Langmuir 1991, 7, 1791-1796
1791
Multilayer Films of Cationic Surfactants on Electrodes. Control of Charge Transport by Phase James F. Rusling' and Heping Zhang Department of Chemistry (U-60), University of Connecticut, Stows, Connecticut 06269-3060 Received December 13, 1990. In Final Form: February 21,1991
Multilayer films of water-insoluble didodecyl- and dioctadecyldimethylammonium bromide (DDAB and DODAB) were cast from solutions in chloroform onto pyrolytic graphite electrodes. These cationic surfactants gave stable coatings of 5000-8000 layers that acted in their liquid crystal states as anion exchangers, incorporating large amounts of ferrocyanide and copper phthalocyaninetetrasulfonate from aqueous 0.1M KBr. Voltammetric peak current vs temperature plots for oxidation of ferrocyanide in the films showed linear branches with intersection points close to gel-to-liquid crystal phase transition temperatures of aqueous bilayer suspensions of the surfactants. Charge,transport involving incorporated anions was gated by phase. It was turned on in the liquid crystal phase and turned off when the film was brought to the gel phase. Charge transport for ferrocyanide incorporated in surfactant films in the liquid crystal state appears faster than in many ion exchange polymers. DDAB films at 30 OC (liquid crystal phase) acted as a barriers for charge transfer ta the hydrophilicRu(NH@ in solution but were permeable to hydrophobic dications. Permeability and fluidity of the films were increased by butanol. In related work using micellar surfactant solutions and dispersions as media for electrochemicalcatalysis,we found significant enhancements in rates of mediated secondorder reductions under conditions where coadsorption of thick surfactant fims with reactants occurred on the surface of the e l e ~ t r o d e . ~ ~However, -~' very negative electrode potentials were required to form coatings useful for reactant preconcentration by spontaneous adsorption of cationic Surfactants from solution. We sought more general surfactant coatings that would be stable a t any potential and capable of binding reactants by coulombic and hydrophobic interactions. Casting films of insoluble surfactants onto electrodes (1)Daikufu, H.; Aoki, K.; Tokuda, K.; Matauda, H. J . Electroanal. seemed a particularly easy method to make multilayer Chem. Interfacial Electrochem. 1986, 183, 1-26. (b) Daikufu, H.; coatings. A few recent reports addressed properties of Yoehhura, I.; Hizata, I.; Aoki, K.; Tokuda, K.; Matauda, H. J. Electroanal. Chem. Interfacial Electrochem. 1986,199,47-68. lipids deposited on electrodes as charge transfer barriers (2)Fujihira, M.; Araki, T. Chem. Lett. 1986,921-922. and preconcentration films. For example, palmitic acid16 (3)Facci, J. S.;Falcigno, P. A.; Gold, J. M. Langmuir 1986,2,732-738. on Pt and phosphatidylcholine (PC) on glassy carbon'' (4)Widrig, C. A.; Miller, C. J.; Majda, M. J . Am. Chem. SOC.1988, 110,2009-2011. inhibited charge transfer with hydrophilic ions in aqueous (5)Zhang, X.;Bard, A. J. J . Am. Chem. SOC.1989,111,8098-8106. solutions. Hydrophobic cations were taken up by the PC (6) Fujihira,M.;Pootsittieak,S. J.Ekctroanu1. Chem.InterfacialElecfilms in about an hour. A PC film on glassy carbon gave trochem. 