Anal. Chem. 1991, 63, 2163-2168
2163
Surfactant-Intercalated Clay Films for Electrochemical Catalysis. Reduction of Trichloroacetic Acid Naifei Hul and J a m e s F.Rusling* Department of Chemistry (U-60), University of Connecticut, Storrs, Connecticut 06269-3060
Composite films made from insoluble diaikyidimethyiammonium surfactants and clay colloids were evaluated for electrochemical catalysis. When cast on pyrolytic graphite (PO) electrodes from chloroform, the films act as chargetransfer barriers toward multivalent eiectroactive ions in solution. They take up lons with hydrophobic ligands, e.g. tris(2,2’-bipyridyl)cobait( I I ) . To make catalytic electrodes, water-insoluble cobalt and iron phthalocyanines (CoPc and FePc) were dissolved in surfactant-clay dispersions in chioroform and cast on PO. Films containing CoPc were 35-fold more active than FePc films for catalytic reduction of trichloroacetic acid. CoPc films decreased overpotential for reduction of trichloroacetic acid by about 0.5 V compared to 0.05 V for FePc. These MPc films gave stable catalytic currents for at lead 10 days. Catalytk current vs temperature plots for reduction of trichloroacetic acid showed linear branches with Intersection points close to reported gei-to-liquid crystal-phase transition temperatures. Charge transport was fader In the llquld crystal than the gel phase of the f l h . Observation of phase transltions suggests a muitlbllayer structure.
In electrochemical catalysis, mediators shuttle electrons between electrodes and substrate molecules. In this way, the potential required to reduce or oxidize recalcitrant substrates is significantly lowered. When applied potential is controlled, current for electrolysis of the mediator (catalyst) is amplified in the presence of substrate. In catalytic reduction, for example, the mediator is reduced at the electrode and transfers electrons to the substrate. This regenerates the oxidized form of the mediator, which is reduced again at the electrode. It is this cycling of mediator between oxidized and reduced forms at potentials near ita Eo’ that amplifies the cathodic current. Electrochemical catalysis has been used frequently as the basis for sensitive electroanalytical methods (I, 2). The amount of current amplification, and the sensitivity of the method, depend on the rate of reaction between reduced mediator and substrate (3). Developing electrode coatings designed for specific tasks has been a major research goal of recent years. Excellent progress has been made with polymers (4,5). However, many common polymers have relatively low rates of charging, substrate permeation, and ion transport, which may limit their use in catalytic applications. Recently, we found that films of water-insoluble, cationic surfactants cast on electrodes from organic solvents are stable and have properties suitable for efficient electrochemical catalysis, i.e. fast charge transport, excellent substrate permeation, and fast diffusion of countenons (6). These films act as anion exchangers, incorporating large amounts of multivalent anions such as ferrocyanide and copper phthalocyanine tetrasulfonate (CuPcTSQ-). Cast cationic surfactant films loaded with ferrocyanide also showed phase transitions, revealed as sharp discontinuities On leave from Beijing Normal University, Beijing, China. 0003-2700/9 1/0363-2163$02.50/0
in plots of peak current vs temperature (6). At temperatures below a critical value (Tc), surfactant bilayers in phospholipid liposomes and surfactant vesicles exist in a highly ordered form with all hydrocarbon chains in trans configurations. This characterizes a solidlike phase called the “gel” (7).An increase in temperature above T,causes the formation of gauche bond orientations, leading to kinks in the hydrocarbon chains and an increase in fluidity of the bilayer. This ”chain melting” at the phase-transition temperature results in a lamellar liquid crystal phase. In multilayer cationic surfactant films (6), the phase transitions suggest a multibilayer structure for the fiis. Films of tetraalkylammonium surfactant bilayers intercalated between colloidal clay layers were recently prepared by Okahata and Shimizu (8) as membranes with controlled permeability. X-ray diffraction and gel to liquid crystal-phase transition data suggested that surfactant molecules in tailto-tail bilayer orientations separated the clay layers. In the liquid crystal phase, surfactant-clay films were permeable to water-soluble organic compounds. Permeability was turned off by bringing the films to the solidlike gel phase. Clays are cation exchangers, so these films must be held together by Coulombic attraction between clay and surfactant head groups combined with hydrophobic interactions between surfactant tails. Stability of surfactant-clay composites to extremes of ionic strength, temperature, and p H was much better than for simple surfactant films and surfactant-polyelectrolyte composites. In this paper, we report electrochmical properties and examples of electrochemical catalysis in films made from clay colloids and dialkyldimethylammonium surfactants. These films with metal phthalocyanine mediators incorporated were extremely stable during catalytic reduction of trichloroacetic acid. Unlike films made entirely from insoluble Surfactants, the surfactant-clay composites are not ion exchangers. EXPERIMENTAL SECTION Chemicals. Didodecyl- (DDAB) and dioctadecyldimethylammonium bromide (DODAB) (99+%) and iron and cobalt phthalocyanine were from Eastman Kodak and used as received. Ruthenium(II1) hexammine was from Strem Chemical Co. Potassium ferrocyanide was from Matheson, Coleman, and Bell. l,l’-Diheptyl-4,4’-bipyridinium (diheptylviologen)dibromide w u from Aldrich. Hydroquinone was from Baker Co. Bentonite clay (Bentolite H) was from Southern Clay Products and had a cation-exchange capacity of 80 mg/100 g (9). Tris(2,2’-bipyridyl)cobalt(I1) was prepared in situ by making solutions 5 mM in cobalt sulfate and 25 mM in 2,2’-bipyridine. The result is 5 mM tris(2,2‘-bipyridyl)cobalt(II)with 10 mM excess 2,2’-bipyridine to suppress dissociationof the ligands (IO). All other chemicalswere reagent grade. Apparatus. Cyclic and square-wave voltammetry were done in three-electrode cells using basal plane pyrolytic graphite disks as working electrodes, a platinum-wire counter electrode, and a saturated calomel electrode (SCE) as reference. Potentiostats were either PARC 270 or 174 systems or the BAS-100 electrochemical analyzer. Cells were thermostated at 25 O C with a circulating water bath unless otherwise noted. Pyrolytic graphite (PG) cylinders were machined from PG blocks (HPG-99,Union Carbide). Dish (geometricA = 0.20 cmz, height ca. 3 mm) were cleaved from these cylinders with a razor 0 1991 American Chemical Soclety
2184
ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991
I
0.50
I
0. i 0
I
I
-0.30
I
1
-0.70
E (VI 0.50
0.70
0.90
1.10
I .30
-E (VI
Flgure 1. Repetitive CV scans (50) at 0.10 V s-’ of a DDAB-clay electrode in 5 mM tris(2,2’-bipyridyl)cobalt(II), 10 mM 2,2’-bipyrldlne, 10 mM sodium sulfate. This electrode was first scanned repetitively in the above solution, and then current was allowed to decay in 10 mM sodium sulfate solution. The above data represent the second immersion in 5 mM tris(2,2’-bipyrktyl)cobalt(II);(a)first scan; (b) last scan.
blade. The PG disks were sealed with a heat gun into the larger end of polypropylene tips from a Pipetman pipetter. The seal was checked with an optical stereomicroscope. Contact to the backside of the electrode was made with conducting epoxy and a copper wire. Excess polypropylene was removed and the PG electrode was polished to a rough finish with 600-grit Sic paper on a polishing wheel. Preparation of Surfactant-Clay Films. The method of Okahata (8)was used. Clay colloids were prepared in the sodium form as described previously (9). A dispersion of 0.5 g of colloidal sodium bentonite in 25 mL of water and an aqueous dispersion of 1.2 mmol of DDAB or DODAB in 10 mL were mixed and stirred at 70 O C for 1h. The white surfactant-clay precipitate was filtered, washed with water, and resuspended in 15 mL of chloroform. About 30 mL of methanol was added until the suspension became cloudy. This mixture was aged at ca. 35 “C for 30 min, allowed to cool to room temperature, and filtered under vacuum. Solvent was removed from the precipitate by keeping a vacuum on the filter funnel. A white powder was obtained. Films were made by depositing 60 pL of a translucent chloroform dispersion of the surfactant-clay composite (2 mg mL-’) onto a freshly polished PG disk. The chloroform was evaporated overnight in air. Films containing iron(I1) or cobalt(I1) phthalocyanine (FePc or CoPc) were made by mixing 1 mL of the above surfactant-clay dispersion with 1 mL of a 10 mM solution of the FePc or CoPc in chloroform. This mixture was ultrasonicated for 0.5 h, and a measured amount (usually 120 pL) was deposited onto a PG disk. Chloroform was evaporated overnight in air. Procedures for Voltammetry. Methods were similar to those reported previously (6,9). Oxygen was removed from solutions before experiments by bubbling for 5 min or more with purified nitrogen. Resistance of cells assembled with coated electrodes was always less than 100 Q. Where appropriate, uncompensated ohmic drop of the cells was compensated for by the BAS-100 resistance feedback system. For electrodes containing MPc’s, the initial potential was held for 30 s before scanning. This was found necessary to achieve optimal peak currents.
RESULTS In most experiments, PG electrodes were coated with 0.12 mg of DDAB-clay composite. Scanning electron microscopy of cleaved films indicated a dry thickness between 20 and 30 pm. From clay-surfactant bilayer thicknesses measured by X-ray diffraction (8) for the composites, the films correspond to 4OOC-7000 surfactant bilayers. These films were not used
Figure 2. Multiple CV scans (5) at 0.10 V s-‘ in 45 mM Ru(NH,I3+, 10 mM sodium sulfate: (a) DDAB-clay electrode; (b) bare PG.
previously for voltammetry. Thus,we examined their behavior toward several eledroactive ions before proceeding to catalytic applications. Response to Water-Soluble Electroactive Species. DDAB-clay electrodes gave chemically reversible cyclic voltammograms (CV) in solutions containing tris(2,2’-bipyridyl)cobalt(II) (Figure 1). Cathodic peak potential a t 0.1 V s-l was -1.15 V, 80 mV positive of that in aqueous micellar cetyltrimethylammonium bromide (10). An increase in anodic and cathodic peak currents (Figure 1)during repetitive scans in solutions of tris(2,2’-bipyridyl)cobalt(II) showed that this cation was taken up by the films. Switching the electrode between analyte and blank electrolyte solutions was repeated many times with the same steady-state CV currents as in Figure 1 measured in solutions of 5 mM tris(2,2’-bipyridyl)cobalt(I1). Cathodic current a t steady state was %fold larger than that for bare PG in the same solution. In blank solutions, current decayed to about 25% of the steady state during 30 min of repetitive scans. CVs in solutions of 45 mM Ru(NH3):+ on clay-DDAB electrodes gave cathodic peaks less than 10% of the height of the peak on bare PG (Figure 2). Repetitive scans gave stable CVs that did not increase in peak height. A blocking effect of about the same magnitude was found for DDAB films in solutions of Fe(CN)64-and diheptylviologen (HV2+). MPc-DDABXlay Films. Metallophthalocyanines (MPc) are quite insoluble in water and have a wide range of catalytic activities for redox reactions (11,12). The reactivity of iron and cobalt phthalocyanines in electrochemical catalysis (12, 13) parallels that of cobalt corrins (14-16). Thus, FePc and CoPc should be efficient catalysts for dechlorination of trichloroacetic acid, similar to the cobalt(I1) corrin vitamin BIzr (16).
FePc and CoPc were dissolved in DDAB-clay dispersions before casting on PG (see Experimental Section). The resulting films contained 0.6 pmol of MPc and 0.12 mg of composite. CV gave very small peaks for MPc-DDAB-clay electrodes in electrolyte solutions. Thus, we studied these films mainly with square-wave voltammetry (SWV) (Figure 3) because of its better sensitivity. SWV of the CoPc film (Figure 3a) gave forward and reverse peaks consistent (17) with reversible reduction of ConPc. Net peak current was proportional to f at f < 200 Hz,as expected for thin-layer voltammetry (17,18). Between 400 and 1200 Hz,net current was linear when plotted against f / 2 (Figure 4a), suggesting diffusion-like behavior within the film (4,18). Peak potential was constant at -0.45 f 0.02 V at frequencies cf) up to 1400 Hz.
ANALYTICAL CHEMISTRY, VOL. 63,NO. 19, OCTOBER 1, 1991 12
20
I
I
II - . \
I
-6'
'
'
'
'
'
'
'
'
-E, V
1 .oo
-200
1000
400
1600
SCE
VS
-E, mV
1
3.00
'.
'
'
0.50
0.00
2165
I
Flgure 5. SWV forward currents at 500 Hz, 25-mV amplitude in 0.1 M KBr: (a) CoPc-DDAB-clay, no trichloroacetic acid; (b) DDAB-clay without CoPc in 10 mM trlchloroacetic acid: (c) CoPc-DDAB-clay, 10 mM trichloroacetic acid.
'
-1.50 ' 0.20
'
'
'
'
'
'
'
'
'
'
-E, V
VS
'
'
'
1.60
0.90
SCE
Figure 3. Forward and reverse SWV current-potential curves in 0.1
M KBr for (a) the CoPc-DDAB-clay electrode at f = 2000 Hz, amplitude 25 mV and (b) the FePc-DDAB-clay elecirode at f = 1500 Hz, amplitude 25 mV.
0 " " ~ " " ~ " ' " ' ~ " ' J 0
500
1500
1000
2000
FREQUENCY 3 )
u
2 -
u5
1 -
--e E
I 0
I
b
a / + +:
I
*I
10
20
40
30
sq. root of f
50
(Hz)
0 ' 0
'
'
'
'
"
500
'
'
' ' 1000
FREQUENCY
Figure 6 . Influence of the SWV frequency (Hz) at 25-mV pulse amplitude in 10 mM trichloroacetic acid/ 0.1 M KBr on the forward c w e n t catalytic efficiency as ic/i., (see text): (a) CoPc-DDAB-clay; (b) FePc-DDAB-clay.
b
0
0
10
20
30
40
50
sq. root of f (Hz)
Flgure 4. Dependence of the SWV net current (amplitude 25 mV) on the square root of the frequency for (a) the peak at -0.45 V for the CoPc-DDAB-clay film and (b) the peak at -1.30 V for the FePcDDAB-clay film.
FePc films gave two sets of reversible SWV forward and reverse peaks (Figure 3b), attributed to reductions of FenPC and Fe'Pc-, respectively (19). For the more negative peak, at which mediation of reduction of trichloroacetic acid was found, net peak current was directly proportional to frequency (f) between 8 and 80 Hz,suggesting thin-film behavior. At f between 100 and 2000 Hz,net peak currents were propor(Figure 4b). Potentials of the two peaks were tional to
constant at -0.61 f 0.01 and -1.30 f 0.01 V in this f range. Catalytic Reduction of Trichloroacetic Acid. When CoPc-DDAB-clay electrodes were used in solutions containing trichloroacetic acid, large increases in cathodic CV and forward SWV current were observed (Figure 5) at about -0.5 V. This peak was about 0.5 V positive of that for unmediated direct reduction of trichloroacetic acid on an electrode coated with DDAB-clay without CoPc. Since SWV forward currents show larger catalytic increases than net currents (20)catalytic reactions were examined mainly with forward currents. Catalytic efficiency expressed as the ratio of catalytic SWV forward current (i,) to the forward current for the CoPc film in solutions not containing trichloroacetic acid (id) decreased with increasing f (Figure 6a), as expected for an electrochemical catalytic reduction (20). A t fixed f , catalytic current increased with the concentration of trichloroacetic acid. FePc-DDAB-clay electrodes also gave peaks of increased height in solutions of trichloroacetic acid. In solutions containing this substrate, amplification of cathodic CV and forward and difference SWV currents occurred at the potential of the second reduction peak of FePc, -1.30 V. This is only
2166
ANALYTICAL CHEMISTRY, VOL. 63,NO. 19, OCTOBER 1, 1991 0
0
I
r X
4
E
r X
4
E
i
t!
8 4o 0
2
5 Y
Y
4zLc-J
Q)
n
n
6
-5
5
15
25
35
20
30
previous value; (+) T decreased from previous value. about 50 mV positive of the direct reduction peak of trichloroacetic acid on DDAB-clay electrodes. Catalytic efficiency decreased with increasing f (Figure 6b). The SWV forward current increased with concentration of trichloroacetic acid. CoPc-DDAB-clay electrodes were examined for stability in solutions of 10 mM trichloroacetic acidl0.l M KBr by periodic CV scans at 0.1 V s-l. No systematic decreases in peak current were found over 10 days, and in fact a small increase was observed. CoPc-DDABday electrodes were also removed from solution, washed, and dried in air. Upon returning to 10 mM trichloroacetic acid in 0.1 M KBr the next day, nearly the same catalytic current was obtained. This excellent stability was in contrast to that of a pure CoPc film on PG without the DDAB-clay composite. To make the pure CoPc film, 60 p L of 10 mM CoPc in chloroform was deposited onto bare PG and dried. This electrode gave good initial catalytic activity in 10 mM trichloroacetic acid in 0.1 M KBr. However, catalytic current decreased somewhat over 'Iz day, and the coating developed cracks and flaked off of the underlying PG. Catalytic reductions with MPc films were examined at a series of temperatures (2'). Plots of forward SWV peak current vs T for MPc-DDAB-clay films in solutions of trichloroacetic acid (Figure 7 ) showed dicontinuities a t 5 "C, below which catalytic current remained nearly constant. Temperature dependence of the catalytic current was also followed in a f i made from clay and diodadecyldimethylammoniumbromide (DODAB), which has a higher gel-to-liquid crystal-phase transition temperature (8) than DDAB-clay. Peak current measured by CV and SWV vs T for FePc-DODAB-clay in solutions of trichloroacetic acid showed a discontinuity a t 54 "C (Figure 8). DISCUSSION Response to Water-Soluble Electroactive Species. Results show that clay-DDAB films do