2920
Langmuir 1996,11, 2920-2925
Polymer Films with Tunable Surface Properties: Separation of an Oil-in-WaterEmulsion at Poly(3-methylthiophene) Walter Torres, John C. Donini,? Anton A. Vlcek, and A. B. P. Lever* Department of Chemistry, York University, North York, Ontario, Canada M3J l P 3 Received February 7, 1995. In Final Form: May 11, 1995@ We describe here the effect of an oil-in-water emulsion on the cyclic voltammetry of a redox probe at a platinum electrode coated with poly(3-methylthiophene), PMeT, a material for which wetting properties can be tuned by controlling its oxidation (charging) level. When a water soluble redox probe, such as potassium ferrocyanide or hexaammineruthenium(II1) chloride, is mixed in an oil-in-wateremulsion, the current response of the probe at W M e T decreases to less than lo%, consistent with adsorption of the disperse phase (oil + surfactant) at the polymer. On switching the partly blocked PMeT to its oxidized state, this polymer turns hydrophilic, decreasing the hydrophobic interaction between the polymer and the adsorbed oil particles. Removal of oil from the film is facilitated by electrostatic repulsion between the charged film and the surfactant bound to the oil phase, as indicated by the reappearance of the characteristic current signal at W M e T in an aqueous solution of the probe.
Introduction Organic conducting polymers have applications in electronics, rechargeable batteries, and sensors. In the past 15 years, much work has been done on understanding the controllable, reversible changes in electronic conductivity and spectral properties that accompany the redox transitions of these polymers. Less attention, however, has been paid to the wettability ofthese materials, a crucial parameter for the applicability of conducting polymers as sensors and separation interfaces. The wettability of solid surfaces is commonly evaluated by measuring the contact angles of probe liquids on the surface. Nevertheless, only a few reports have dealt with contact angle measurements on conducting polymers.l-' In qualitative agreement with Niemitz and Kossmehl,2 we have found that the wettabilities of some poly(3-alkylthiophene) films can be tuned by controlling the oxidation (charging) level on the polymer surface. (Alkyl groups = methyl, ethyl, and butyl).s We are interested in conducting polymers with tunable wettability because they would be useful for electrochemical breaking of emulsions. We
* Author to whom correspondence should be addressed.
Currentaddress: Western Researchcentre,CANMET. Devon. Alta., Canada. Abstract Dublished in Advance A C S Abstracts. Julv 1. 1995. (1)To the be'st of our knowledge, systematic, comprehensive studies @
of the surface energetics of these materials are not available. For relevant studies on some common conducting polymers, see refs 2-7. (2)(a) Niemitz, M.;Kossmehl, G. Angeu. Makromol. Chem. 1991, 185/186,147.cb) Kossmehl, G.;Niemitz, M. Synth. Met. 1991,41-43, 106. (c) Baughman, R. H. Makromol. Chem. Macromol. Symp. 1991, 51, 193. (3)The differences betweenthe advancingandrecedingcontact angles of water on (neutraland oxidized)polyaniline, under dynamic conditions, were measured by the Wilhelmy plate technique: Habib, M. A. Langmuir 1988,4 , 1302. (4)( a )Guiseppi-Ellie,A,; Wnek, G. E.; Wesson, S. P. Langmuir 1986, 2,508.For comments on the interpretation ofthe data, see: (b)Kloubek, J. Langmuir 1992,8,744. (c) van Oss, C. J.; Good, R. J.;Giese, R. F. Langmuir 1992,8,748. (5) Chemical treatment with KMnOl improves the hydrophilicity of polyacetylene, PA. The contact angle ofwater on neutral PA decreased from 72" to lo",without appreciably altering the conductivity of this material. Guiseppi-Elie, A,; Wnek, G. E. J . Polym. Sci. Polym. Chem. E d . 1986,23,2601. (6)Schonhorn, H.; Baker, G. L.; Bates, F. S. J . Polym. SCL.:Polvm. Phys. Ed. 1986,23,1555. (7, Hernandez, R.;Diaz,A. F.; Wa1tman.R.; Bargon, J.J.Phys. Chem. 1984,88,3333. ( 8 )!a) Torres, W.; Jin, Z.; Lever, A. B. P., unpublished results. (b) Jin, 2. Ph.D. Thesis, York University, 1994.
