2632
J. Phys. Chem. 1982, 86,2632-2640
ilarly, in the shell of H+(CH,CN),, a methyl hydrogen welcomes the approach of the nitrogen lone-pair electrons of the third CH3CN. Therefore, in these shells the most reactive site for the third body is not the central proton, but the terminal hydrogen on account of the exchangerepulsion block at the former position. In these clusters, the third and more neutral molecules may be regarded as the “surroundings” which are nonequivalent to the first and the second ones. In Li+(CH,CN), and Na+(CH,CN),, the most symmetrical structures are found to be favorable. There is no
tight covalent and directional bonding. This is in contrast to the case of H+(CH,CN),. Different from the proton, Li+ and Na+ have the ionic radii and serve merely as the origin of the electrostatic field due to their inert electronic structure. Acknowledgment. We thank the Institute for Molecular Science for allotment of the CPU time of the HITAC M-200H computer. This study is supported in part by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Science, and Culture.
Electrochemistry in Ordered Systems. 2. Electrochemical and Spectroscopic Examination of the Interactions between Nitrobenzene and Anlonic, Cationic, and Nonionic Micelles Gregory L. McIntire,+ Donna Mark Chiappardi, Robert L. Casselberry, and Henry N. Blount’ Center for Catalyiic Science and Technolcgy and Brown Chemical Laboratoty, The Universi!y of Delaware, Newark, Delaware 1971 1 (Received: August 13, 1981; In Final Form: February 8, 1982)
The reductive electrochemistryof nitrobenzene (NB) to the corresponding anion radical (NB-.) and dianion has been examined in anionic, cationic, and nonionic micelles. The observed voltammetry of NB in these systems has been related to the nature of the interactions of NB and NB-with the respective micelles. These interactions have been interpreted in terms of models of various micelle/substrate interactions which were tested by using values of the diffusion coefficient of NB in each micelle system over a range of NB concentrations. Results indicate that the stability of NB-. in anionic micelles arises from a relatively strong surface interaction. The stability of the nitrobenzene anion radical in nonionic micelles suggested by voltammetry and supported by ESR also derives from a surface interaction. Cationic micelles, however, provide a pseudophase into which NB partitions such that NB-. resulting from reduction of NB is concentrated within the small volume of the micelle and therein undergoes homogeneous chemical reactions to yield phenylhydroxylamine. These results point to the importance of understanding potential interactions between substrates and micelles in describing the behavior of micelle-solubilized substrates.
Introduction Micelles, dynamic aggregates of amphiphilic molecules, possess regions of hydrophilic and hydrophobic character in which normally water-insoluble species may be solubi1ized.l This unique property of micellar solutions, as well as the short-range order afforded to the reaction environment by micelles, has given rise to the growing use of these media in a broad spectrum of applications. The microscopic order of the micelle system is known to be of significance in biological,2synthetic,, and energy-transfer systems4 wherein the solubilized species can, under appropriate conditions, serve either as an electron acceptor or as a n electron donor. There are a variety of interactions which may be operative between solubilized substrates and host micelles including electrostatic a t t r a ~ t i o nsurface ,~ adsorption: pseudophase extraction,7 and substratelamphiphile coassembly to yield a unique micelle composit i ~ n .Understanding ~ ~ ~ the specific nature of the interaction between a solubilized species and the host micelle is critical to describing the impact of the micellar microenvironment on the functional behavior of the substrate. Work in these laboratories has addressed the effects of micelle solubilization on the redox behavior (formal pot Research Laboratories, Eastman Kodak co., Rochester, NY 14650.