not act as ion exchangers for hydrophilic multivalent ions such as Ru(NH&~+ and Fe(CN):- in solution. Films were stable even at relatively high concentrations (45 mM) of the multiply charged ions. Nor was a moderately hydrophobic dication such as diheptylviologen incorporated into these films. Charge transfer between the electrode and the above ions was blocked by about 90%. On the other hand, tris(Z,Z'-bipyridyl)cobalt(II) was incorporated into the film (cf. Figure 1). Presumably, hydrophobicity or attenuated charge/size (suggested by a reviewer) facilitated by the bipyridyl ligands allow tris(2,Z'bipyridyl)cobalt(II) to enter the DDAB-clay film. If tris(2,2'-bipyridyl)cobalt(II) were bound directly to clay by ion exchange in the composites, Ru(NHJ:+ should also be taken up from solution into the film, but it is not. Thus,
50
60
70
0
T, O C
Flgure 7. Influence of temperature on the catalytic forward SWV current at 100 Hz, 25-mV amplitude, for the CoPc-DDAB-clay electrode in 10 mM trichloroacetic acid/O.1 M KBr: (0)T increased from
40 T,
C
Figure 8. Influence of temperature on the catalytic current of the FePc-DODAB-clay electrode in 10 mM Mchloroacetlc ackVO.1 M KBr: (A)cathodic CV current at 0.10 V s-'; (0)forward SWV current at 15 Hz, 25-mV amplitude.
tris(2,2'-bipyridyl)cobalt(II) may enter into hydrophobic surfactant bilayer regions between clay layers. Films Containing MPcs. Both CoPc and FePc films showed nearly reversible SWV peaks assigned by comparing with their reductions on PG in DMSO (19): M"Pc M'Pc-
+ e = M'Pc+ e = M'Pc2-
(1) (2)
The CoPc peak a t -0.45 V and the FePc peak a t -0.61 V in the films are presumably associated with eq 1. The peak at -1.30 V for FePc is most likely caused by the reduction in eq 2. Sites at which the MPc's reside in the film cannot be identified unambiguously without further experiments. Possibilities include adsorption onto the clay and/or underlying PG, association with the surfactant bilayers, or presence as crystallites in the fii. SWV net peak currents proportional to f at lower f suggest thin-layer electrochemical behavior for these slower experiments. That is, all electroactive MPc in the film has probably been reduced a t the end of the scan. Dependence of net peak current on PI2 a t higher frequencies (cf. Figure 4) indicates diffusion-like behavior (17) and only partial reduction of MPc in the film during the faster experiments. Such an evolution from thin-layer to diffusion-like behavior with increasing speed of the experiment is characteristic for electrodes coated with films containing redox sites ( 4 ) . Such results suggest some distribution of electroactive MPc's throughout the films. In previous studies of clay-coated electrodes (9, 21, 22), interlayer spacing was always much smaller than the 3.0 nm measured for the clay-DDAB composite (8). Metal bipyridyl complexes intercalated directly between clay layers were considered to be electrochemically silent (21,22). The electroactive forms of the complexes resided on outer clay surfaces only. In contrast, DDAB bilayers in our composites a t room temperature act as large fluid spacers between the clay layers. In the liquid crystal phase, these films have bulk permeability to water-soluble organics (8),probably involving passage of the probe compound through the fluid bilayers. Thus, surfactant-clay composites may provide systems in which it is possible to electrochemically address metal bipyridyl or phthalocyanine redox centers between the clay layers. Electrochemical Catalysis. In solutions of trichloroacetic acid, the amplification of current a t potentials of reduction of Co"Pc and Fe'Pc- in the films and the decrease in catalytic efficiency as f increases are consistent with catalytic reduction of trichloroacetic acid. Catalytic reduction of trichloroacetic acid by water-soluble vitamin BIzrinvolves stepwise dechlorination (16). Similar processes are likely to be mediated by
ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991
Table 1. Apparent Pseudo-First-Order Rate Constants for Reduction of Trichloroacetic Acida
io-%, SWV f, Hz
CoPc film
5-1
FePc film
0.19
100 150 200 250 300 400 500 600 700 800 900 1000 1200 1400
0.13
4.3 6.2
6.1 5.6 5.8 5.8 5.8 5.8 5.7 f 0.6
mean f s
0.12 0.083 0.15 0.16 0.17 0.17 0.20 0.24
0.16
0.04
OFor electrodes coated w i t h 0.6 jtmol of M P c and 0.12 m g of clav-DDAB a t 25 O C in 10 mM trichloroacetic acid/O.l M KBr.
MPc's. The following two-electron pathway, illustrated for CoIIPc, describes the kinetics:
Scheme I CoIIPc CoIPc-
Co'Pc-
+ RCl + R'
-
+ e = CoIPcC1- + CoIIPc + R' CoIIPc + R(fast)
(3) (4)
(5) The rate-determining step is assumed to be reaction of Co'Pcwith substrate RC1 (eq 4), as in other reactions of this type (14-16). Equation 5 is a source of a second electron. A kinetically equivalent alternative (14) is reaction of CoIPcand RCl in a concerted E2 elimination to give R- directly. Rpresumably undergoes rapid protonation to give product RH. Analogous pathways with FeIPc- as mediator are envisioned as starting with eq 2. Using SWV theory (20) for one-electron catalysis, we computed a theoretical working curve of ic/& vs log k r for our experimental conditions, where r is the width of the squarewave cycle in seconds. This was used to estimate apparent pseudo-first-order rate constants ( k = k,[RCl]) for the two catalytic electrodes from the catalytic efficiency ratios measured under diffusion-like conditions, where id is proportional to PI2. We compared ic/2id to the working curve to account for the two electrons needed to reduce the C-C1 bond. This approach is based on Scheme I involving a fast second electron addition and is called the solution electron-transfer approximation (23). Values of k were reasonably constant in the diffusion-controlled frequency range (Table I). Since exact concentrations of reactants at the reaction site are unknown, k values in Table I are apparent rate parameters only. Nevertheless, they can be used for comparisons. The apparent k for the CoPc film is 35-fold larger than that of the FePc film. Also, the CoPc film lowers the overpotential for reduction of trichloroacetic acid by about 0.5 V. Thus, the reduction of trichloroacetic acid seems to be mediated by Co"Pc in a way similar to that of the Co(I1) corrin vitamin Blzr(16). If the apparent pseudo-first-order constant k is divided by substrate concentration (0.01 M), an apparent k , estimated for the CoPc film is in the same range as the rate constant for vitamin BlZr(1.5 X lo5 M-ls-l) in homogeneous solution. The latter reaction is considered to be fast because of an inner-sphere mechanism (16). Mediation of reduction of trichloroacetic acid by the FePc f i m is somewhat different and slower than that of CoPc films. Catalysis occurs at potentials of the second peak, where FeIPc2-
2167
is formed in the film. Overpotential for reduction of trichloroacetic acid by FePc films is decreased by less than 0.1 V. Thus,CoPc f i are much more active catalysts than FePc films and should be more useful for analytical applications. Structural and charge transport properties of the surfactant-clay composites are governed by temperature. Transitions between gel and liquid crystal phases in surfactant-clay films were previously measured by differential scanning calorimetry and permeability (8). Values of Tcwere 15 "C for DDAB-clay and 54 "C for DODAB-clay. (These values were 5-10 "C higher than for aqueous bilayer dispersions of the surfactants (8)). Tcvalues for the composites are similar to temperatures of breaks in catalytic current vs T curves (Figures 7 and 8) found for DDAB-clay (5 "C) and DODABclay (54 "C). Although the former value is somewhat smaller than previously found, Tcvalues are known to be influenced by salts and other additives in solution (7)which were different in ref 8 than in our experiments. Also, the value of 5 "C for the DDAB-clay film is close to that of 9 "C found for pure DDAB films containing ferrocyanide ions (6). We consider the discontinuous temperature dependence of catalytic current to result from phase transitions. This suggests at least partly ordered bilayer structures for the surfactant-clay f i ,as reported by Okahata (8). The underlying rough PG (roughness factor 3.2 (6)) may promote some disorder, as could the MPc's. Breaks in current vs T curves were also found with cast films of pure DDAB and DODAB containing electroactive anions (6). Results of these types of experiments suggest that charge transport through these films is considerably faster in the liquid crystal phase than in the gel phase. Conclusions. The ability to incorporate efficient mediators such as CoPc into these stable, organized fluid coatings on electrodes suggests that composite films of cationic surfactants and clay have promise for applications in electroanalysis. Films containing CoPc should be especially useful for rapid catalytic reductions of activated organohalides such as halogenated acetic acids, vicinal dihalides (24),and a variety of organic pesticides with halogens on adjacent carbons, as well as for catalytic oxidations (I2,25-27). The films are easy to make and should be able to incorporate other water-insoluble mediators. DDAB-clay films are charge-transfer barriers to hydrophilic multivalent ions in solution, which may provide selectivity in some applications. Phase transitions provide for thermal switching; i.e. current is turned on in the higher temperature liquid crystal phase but is decreased considerably in the lower temperature gel state.
ACKNOWLEDGMENT This paper is part 8 of the series "Electrocatalysis in Organized Assemblies". LITERATURE CITED (1) Meltes, L. Polarographic Techniques, 2nd ed.; Wlley: New York. 1965. (2) Bond, A. M. Modern Polarographic Methods in AnalyNcel Chemise; Marcel Dekker: New York, 1980. (3) Rusllng, J. F. Trends Anal. Chem. 1988, 7 , 266-269. (4) Murray, R. W. In Nectroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, pp 191-368. (5) Fujihira, M. In Topics in Organic Electrochemlstry;Fry, A. J., BrHton, W. E., Eds.; Plenum: New York, 1986; pp 255-294. (6) Rusling, J. F.; Zhang, H. Langmuir, In press. ( 7 ) Lee. A. J. Biochim. Bbphys. Acta 1977, 472, 237-281, 285-344. 18) Okahata Y.: Shlmizu. A. Lanamuir 1989, 5 , 954-959. . .
S.L. Langmui; 1989,5,650-660. (10) Kamanu, G. N.; Leipert, T.; Shukla, S.S.;Rusling, J. F. J . Electroanel. Chem. Interfac&l Electrochem. 1987. 233. 173-187. (1 . 1). Moser. F. H.: Thomas, A. L. Ths Phthalocyanines; CRC Press: Boca Raton, FL, 1983; Vol. I. (12) Scheffokl, R. I n Modern Synthetic Methods; Scheffokl, R., Ed.; Wlley: New York; 1983; Vol. 3, pp 355-439. J. F.: Shi. C.: Couture, E. C.: Kumoslnskl. T. F. In Redox (13) ~. Ruslina. Chemktry and Interfacial Behavior of Biological Molecules; Dryhurst, G., Niki. K., Eds.; Plenum: New York, 1988; pp 565-581.
2168
Anal. Chem. W Q I , 63,2168-2174
(14) Connors, T. F.; Arena, J. V.; Rusllng, J. F. J . mys. Chem. 1988, 92, 28 10-28 18. (15) Owlla, A.; Wang, 2.; Rusllng, J. F. J . Am. Chem. SOC. 1989, 7 1 7 , 5901-5908. (16) Rusling, J. F.; Miaw, C. L.; Couture, E. C. Inwg. Chem. 1990, 29, 2025-2027. (17) Osteryoung, J.; O’Dea, J. J. I n Electroanalyticel Chem/stty;Bard, A. J., Ed.; Marcel Dekker: New York, 1986; Vol. 14, pp 209-308. (18) Webber, A.; Shah, M.; Osteryoung, J. Anal. Chim. Acta 1984, 757, 1-16. (19) Owlla, A,; Rusling, J. F. J . Nectroanal. Chem. Interfaclal Nectrochem. ioa?, 234, 297-314. (20) Zeng, J.; Osteryoung, R. Anal. Chem. 1988, 5 8 , 2766-2771. (21) Rudzinski, W. E.; Bard, A. J. J . Electroanal. Chem. InterfacielElectrochem. 1988, 799, 323-340. (22) King, R . D.; Nocera, D. G.; Pinnavaia, T. J. J . Nectroanal. Chem.
Interfackl Ekhochem. 1987, 236, 43-53. (23) Andrieux, C. P.; Blocman, C.; Dumas-Bouchlat, J.-M.; M’Halla, F.; Saveant, J. M. J . Electroanel. Chem. Inteffackrl Electrochem. 1980, 773, 19-40. (24) Rusling, J. F. Acc. Chem. Res. 1991, 24, 75-81. (25) Halbert, M. K.; Baldwin, R. P. J . Chromatogr. 1985, 345, 43-49. (26) Halbert, M. K.;Baldwin, R. P. Anal. Chem. 1985, 57, 591-595. (27) Santos, L. M.; Baldwin, R. P. Anal. Chem. 1988, 58, 848-852.