reasoned that, for a tunable polymer in contact with an oil-in-water emulsion, an interplay of hydrophobic and electrostatic interactions would produce oil spreading on the removal from the polymer by switching the oxidation state of the film. Here we describe electrochemical experiments with water soluble probes mixed in oil-in-water emulsions to probe reversible spreading of oil on Pt electrodes modified with PMeT. In aqueous solutions, the probes, ferrocyanide and hexaammine ruthenium(III), are electrochemically reversible a t Pt and PtPMeT electrodes. The redox potentials of these probes are in a region where PMeT is in its neutral f ~ r m . ~ J O For PMeT in 0.10 M LiC104 (aqueous), voltammetric oxidation and reduction waves occur a t f0.68 V and +0.42 V vs AgCVAg (Figure 1).By putting a W M e T electrode in contact with the emulsion, the current response of the probes is blocked. The current signal can be partly recovered after oxidizingthe electrode and transferring it to the initial probe solution. Thus, because ofits tunable hydrophobic-hydrophilic properties, PMeT can be used as a recyclable filter for oil-in-water emulsions and is potentially useful for related separation technologies.
Experimental Section Reagents and Solvents. 3-Methylthiophene (MeT, Fluka) was passed over neutral alumina. Tetrabutylammoniumperchlorate (TBAP, Southwestern Analytical Chemicals) was recrystallized from chloroformlethyl acetate, dried, and stored (9)Four limiting mechanisms of electron transfer between soluble, electroactive probes and polymer-modified electrodes are known: electron transfer a t the electrode/polymer interface with diffusion of the probe through (i)pinholes or (ii)the film itself, and electron transfer at the polymer/solutipn interface with (iii) electron (or hole) conduction through the polymer or (iv) electron exchange between the probe and electroactive sites of the polymer. See: (a) Laviron, E. J.Electroanal. Chem. 1982,131,61.(b) Peerce, P. J.;Bard, A. J . Electroanal. Chem. 1980,114,89.( c ) Peerce, P.J.;Bard, A. J. J . Electround. Chem. 1980, 112,97. (10)Control experiments suggest that electron transfer between either ferrocyanide or hexaammineruthenium(II1) ions and W M e T occurs at the electroddpolymer interface with probe diffusion through pin holes in the film (ref 9,mechanism i). However, electron (or hole) conduction through the polymer (mechanism iii in ref 9) cannot be ruled out. The oxidation of ferrocene and the reduction of tetracyanoquinodimethane at Ptlpolythiophene in acetonitrile have been explained in terms of mechanism iii. See: (a) Levi, M. D.; Vorotyntsev, M. A.; Skundin,A. M.; Kazarinov, V. E. J . Electroanal. Chem. 1991,319,243. (b) Kazarinov, V. E.; Levi, M. D.; Skundin, A. M.; Vorotynsev, M. A. J. Electroanal. Chem. 1989,271, 193.