tentials) and reactivities of the one-electron oxidation and reduction products (radical ions) of solubilized sub~trates.~JOA previous report has detailed the redox behavior of sodium dodecyl sulfate (SDS) micelle-solubilized 10-methylphenothiazine (MPTH).g The product of the monoelectronic oxidation of MPTH, the cation radical, associates both with free dodecyl sulfate anions (DS-) and with the micellar phase.g Observation of an interaction between the cation radical and the anionic dodecyl sulfate suggested that negatiuely charged species, arising from the reduction of solubilized neutral substrates, might prefer(1) Fendler, J. H.; Fendler, E. J. ‘Catalysis in Micellar and Macromolecular Systems”; Academic Press: New York, 1975. (2) For example: (a) Helenius, A.; Simons, K. Biochim. Biophys. Acta 1975,415,29. (b) Mukherjee, T.; Sapre, A. V.; Mittal, J. P. Photochem. Photobiol. 1978, 28, 95. (3) Cordes, E. H. Pure Appl. Chem. 1978,50, 617. ( 4 ) Brugger, P.-A.; Infelta, P. P.; Braun, A. M.; Gratzel, M. J. Am. Chem. SOC.1981,103, 320. (5) Quina, F. H.; Politi, M. J.; Cuccovia, J. M.; Baumgarten, E.; Martins-Franchetti,S.M.; Chaimovich, H. J. Phys. Chem. 1980,84,361. (6) Eriksson, J. C.; Gillberg, G. Acta Chem. Scand. 1966, 20, 2019. (7) Bunton, C.A.; Romsted, L. S.;Savelli, G. J. Am. Chem. SOC.1979, 101, 1253. (8) Funasuki, N.; Hada, S. J. Phys. Chem. 1980,84, 736. (9) McIntire, G. L.;Blount, H. N. J . Am. Chem. SOC.1979,101,7720. (10) McIntire, G. L.; Blount, H. N. In “Solution Behavior of Surfactants: Theoretical and Applied Aspects”; Mittal, K. L, Fendler, E. J., Eds.; Plenum Press: New York, 1982.
0022-3654/82/2086-2632$01.25/00 1982 American Chemical Society
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The Journal of Physical Chemistry, Vol. 86, No. 14, 1982
2833
entially interact with cationic micelles. Thus, the reduction of nitrobenzene (NB) to the anion radical (NB-0) has been examined in the presence of cationic, nonionic, and anionic micelles to determine the effect of these media on the heterogeneous electron-transfer properties of NB and on the kinetic behavior of NB-.. In contrast to the expected behavior, electrochemical and spectroscopic results indicate that the anion radical is more persistent in the presence of anionic SDS micelles than in either isotropic aqueous solution or cationic micelles. While electrochemical results do not clearly demonstrate the existence of NB-e in the nonionic micelle system examined in this work, electron spin resonance (ESR) spectroscopy reveals the presence of this species during the reduction of NB in this medium. Models for binding of NB to micelles have been developed to provide an indication of the nature of the interaction between NB and the respective micelles. Results derived from measurements of the diffusion coefficients of NB in these media suggest that the nature of the interaction between NB and the micelle is dependent upon the type of micelle involved. The data are consistent with a surface interaction between NB and the anionic (SDS) micelles. Support for this conclusion is provided by studies with varying counterions and counterion concentrations. In contrast, the association of NB with cationic micelles is best described as a partitioning between two phases. The nature of the interaction between NB and nonionic micelles is dependent upon the NB concentration. At lower NB concentrations, an interfacial interaction is suggested in a region of the micelle where the micropolarity is similar to that of ethanol. These results point to the importance of the nature of the interaction between solubilized substrate (NB) and micelle with respect to the observed functional behavior of the solubilized species.
TMADS, and ADS were further purified by recrystallization from isopropyl alcohol and were dried in vacuo overnight at room temperature. Apparatus and Techniques. Cyclic voltammetry and potential step experiments were performed with a threeelectrode potentiostat equipped with circuitry for the compensation of solution resistance.12 Hanging mercury drop electrodes (HMDEs) were produced with a Metrohm E410 drop extruder using triply distilled mercury. Measurements of solution pH were made by using a glass electrode, a saturated calomel reference electrode, and a Perkin-Elmer Metrion IV pH meter. Spectrophotometric measurements were performed with a Hitachi 100-80 double-beam, microprocessor-controlled, scanning spectrophotometer. ESR measurements were made on a Varian E109E spectrometer equipped with an E-231 cavity using conventional quartz flat cells. ESR spectra of the nitrobenzene anion radical were obtained by in situ electroreductions at a mercury pool electrode. All solutions for ESR spectrometry and reductive electrochemistry were purged of oxygen before use. All electrode potentials are reported relative to the aqueous saturated calomel electrode (SCE). The distribution coefficient of NB between l-dodecanol and water was estimated in the following manner. An aqueous solution of NB was prepared and the concentration determined spectr~photometrically.'~Aliquots of this solution were then equilibrated with equal volumes of l-dodecanol at 30 OC for 1and 2 h to ensure attainment of equilibrium. The concentration of NB in the aqueous layer was redetermined and the concentration of NB in the alcohol calculated from the corresponding change in the aqueous solution. KD, calculated from
Experimental Section Materials. Sodium dodecyl sulfate (SDS, Aldrich) was washed repeatedly with ether and then thrice recrystallized from 95% ethanol. Cetyltrimethylammonium bromide (CTAB, Aldrich) was recrystallized from absolute ethanol. Cetyltrimethylammonium chloride (CTAC, Eastman) was recrystallized from ethyl acetate and dried in vacuo for 24 h at room temperature. The nonionic poly(oxyethylene(23) lauryl ether) (Brij-35, Aldrich) was used as received. The critical micelle concentrations of SDS, CTAB, and Brij-35 have been r e p ~ r t e d .The ~ cmc of CTAC in the presence of 0.05 M LiCl was evaluated by the method of dye solubilization' and found to be (7.3 f 2.0) X M. Nitrobenzene (Baker) was passed through a column of activated alumina (400 OC, 48 h) before use. All other chemicals were reagent grade or equivalent. All solutions were prepared with deionized, distilled water. Dodecyl sulfate surfactants containing various counterions were synthesized as follows. To a stirred solution of 100 mL of ether at 5 "C was added 10 mL (0.15 mol) of chlorosulfonic acid (Aldrich) and 28.0 g (0.15 mol) of l-dodecanol (Aldrich). The solution volume was then doubled with ethanol and a sufficient amount of the requisite hydroxide (e.g., LiOH to yield lithium dodecyl sulfate) added to ensure complete neutralization. The white, heterogeneous mixture was stirred at room temperature for 1h and then filtered and washed repeatedly with ether. Lithium dodecyl sulfate (LDS), potassium dodecyl sulfate (KDS), tetramethylammonium dodecyl sulfate (TMADS), and ammonium dodecyl sulfate (ADS) were all synthesized by the above procedure.'l LDS, KDS,
was determined to be 15 f 3 for NB between l-dodecanol and water. Measurements of the diffusion coefficient of NB in the various micellar systems and in supporting electrolyte alone were performed at HMDEs. The area of the HMDE was routinely calibrated by measuring the charge uptake with time during a potential step experiment for the reduction of Pb(NO& in 1.0 M KNOB(D = 6.698 X lo+ cm2 s-l)14using the integrated form of the Cottrell equation15
(11) Schick, M. J. J. Phys. Chem. 1964, 68, 3585.
KD
Q
= [NBlorg/[NBlaq
= 2nFAD'/2COt'/2/n'/2
(1)
(2)
Similarly, the time-dependent charge uptake for the reduction of NB was measured and the value of the diffusion coefficient determined. The data were acquired and reduced by using a Data General Corp. NOVA 1200 computer system and conventional operational amplifier circuitry. All diffusion coefficients were determined at 25 f 0.2 OC unless otherwise noted. Viscosity values were determined with an Ubbelohdetype viscometer (Cannon Instrument Co.) having a constant of 2.801 X cSt s-l. All values were determined at 25 f 0.2 "C. Values for the diffusion coefficients of NB in the aqueous phase of the micellar systems were adjusted (12) Pilla, A. A. J . Electrochem. SOC.1971, 118, 702. (13) cNBH,/266 nm = 7900 M-l cm-'. Rao, C. N. In "The Chemistry of the Nitro and Nitroso Groups"; Feuer, H., Ed.; Interscience: New York, 1969; Part I, p 95. (14) Rulfs, C. L. J. Am. Chem. SOC. 1954, 76, 2071. (15) The contribution to the charge arising from diffusion to a spherical electrode was evaluated to be less than 2% of the measured ~ considered during data reduction for evaluation of the value and w a not diffusion Coefficients of NB in these systems.16 (16) Bard, A. J.; Faulkner, L. R. "ElectrochemicalMethods";Wiley: New York, 1980; p 433.
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The Journal of phvsical Chemistty, Vol. 86, No. 14, 1982
McIntire et al.
A
A
30pA
I
.
.
.
.
I
.
- 1 ODD
0
.
.
.
]
-2000
mV vs SCE
0
B
O
l
SCE
-1 000
Figure 2. Cycllc voltammeby of 0.50 mM NB in 50 mM Llcl containing 50 mM SDS. HMDE radius = 0.046 cm, scan rate = 50 mV s-'. Arrow lndlcates lnltlal potential and direction of sweep.
0
0
mV vs
l
l
.
I
I
I
-1000
I
.
.
l
-2000
mV vs SCE
Figure 1. Cyclic voltammetry of 0.50 mM NB In (A) 50 mM LiCI, (B) 50 mM LCl contahing 7.0 mM CTAB, and (C) 50 mM LiCl contalnin 5 % (w/v) Brij-35. HMDE radius -5 0.046 cm, scan rate = 50 mV s- . Arrow indicates inltlal potential and dlrection of sweep.
