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Anal. Chem. 1992, 64, 3180-3188
Stable Films of Cationic Surfactants and Phthalocyaninetetrasulfonate Catalysts Naifei Hu$ David J. Howe, Maryam F. Ahmadi, and James F. Rusling' Department of Chemistry, Box U-60,University of Connecticut, Storrs, Connecticut 06269-3060
Films made from catlonlc rurfactak and wdketalned redox catalyst8 were Investlgated. Full kadlng of metal phthalocyanlnetetrarulfonates (MPcTSC) Into watwlnsoluble dC alkyldhethylammonlum rurfactak by Ion exchange from aqueous solutkns yielded coatlng8 on electrodes that retain these catalyst ions for 1-2 weeks In electrolyte solutions. I n contrast, partly loaded flknr loat most MPcTSC Ion8 In a few houru All film8 showed gel-tdlqukl crystal phau tradtions at temperatures characterktlc of wrfactanl Mlayen. Crosswctknal vhws by SEM showed layers of 0.1-0.2 pm, as w d l as m e dhderod regions. Each larger layer is probably made up of stacks of many molecular Wayen. Retention of MPcTSC Ion8 seems related io thdr dimerkatlon. Dinwa of MPcTSC amoclaled with ammoniumhead groups may crosslink adlacent surfactant Mayors. The MPcW- Ion8 that enhance stablllty In thou f l h are ako goad redox catalysts.
INTRODUCTION Films of water-insoluble surfactants1 can be prepared by casting their solutions onto a solid support and evaporating the organic solvent. Casting offers a simple means to prepare relatively thick multiple bilayer surfactant films compared to the reliable but tedious Langmuir-Blodgett film transfer.2 Surfactant bilayer films intercalated between clay layers3 or linear ionic polymers,496 as well as f i s of polymerized surfactanta,4d-f have also been prepared by casting. The surfactants used typically have two or three hydrocarbon chains of 12 or more carbons. They do not form micelles which would tend to dissolve the f i s in water. Permeability of cast surfactant fiis is controlled by their phase. Neutral, water-soluble solutes pass through filmsthat are in the liquid crystal state, but permeability is turned off when the films are brought to the solid-like gel phase.3-5 + Permanent address: Beijing Normal University, Beijing, China.
(1)(a) Nakaahima, N.; Ando,R.; Kunitake, T. Chem. Lett. 1983,15771580. (b) Kunitake, T.; Shimomura, M.; Kajiyama, T.; Harada, A.; Okuyama, K.; Takayanagi, M. Thin Solid Film 1984,121,L89-91. (c) Iehikawa, Y.;Kunitake, T. J. Am. Chem. SOC. 1986,108,8300-8302. (d) Hamachi, I.; Noda, S.; Kunitake, M.; Hamachi, I.; Honda, T.; Noda, S.; Kunitake, T. J. Am. Chem. SOC. 1990,112,6744-6745. (e) Hamachi, I.; Honda, T.; Noda, S.; Kunitake, T. Chem. Lett. 1991, 1121-1124. (0 Hamachi, I.; Noda, S.; Kunitake, T. J.Am. Chem. SOC. 1991,113,96251991,113,6219630. (9) Ishikawa, Y.; Kunitake, T. J. Am. Chem. SOC. 630. (2)Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982. (3)Okahata, Y.;Shimizu, A. Langmuir 1989,5,954-959. (4)(a) Shimomura, M.; Kunitake, T. Polym. J. 1984,16,187-190.(b) Kunitake, T.; Tsuge, A.; Nakaahima, N. Chem. Lett. 1984,1783-1786.(c) Nakaahima, N.; Kunitake, M.; Kunitake, T.; Tone, S.; Kajiyama, T. Macromolecules 1981, 18, 1515-1516. (d) Higashi, N.; Kajiyama, T.; Kunitake, T.; Prass, W.; Rmgadorf, H.; Takahara, A. Macromolecules 1987,20,29-33.(e) Nakashima, N.; Eda, H.; Kunitake, M.; Manabe, 0.; Nakano, K. J.Chem. SOC.,Chem. Cornrnun. 1990,443-444. (0Kunitake, T. Polym. J. 1991,23,613-618. (5)(a) Okahata,Y.; Enna, G.; Taguchi, K.; Seki, T. J.Am. Chem. SOC. 1985,107,5300-5301.(b) Okahata, Y.; Enna, G. J. Phys. Chem. 1988, 92,4546-4551. (c) Okahata,Y.;Enna, G.; Takenouchi, K. J. Chem. SOC., Perkrn Trans. 2 1989.835-843. 0003-2700/92/0384-3 180$03.OOlO