0743-746319512411-2920$09.00/0 0 1995 American Chemical Society
Polymer Films with Tunable Surface Properties
Langmuir, Vol. 11, No. 8, 1995 2921
0.40
0.20
4.00 4
e
ga
u
-0.30
the anodic cyclic voltammetric peaks of the polymers in 0.10 M LiC104 a t 5 mV/s. To study the variation of contact angles on PMeT as a function of charging and discharging level, films were held at various increasing (and decreasing) potentials in the range between 0 and 1.0 V (and from 1.0 to 0 V) vs AgCVAg in water containing 0.2 M LiC104 for 1 min a t each selected potential, disconnected, and rinsed (and dried) as above. Diluted Emulsions. Oil in water emulsions were prepared by sonicating mixtures of a linear alkane and aqueous solutions containing a redox probe, a surfactant, and support electrolyte a t room temperature for 2 h. Three solutions were prepared: M &[Fe(CN)61 1.0 x (a) solution I, 5.0 x M CTAB 1.0 x M HC1+ 0.010 M NaC1; (b) solution 11, 5.0 x M &[Fe(CN)6] 1.0 x M SDS 1.0 x M NaOH 0.10 M Nacl; and (c) solution 111, 5.0 x loe4 M [Ru(NH&]C13 1.0 x M CTAB 1.0 x M HCl 0.010 M KBr. Emulsion I contained a mix of 10g L-l n-hexadecane in solution I. Emulsion I1 was prepared by adding 10 g L-l n-dodecane to solution 11. Emulsion I11contained amix of 10 g L-l n-hexadecane in solution 111. Samples of emulsions 1-111 were placed on clean glass slides at room temperature and observed with an optical microscope. The particle size distribution ranged from a few microns to nearly 1 mm. These emulsions were stable for at least 2 h. To avoid complications in the electrochemical measurements, the concentrations of SDS and CTAB were kept below the critical micelle concentrations (cmc) for these surfactants for a given ionic strength. Reported cmc for SDS in 0.010 M NaCl is 1.5 x M.lZa The cmc for CTAB in 0.010 M KBr is 3.4 x M.lZb
+
4.40
4.60
4.80
-1.00 1
0.8
0.6
a4
0.2
o
Volts vs. Ae/AgCl
Figure 1. Cyclic voltammogram of a Pt/PMeT electrode in 0.10 M LiC104 a t 50 mV/s at room temperature. Continuous and dashed lines are for the first and "equilibrium" scans, respectively. Film thickness = 0.1 pM. under vacuum. Lithium perchlorate (LiC104, J. T. Baker), potassium ferrocyanide (&[Fe(CN)6], Anachemia), hexaammineruthenium(II1) chloride ([Ru(NH3)6]C13,Aldrich), sodium dodecyl sulfate (SDS,Aldrich),cetyltrimethylammonium bromide (CTAB,Aldrich), n-hexadecane (Aldrich), n-dodecane (Aldrich), potassium bromide (KBr, BDH), and potassium chloride (KCl, BDH) were used as received. Acetonitrile was distilled over PzO5 and then stored over 3 A molecular sieves. Water was doubly distilled. 3-Ethylthiophene (EtT) and 3-butylthiophene (BUT) were synthesized as in 1iterature.ll Electrodes and Cells. Electrochemical measurements were controlled with a PAR 263 system and conducted in a onecompartment cell in a three-electrode configuration. Platinum disks (0.44 cm2 and 0.7 mm2)and stainless steel mesh (area = 6.0 cm2)were used as working and auxiliary electrodes, respectively. Potentials were measured against either AgCVAg (in aqueous solutions and emulsions) or Ag (in acetonitrile). E m for the redox couple ferrocenium/ferrocene is 0.31 V vs Ag wire in M ferrocene 0.2 M LiC104. acetonitrile containing 1.0 x All solutions and emulsions were purged with argon before the experiments, which were run under an Ar blanket a t room temperature and pressure. Electropolymerizations of alkylthiophenes were performed by cycling the electrode potential between 0.0 and 1.75 V vs Ag at 100 mV s-l in acetonitrile containing 0.050 M monomer 0.20 M LiC104. The thicknesses ofthe dry films were estimated from the total anodic charge passed during electrodeposition. Roughly, a charge density of 340 mC/cm2gives a 1pm thick film. Contact Angles. Measurements were performed on a homemade contact angle goniometer. Three drops of doubly distilled, deionized water (5 pL) were placed at three random spots on each polymer film (geometric electrode area = 0.44 cm2) with a microliter syringe. The humidity in the measuring chamber was kept at 60(+3)%. Experiments were carried out at room temperature (22 f 2) "C and pressure. Neutral and oxidized polyalkylthiophenes were obtained by holding the filmpotentialat -0.2Vand fl.OVvsAg, respectively (in 0.10 M LiC104 acetonitrile) for ca. 1min, disconnecting the electrode, rinsing with copious amounts of acetonitrile and acetone, and drying in air for 30 min. The charging level, reflecting the amount of Clod- ions incorporated into the fully oxidized films, varied between 18 and 27% (1anion per 5.6 and 3.7 monomer rings) and was calculated from the charge under
+
+
(11)Lemaire, M.; Roncali, J.;Garreau, R.; Gamier,F. New J.Chem. 1987,11, 703.
+
+ +
+ +
+ +
Results and Discussion Contact Angles of Water on Conducting Polymers. For a homogeneous, flat, smooth, isotropic, and nondeformable solid surface in equilibrium with a liquid and its vapor, the contact angle of the liquid on the solid is related to equilibrium thermodynamic functions of the solidliquid-vapor interface through the Young (1)and Dupr6 (2) equations.13J4 However, real surfaces rarely satisfy all these conditions and, normally, contact angles show hysteresis.15-17 Further complications may arise when studying contact angles at polymer surfaces.18J9 Studies of the wettability of conducting polymers are scarce. Those available, with one e ~ c e p t i o nonly , ~ discuss advancing contact a n g l e ~ . ~ Table , ~ - ~ 1 shows reported (12)(a) Mukejee, P.; Mysels, K. Critical Micelle Concentrations of Aqueous Surfactant Systems; NSRDS-NBS 36; U. S. Government Printing Office: Washington, D. C., 1971;p 52. (b) Beg, A.E.; Meakin, B. J.; Davies, D. J. G. Pakistan J.Sei. Ind. Res. 1979,22,283. (13) (1) Y S " - Ysl = Y1" cos(@)
(2) Wa = Y,O + ~ 1 v el where Wa is the work of adhesion, yaois the surface tension in vacuum, y is the interfacial tension, and s, v, and 1 represent the solid, vapor, and liquid phase, respectively. (14)When the liquid completely wets the surface,the contact angle is zero. The wettability of the solid by the test liquid decreases as the contact angle increases. (15)(a)The value of the contact angle depends on whether the drop of the liquid advances or recedes on the solid surface. (b) There's an on-going debate on the thermodynamic validity of the Young equation. For a recent discussion, see: Merchant,G .J.;Keller, J. B. Phys. Fluids,
A 1992,4,477. (16)Although contact angle hysteresisis not fully understood, it has been classically attributed to macroscopiccharacteristicsof the surface
such as roughness (the ratio between the real and apparent surface area) and heterogeneity (domains of different surface energy). More recently, however, the contribution of molecular interactions at the solid-liquid interfacehas been investigated. For recent contributions, see, for instance: (a) Cazabat, A. M. Adu. Colloid Interface Sci. 1992, 42,65. (b) Ong, T. H.; Ward, R. N.; Davies, P. B. J.Am. Chem. SOC. 1992,114,6243. ( c ) Chen, Y.L.; Helm, C. A.; Israelachvili,J. N. J. Phys. Chem. 1991,95,10736. (d) Chaudhury, M.; Whitesides, G. M. Langmuir 1991,7 , 1013. (17)For comparison ofdifferenttechniquesto measure contact angles, see: Lander, L. M.; Siewierski, L. M.; Brittain, W. J.; Vogler, E. A. Langmuir 1993,9, 2237. (18)Polymers are known to rearrange after their surfaces come into contact with a liquid. This surface relaxation can take seconds,hours, or even days, depending on the rigidity of the polymer.
2922 Langmuir, Vol. 11, No. 8, 1995
Torres et al.