8
from the value measured in 0.05 M LiCl alone (DmLicl= 7.76 X lo4 cm2s-l) using the Stokes-Einstein equation"
D = kT/(6aqr)
(3)
Treating the quantity kTl(6ar) as an empirical constant, C', eq 3 was rewritten as
D = C'/q
(4)
where C'was evaluated as 6.94 X lo4 cm2 cSt s-l for NB in 0.05 M LiC1.
Results and Discussion Voltammetry. Meyer et have reported that cationic micelles enhance the kinetic stability of the electrogenerated anion radicals of phthalonitrile and fluorenone. Their voltammetric investigation revealed that the reduction of phthalonitrile to the corresponding anion radical becomes chemically reversible a t scan rates greater than 200 mV s-l in the presence of cationic CTAB micelles. Scan rates in excess of 50 V s-l are required to observe chemical reversibility in isotropic aqueous media alone.18 The authors concluded that the effect of association of the anion radical with cationic CTAB micelles is to decrease the rate of protonation of the phthalonitrile anion radical 250-fold. In marked contrast both to the report of Meyer et al.18 and to behavior predicted on the basis of the mi(17)Yeh, P.;Kuwana, T. J. Electrochem. SOC.1976, 123, 1334. (18)Meyer, G.; Nadjo, L.; Saveant, J. M. J.Electrwnal. Chem. 1981, 119, 417.
cellar electrochemistry of MPTH,S it was found that in cationic CTAB micelles the reductive electrochemistry of NB (Figure 1B) was unchanged from that noted at a mercury electrode in isotropic aqueous solutions at scan r a t e up to 500 mV s-l (Figure 1A). The reductive behavior of NB in CTAC micelles is identical with that observed in CTAB micelles and is characterized by a single, chemically irreversible, four-electron cathodic process which gives rise to phenylhydroxylamine. Upon scan reversal, the oxidation of phenylhydroxylamine to nitrosobenzene is observed; this species can be re-reduced to phenylhydroxylamine upon subsequent cathodic sweep. In nonionic Brij-35 micelles, the previously observed fourelectron wave is significantly broadened as shown in Figure lC, but multiple processes cannot be definitively resolved. The redox process seen at more extreme potentials in the Brij-35 medium corresponds to the reduction of azobenzene, a product of the coupling reaction between phenylhydroxylamine and nitros~benzene.'~ The cathodic voltammetry of nitrobenzene in SDS micelles is reflected in Figure 2A and shows that the overall four-electron process apparent on the first cathodic sweep in water, CTAB, CTAC, and Brij media has now been split into two well-resolved reductive processes. If the cathodic sweep is reversed after only the first peak has been traversed, then this process is seen to be chemically reversible on the 5-s time scale of the experiment (Figure 2B). In addition, phenylhydroxylamine formation is shown to be coupled to the more cathodic wave. The relative charge uptakes of these two cathodic processes observed in SDS media were characterized by using single potential step chronocoulometry. Charge uptake was monitored as a function of time for two series of experiments in which the potential of the HMDE was stepped to -900 and -1300 mV, respectively. These potentials are sufficient to drive the two successive cathodic processes at diffusion-controlled rates. The charge uptakes at these two potentials were found to be lineraly dependent upon the square root (19)This reaction occurs to a significant extent in alkaline isotropic aqueous media. Concentation of both the intermediate nitrosobenzene and product phenylhydroxylamine within the micelle phase results in micellar catalysis of the coupling reaction at pH levels employed in these studies (pH 57.0). Fry, A. J. 'Synthetic Organic Electrochemistry"; Harper and Row: New York, 1972;p 285.