1986,199, 481484. (b) Fujihira, M.; Pootaittisak, S. Chem. Lett. 1986,251-252. a 4-fold enhancement in current in determining the an(7)Zhang, X.;Bard, A. J. J. Phys. Chem. 1988,92,5566-5569. ticancer drug marcellomycin in urine.I8 The ammonium (8)Okahata, Y.; Teuruta, T.; Ijiro, K.; Ariga, K. Langmuir 1988,4 , form of marcellomycin forms acomplex with the phosphate 1373-1375. (9)(a) Zaba,B. N.;Wilkineon, M.C.; Taylor, D. M.; Lewis, T. J.; Laidgroups of the lipids. Maximum uptake of the cationic man, D. L.FEBS Lett. 1987,213,4944. (b) Yokota, T.; Itoh, K.; Fudrug occurred in 3 min, with loss of lipid from the electrode jhhima, A. J . Electroanal. Chem. Interfacial Electrochem. 1987,216, at >4 min. PCs incorporated in carbon paste electrodes 289-292. showed better stability than the films.21 Carbon paste (lO)Porter,M.D.;Bright,T.B.;Allara,D.L.;Chideey,C.E.D.J.Am. Chem. SOC.1987,109,8654-3568. with stearic acid incorporated has been investigated for (11)(a) Finklea, H. 0.; Avery, S.; Lynch, M.; Furtach, T. Lcrngmuir detection of dopamine in the brain in the presence of ascor1987,3,-13. (b) Bundi Lee, K. A.; M o m , R.; Mcbnnan, G.; babez0 Films of PC and cholesterol on electrodes were Finkka, H.0.J.Electroanal.%em. Interfacial Electrochem. 1988,246, 2171224. more stable than PC films, but gave smaller signals.lg (12)Laibinia, P.E.;Hickman, J. J.; Wrighton, M. S.; Whiteeides, G. Multilayer films can be cast onto solid supports from M. Science 1989,295,845-847. solutions or dispersions of many water-insoluble surfac(13)Fujihira, M.; Muralti, H.;Aoy@, S. Bull. Chem. SOC.Jpn. 1986, 59,975-980. tants. Observations of sharp phase transitions suggested (14)Finklea, H.O.;Robimn, L. R;Blackburn, A.; Richter, B.; Allara,
Introduction There has been considerable interest recently in functional coatings of water-insoluble amphiphilic molecules on electrodes for Catalysis, preconcentration, and inhibiting charge transfer. Specific methods to prepare these coatings include transfer of Langmuir-Blodgett (LB) fiimsto electrodes,'* covalent bonding of monolayers through sulfur'*'2 or silicon's16 linkages, and casting lipids onto electrodes from organic solvents.18-'9 Incorporation of amphiphilic molecules in carbon paste electrodes has been used for analytical purposes.MVz1 ~~
D.; Bright, T. Langmuir 1986,2,23s244. (15)(a) Sabatani, E.;Rubinnbin, I.; Moaz, R.; Sagiv, J.J. Electroanal. Chem. Interfacial Electrochem. 1987,219, 365-371. (b) Sabatani, E.; Rubinatein, I. J. Phys. Chem. 1987,9I,686343669. (16)Tanaka, K.; Tamamuehi, R. J . Electroanal. Chem. Interfacial Electrochem. 1987,236,305-107. (17)Garcia, 0.J.; Quintela, P. A.; Kaifer, A. E. Anal. Chem. 1989,61, 97-1. Kauffmann, J.-M.; Patriarche, G. J.; Christian, G. D. (18)C h t e l , 0.; Anal. Chem. 1989,61,170-173. (19)Wang, J.; Lu, Z.A d . Chem. 1990,62,826-829. (20)Lyne, P. D.; O'Neill, R D. Anal. Chem. 1990,62,2347-2351.
(21)Kaufmann, J.-M.; El Jamal, A.; Vire, J. C.; Patriarche, 0. J. (Extended Abstracts), 177thMeeting of the ElectrochemicalSociety,Montreal, May 1990; Paper No. 641. (22)Rusling, J. F.; Shi, C.-N.; Goeeer, D. K.;Shukla, 5. S. J. Electroanal.Chem.InterfacialElectrochem. 1988,240,201-216,andrelbrencee therein. (23)Rueling, J. F.;Shi, C.-N.; Couture, E. C.; Kumoainnki, T. F. In Redox Chemistry and Interfacial Behavior of Biological Molecuks;Dryhurst, G., Niki, K.,me.;Plenum: New York, 1988;pp 565-581. (24)Iwunze, M. 0.; Rusling, J. F.J . Electroanul. Chem. Interfacial Electrochem. 1989,266,197-201.
0 1991 American Chemical Society
Ruling and Zhang
1792 Langmuir, Vol. 7, No. 8, 1991 that such films have ordered multibilayer structures.26v26 In this paper, we describe the properties of films of the water-insoluble double chain surfactants didodecyl- and dioctadecyldimethylammonium bromide cast from chloroform solutionsonto pyrolytic graphite electrodes. These surfactant coatings are stable for up to 5 days when used in water. They take up large amounts of electroactive multivalent anions from aqueous solution a t temperatures above their gel-&liquid crystal phase transitions. They reject hydrophilic electroactive cations and are permeable to hydrophobic cations. Electrochemistry of incorporated anions can be turned off by bringing the films to the gel phase, and turned on again by returning to the liquid crystal phase.
Experimental Section Didodecyl- (DDAB) and dioctadecyldimethylammonium bromide (DODAB) were from Eastman (99+% ); diheptylviologen chloride and copper phthalocyaninetetrasulfonatetetrasodium salt were from Aldrichandwere used asreceived. Other chemicals were reagent grade from sources cited previously."' Cyclic voltammetry (CV) was done by using a BAS-100 electrochemical analyzer with three electrode cells, a saturated calomel electrode as reference, and a Pt wire as counter. To make working electrodes, basal plane pyrolytic graphite (HPG, Union Carbide) disks (A = 0.20 cm*)were cleaved with a razor blade from a cylinder of PG and held onto a steel rod with a Teflon holder as described previously." Roughness factors of uncoated PG electrodes, expressed as [electrochemically measured area]/ [geometricarea]averaged 3.2 as estimated from peak currents for CV of ferrocyanide ion in 0.1 M KBr. PG electrodes were coated with ca. 1.3 pmol of surfactant by pipetting from a 0.1 M solution in CHCb onto a freshly cleaved PG disk. Solventwas evaporated in air, usuallyovernight,before usingthe electrodes. Cellswere thermostatad to f0.1 "C. Oxygen was removed from solutions by bubbling with purified nitrogen. Ohmic drop of the cells was compensated to 185% for all CVs by the BAS-100's resistance feedback method.
- l L' 0.70
'
'
'
'
0.20
E,
O'?O
'
'
V
'
'
'
-0.3 0
v8 SCE
> t b
-0.20 0.70
-0.30
0.20
E, V vs SCE Figure 1. Cyclic voltammograms at 0.10 V s-l in aqueous0.1 M
KBr containing1.0mM ferrocyanide at (a)a PG electrodecoated with 1.3 pmol of DDAB after multiple scans at 30 "C and (b) a bare PG electrode (scale 5X more sensitive than in part a).
Rssults Uptake of Ferrocyanide. Deposition of 1.3 pmol of DDAB on a PG disk results in about 5000-8000 monolayers, as estimated from geometric considerations.28 Assuming an ideal ordered multibilayer structure for the films,%* thicknesses of about 20 pm are estimated. When DDAB-coated PG electrodes were placed in a solution of 0.1 M KBr containing 1 mM ferrocyanide ion at 25 or 30 OC, uptake of ferrocyanide was detected by a rapidly growing CV oxidation peak a t about -0.02 V vs SCE (Figure 1). The largest peak oxidation currents in DDAB films were on the order of 50-fold larger than for 1 mM ferrocyanide on bare PG, and peaks were shifted 240 mV negative of the bare PG value. Peak currents increased faster under repetitive scanning than if the electrode was held at open circuit for an equivalent time. At low scan rates (50.1 V s-l), CVs had roughly symmetric shapes characteristic of electrochemistry confined to a thin layer on the e1ectrode.m However, peak shapes were nonideal in that peak width was smaller for anodic than for cathodic scans and separation between peaks was about 70 mV. (25) Nakashima, N.; Ando, R.; Kunitake, T. Chem. Lett. 1989,15771680. (28) Kunitake,T.;Shi.momua,M.; Kajiyama,T.;Harada, A.; Okuyama, K.; Takayanagi, M. Thin Solid F i l m 1984,121, L89-91. (27) Ruling, J. F.;Zhang, H.; Willia, W. 5.A w l . Chim. Acta 1990,235, 307-316. (28)(a) Molecular volumes and le respectively: DDAB, 753 As,20 A;DODAB, 1067 As,26 A. G.; Sen, R.;Evans, D. F.; Trend, J. E. J. Phys. Chem. 1968,92,74&783. (29) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980.