Results of X-ray diffraction and electron microscopy, as well as phase transitions for surfactant f i s at temperatures close to those of bilayer vesicle suspensionsof the same surfactants, have been used to propose multiple bilayer structures.1.3-6 Stable, ordered surfactant films have a wide range of potential applications. Possibilitiesinclude membranes with controllable permeability?+ coatings for piezoelectric6 or amperometric sensors,Qand kinetic control of catalytic chemical or electrochemical reaction^.^^ Surfactant molecules in these f i e are arranged in bilayers resembling those of lipid membranes in living cells. Thus, additional applications include biomembrane-like supports for ordering biological macromoleculesld-fand inorganic complexeslwand for designing systems with vectorial electron transport.10 We are currently evaluating insoluble surfactant f i s containing redox mediators for electrochemical catalysis, specifically for dehalogenations of organohalide pollutants.8 Films of didodecyl- and dioctadecyldimethylammonium bromide (DDABand DODAB)cast onto pyrolytic graphite electrodes readily incorporated multivalent anions from solution. When used in aqueous solutions, these films excluded hydrophilic multivalent cations but were able to preconcentrate hydrophobic ions and neutral molecules.7 Thus, such fiis should be useful in exerting selectivity and control of catalytic reactions. Anionic redox catalysts can be introduced into liquid crystalline DDAB and DODAB films on electrodes by ion exchange from aqueous solutions. Anionic macrocyclic complexes such as metal phthalocyaninetetrasulfonates have a wide range of catalytic activity." Films incorporating such complexes catalyzed dehalogenation of organohalide pollutants, converting vicinal dibromides to olefins and trichloroacetic acid to acetic acid.8 Clay-surfactant composite films containing neutral metal phthalocyanines catalyzed similar reductions.9 Charge transport rates are much better for both types of films in liquid crystal phases than in solid-like gel states. Gel-to-liquid crystal phase transitions were detected by voltammetry and differential scanning calorimetry.7-9 Composite clay-surfactant f i b s containing metal phthalocyanines showed excellent stability, retaining catalytic activityfor 1month or more.9 However,maintenance of stable amounts of anionic catalysts in pure DDAB and DODAB f i i s depends strongly on the type of anion incorporated. Hexacyanoferrate(4) ion and cobalt(II1) corrinhexacarboxylate are readily incorporated into DDAB and DODAB films. However, when f i e loaded with these anions are placed in a solution containing only supporting electrolyte, 50-75 % of these electroactive anions are leached out in several houre.8*B (6)Okahata,Y.;Ebato, H. Anal. Chem. 1991,63,203-207. (7)Ruling, J. F.; Zhang, H. Langmuir 1991,7, 1791-1796. (8)Ruling, J. F.; Hu, N.; Zhang, H.; Howe, D.; Mmw, C.-L.; Couture, E. In Electrochemistry in Microheterogeneoua Fluids; Mackay, R. A., Texter, J., Eds.; VCH Publishers: New York, 1992. (9) Hu, N.; Ruling, J. F. Anal. Chem. 1991,63,2163-2168. (10)Gratzel, M.Heterogeneous Photochemical Electron Transfer; CRC Preee: Boca Raton, FL,1989. (11)Mom, F. H.;Thomas, A. L. The Phthalocyanines; CRC Press: Boca Raton, FL, 1983;Vol. I, pp 79-100. 0 1992 Amerlcan Chemical Soclety
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On the other hand, when sufficient metal phthalocyaninetetrasulfonates (MPcTS4-)are incorporated, DDAB films retain most of these catalytic anions for 10 days or more in 0.1 M KBr solutions.8 In this paper, we present results of differential scanning calorimetry, molecular spectroscopy, voltammetry, and scanning electronmicroscopy (SEMI with energy-dispersive X-ray (EDX) analysis which provide insight into the structure and causes of stability of cationic surfactant films containing metal phthalocyaninetetrasulfonates. Absorbance spectra suggest that dimerization of metal phthalocyaninetetrasulfonateslz contributes to the remarkable stability of catalytic films containing these complexes. These films are microscopically ordered in multiple bilayers of surfactants, but are macroscopically heterogeneous. EXPERIMENTAL SECTION Chemicals and Solutions. Didodecyldimethylammonium bromide (DDAB) and dioctadecyldimethylammoniumbromide (DODAB) were 99+ % from Eastman Kodak. Cetyltrimethylammonium bromide (CTAB, hexadecyltrimethylammonium bromide) was Fisher certified (99.8%). Copper phthalocyanine3,4',4",4"'-tetrasulfonic acid and nickel phthalocyaninetetrasulfonic acid (mixture of isomers) were obtained as tetrasodium salts from Aldrich. All other chemicals were reagent grade. Apparatus and Procedures. A Bioanalytical SystemsBAS100and PARC Model 273 electrochemistry system were used for cyclic voltammetry (CV). The working electrode was a basal plane pyrolytic graphite (HPG-99,Union Carbide)disk (geometric A = 0.2 cm2). Electrodes were prepared by sealing pyrolytic graphite (PG) disks into the large end of a polypropylene pipette tip as described previouslyQor by sealing to a glass tube with heat shrinkable tubing. Electrodes were abraded with 600-grit Sic paper on a metallographic polishing wheel prior to coating with surfactant. Twofilm thicknesses were used. PG electrodes were coated by pipeting 20 p L of 0.1 M solution of DDAB in chloroform (thick film) or 6 p L of 0.01 M solution of DDAB (thin film) onto PG disks. Chloroform was allowed to evaporate for 24 h after fitting a small bottle tightly over the electrode to serve as a closed evaporationchamber. Coatedelectrodeswere subsequentlycured for at least 24 h in air. This method gave reasonably uniform and reproducible coatings as evaluated by light microscopy, SEM, and CV of incorporated ferrocyanide and MPcTSC ions. Approximate film thicknesses estimated from geometric factors' and confirmed by SEM cross-sectionalviews were 1-2 pm for thin films and 30-50 pm for thick films. Thin DDAB films were used for most of the electrochemical experiments. The three electrode cell for CV studies included the surfactantcoated PG working electrode, a platinum wire counter electrode, and a saturated calomel electrode (SCE) or a Ag/AgBr wire as reference. In 0.1 M KBr, the Ag/AgBr reference half-cell had a potential of 0.086 V vs SCE. The ohmic drop of cells was compensated 290% by the electrochemicalanalyzers;the typical uncompensated resistance was 10-20 Q. Experiments were thermostated at 25.0 f 0.1 "C. All solutions were purged with purified nitrogen to remove oxygen before voltammetry. Absorptionspectroscopywas done using either a Perkin-Elmer Model X3B or a Milton Roy Spectronic 3000 Array UV-vis spectrophotometer. Films were deposited as above onto glass microscope slides at a sufficient thickness to give measurable absorbance. These films were equilibrated with aqueous 0.1 M KBr solutions containing 0.5 mM of the metal phthalocyaninetetrasulfonates (MPcTS4-). Films were removed from these (12)(a) Schelly, Z.A.; Farina, R. D.; Eyring, E. M. J. Phys. Chem. 1970,74,617-620. (b)Schelly,Z.A.; Harward,D. J.;Hemmes,P.;Eyring, E. M. J. Phys. Chem. 1970,74,3040-3042. (c) Gruen, L. C.; Blagrove, R. J. A u t . J. Chem. 1972,25,2553-2558; 1973,26,319-323. (d) Boyd, P. D. W.; Smith, T. D. J. Chem. SOC.,DaZton Tram. 1972,839-843. (e) Farina, R. D.; Halko, D. J.; Swinehard, J. H. J. Phys. Chem. 1972, 76, 2343-2348. (0Abe1,E. W.;Pratt, J. M.;Whelan,R. J. Chem.Soc.,Dalton Trans. 1976,509-514. (9) Yang, Y.; Ward, J. R.; Seiders, R. P. Znorg. Chem. 1988,24,1765-1769.
9181
Table I. Phase-Transition Temperatures ("C)
sample dry DDAB powdep wet DDAB powderb aqueous DDAB dispersion' DDAB f i s d + NiPcTS4+ CuPcTS4+ Fe(CN)& aqueous DODAB dispersionc DODAB filmsd + NiPcTS' + CuPcTS4+ Fe(CN)e4-
diff scanning calorimetry this work lit. [refl
voltammetry
prev work [refl
a Dried in a vacuum over cas04 dessicant for 2 days. * Stored in a closed chamber with saturated water vapor for several days. 10 mM aqueous solution sonicated for 4 at 50 "C.d After full equilibration in solutionof 0.1 M KBr or 0.1 M KBr + 1mM of ion stated. Duplicate values are for separate f i i .
solutions, dipped in pure water several times, and then placed in the spectrometer to obtain the spectra. Scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX)were done with an Amray 1810microscope using a tungsten filament as the electron source. DDAB films for SEM/EDX analysis were coated onto PG electrodes using the same method as for voltammetry. The entire electrode assembly could be attached to the mounting stage of the SEM with electrical connection through the electrode lead wire. Prior to analysis by SEM, 5 nm of gold was coated onto samples with a Model SC 500 sputter coater (Bio-Rad). For cross-sectional SEM views, coatings were cast onto very thin disks of pyrolytic graphite and freeze-fractured after immersion in liquid nitrogen. EDX was done using a Phillips North American EDAX Model PV-9800 system. The beam diameter for spot analyseswas 2 pm. Sensitivity factors provided with system software were used to convert counts to relative atom percent. Differential scanning calorimetry was done with a PerkinElmer Model DSC 7 calorimeter calibrated with water (0.0 "C) and indium (156.6 "C). Thick DDAB films (5-10 mg) were prepared as above on PG electrodesand equilibrated until steadystate CVs were obtained. The films were kept in solution until immediately before the DSC run and then scraped off of the PG into aluminum sample pans which were then crimped shut. Samples were held at -30 "C for 10 min and then scanned at 10 "C min-l to 0.0 "C, where they were held for 10 min. Analytical scans at 10 "C m i d were subsequently initiated. Phase transitions are reported as onset temperatures of peaks in the thermograms.
RESULTS Calorimetry. Previous work showed that gel-to-liquid crystal phase transitions measured for surfactant films by differential scanning calorimetry (DSC) corresponded to breaks in cyclic voltammetric peak current vs temperature curves.7-9 More extensive calorimetric studies are reported herein. A phase transition at 55 "C was observed for dry DDAB powder (Table I). When the powder was allowed to equilibrate with saturated water vapor under ambient temperature, a transition (T,) was observed at 10 OC with no peak at 65 OC. The 10 "C transition most likely corresponds to the so called Lam2phase, a lamellar liquid crystal system observed13when there is about 7-11 % water present in DDAB. An aqueous (13)Fontell,K.;Ceglie,A.;Lindman,B.;Ninham,B.ActaChem.Scand. 1986,A40,247-256.
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 24, DECEMBER 15, 1992 15 I
15 10
0
0
10
I
-
r
2E
5
5 -
0
-5 -10 0.20
0.40
0.60
0.80
1 .oo
-E, V us. AgIAgBr
F b v o 1. CydicvottammogramsatO.lOVs-l: (a)atbareWelectrode in 0.1 M KBr10.2 mM NIPcTSC; (b) DDABaated Po fully loaded with NIPcTSC in 0.1 KBr with no NIPcTSC in solutlon.
sonicated dispersion of DDAB gave a T,of 15 "C, slightly larger than values previously reported for sonicated vesicle dispersions,13 but similar to that reported for the lamellar liquid crystal phase Laml observed at low DDAB concentrations in water.14 Similar DSC behavior for dry dioctadecyldimethylammonium chloride (DODAC)has also been found. A gel-to-liquid crystal transition at 52 "C was observed only after addition of sufficient water and persisted between 9 and 86%water.15 This higher value of T,is consistent with the longer chain length compared to DDAB and is similar to that reported for DODAB (Table I), despite the difference in anion. Films of DDAB fully loaded with MPcTS" or ferrocyanide ions showed phase transitions by DSC and CV at similar temperatures(TableI)to those of cast DDAB films containing Br- ions and the Laml and Lam2 phases. Data for DODAB support similar conclusions. Electrochemistry. Voltammetry was done at 25 O C with DDAB films in a lamellar liquid crystal state. Films were loaded with MPcTS4-ions by placing freshly prepared DDABcoated electrodes into 0.2-1.0 mM MPcTSN@/0.1 M KBr solutions and scanning repeatedly over the potential range of the first CV peak, usually -0.2 to -1 V. Full incorporation of MPcTS4- ions into DDAB films, as detected by reproducible CVs, occurs in about 30 min for thin films. Loading also occurs without scanning, but at a slightly slower rate. In water, MPcTS" ions are heavily aggregated,12and only very small, broad irreversible CV peaks are observed at PG electrodes.16 When DDAB films fully loaded with MPcTS4ions are rinsed with water and placed into 0.1 M KBr without MPcTS", chemically reversible cyclicvoltammograms (Figure 1)are observed. At 100 mV s-l, the first cathodic peak for NiPcTSb is at -0.75 V vs Ag/AgBr and that of CuPcTS4- is at -0.4V. These cathodic peak potentials are similar to those reported for the one-electronM(II)PcTS4-/M(II)PcTSsredox couples in noncoordinating organic solventa.17 As discussed below, these fully loaded fiis have excellent stability in water. When DDAB f i i s fully loaded with MPcTSQ-were sonicated for several seconds in chloroform, the solvent took on the characteristic color of the MPcTS4-. Since MPcTSNm is completely insoluble in chloroform, this suggests that the MPcTS" removed from the electrode was (14)Evans, D. F.;Mitchell, D. J.; Ninham, B. W. J.Phys. Chem. 1986, 90.2817-2825. (15)Komada, M.; Kuwabara, M.; Seki, S. Thermochim. Acta 1981,50, 81-91. (16)Zevecic, S.;Simic-Glavaski, B.; Yeager, E.; Lever, A. B. P.; Minor, P. C. J.Electroanal. Chem.Interfacial Electrochem. 1985,196,339-358 and references therein. (17)Lever, A. B.P.; Licoccia, S.; Magnell, K.; Minor, P. C. Aduances in Chemistry Series No. 201;American Chemical Society: Washington, DC, 1982;pp 237-251.
,
I
-10 0.20
0.40
'\Y
0.60
, 0.80
1.oo
-E, V vs. AglAgBr
Figuro 2. Series of cyclic voltammograms at 0.10 V s-I in 0.1 M KBr showlng uptake of NiPcTSC by a DDAB film from Its 0.2 mM solutlon during continuous scanning after (a) 1, (b) 3, (c)8, (d) 10, (e) 18, and (f) 48 min.
associated with surfactant head groups. Moreover, no CV peaks were observed after returning the chloroform-washed electrodes to 0.5 mM MPcTS"/O.l M KBr. Chloroform apparentlysolubilizesmost of the electroactiveMPcTS4-from the electrode. As mentioned, diffusion-controlled peaks of MPcTS4- are difficultto detect in water on bare PG (cf. Figure la). Scan rate studies were done on thin films fully loaded with NiPcTS4- and CuPcTS4-ions. Peak current vs scan rate (v) plots were linear only at Y < 3 mV s-l, where nearly symmetric peaks were observed. Between 0.06 and 10 V 8-1, the peak had the characteristic unsymmetrical diffusion-controlled shape (Figure 1)and cathodic peak currents were proportional to v1/2. Cathodic and anodic peak currents were equal throughout this range of scan rates. These observations are consistent with the CV behavior expected1*for an electroactive species confined to a relatively thick film on an electrode surface. SymmetricCV peaks found at very low scan rates suggested that nearly all the electroactive material was reduced at the cathodic end of these scans. Integration under this oneelectron cathodic peak gives the charge (8)proportional to the so-called surface concentration of electroactive material (FJ, given byla
ro= QIFA where A is the area of the electrode and F is Faraday's constant. Surface concentrations for thin films were estimated in two different ways: (1)integration of the cathodic CV at 1 mV s-1 and (2) integration of scam made at 6 mV 8-1 through the potential range of the peak, but stopped and held at a potential corresponding to about 60 % of peak current on the negative side of the peak for 600 8. The latter method might measure charge from MPcTS4- monomers slowly dissociated from dimers in the film. Results of these two experiments were in good agreement and consistently gave surface concentrationsof (4-5)X 10-8 mol cm-2 for the two MPcTS4ions. Using the average of these surface concentrationsand 1.5 pm for the estimated film thickness to obtain f i i volume, we estimate a concentration of about 0.3 M for the electroactive species in fully loaded thin films. The CVs show interesting changes as the film is loaded by repetitive scanning in MPcTS4- solutions. Shortly after a freshly prepared thin film DDAB-PG electrode is placed into aqueous 0.2 mM NiPcTS", a reversible CV is observed with cathodic peak potential of -0.67 V (Figure 2a). With (18)Murray, R.W. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984;Vol. 13,pp 191-368.
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 24, M E M B E R 15, 1002 51 0 r
I
21
I
4 X
o L ' " " ' ' ' " " ' ' ~ 0
100
2 00
- 2 " " ' " " " " " " " " 0.40 0.00 -0.40 -0.80
900
t, min.
thick film
A
-1.20
-1.60
-1.20
-1.60
E, V vs. AgIAgBr
1.00
thin film
1.80 I
I
I
< E
c-
e
3
0 Y
0
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0.00 0
90
180
270
1, hra Flgure 3. Influence of time In a 0.1 M KBr solutlon on cathodlc voltammetric peak current of DDAB fllms at 0.10 V s-I: (a) loaded wlth NIPcTSC by continuous CV cycllng for 5 mln; (b) fully loaded with NIPcTSC. Note tlme scale of minutes In a and hours In b. (Llnes drawn arbitrarily.)
increasing time, during which repetitive scanning was done, both cathodic and anodic peaks shift negative. Between 7 and 10 min, a new, slightly more negative cathodic peak (Figure 2c,d) grows in and replaces the original peak. The cathodic peak potential shifts negative to about -0.78 V in the first 20minandthenremainsnearlyconstant. The overall potential shift during loading was typically -110 mV. The anodic peaks shift comparably. A steady-state CV is reached after about 30 min of scanning, after which the CV remains constant with time (Figure 2g). Similar resulta were obtained when incorporating CuPcTS". An increase in rigidity of the films was also observed upon loading them with MPcTS4-. DDAB films soaked in aqueous solutions are soft and malleable. Fully loaded films are rather brittle, and pieces can be chipped away from the PG surface with a spatula. Retention of incorporated MPcTS4- ions in DDAB films depends strongly on the amount of loading. In thick films that were removed from solution before full loading was reached (cf. Figure 2a-c), washed, and placed in fresh 0.1 M KBr, about 75 % of the MPcTS4-is lost to the solution within several hours (Figure 3a). Thin films show a more gradual decay, but show a 75% loss in about 3 h. In contrast, fully loaded films have much smaller rates of decreaee in CV currenta over 270 h in 0.1 M KBr. Thus, a sufficiently large concentration of incorporated MPcTSQ-produces films that retain MPcTS4- quite effectively. Ion-exchange properties of f i i s fully loaded with MPcTS4are different from the original films containing B r ions. For example, when a DDAB film loaded with CuPcTS' and used for several days was placed in 5 mM ferrocyanide/O.l M KBr, a quite small sharp oxidation peak for ferrocyanide was found
0.40
0.00
-0.40
E, V
VO.
-0.80 AgIAgBr
Fbufr 4. Cyclic voltammograms at 0.10 V s-I of WA&coated W electrodes: (a) film fully loaded wlth CuPcTSC, used for several days In 0.1 M KBr solutions, then scanned after 3 h In 5 mM K,Fe(CN)e/O. 1 M KBr; (b) fresh DDAB film on W electrode fully equilibrated wlth 5 mM K,Fe(CN)e/O.l M KBr. Note different current scales In a and b.
at 0.1V (Figure4a), even after 2.5 h of soaking and intermittent CV scans. (An additional small oxidation peak near 0.3 V is probably caused by diffusion of ferrocyanideto small uncoated sites on this electrode.) In contrast, a fresh DDAB f i i loaded in 5 mM ferrocyanide for about 30 mingave a nearly symmetric oxidation peak at 0.05 V that was about 100 times larger (Figure 4b) than on the DDAB-CuPcTS' electrode. Molecular Spectroscopy. Electronic absorption spectra of MPcTS4- ions give characteristic peaks for dimers in the visible region.12 This is illustrated by spectra of CuPcTS4in water (Figure 5). Peaks at 668 nm correspond to the monomer, and those at 630 nm are for dimers. Similar resulta were obtained in water for NiPcTS'. These spectra show that MPcTS4- ions are heavily dimerized in water even at quite low concentrations. Analyses of A (668 nm) vs concentration data with the appropriate Beer's law models'* were consistent with monomer-dimer equilibrium between 0.3 and 5 r M MPcTP-, as reported previously.12 DDAB films fully equilibrated with MPcTS4- solutions, removed from solution, and washed with water (Figure 6) had relatively small dimer peaks. Their spectra suggested similar degrees of dimerization in equilibrated films and in aqueous solutions with concentrations of 0.3 pM, assuming that extinction coefficients are similar in the two media. Also, spectra of 1 p M MPcTSC in 0.1 M micellar solutions of cetyltrimethylammoniumbromide showed only a monomer peak, with no evidence for dimerization. Spectra of films soaked in MPcTSC solutions for different times were also measured. The ratio of monomer to dimer peak absorbance decreased with time, reaching a constant value of 1.3-1.5 in