The Journal of Physicai Chemistry, Vol. 86, No. 14, 1982 2635
Electrochemistry in Ordered Systems
TABLE I: Ultraviolet Absorption Maxima of Nitrobenzene"
A
I
10 G
B
C
Figure 3. ESR spectra observed for in situ electroreduction of 0.50 mM NB in (A) 50 mM WI,(6) 50 mM LlCl containing 50 mM SDS,and (C) 50 mM LiCl containing 6 % (w/v) Brlj-35.
of time as predicted by the integrated form of the Cottrell equation for kinetically stable systems (eq 2). The ratio of the slope of the Q vs. t1I2dependence observed at -1300 mV to that observed at -900 mV provided a measure of the relative "n values" for the two processes. This ratio was experimentally determined to be 3.9 f 0.1, which corresponds to charge uptakes of one and three electrons, respectively, for the first and second cathodic processes. These data suggest that the reversible electrochemical process at -766 mV corresponds to the formation of NB-in SDS micellar systems. Electroreduction in the ESR cavity of NB in these various media has provided confirming evidence for the formation of NB-.. As seen in Figure lA, there is no definitive electrochemical evidence for the formation of NB-. in aqueous lithium chloride solution. However, the presence of NB-. during the electroreduction of NB in aqueous solutions containing lithium salts has been detected by ESR.20 In situ electroreduction of NB at a mercury electrode in SDS micellar systems gives rise to a well-resolved 54-line spectrum (Figure 3B) which is unchanged from that observed in aqueous LiCl (Figure 3A). In nonionic Brij-35 micelles, the ESR spectrum obtained upon electroreduction of NB is anisotropically broadened, most notably in the high-field portion of the spectrum (Figure 3C). Contrasting these results, in situ reduction of NB in CTAB micelles did not give rise to the anion radical at concentrations which could be detected by ESR. This was true even in the presence of those same con(20) Piette, L. H.; Ludwig, P.; Adams, R. N. J.Am. Chem. SOC.1961, 83,3909.
medium cmc hma.x,b nm heptane 250.0 t 0.2 cyclohexane 251.9 t 0.5 1 -propanol 254.5 t 0.5 ethanol 257.0 t 0.1 50 mM LiCl 265.7 t 0 . 3 20 mM SDSC 1 mM 264.8 t 0.2 5 mM CTAW 1 mM 264.9 t 0.1 6% B1ij-35~9~ 1.1%d 258.8 r 0.3 " NB concentration controlled at 40 pM in all media. Mean of four determinations t standard deviation. Contains 50 mM LiCl. Wt/vol %. TABLE 11: Nitrobenzene Anion Radical Nitrogen Hyperfine Splitting Constants medium aN,G ref DMP 9.83 35 20%H,O/DMP 12.06 35 50%H,O/DMP 13.15 35 70%H,O/DMP 13.55 35 90%HzO/DMFa 13.87 35 6% Brij-35b,c 13.92 2 0.05d this work 14.06 t 0.05d 30 mM SDSb this work 14.06 t 0.05d 50 mM LiCl this work " N,N-Dimethylformamide;contains 100 mM tetraethylammonium perchlorate. Contains 50 mM LiCl. Mean of 8 determinations t standard Wt/vol %. deviation.
centrations of LiCl which, in the absence of CTAB, rendered NB-. perceptible. The pronounced stability of the nitrobenzene anion radical in SDS micelles is a direct consequence of the unique interaction between NB-. and the micellar environment. Solubility studies of NB and analogous aromatic compounds indicate that the anion radical precursor is associated with the micellar phase of each of the surfactants examined in this work.6p21 Ultraviolet absorption maxima of NB in various polar, nonpolar, and micellar media are shown in Table I and indicate that, in ionic surfactants, NB resides in a highly polar environment such as would be expected in the Stern region of these micelles. The nitrogen hyperfine splitting constants of NB-. in media of varying polarity are shown in Table I1 and indicate that, like its precursor (Table I), NB-.is also resident in the highly polar Stern layer of SDS micelles. The concentration of alkali metal ions in this local microscopic environment has been estimated to be between 3.5 and 4.5 M.22 Krygowski and co-workers have shown that, in nonpolar solvents, the anion radical of nitrobenzene undergoes ion pairing with the cation of the supporting e l e ~ t r o l y t e .Additionally, ~~ the second reduction process has been reported to be sensitive to cations in solution as well.24 Thus, the concentration enhancement of counterions in this unique local region of solution as well as residence of the NB-- in this same region may give rise to micelle-enhanced ion pairing of the anion radical of nitrobenzene in SDS micelles. (21) (a) Donbrow, M.; Rhodes, C. T. J.Pharm. Pharmacol. 1966,18, 424. (b) Fendler, E. J.; Day, C. L.; Fendler, J. H. J.Phys. Chem. 1972, 76, 1460. (22) Romsted, L. S. In 'Micellization, Solubilization and Microemulsions";Mittal, K. L., Ed.;Plenum Press: New York, 1977; Vol. 2, pp 509-30. (23) Krygowski, T.M.; Lipsztajn, M.; Galus, