experiments, where the thickness of the Teflon film is 10 µm instead of tenths of micrometers as in Saphiro’s device. Our experimental results showing that the most resistant liquid [OMIM][BF4] is the one which presents the highest electrowetting efficiency (largest difference between θ0 and θs) confirm that the effect of liquid resistance is negligible. The correlation found in our work between the liquid surface tension and electrowetting behavior would suggest the use of the model proposed by Quinn et al.9 on the basis of thermodynamic considerations. According to these authors, “electrowetting saturation is not a defective phenomenon but rather the consequence of reaching a thermodynamic limit of stability”. The contact angle at saturation, θs, is reached when the interfacial tension of the solid/liquid interface, γSL, vanishes. Then, substitution in the Young equation leads to the following relation between θs and the interfacial tensions of the solid/gas (γsol) and the liquid/gas (γ) interfaces

θS ) arccos

γsol γ

(5)

Application of this equation to our experimental results yields values for the surface tension of PTFE varying from 20 mJ · m-2 for [BMIM][BF4] to 27 mJ · m-2 for [OMIM][BF4] and [EMIM][NTf2]. The discrepancy among these values and the

Electrowetting of Ionic Liquids fact that they are higher than the typical value for PTFE (17.5 mJ · m-2 for PTFE from Goodfellow21) demonstrates that this model is too simplistic. Millefiorini et al.6 also verified that eq 5 did not hold for the contact angle saturation of several ionic liquids. A particularly interesting model with a sound physical basis was proposed by Verheijen and Prins.11 These authors attributed the contact angle saturation to the charge trapping that occurs in or on the insulating layer when the interaction of ions with the solid exceeds the cohesive forces in the liquid. The same mechanism was claimed for charge transfer to a polymer surface by contact with conducting liquids.22 The presence of the trapped charge could be accounted for through the following modification of the Young-Lippmann equation

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cos θ ) cos θ0 +

εε0 (V - VT)2 2γd

(6)

where VT is the potential at the trapped charge layer and the remaining symbols have the same meaning as in eq 1. The existence of this trapped charge implies that the charge density in the interface of the liquid drop with the polymer decreases according to Gauss’s law and functions as a Faraday shield reducing the effect of the applied voltage over the contact angle. Figure 5a shows the plots of VT vs V. VT was obtained fitting eq 6 to our experimental data. For applied voltages below the saturation threshold values (approximately (100 V), the voltage due the trapped charge is close to zero. Outside this interval, VT (*0) increases with V according to a third-order polynomial.

Figure 3. Contact angle, θ, vs applied voltage, V, for [BMIM][TFA] (a), [BMIM][BF4] (b), [EMPy][EtSO4] (c), [OMIM][BF4] (d), [EMIM][EtSO4] (e), and [EMIM][NTf2] (f). The black symbols represent the experimental values obtained in several experiments by increasing the applied voltage in increments of 25 V. The open symbols represent the values measured when the voltage was decreased from the maximum applied value to 0 V and refer to a single experiment for either positive or negative voltages. The solid lines represent the fittings to the Young-Lippmann equation for voltages -100 V e V e 100 V.

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Figure 4. Plots of cos θ vs V2 obtained from the fittings of the experimental data to the Langevin function (eq 2). The maximum error in the evaluation of cos θ varies between 0.03 for initial values of cos θ and 0.08 for saturation values.

Figure 5. (a) Voltage due to trapped charge, VT, vs V. VT is derived from fitting the data of Figure 3 to eq 6. For potentials below the threshold value ca. (100 V, the voltage due to trapped charge is close to zero; for higher potentials, VT exhibits a cubic dependence on V. (b) Plots of cos θ vs V2 obtained from the modified Young-Lippmann equation using the dependence VT ) f(V) represented in Figure 5a.