0
40
80
120
160
Time. hr
Figure 2. Influence of time on steady-stateanodic peak current at 0.1 V 8-1 at 25 "C for 1.0 mM ferrocyanide at DDAB-PG electrodes in solutions of (A)0.1 M KBr and (0)0.1 M KBr and 0.2 M butanol.
Similar results were found for DODAB-PG electrodes in 0.1 M KBr/l mM ferrocyanide a t T > 50 OC. Although results varied somewhat from electrode to electrode, the uptake of ferrocyanide under intermittent scanning conditions usually peaked after an hour or two, subsequentlydecreased to about half of its maximum value (Figure 2), and then remained stable for about 5 days. When an electrode was loaded with ferrocyanide during repetitive scanning, then allowed to sit in ferrocyanide solution at open circuit for 3 or more hours, the current decayed slowly. However, a second series of repetitive scans after this decay in current caused a second growth of the oxidation peak. This was similar to the scanningenhanced rate of initial uptake mentioned above. Thus, data in Figure 2 reflect CV peak currents after multiple scans had reached steady-state values. Addition of 0.2 M butanol to the 0.1 M KBr solution
Langmuir, Vol. 7, No.8,1991 1793
Cationic Surfactants on Electrodes Temperature Increase
Temperature Decrease
Current
A
Temperatwo
0
1 30
0
5
3
E
4 1
a
1
t
A
J 0 A
-
n o 10
20
h
30
40
50
60
E
70
3 '
a
2
Temperature ( * C )
Figure 3. Influence of temperatureon steady-stateanodic peak current for 1.0 mM ferrocyanide in 0.1 M KBr/0.2 M butanol at a DODAB-PG electrode. Experiments were begun at lowest T, equilibrated for about 30 min with scanning at each T, and then advanced to the next T. After reaching 64 "C,T was then decreased in a similar stepwise fashion. gave larger steady-state ferrocyanide currents after the initial growth period (Figure 2b). With butanol present, the decreasefrom maximum anodic current was only about 20%. When coated electrodes loaded with ferrocyanide were washed with water and placed in ferrocyanide-free 0.1 M KBr, the current dropped over 6 h to about 2&30% of the initial value and then remained stable. Thinner surfactant coatings were less stable than those described above. This was illustrated by electrodescoated by dipping in a 0.1 M DDAB solution in CHCls, shaking off excess solution, then evaporating the solvent. Large anodic currents were measured initially when these electrodes were placed into 1mM ferrocyanide in 0.1 M KBr, but current decreased to 25% of the maximum value overnight. Also, diffusion-controlled oxidation peaks characteristic of oxidation on the bare electrode at about 0.22 V vs SCE were observed after this 30-h period. Effect of Temperature. After DDAB- and DODABcoated electrodes had attained a stable steady-state CV signal in 1 mM ferrocyanide/O.l M KBr, temperature was changed, 30 min of equilibration was allowed, and steadystate CVs were measured after multiple scans at the new temperature. For DODAB films in 0.1 M KBr/0.2 M butanol, peak oxidation current measured in this way was very small at temperatures less than 42 OC but increased dramatically at T > 42 "C (Figure 3). At T < 42 "C, ferrocyanide peak current on DODAB-PG was smaller than on bare PG. At 64 "C,anodic current on DODAB-PG was >10 times that on bare PG. Similar temperature effects were found for electrodes coated with DDAB. The break point in peak current vs T (i-T) plots in 1mM ferrocyanide was 9 (*1) OC in 0.1 M KBr. The break point was 2 (h1) "C when 0.2 M butanol was present in solution. Peak currents were reproduced reasonably well during cycling between temperatures above and close to i-T break points (Figure 4). To study causes of the current decrease at low T, coated electrodes were loaded with ferrocyanide at a temperature above the break point, then T was decreased (25 O C for DODAB and
