3537
J . Phys. Chem. 1988, 92, 3537-3542
between lipid bilayers observed in both water and ethylammonium nitrate, but it is clear from the present measurements that neither of the pure liquids solvates the mica surface in the same way. Acknowledgment. These experiments commenced while R.G.H. was on leave at the University of Minnesota, and he acknowledges
the hospitality there of Matt Tirrell. We thank Hal Beesley and Patty McGuiggan for preparing the ethylammonium nitrate samples and Stan Miklavic and Hugo Christenson for helpful discussions. Registry No. EAN, 22113-86-6.
Complexation of Redox-Active Surfactants by Cyclodextrins Abigail Diaz, Pablo A. Quintela, Jodi M. Schuette, and Angel E. Kaifer* Department of Chemistry, University of Miami, Coral Gables, Florida 331 24 (Received: August 7 , 1987; In Final Form: October 27, 1987)
The effects of cyclodextrins on the aggregation behavior and electrochemicalproperties of N-ethyl-N’-hexadecyl-4,4’-bipyridinium bromide (CI6VBr2)and N-ethyl-N’-octadecyl-4,4’-bipyridinium bromide (ClsVBr2)were assessed by using electrochemical, optical, and surface tension techniques. The association constants of these two amphiphilic viologens with a- and @-cyclodextrin (ACD and BCD) were determined with a conductance method. Values in the range 103-104 M-I were obtained for all four complexes. In the presence of ACD, the first reduction couple of both viologens exhibits reversible, diffusion-controlled voltammetric behavior. ACD also inhibits completely the formation of dimers from the corresponding cation radicals. In striking contrast, the presence of BCD does not eliminate the precipitation on the electrode surface of the hydrophobic, reduced viologen species. Furthermore, BCD is unable to prevent the extensive dimerization of the viologen cation radicals.
Introduction Cy~lodextrinsl-~ are cyclic glucopyranose oligomers having a characteristic toroidal shape. These compounds are soluble in aqueous media because of the hydrophilic nature of the outer surface of the torus. The inner surface is more hydrophobic, and thus hydrated cyclodextrins represent a high-energy state that can readily accept guest molecules in place of the inner water molecules.’ Indeed, the resulting host-guest complexes are more stable as the hydrophobicity of the guest molecule increases. As expected for inclusion complexes, the better the fit of the guest molecule in the inner cavity of the cyclodextrin, the more stable the host-guest complex will @-Cyclodextrin (BCD, seven glucopyranose units) forms complexes with many organic molecules because its cavity is well suited to bind phenyl moieties and other functional groups. a-Cyclodextrin (ACD, six glucopyranose units) does not form so many complexes because of its smaller inner cavity. However, simple size considerations indicate that ACD is well suited to bind compounds having long alkyl chains such as surfactants. There have been some scattered reports on the interactions of cyclodextrins and surfactants. For instance, Ise and co-workers have reported on the association of cyclodextrins and common surfactants like sodium dodecyl sulfate and cetyltrimethylammonium bromide.4a They found that the addition of cyclodextrins increases the apparent critical micelle concentrations (cmc) of both surfactants. Using conductometric techniques, they determined equilibrium constants between ACD, BCD, and the two surfactants. Similar results have been reported for ACD by Satake et al.4b Thomas and co-workers have studied the formation of CD-surfactant-aromatic fluorophore ternary c o m p l e x e ~ . ~ Quite recently, Kusumoto et al. have also reported on the interaction of pyrene with BCD in aqueous surfactant solutions.6 Yasuda et al. have studied the interactions of several rather hy(1) Szejtli, J. Cyclodextrins and their Inclusion Complexes; Akademiai Kiado: Budapest, 1982. (2) Bender, M. L.; Komiyama, M. Cyclodextrin Chemistry; SpringerVerlag: Berlin, 1978. (3) Saenger, W. Angew. Chem., In?. Ed. Engl. 1980, 19, 344. (4) (a) Okubo, T.; Kitano, H.; Ise, N. J . Phys. Chem. 1976,80, 2661. (b) Satake, I.; Ikenoue, T.; Takeshita, T.; Hayakawa, K.; Maeda, T. Bull. Chem. SOC.Jpn. 1985, 58, 2746. ( 5 ) Hashimoto, S.; Thomas, J. K. J. Am. Chem. SOC.1985, 107, 4655. (6) Kusumoto, Y.; Shizuka, M.; Satake, I. Chem. Phys. Lett. 1986, 125, 64.