Electrowetting of Ionic Liquids Figure 5b shows the theoretical curves cos θ vs V 2 obtained through the substitution in eq 6 of VT by its value represented in Figure 5a. Clearly, the model fails for potentials higher than those corresponding to the maximum difference (V - VT) where the value of cos θ reaches a maximum instead of tending to a constant value. Among the six ionic liquids studied, [EMIM][EtSO4], [EMPy][EtSO4], and [BMIM][BF4] present the lowest values for VT. The reason why the trapped charge is smaller for these ionic liquids must be related with their higher surface tension. The surface tension is known to depend on the strength of the interactions between the anion and the cation rather than the interactions between ion pairs.23 This is the reason why increasing the size of the alkyl substituent in the imidazole ring, for a common anion, results in the reduction of the hydrogenbond strength between anion and cation, with a concomitant lowering of the surface tension. The effect of the size of the anion is less clear. In our case, the liquids with higher surface tension are those with smaller cations and/or anions. If the surface tension of [EMIM][EtSO4], [EMPy][EtSO4], and [BMIM][BF4] is higher, a stronger electrostatic force is necessary to overcome the interaction anion-cation, and as a result, the charge trapped inside the polymer film should be smaller. The irreversibility of the electrowetting process was demonstrated for all ionic liquids and, in the case of [OMIM][BF4], for all applied voltages. Hysteresis of the contact angles measured at zero voltage varied between 18° for [EMIM][NTf2] and 42° for [OMIM][BF4]. According to Vallet et al.,2 the origin of this irreversibility is the modification of the polymer film at the edge of the drop. We confirm this hypothesis because in various experiments it was possible to detect visually the pinning of the three-phase contact line when the voltage is decreased from the maximum applied value. We repeated the procedure used by Vallet et al.2 to investigate the presence of a hydrophilic ring that could justify this behavior. After one experiment, we blew humid air over the area of the PTFE film previously covered by the drop. Observation of the polymer with a microscope showed the presence of water droplets only along the drop perimeter. In contrast with Dubois et al.,7 we did not find any correlation between the value of hysteresis and the nature of the anion. Ionic liquids sharing the same anion, e.g., [OMIM][BF4] and [BMIM][BF4], have quite different hysteresis values (42° and 23°, respectively). We should stress here that a contribution to the irreversibility of the electrowetting process from the difference between advancing and receding contact angles is expected. In fact, when the applied voltage increases (and the contact angle decreases), the liquid advances on the surface. The opposite is true when the voltage decreases. The difference between advancing, θa, and receding, θr, contact angles depends on the surface heterogeneities, both chemical and physical. We measured θa - θr (at 0 V) with [OMIM][BF4] and found 27°, which may justify the irreversibility in the contact angle observed at small applied potentials. Our findings suggest that the irreversibility of the electrowetting process is a consequence of the experimental procedure and does not reflect an intrinsic characteristic of the ionic liquid. Conclusions The main conclusion of this work is that the six ionic liquids investigated exhibited the electrowetting behavior typical of aqueous electrolyte solutions. In particular, high electrowetting efficiency was achieved, namely, a maximum contact angle