RECEIVED for review March 1, 1991. Accepted July 1, 1991. This work was supported by U.S. PHS Grant ES03154 awarded by the National Institute of Environmental Health Sciences.
Measurement of Concentration Profiles inside a Nitrite Ion Selective Electrode Membrane Xizhong Li and D. Jed Harrison* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
The behavlor of bromo(pyrldlne)(5,10,15,20-tetraphenylporphyrlnato)cobattate(CoTPP(py)Br) as a NO,--selectlve Ion carrier Is reported. I n dloctyl adlpate plastlclred poly(vlnylchlorlde) membranes the carrier Induces NO,- sensltlvlty wlth a slope of -57 mV/decade and selectlvlty coefflclents for CI-, Br-, H,P04-, and HP04?-of 5 X lo4, 1.6 X 2.1 X and 6.1 X lo4, respectively. These membranes respond to pH changes above pH 6 wlth a slope of -25 to -34 mV/pH unit. A spatlal Imaging photometer has been developed to Image the concentratlon proflle changes Inside the membrane In the dlrectlon of Ion transport wlth a spatlal resolutlon of less than 5 pm. The uptake of H20 Is observed by the formatlon of llght scattering centers and exhlblts a nonunlform dlstrlbutlon across the membrane. The transport of NO2- Is also observed when a CoTPP(py)Br-contalnlng membrane Is exposed to NO2- at one membrane/solutlon Interface. The Internal concentration proflle Is descrlbed by Flck’s laws of dlffuslon, glvlng a dlffuslon coefficient of 5 X lo-’ cm2/s In the membrane. Evldence for lnhomogenelty In the membrane In the dlrectlon of charge transport Is also obtalned.
INTRODUCTION Considerable effort has been invested in theoretical analysis of membrane potentials during the past century (1-17). Much of the behavior of membrane potentials can be satisfactorily described; however, a number of somewhat different assumptions can be made about the internal concentration profile of electroactive species within a membrane (9,12).In the case of ion-sensitive membranes differentiation between the models is based on deductions from the membrane potential and current response to external stimuli (9,12, 13, 15-1 7). There is very little direct evidence from measurements of the internal concentration profiles themselves, and this means the extent to which a membrane obeys the ideal models is difficult to determine. Transport of K+ and H+ across a HzO/CHC13/Hz0liquid membrane has been examined by
* To whom correspondence should be addressed. 0003-2700/91/0363-2168$02.50/0
O’Brien et al. using a holographic technique based on the dependence of refractive index on solute concentration (18). Their study suggested convective effects were present during the initial stage of transport, but the steady-state distribution showed a linear concentration gradient across the CHC13 phase, indicating Ficks laws of diffusion adequately describe transport in the liquid membrane. Most liquid membrane electrodes in active use are based on a polymer membrane structure consisting of poly(viny1 chloride) (PVC), a H20-immiscible organic solvent such as dioctyl adipate, an ionophore such as valinomycin, and lipophilic salts to enhance Donnan exclusion (12).These materials are far more complex than a pure liquid membrane, but there is very little detail on the internal concentration profiles. Simon’s group have used a radiotracer technique to analyze transport across a membrane divided into five 40 pm thick segments, providing a spatial resolution of -40 pm (11). In that work an applied voltage was used to drive ions across a valinomycin-containing membrane. The distribution of K+ and valinomycin observed were qualitatively in agreement with the models developed by Morf et al. (10)and later by Buck, Pungor, and their co-workers (14-17). However, the radiotracer study offered insufficient spatial resolution and precision for quantitative comparison to theory. Further, Thoma et al. (11)did not obtain data under open circuit conditions with asymmetrical bathing solutions, which is in fact the common mode of use of these membranes. We have developed a spatial imaging photometer (SIP)to measure changes in absorbance in the direction of transport across an ion-sensitive membrane, with a nominal spatial resolution of 1.25 pm. The apparatus is similar to those described by Fukanaka et al. (19)and later by McCreery and co-workers (20,21)for imaging the electrochemical diffusion layer at a solid electrode. Those authors have reported the spatial resolution due to diffraction from the lenses and refractive index effects is limited to 2 pm (19,21), and due to edge diffraction effects a resolution of less than 5 pm is estimated near the electrode edge (21). We have recently reported the details of the design of the instrument and the electrochemical cells used to probe the distribution and diffusion rate of HzO inside a PVC-based membrane, using water-sensitive (solvatochromic) dyes (22).In the study of the distribution of electroactive species, the method is limited 0 1991 American Chemical Society