0022-3654/88/2092-3537$01 S O / O
drophobic viologens’ with cyclodextrins, aiming at the development of a practical electrochromic display system. Our group is interested in the redox properties of viologens solubilized in hydrophobic, membrane mimetic environments.8-’0 Therefore, we decided to investigate the effects of cyclodextrins on the electrochemical behavior and aggregation properties of the two viologen compounds CI6VBr2and C,,VBr2. These two
‘1fjVBr2
6-CH2-t
“3-c \
\
/N+-CH2-CH3
2 Br-
C18VBr2
compounds (CI6VBr2and C18VBr2)were selected because of their amphiphilic nature that provides two possible binding sites for the cyclodextrin hosts: the aromatic viologen group or the alkyl chain. We report here the results of this study.
Experimental Section Materials. The surfactant viologen bromides, CI6VBr2and C,,VBr2, were prepared according to published procedures for the asymmetric quaternization of 4,4’-bipyridine.” 1-Ethyl4-(4’-pyridyl)pyridinium bromide was first synthetized by mixing 4.0 g of 4,4’-bipyridine (Aldrich) with 20 mL of ethyl bromide. The reaction mixture was kept at room temperature for 4 days and then at 40-50 OC for 3 more days. The resulting mixture was stirred in. 100 mL of toluene to remove the unreacted bipyridine and filtered to separate the solid product. Recrystallization of this solid from hot acetonitrile yielded a material with an N M R spectrum (DMsO-d,) consistent with the product structure. The second quaternization was performed in acetonitrile by reacting the ethylated intermediate with either hexadecyl or (7) Yasuda, A,; Kondo, H.; Itabashi, M.; Seto, J. J. Electroanal. Chem. 1986, 210, 265.
(8) Kaifer, A. E.; Bard, A. J. J. Phys. Chem. 1985, 89, 4876 (9) Kaifer, A. E. J. Am. Chem. SOC.1986. 108. 6837. (10) Quintela, P. A,; Kaifer, A. E. Langmuir 1987, 3, 769. (1 1) Pileni, M. P.; Braun, A. M.; Gratzel, M. Photochem. Photobiol. 1980, 31, 423.
0 1988 American Chemical Society
3538
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988
octadecyl bromide (Aldrich). The reaction mixtures were stirred under reflux for 18 h, and the precipitates were filtered off and washed with toluene. The collected yellow solids were recrystallized from absolute ethanol (CI6VBr2)and water (CI8VBrZ). After two recrystallizations both compounds were dried overnight at 80 OC in a vacuum oven. N M R and IR spectra were consistent with the structures. ACD and BCD were obtained from Aldrich or Fluka. Both materials were used as received without further purification. No difference in behavior was observed with cyclodextrins purchased from different suppliers. Pyrene was recrystallized from ethanol. All solutions were freshly prepared with distilled water that had been further purified by passage through a Barnstead Nanopure ion-exchange system. Equipment. Electrochemical experiments were performed with a Princeton Applied Research (PAR) Model 175 universal programmer, a Model 173 potentiostat, and a Model 179 digital coulometer equipped with positive feedback circuitry for IR compensation. Current-potential curves were recorded on a Soltec VP-6423s X-Y recorder. Absorption spectra were measured with either a Perkin-Elmer Lambda 9 or a Hewlett-Packard 8452A spectrophotometer. Conductance measurements were done with a Jenway PCM3 meter. Steady-state emission spectra were recorded with an Aminco-Bowman spectrophotofluorimeter. Surface tension measurements were performed on a Fisher Model 20 tensiometer by using the du Nuoy method with platinum-iridium rings. Methods. Cyclic voltammograms were obtained by using a two-compartment cell of conventional design. All solutions were thoroughly deoxygenated by purging with purified nitrogen. A nitrogen blanket was maintained above the solution during the electrochemical experiments. Glassy carbon and platinum working electrodes (Bioanalytical Systems, IN) were customarily polished with 0.05-rm alumina and sonicated in ethanol prior to use. All potentials are reported against the sodium chloride saturated calomel electrode (SSCE) unless otherwise specified. Samples of the viologen cation radicals were prepared by controlled-potential electrolysis of solutions containing adequate concentrations of the parent dication. These electrolyses were performed in a vacuum cell by using carbon cloth as the working electrode material. When the reduction reaction was completed, the solution was transferred to an attached quartz cell (0.1 cm), which was then flame-sealed and detached from the electrochemical cell for spectroscopic analysis. Alternatively, a second, experimentally simple procedure was also employed. A strip of glass covered with conductive SnOz (transmittance 80%, Delta Technologies) was affixed to one of the optical windows of a 1 .O-cm visible cell in such a way that the conductive surface faced the solution in the cell. This cell was then placed in the cell holder of the Hewlett-Packard 8452A spectrophotometer; a Pt foil (auxiliary electrode) and a Ag wire (reference electrode) were immersed in the viologen solution, filling the cell, and kept out of the optical pathway. A blank spectrum was recorded at this point. Then, a negative potential was applied to the SnOz surface to begin the generation of the cation radical. Spectral acquisition (integration time 0.5 s) at this point yielded visible spectra identical with those previously recorded for the same system by using the more cumbersome vacuum procedure. Determination of Association Constants. Association constants between the cyclodextrins and the surfactant viologens were determined by using a conductometric method.12 Initially the specific conductance of a 0.5-1.0 mM solution of the surfactant viologen was measured. Cyclodextrin was added to this solution, and the conductance measured until a constant value was obtained. If the concentration of surfactant is kept below the cmc, it can be shown that ( L F- L)/[CD], = KL - KLB where LF and LB are the equivalent conductances of the free (12)
Schori, E.; Jagur-Grcdzinski, J.; Luz, Z.; Shporer, M. J . Am. Chem.
SOC.1971, 93, 7133.
Diaz et al. 20
152
10-
05-
01
I
I
208
2135
2 19
2 245
1 0 5 x EQUIVALENT CONDUCTANCE
Figure 1. Plot of (LF - L)/[CD], versus L for the C,,VBr, system at 25 OC.
2 30
+ ACD
’1 Concentration,
mM
Figure 2. Absorbance at 310 nm (optical pathway 1.0 cm) versus concentration of CI6VBrZ.
surfactant and the cyclodextrin-bound surfactant, respectively, L stands for the measured equivalent conductance, and K (L/mol) represents the association constant between the cyclodextrin and the surfactant viologen. [CD], is the equilibrium concentration of free cyclodextrin and is given by the equation iCD1c = [CD1O - [svlO(LF - L ) / ( L - LB) (2) According to eq 1 , plots of (LF- L)/[CD], versus L yielded straight lines from whose slopes the K values were obtained. A typical plot is shown in Figure 1. The accuracy of these determinations was adversely affected by the small conductance differences observed between the free and cyclodextrin-bound surfactant viologens.I3 The reported values are averages of two or three independent determinations.
Results Aggregation Properties of cI6p+and C 1 8 P + .Several asymmetric viologens with a long alkyl chain have been reported in the literature. Gratzel and co-workers have published the synthesis and aggregation properties of a series of asymmetric, amphiphilic vi~logens.’~This work and other data from the literature strongly suggested that c16vz+and C18V2+must also possess amphiphilic properties. Indeed, this was quickly verified through surface tension and UV absorption measurements. For instance, the main absorption band of monomer viologens has its maximum at 260 nm. This absorption band is red shifted when the viologens aggregate; the red shift can even be observed at wavelengths within the lower energy slope of the absorption band.I4 A plot of absorbance (at 310 nm) versus concentration of C,,VBr, is shown in Figure 2. The change of slope in the graph is due to the red shift caused by the aggregation of the viologen molecules. Therefore, the cmc can be obtained from the intercept of the two (1 3) This difficulty was also noted by Ise and co-workers!a (14) Krieg, M.; Pileni, M. P.; Braun, A. M.; Gratzel, M. J . Colloid Interface Sci. 1981, 83,210.
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988 3539
Complexation of Redox-Active Surfactants
TABLE I: Association Constants at 25 OC of Surfactant Viologens with Cvclodextrins
viologen
I 15i
C16VBr2
Cl8VBr2
A
“ I 2
ACD Concentration,
4
cyclodextrin ACD BCD
K , M-’ (1.6 f 0.8) (9.9 f 2.0) (1.7 f 0.5) (9.5 f 2.5)
ACD
BCD
X
lo4
x 103 X
lo4
x 103
n
6
mM
Figure 3. Dependence of the pyrene’s relative fluorescence intensity on ACD concentration: [pyrene] = lo-’ M; [C,6VBrZ]= 1.0 mM. The medium was 50 mM NaCl.
linear segments. Thus, the cmc of CI6VBr2in pure water was determined to be 2.8 X M. The cmc of CIBVBrZ in pure water could not be accurately determined because it is quite close to its solubility limit; however, it seems to be about 1 X M. These values agree well with those reported by Gratzel et al. for structurally similar v i ~ l o g e n s . ’The ~ cmc values were also determined in 50 mM NaCl because the electrochemical experiments (vide infra) were performed in this aqueous medium. Using surface tension measurements, we obtained values of 4 X lo4 and 3X M for C16VBrzand CI8VBr2,respectively. Association of the Surfactant Viologens with Cyclodextrins. The possibility of formation of inclusion complexes between the surfactant viologens and cyclodextrins was initially assessed with surface tension measurements. For instance, a 1.O m M solution of c,6v2+ (in 50 mM NaC1) shows a surface tension of 44 dyn/cm, which is characteristic of ionic surfactants at concentrations above the cmc.I5 The addition of ACD to this solution causes a quick increase of the surface tension. After 3-4 equiv of ACD is added, the surface tension reaches values characteristic of solutions without surfactants (65-73 dyn/cm). Similar observations were Since ACD does not have made with 1.0mM solutions of c]8v2+. any surface activity,I6the increase in surface tension brought about by its addition can be interpreted as a result of the formation of a surfactant-cyclodextrin complex. Thus, as the concentration of cyclodextrin increases in the solution, the viologen molecules are removed from the air-solution interface, and, therefore, the surface tension of the solution increases. In the case of c16vz+and ACD we performed another experiment to prove qualitatively the formation of a complex. A 1.0 m M solution of the surfactant viologen was prepared in 50 mM NaCl that had been previously saturated with pyrene (the final concentration of pyrene in the solution was then about lo-’ M). The fluorescence spectrum of this solution (excitation wavelength 332 nm) corresponds to that of pyrene, but the recorded intensities are very low. Indeed, this is likely the result of electron-transfer quenching of the excited-state pyrene molecules by the viologen acceptors. The efficient quenching can be explained by the proximity of the pyrene molecules to the viologen head groups if one accepts the increasingly widespread view that nonpolar molecules like pyrene are solubilized near the Stern layer in ionic micelles.” As can be seen in Figure 3, addition of ACD to the solution increases the intensity of fluorescence emission, leveling off after the addition of 2-3 equiv of ACD. Again, these results suggest the formation of a rather stable complex between ACD and CI6V2+.ACD associates with the surfactant viologen molecules, causing the concentration of free viologen to decrease below the cmc (in effect destroying the micelles) and releasing the pyrene molecules to the bulk solution where the quenching (1 5 ) Shinoda, K.; Nakagawa, T.; Tamamushi, B.-I.; Isemura, T. Colloidal Surfactants; Academic: New York, 1963. (16) Cserhati, T.; Szejtli, J. Tenside Deterg. 1985, 22, 5 . (17) Menger, F. M. Arc. Chem. Res. 1979, 12, 1 1 1 .
pOTENTlPL
( V
V I
SSCE)
Figure 4. Cyclic voltammograms on glassy carbon of 1 .O mM solutions of CI6VBr2also containing (A) 50 mM NaCl and (B) 10 mM ACD + 50 mM NaCl (scan rate 50 mV/s).
of the excited pyrene molecules would proceed more slowly at diffusion-controlled rates. An alternative rationalization of the data in Figure 3 would assume the formation of pyrene-ACD complex; however, this possibility must be ruled out because the pyrene molecule is too large to fit inside the ACD cavity.6 After these exploratory experiments, we set up to determine the equilibrium constants for the association of the surfactant viologens with the cyclodextrins. The values obtained with the conductance method discussed in the Experimental Section are given in Table I. These equilibrium constant values were calculated by assuming the formation of l :l complexes. N o evidence could be gathered for the existence of 2:l or 1:2 complexes. Although all the association constants are relatively similar, notice that both C16VZ+and c18v2+exhibit slightly larger values with ACD than with BCD. It must be noted that although the reported values are high, they are consistent with the pronounced effect that 2-3 equiv of cyclodextrin exert on the properties of dilute (1.0 mM) solutions of the surfactant viologens. For the cyclodextrin host to complex a substantial fraction of surfactant viologen at the millimolar level, the association constant must be in the range 103-104. Electrochemistry of the Surfactant Viologens in the Presence of Cyclodextrins. The selection of redox-active surfactants as guests for the cyclodextrin hosts affords the possibility of monitoring the complexation reactions through the use of electrochemical techniques. The electrochemistry of any viologen (Vz+) is characterized by two consecutive monoelectronic reductions to yield first a cation radical (V’) and, then, a neutral molecule (V).18 This general behavior is complicated in the case of the selected surfactant viologens by the hydrophobic character of the reduced species that precipitate at the electrode surface, resulting in voltammetric behavior that is not diffusion-controlled. For instance, a cyclic voltammogram of Cl,V2+ in aqueous solution is shown in Figure 4A. The shape of all peaks is characteristic of processes in which heterogeneous electron-transfer reactions are coupled to phase changes (like precipitation and redissolution). The voltammogram clearly indicates that the cation radical, c,6v+, and the neutral molecule, c16v, are insoluble and precipitate on the electrode surface. The longer chain analogue, CI8V2+,also shows similar voltammetric behavior. The first reduction of c16vz+in the presence of a 10-fold excess of ACD appears to be free of these precipitation effects, although the second reduction process is again coupled to the formation -
~
(18) Bird, C L.; Kuhn, A T. Chem. SOCReo 1981, IO, 49.
3540
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988 A
B
0
03
POTENTIAL I
5
- 0
Square
;5
10
ROO^ a t
Scan
Rate
ImY
15
10
11'
Figure 5. Scan rate dependence of the cathodic peak current for the first reduction on Pt of 1.0 mM C,6VBr2in the presence of 10 mM ACD. Inset: cyclic voltammogram on Pt at SO mV/s.
of precipitates on the electrode surface (see Figure 4B). When the direction of potential scan is switched around -0.8 V versus SSCE, so that the second reduction process is avoided and a platinum working electrode can be used, the cyclic voltammogram in Figure 5 (see inset) is obtained. Similar behavior was also recorded on glassy carbon surfaces, but the background currents were larger. The separation between the reduction and oxidation peaks is 60 mV at moderate scan rates, indicating a nernstian redox couple. A plot of reduction peak currents versus the square root of scan rates is linear (see Figure 5), as expected for a diffusion-controlled process. The diffusion coefficient calculated from the slope of this plot is 2.7 X 10" cm2/s, in good agreement with previously r&portedvalues for substrate-cyclodextrin complexes.19 The apparent reduction potential (as obtained from the average of the reduction and oxidation peak potentials) is -0.66 V versus SSCE. This reversible, diffusion-controlled electrochemical process was assigned to the first one-electron reduction of the ACDC16V2+complex, that is ACD-C16V2+ 4- e-
ACD-CI~V'
Eo' = -0.66
v
(3)
The two species involved in this electrochemical process appear to be soluble in the aqueous medium employed for the voltammetric experiments. If the negative potential scan is extended to -1.0 V versus SSCE, a second reduction wave is observed (see Figure 4B). The distorted shape of this reduction peak clearly suggests that diffusion is no longer the only process controlling the current. This is clearly verified on the reverse scan due to the large oxidation peak associated with the second redox couple, indicating that the reduction of the ACD-C16V+ species yields an insoluble compound that immediately precipitates on the electrode surface. The reduction of ACD-C16V+ generates ACD-C16V that could either deposit on the electrode surface or dissociate, releasing neutral c16v, which would then precipitate on the electrode. The present data do not allow us to discern which one is the predominant pathway. The electrochemistry of c16v2+in the presence of ACD at lower concentrations is similar to that observed for the viologen in the absence of ACD because of the precipitation of the residual uncomplexed C16V2+upon reduction. It is thus necessary to add an excess of cyclodextrin to the solution to decrease substantially the concentration of free surfactant viologen and allow a clean observation of the electrochemistry of the ACD-CI6V2+complex. The voltammetric behavior of CI8V2+in the presence of a 10-fold excess of ACD is close to that observed for the shorter chain analogue. However, the first reduction couple is not so free of complications, as can be seen in Figure 6. At fast scan rates the voltammetric response is essentially diffusion-controlled, while
085
( V
is
07
05
03
SSCE)
Figure 6. Cyclic voltammograms on Pt of a 1.0 mM solution of C,,VBr, in the presence of 10 mM ACD; supporing electrolyte 50 mM NaCI; (A) scan rate 20 mV/s; (B) scan rate 200 mV/s.
I " "
A ]
-Figure 7. Absorption spectra (300-800 nm; absorbance scale 0-0.2) of reduced surfactant viologen solutions: (A) 1.0 mM CI6V+in 50 mM NaC1; (B) A 10 mM ACD; (C) A + 10 mM BCD; (D) 1.0 mM CI8V+ in 50 mM NaCI; (E) D + 10 mM ACD; (F) D + 10 mM BCD.
+
at slower scan rates a small desorption spike can be seen in the oxidative scan. This result suggests that the ACD-CI8V+complex is less soluble in the reaction medium than the ACD-C16V+ complex, which is probably a reflection of the lower solubility of the longer chain viologen. A 10-fold excess of BCD also changes the voltammetric behavior of either c16v2+or C18V2+.However, the first reduction couple of the BCD complexes does not exhibit reversible behavior at the scan rates surveyed (up to 1000 mV/s). In both cases, the reduction of the complex appears to be coupled to other processes, such as precipitation and dissociation of the complexes. An important aspect of the aqueous chemistry of viologen cation radicals is their tendency to dimerize.'8-20 The extent of the dimerization reaction can be easily estimated through the visible absorption characteristics of cation radical solutions. Using the procedures detailed in the Experimental Section, we obtained the visible spectra of CI6V+and C18V+in the absence of cyclodextrin and in the presence of 10-fold excesses of either ACD or BCD. The resulting spectra are shown in Figure 7. The spectra of cI6v+ and C18V+(see Figure 7, parts A and D) show strong absorption maxima at 365 and 560 nm, corresponding to dimer absorptions.2o In both spectra, the typical monomer absorption maxima (at 395 and 602 nm) are completely obscured by the dimer absorptions. Furthermore, we observed that the electrolysis of the parent di-
(19) Matsue, T.; Evans, D. H.; Osa, T.; Kobayashi, N. J . Am. Chem. SOC. 1985, 107, 3411.
(20) Kosower, E. M.; Cotter, J. L. J . Am. Chem. SOC.1964, 86, 5524
Complexation of Redox-Active Surfactants cation solutions produced a purple film on the working electrode surface. The purple color (characteristic of the cation radical) did not extend to the bulk solution; that is, the electrogenerated cation radical species are insoluble in the electrolysis medium and form solid films at the electrode-solution interface. Indeed, we were forced to use the SnOz-modified optical cell to record these visible spectra. Since the cation radicals precipitate as solids, the extent of dimerization is very high. This behavior is characteristic of hydrophobic viologens, like the commercially available heptylviologen, which has been proposed as a candidate for electrochromic display systems.21*22 The presence of a 10-fold excess of BCD does not affect the deposition of purple films on the electrode surface; the spectra (see Figure 7 , parts C and F) reveal also a large extent of dimerization and are essentially indistinguishable from those recorded in the absence of cyclodextrin. This indicates that the vilogen-BCD complexes either precipitate upon reduction or dissociate to release viologen cation radicals, which then deposit. In striking contrast with BCD, the addition of a 10-fold excess of ACD to either c16vz+or c18v2+solutions has a remarkable effect on the visible spectra of the corresponding cation radicals. The spectra are shown in Figure 7, parts B and E; it is clearly evident that ACD prevents the dimerization of the electrogenerated cation radicals. The observed spectra are identical with those reported for viologen cation radicals in nonaqueous solvents where dimerization is known to be entirely s ~ p p r e s s e d . ~The ~ electrogeneration of c]6v+in the presence of ACD proceeds without the formation of purple deposits on the electrode surface. In the case of ClsV’, some filming takes place but the purple color mostly spreads to the bulk solution. This is in excellent agreement with the voltammetric results for these systems. The suppression of dimerization that is observed in the presence of ACD appears to be related to the solubilization of the cation radicals upon cyclodextrin complexation. However, the ACD-C18V+ system is puzzling because the cyclodextrin does not solubilize completely the cation radical and, strikingly, the visible spectrum does not show any dimer absorption. The reasons for this are still unclear.
Discussion Evans and collaborators have published an interesting study of the electrochemical behavior of ferrocene-carboxylic acid in the presence of BCDI9 and a summary of general guidelines on the electrochemical methodology that can be used to assess the complexation of redox-active molecules by cyclodextrin hosts.z4 However, the amphiphilic character of the surfactant viologens used in this work prevented us from applying most of these methods. For instance, Evans et al. determined the association constant between ferrocene-carboxylic acid and BCD by two methods: the first is based on monitoring the apparent diffusion coefficient of the ferrocene derivative as a function of the concentration of added BCD, and the second requires the measurement of the half-wave potential of the ferrocene derivative couple as a function of the concentration of added cyclodextrin. Neither method can be applied to our systems because of the insolubility of the c]8v+and cl6v+ species that complicates greatly the voltammetric behavior, making impossible the determination of either diffusion coefficients or half-wave potentials under most of the experimental conditions surveyed in this work. We could only determine accurately these two parameters for the ACDc]6v2+complex, obtaining values in good agreement with literature values. Thus, the diffusion coefficient calculated for this complex (2.7 X lo4 cm2/s) is reasonably close to that calculated by Evans and co-workers for the BCD-ferrocene-carboxylic acid complex (2.2 X cm2/s).I9 The half-wave potential for the electrochemical reaction of eq 3 (-0.66 V versus SSCE) is also in good agreement with that reported for the first reduction of Van Dam, H. T.; Ponjee, J. J. J . Electrochem. SOC.1974,121, 1555. Jasinski, R. J. J . Electrochem. SOC.1977, 124, 637. Watanabe, T.;Honda, K. J . Phys. Chem. 1982,86, 2617. Matsue, T.; Osa, T.; Evans, D. H. J . Inclusion Phenom. 1984, 2, 547. (25) Okuno, Y.; Chiba, Y . ;Yonemitsu, 0.J . Chem. Sm., Chem. Commun. 1984, 1638. (21) (22) (23) (24)
The Journal of Physical Chemistry, Val. 92, NO. 12, 1988 3541 methylviologen in aqueous media (-0.69 V versus SCE).8 This fact probably reflects that the complex between ACD and the surfactant viologen is of the inclusion type with the long alkyl chain of the viologen interacting with the hydrophobic cavity of the cyclodextrin. Hence, the redox properties of the viologen moiety are essentially unaltered by ACD complexation. The conductometric method used to determine the association constants is similar to that employed by Ise et al. to obtain the association constants of BCD and ACD with two electroinactive s ~ r f a c t a n t s .The ~ ~ values reported in this work are higher than theirs. The differences are moderate for BCD complexes (our and c18vz+are both in the order of 9000 M-’, values for c]6v2+ while their value for CTABr was 2240 M-I) and larger for ACD complexes. However, Satake and c o - ~ o r k e r shave ~ ~ recently reported higher values (in the lo3 range) for the association constants of ACD complexes with dodecyl surfactants, in better agreement with our values. Although the error margin of these determinations is considerable, our results seem to indicate that the ACD complexes are slightly more stable than the corresponding BCD complexes. Ise and co-workers’ values suggest the opposite. It must be remembered that the surfactants under study are structurally