J. Phys. Chem. C, Vol. 113, No. 21, 2009 9327 decrease of ∼48° was measured for [OMIM][BF4]. This observation agrees with experimental results obtained with ionic liquids in high-vacuum conditions, but it is in contradiction with the findings of other authors who claimed that ionic liquids have some peculiar behavior. A second outcome of this work is the strong indication that the contact angle saturation obtained at high voltages derives from the existence of trapped charge. Other common explanations were analyzed and discarded, while the hypothesis of trapped charge leads to a modified Young-Lippmann equation that fits reasonably well our experimental data. The Langevin function also provides a good fitting of our experimental results in the whole range of applied voltages. The electrowetting process was found to be irreversible, even at low applied voltages. From a practical point of view, our results demonstrate that electroweting may be successfully applied to ionic liquids, although the experimental conditions should be optimized in order to reduce the contact angle hysteresis. Acknowledgment. The authors thank Dr. Terry Blake for helpful advice and Prof. Jose´ Nuno Canongia Lopes for the kind offer of [EMIM][NTf2]. This study was financially supported by the project PCDT/QUI/66211/2006. Jose´ Restolho was awarded with a research grant of the same project. References and Notes (1) Mugele, F.; Baret, J. J. Phys.: Condens. Matter 2005, 17, 705– 774. (2) Vallet, M.; Berge, B.; Vovele, L. Polymer 1996, 37, 2465–2470. (3) Quinn, A.; Sedev, R.; Ralston, J. J. Phys. Chem. B 2003, 107, 1163– 1169. (4) Verheijen, H.; Prins, M. ReV. Sci. Instrum. 1999, 70, 3668–3673. (5) Galin˜ski, M.; Lewandowski, A.; Steˆpniak, I. Electrochim. Acta 2006, 51, 5567–5580. (6) Millefiorini, S.; Tkaczyk, A. H.; Sedev, R.; Efthimiadis, J.; Ralston, J. J. Am. Chem. Soc. 2006, 128, 3098–3101. (7) Dubois, P.; Marchand, G.; Fouillet, Y.; Berthier, J.; Douki, T.; Hassine, F.; Gmouh, S.; Vaultier, M. Anal. Chem. 2006, 78, 4909–4917. (8) Halka, V.; Freyland, W. Z. Phys. Chem. 2007, 221, 1–11. (9) Quinn, A.; Sedev, R.; Ralston, J. J. Phys. Chem. B 2005, 109, 6268– 6275. (10) Bo¨ttcher, C. J. F. Theory of Electric Polarization; Elsevier: Amsterdam, 1973; Vol. 1. (11) Verheijen, H.; Prins, M. Langmuir 1999, 15, 6616–6620. (12) Go´mez, E.; Gonza´lez, B.; Calvar, N.; Tojo, E.; Domı´nguez, A. J. Chem. Eng. Data 2006, 51, 2096–2102. (13) Gonza´lez, B.; Calvar, N.; Go´mez, E.; Macedo, E. A.; Domı´nguez, A. J. Chem. Eng. Data 2008, 53, 1824–1828. (14) Tokuda, H.; Tsuzuki, S.; Susan, A.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2006, 110, 19593–19600. (15) Harris, K.; Kanakubo, M.; Woolf, L. A. J. Chem. Eng. Data 2007, 52, 2425–2430. (16) Restolho, J.; Mata, J. L.; Saramago, B. J. Chem. Eng. Data 2009, 54, 950–955. (17) Blake, T. D.; Clarke, A.; Stattersfield, E. Langmuir 2000, 16, 2928– 2935. (18) http://www.texloc.com/closet/cl_ptfe_properties.htm, retrieved in March 9, 2009. (19) Ricks-Laskoski, H.; Snow, A. J. Am. Chem. Soc. 2006, 128, 12402– 12403. (20) Shapiro, B.; Garrell, H.; Kim, C. J. Appl. Phys. 2003, 93, 5794– 5811. (21) Liu, F.; Shen, W. Colloids Surf., A 2008, 316, 62–69. (22) Chudleigh, P. W. J. Appl. Phys. 1976, 47, 4475–4483. (23) Freire, M.; Carvalho, P.; Fernandes, A.; Marrucho, I.; Queimada, A.; Coutinho, J. J. Colloid Interface Sci. 2007, 314, 621–630. (24) Vila, J.; Gine´s, P.; Pico, J. M.; Franjo, C.; Jime´nez, E.; Varela, L. M.; Cabeza, O. Fluid Phase Equilib. 2006, 242, 141–146. (25) Widegren, J. A.; Saurer, E. M.; Marsh, K. N.; Magee, J. W. J. Chem. Thermodynamics 2005, 37, 569–575. (26) Nishida, T.; Tashiro, Y.; Yamamoto, M. J. Fluorine Chem. 2003, 120, 135–141. (27) Kanakubo, M.; Harris, K. R.; Tsuchihashi, N.; Ibuki, K.; Ueno, M. Fluid Phase Equilib. 2007, 261, 414–420.

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