Table 1. Reported Contact Angles of Water on Neutral and Oxidized Conducting Polymer Films contact angle of water
Table 2. Contact Angles of Water on MetaYPolymersa Held in Oxidized and N e u t r a l States
polymer
neutral film
oxidized film
ref
electrode
n
cis-polyacetylene cis-polyacetylene polyaniline polypyrrole poly(N-methylpyrrole) polythiophene polythiophene poly(3-methylthiophene)
72.0 89 73 62 63 86 180 90
49.6
4,5
Pt"T
77 72 55 52 83 0 88
6 7 7 7 7 2 7
3 3
contact angles of water on conducting polymer films. An increase in wettability with film charging has been observed for ~is-polyacetylene~~~ and polythiophene.2 However, these data have not been r e p r ~ d u c e d . ~We , ~ have found that the measured angles of water on electrochemically prepared films strongly depend on film thickness and on polymerization variables such as applied electrode potential (or current density), electrolyte, solvent, and temperatureO8 Measurements on Poly(S-alkyl)thiophenes. Table 2 shows our measurements of advancing contact angles on neutral and oxidized films of polythiophene and poly(3-alkylthiophenes). Average values are for 3n measurements, where n is the number offilms and a i s the standard deviation. The thicknesses of the dry films were between 0.1 and 0.2 pm. Neutral films are markedly hydrophobic (contact angles higher than 90') while oxidized films are hydrophilic. Apparently, the angles on the oxidized films increase with the length of the alkyl substituent, but this trend may also be related to differences in film morphology. The contact angles change gradually between the limiting values shown in Table 2, as these films are charged or discharged. For example, Figure 2 shows contact angles ofwater on PMeT as a function of electrode potential. White (and black) squares are for measurements while continuously increasing (and decreasing) the electrode potential to the indicated value. The data, however, show hysteresis and, consequently, these instantaneous contact angles cannot be used to calculate the energetics a t the polymer-liquid interface and only have qualitative value in determining the hydrophobicity or hydrophilicity of these films. Our results for PT and PMeT cannot be directly compared to the data in Table 1because the experimental conditions for contact angle measurements (such as surface pretreatment, water drop size, and vapor composition) in other reports were not clearly specified and might have been different from ours. Contrary to polythiophene, polyalkylthiophenes can be electrochemically cycled in aqueous media. We chose PMeT for our experiments to probe reversible oil spreading because it shows the highest change in contact angle among the polyalkylthiophenes tested. Ferrocyanide Ion at W M e T Electrodesin Media Containing CTAB. Curve a of Figure 3 is a voltammogram of solution I a t the Pt electrode. The redox transition Fe3+/2+is a t E112 = 0.08(*0.02) V vs AgCUAg. The features of this voltammogram are as expected for a reversible, one-electron transfer?O the anodidcathodic current ratio, IpaApc, is near 1 and the current peaks (19)(a) Yasuda, T.; Okuno, T. Langmuir 1994,10,2435.(b) YektaFard, M.; Ponter, A. B. J.Adhesion Sci. Technol. 1992,6,263. (c) Ruckenstein, E.; Gourisankar, S. V. J.Colloid Interface Sci. 1986,107, Sharma, A. K. J. Polym. Sci. Polym. Phys. Ed. 488. (d) Yasuda, H.; 1981,19,1285. (e) For comments of the effect of electrostatic charge on the contact angles on polymer surfaces, see: Ponter, A. B.; YektaFard, M. J. Colloid Interface Sci. 1984,101,282.
WMeT WeT WBuT
contact angle oxidized film
neutral film 122i7 116 i 8 117f.5 113 f 2
4
2
film percentage charged*
22
13 5 4 18 i 3 22 2 33 i 4
19 18 18
*
a Polymer films, in the thickness range from 0.1 to 0.2 pm, were synthesized on Pt disk electrodes (F't electrode, geometric area = 0.44 cm?. See Experimental Section for details. See Experimental Section.
I
0'
' 0
.
'
'
Volts
'
"
0.4
0.2 V.I
0.6
'
' 0.8
I
Ag/AgCi
Figure 2. Contact angles of water on dry PMeT films as a function of charging level of PMeT. Films were held at
increasing and decreasingpotentials (white a n d black squares, M LiC104 for 1 min, disconnected, and dried. Error bars represent the standard deviation of the measurements.
respectively) on 0.10
increase linearly with the bulk concentration of ferrocyanide (inthe range from 1.0 x to 2.0 x M) and with the square root of the potential scan rate (from 5 to 75 mV/s). The somewhat large peak-to-peak separation, AEp= 95 mV, canbe attributed to uncompensated solution resistance. The expected value is 60 mVeZ0This voltammogram is essentially superimposable on that of a CTAB-free solution of ferrocyanide a t pH 4, indicating that the surfactant does not perturb the response of the probe.21-2s In solution I, in order to minimize the effect of electrostatic attraction between the surfactant and the (20)Bard, A. J.;Faulkner, R. C. Electrochemical Methods; Wiley: New York, 1980. (21)Voltammograms of ferrocyanide in micellar solutions of 0.10M CTAB show lower peak currenta and E m shifts of ca. 100 mV to more negative potentials with respect to those in surfactant-free solutions as a result of strong interaction between the probe and the surfactant. See: Rusling, J.F.; B a n g , H.; Willis, W. S.Anal. Chim.Acta 1990,235, 307. (22)For strong surfactant-probe interactions, see: (a) Mackay, R. A,; Texter, J.,Eds. Electrochemistry in Colloids and Dispersions; VCH: New York, 1992;Chapters 5 and 13. (b) Mackay, R.;Myers, S. A,; Bodalbhai, L.; Brajter-Toth, A.Ana1. Chem. 1990,62,1084.(c)Rusling, J. F.;Shi, C.-N.; Kumosinski, T. F.Anal. Chem. 1988,60, 1260. (d) Chen, J.-W.; Georges, J.J.Electroanal. C h m . 1986,210,205.(e)Kaifer, A. E.; Bard, A. J.J.Phys. Chem. 1986,89,4876.(0Georges, J.;Berthod, A. Electrochim. Acta l98S,28, 735. (e) Ohsawa, Y.; Aoyagui, S. J. Electroanal. Chem. 1983,145, 109. (g) Ohsawa, Y.; Shimazaki, Y.; Aoyagui, S. J. Electroanal. Chem. 1980,114, 235. (23)For voltammograms of ferrocyanide in 0.010 M NaCl, we found Eyz = 0.08V (at pH 4)and 0.15V vs AgCVAg (at pH 9).
Langmuir, Vol. 11, NO.8, 1995 2923
Polymer Films with Tunable Surface Properties 6.0 8
.
0.40
azo Volts
V.I
-
AplAgCl
I
.
QIO
0.00
Volts
V.I
0.m
.-
AplAgCl
Figure 3. Cyclic voltammograms at 50 mVls of [Fe(CN)d4-:(a) at Pt in solution I; (b) at Pt/PMeT in solution I; (c) at Pt"MeT in emulsion I; (d) at the same Pt"MeT electrode, in solution I, after transferring the electrode from emulsion I to 0.10 M LiC104, holding the electrode potential at +0.9 V vs AgCUAg for 1 min, and disconnecting. PMet thickness = 0.2 pm. Solution I is 5.0 M HC1. Emulsion I is 10 g/L M CTAB + 0.010 M NaCl + 1.0 x x M ferrocyanide in water containing 1.0 x n-hexadecane in solution I. probe on the response of the latter, the molar ratio CTAB/ ferrocyanide was made 115. Curve b of Figure 3 is for solution I at a Pt/PMeT electrode. Ev2 = 0.12 f 0.02 V vs AgCVAg, AE, = 120 mV, Ipa/Ipc = 1.04, Ipa = (scan rate)", and Ipa = concentration of ferrocyanide. The peak heights (which are 95% of those in curve a) are independent of PMeT thickness, in the thickness range from 50 nm to 0.3 pm. In the same thickness range, AE, increases from 90 to 155 mV. These features are consistent with ferrocyanide diffusing through pinholes in the film and reacting at the W M e T interfa~e.~JO This same (as in b) Pt/PMeT electrode was transferred to emulsion I. After two or three potential scans a t 50 mVIs, the current decayed to nearly background level (Figure 3, curve c). Further, a voltammogram recorded after transferring this electrode back to solution I (no emulsion) is still superimposable on curve c. At a bare Pt electrode, the current peak of [Fe(CN),l4- in emulsion I (not shown) is ca. 85% of that in solution I; therefore, the passivation of the W M e T electrode (current blocking) in curve c is consistent with spreading of the oil phase on the hydrophobic surface of neutral PMeT. Curve d of Figure 3 shows the reappearance of the current signal of ferrocyanide, after transferring the W PMeT electrode from the conditions used to generate curve c to 0.10 M LiC104(aqueous), holding a t +0.9 V vs AgCV Ag for 1 min, disconnecting, transferring to solution I, and holding a t -0.2 V for 1 min. The two prominent features ofthe recovered current signal are (1)the current maximum is, typically, 40-60% of that in curve b and (2) both oxidation and the reduction waves reach a plateau, that is, the redox waves do not have the peak shape characteristic of voltammograms of species in solution a t macroelectrodes. Holding the electrode from curve d in 0.10 M LiC104 a t 0.9 V for another 5 min did not result in a further increase of the current maximum. The recovered current signal of ferrocyanide suggests that film oxidation-which must start at surface sites not coated with oil-induces, a t least partly, ejection of oil from PMeT.24 (24)A "deblocked" W M e T electrode in 0.10 M LiC104 shows oxidation and reduction waves a t 0.65and 0.35 V vs AgCYAg. The current peaks, however, are less than 30% of those in Figure 1.
Ferrocyanide Ion at Pt/PMeTElectrodes,in Media Containing SDS. Voltammograms of solution I1 a t Pt andPt/PMeTelectrodesshowEm=O.E f0.02VvsAgCV Ag, I p d p c = 1.1f 0.1, AE, = 90 mV, in analogy to the features in Figure 3, curves a and b. Blocking of the current signal, analogous to Figure 3, curve c, is observed for emulsion I1 a t the Pt/PMeT electrodes: the current response of emulsion I1 is less than 5% of that in solution 11. (In this case, blocking of the current a t PtDMeT can be unambiguously ascribed to spreading of the oil droplets on PMeT because there is no electrostatic attraction between the probe and the surfactant.) After transferring the W M e T electrodes (treated with emulsion 11)to 0.10 M LiC104, holding +0.9 V vs AgCVAg for 5 min, disconnecting, and transferring to solution 11, the electrodes remained "blocked", in contrast to Figure 3, curved. Thus, avoltammogramrecordedbetween -0.2 and 0.4 V vs AgCVAg shows background current only, indicating that an interplay between hydrophobic and electrostatic forces (between film and surfactant) is necessary to remove the oil from the polymer. Hexaammineruthenium(II1)Ion at M M e T Electrodes. Blocking and deblocking of Pt/PMeT were also observed by using a cationic probe in media containing CTAB. Curves a and b of Figure 4 are cyclic voltammograms of [Ru(NH&I3+ a t Pt/PMeT in solution I11 and emulsion 111, respectively. (PMeT thickness = 0.18 pm.) The redox transition Ru3+12+is at Ell2 = -0.17(i0.03) V vs AgCVAg. Curve c of Figure 4 (deblocking effect) was recorded after immersing the blocked electrode in aqueous 0.10 M LiC104,holding its potential a t +0.9 V vs AgCVAg for 5 min, disconnecting, and transferring to the pristine solution 111. As in Figure 3, curve d, recovery of the signal of hexaammineruthenium(II1) is not complete. The current maximum (of the reduction wave) in curve c ia about 60%of that in curve a. The effect of continuous blockingdeblocking of PMeT on the current maximum for Ru3+"+ is shown in Figure 5. After the first three cycles, the intensity of the recovered signal becomes constant, indicating that a part of the film is reversible with respect to oil sorption. Wetting vs Dewetting. Spreading of the oil droplets (actually oil surfactant) is favored by the hydrophobic nature of the neutral polymer. The oil may also spread
+
Torres et al.
2924 Langmuir, Vol. 11, No. 8, 1995 C
-2.0 0.20
0.00 Volts
V.I
-t
azo
0.00
AdAgCI
Volts
VR
Ag/AgCI
4.20
4.40
Volts vs. Ap/AgCl
Figure 4. Cyclic voltammograms at 25 mV/s of [Ru(NH3)d3+:(a) at Pt/PMeT in solution 111; (b) at Pt/PMeT in emulsion 111; (c) at Pt/PMeT in solution 111, after transferring the electrode from emulsion I11 to 0.10 M LiC104,holding the electrode potential at M CTAB M [Ru(NH&I3++ 0.010 M Kl3r + 1.0 x 0.9V vs AgCWAg for 1min, and disconnecting. Solution I11 is 5.0 x M HC1. Emulsion I11 is 10 g/L n-hexadecane in solution 111. 1.0 x
+
in the film, presumably forming pools of bulk oilz6that slow the flow of the aqueous phase to the underlying electrode and contribute to block the current response of the probe. Once film oxidation starts, the hydrophobic interaction between the polymer and the adsorbed oil decreases and removal of oil (peeling of oil from the polymer) is driven by electrostatic repulsion between the charged polymer and the surfactant (that resides a t the oil/water interface).27 Thus, in the presence of SDS, oil removal is not favorable presumably because the anionic head of the surfactant remains adsorbed a t the charged film.2s However, even in the case of the systems containing CTAB, oil removal is not complete, as indicated by the voltammetry of the probes in Figures 3 (curve d), 4 (curve c), and 5.
m
m
0
1
2
3
4
m
5
Conclusions
6
Nnmbcr of Cpclr
Figure 5. Voltammetric, cathodic current maxima for [Ru(NH&I3+at the Pt/PMeT electrode from Figure 4 as a function of number of blocking-deblocking cycles. Scan rate = 25 mV1 s. The current at zero cycle (3.9 PA) is the pristine, cathodic peak of the probe in solution 111.
onto the internal surface of the polymer. One envisions that the effective permeabilities for the oil and water phases through the bulk of the polymer are necessarily different and that the internodular region of the film limits the flow of the emulsion. In macroemulsions, the diameter of the disperse phase droplets varies in the range from 10-1pm to 0.1 mm. In turn, submicrometerthick PMeT films have a compact, nodular structure with typical internodular separation in the submicrometer range, as shown by electronic microscopy.26 Thus, only some of the smallest oil droplets can move into the film and spread spontaneously on the internal polymer surface. Droplets of appropriate size may get irreversibly trapped (25)Although we did not measure the porosity of these films by independent methods such as neutron scattering or ellipsometry, our electronic micrographs are consistent with those by others. See, for instance: (a) Reynolds, J. R.; Hsu, S-G.; Amott, H. J. J . Polym. Sci., Garnier, F. J . Part B: Polym. Phys. 1989,27,2081. (b) Tourillon, G.; Polym. Sci., Polym. Phys. Ed. 1984,22,33.
We have addressed the possibility of reversible spreading of the disperse phase of oil-in-water emulsions at PMeT. Electrochemical experiments with two differently charged probes in the emulsions indicate that adsorption of the disperse phase a t the neutral, hydrophobic polymer is fast (on the scale of seconds). Desorption and removal of oil are possible by charging the polymer to its oxidized,hydrophilic form, providing the adsorbed droplets contain a cationic surfactant. Droplets that contain a n anionic surfactant, SDS, cannot readily be removed presumably because of strong adsorption of the surfactant a t both the neutral and charged forms of the polymer. Thus, removal of oil from the surface requires a combination of a decrease in the hydrophobic attraction between the charged surface and the oil and electrostatic repulsion between the film and the surfactant. Redox switching of the film in an electrolyte solution does not result in complete removal of the oil. We believe that the morphology of the film allows for irreversible trapping of oil particles. (26)For flow of emulsions in porous media, see: Kokal, S. L.; Maini, B. B.; Woo, R. In Emulsions Fundamentals and Applications in the Petroleum Industry; Schramm, L. L., Ed.; American Chemical Society: Washington, DC, 1992; Chapter 6. (27)For relevant discussions on forces between particles and surfaces, see: Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, CA, 1992. (28)We assume that because, in oxidized PMeT, the positive charges are delocalized along the polymer chains, the polymer is highly polarizable. As a result, PMeT can interact strongly with SDS. See: Ren, X.; Pickup, P. G. J . Phys. Chem. 1993,97,5356.
Langmuir, Vol. 11, No. 8, 1995 2925
Polymer Films with Tunable Surface Properties We are currently exploring the use of PMeT as a stationary phase for mixed-mode c h r o m a t ~ g r a p h y a, ~ ~ novel technique for separations of mixtures that contain both polar and nonpolar compounds.
Carlos Jorge da Cunha for his assistance with synthesis, Jin Zhe for earlier contact angle measurements, and Drs. Stephen Lowen and Hitoshi Masui for helpful discussions.
Acknowledgment. We acknowledge the financial support of The Department of Supplies and Services and CANMET, Canada. We thank Lori Payne for herassistance with the contact angle measurements,
LA9500944 (29) For a recent review, see: Henry, M. P. In HPLC: Proteins, Peptides,and Polynucleotides;Hearn, M.T.W., Ed.; VCH: New York, 1991; pp 149-175.