different; however, if the complexation process occurs by the inclusion of the surfactant’s alkyl chain into the cyclodextrin cavity, one would expect to observe certain common trends regardless of the structure of the surfactant’s head group. The examination of CPK models suggests that ACD offers an optimum cavity size for the inclusion of an alkyl chain. BCD has a larger cavity, and the fit between the cavity and the hydrocarbon chain is more loose. These considerations indicate that ACDsurfactant complexes should be more stable than the corresponding BCD-surfactant complexes. This also agrees with the electrochemical results of this work since, at the same concentration level, ACD has a more pronounced effect than BCD on the electrochemical properties of both surfactant viologens. In addition, we have recently reported the disrupting effect of ACD on the aggregation properties of vesicle-forming, double-chain surfactants.M BCD was found not to affect these aggregation properties. All these facts suggest that ACD forms more stable complexes than BCD with surfactant molecules; however, additional work is required to unravel all the aspects of the complexation of surfactants by cyclodextrins. The electrochemical behavior of both c16vz+and c]8v2+ in the presence of excess ACD is rationalized by the formation of a stable inclusion complex with either surfactant. The presence of ACD essentially precludes the precipitation of the cation radical after its electrogeneration. It is thus concluded that the ACD-V+ complexes are soluble in the 50 mM NaCl electrolyte. However, the solubility of ACD-C16V+is higher than that of ACD-CI~V’, as proven by the small desorption spikes still visible in the cyclic voltammograms of the latter. Indeed, the solubility of the ACD-cation radical complexes agrees very well with the spectroscopic results on the dimerization process. The extent of dimerization in ACD-V+ solutions is negligible because the cation radicals are solubilized and kept apart from each other by the cyclodextrin hosts, thus preventing dimerization. In the absence of cyclodextrin the visible spectrum of both reduced surfactant viologens reveal extensive dimerization or aggregation. Cyclic voltammograms and visual observations indicate that the electrogenerated cation radicals precipitate on the electrode surface. Indeed, the formation of a solid deposit of cation radical salts on the electrode surface enhances considerably the extent of dimerization. It must be pointed out that the first report on the dimerization control of viologen cation radicals via cyclodextrin inclusion was recently published by Okuno et aLZ5 BCD forms also stable complexes with the two surfactant viologens. However, cyclic voltammograms of either C16VZ+or C18V2+in the presence of 10-fold excesses of BCD failed to show diffusion-controlled behavior for the first reduction couple of the viologen. This is coherent with the spectral characteristics of the BCD-cation radical solutions, which indicate extensive dimeri(26) Quintela, P. A.; Kaifer, A. E., submitted for publication.
3542
J . Phys. Chem. 1988, 92, 3542-3546
zation. Although some valid arguments could be given to explain this phenomenon on the basis of thermodynamic and kinetic differences between ACD and BCD complexes with the surfactant viologens, it is quite likely that the extensive dimerization of BCD-V+ complexes is mostly due to the lower solubility of these complexes compared to the corresponding ACD-V+ complexes. The lower solubility of the former must be the result of the lower solubility of BCD compared to ACD. The precipitation of the BCD-V+ complexes facilitates the interactions among the exposed viologen moieties that aggregate extensively into dimers. The cyclodextrin hosts, interacting with the alkyl chains, are thus unable to decrease the probability of dimer formation. Another alternative to explain the failure of BCD complexation to prevent viologen cation radical dimerization would be based on the possibility of the BCD molecule hosting two surfactant viologens. However, this alternative does not appear likely because of the 10-fold excesses of BCD used in these experiments that would strongly disfavor 2: 1 complexes. All our data suggest that the single most important factor determining the effect of cyclodextrin complexation on the extent of dimerization is the solubility of the viologen-CD complexes. Conclusions We have shown that both ACD and BCD form stable complexes with the asymmetric, amphiphilic viologens surveyed. The
formation of these complexes has a remarkable effect on the aggregation properties of the viologens. For instance, at the millimolar level, the addition of 1-2 equiv of cyclodextrin to a surfactant viologen solution results in the destruction of the viologen micelles. The voltammetric behavior of the viologens is strongly altered by a 10-fold excess of ACD because the corresponding cation radicals are solubilized upon complexation. This solubilization effect appears to be determinant to suppress the dimerization of the electrogenerated cation radicals. Thus, it is not surprising that BCD, which is substantially less water soluble than ACD, neither solubilizes the cation radicals nor prevents their dimerization. All these findings can be interpreted by assuming that the cyclodextrins interact with the long alkyl chain of the viologens forming inclusion complexes. No evidence was obtained to indicate the interaction of either of the cyclodextrins with the aromatic viologen moieties. The cyclodextrin-viologen complexes appear to be mere examples of a rather unexplored, general class of complexes between cyclodextrins and surfactants. Acknowledgment. This research was supported by a BristolMyers Co. Grant of Research Corp. Registry No. C,,VBr,, 76794-29-1; C,,VBr,, 114094-51-8;ACD, 10016-20-3;BCD, 7585-39-9;C16V2*, 78769-77-4;CI8V2+, 114094-52-9.
Anomalous Wide-Angle X-ray Scattering and X-ray Absorption Spectroscopy of Supported Pt-Mo Bimetallic Clusters. 1. Experimental Technique Mahesh G. Samant, Gerard Bergeret,+George Meitzner, and Michel Boudart* Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025 (Received: August 19, 1987; In Final Form: December 28, 1987)
Anomalous wide-angle X-ray scattering and a controlled atmosphere cell for collection of data on supported Pt-Mo bimetallic clusters are described. The dispersion corrections to the atomic scattering factors were evaluated by X-ray absorption spectroscopy for Pt and Mo near the Pt Llll and Mo K absorption edges, respectively.
Introduction Supported bimetallic clusters are used in catalytic reforming of hydrocarbons to obtain high-octane fuels. To understand such catalysts, it is useful to obtain information on their structure. To this end, extended X-ray absorption fine structure spectroscopy (EXAFS) has been applied to numerous bimetallic systems.’ However, EXAFS gives reliable information only about the first coordination shell around the absorbing atom, and to obtain real distances from EXAFS requires correction for phase shifts.* On the other hand, wide-angle X-ray scattering (WAXS) gives structural information on all successive coordination shells and yields interatomic distances without any correction. Gallezot et aL3 used WAXS to determine the atomic structure of platinum clusters trapped in the supercages of a Y zeolite. The success of the method was greatly helped by the crystallinity of the Y zeolite, which has a sharp diffraction pattern. The Fourier transform of the diffraction pattern gave a featureless background in the radial electron distribution function (REDF), which facilitated the determination of the Pt-Pt distances from the peaks in the REDF. One disadvantage of WAXS is that the REDF includes peaks corresponding to all interatomic distances present in the scattering matter. Thus the information is not element specific. A n ele‘On leave from Institut de Recherches sur la Catalyse, CNRS, F-69626 Villeurbanne Cedex, France. *To whom correspondence should be addressed.
0022-3654/88/2092-3542$01.50/0
ment-specific REDF can be obtained by the “anomalous” wideangle X-ray scattering method (AWAXS) as shown by Fuoss et aL4 in a study of amorphous GeSez bulk alloys. However, AWAXS has never been applied to bimetallic clusters. This series of papers [this paper (part 1) and the paper following in this issue (part 2)] describes the study of bimetallic clusters of platinum and molybdenum contained in Y zeolite with AWAXS and X-ray absorption spectroscopy (XAS). Theory of Anomalous Wide-Angle X-ray Scattering (AWAXS) For a multiatom system the intensity of coherently scattered X-rays is given’ by
I(s) = CCxp,f,(s)fn*(S)Smn(S) m n
(1)
where I is the scattered intensity in electron units, s is the wave 6’ is the Bragg angle, X is the vector defined as s = 4 7 ~(sin ( 1 ) Sinfelt, J. H. Bimetallic Catalysts: Discooeries, Concepts, and Applications; Exxon Monograph; Wiley: New York, 1983. ( 2 ) Lee, P. A,; Citrin, P. H.; Eisenberger, P.; Kincaid, B. M. Reu. Mod.
Phys. 1981, 53, 769. (3) Gallezot, P.; Bienenstock, A. I.; Boudart, M. Nouv. J. Chitn. 1978, 2, 263. (4) FUOSS, P. H.; Eisenberger, P.; Warburton, W. K.; Bienenstock, A. Phjis. Rev. Lett. 1981, 46, 1537. (5) Warren, B. E. X-ray Diffraction; Addison-Wesley: Reading, PA, 1969.
0 1988 American Chemical Society
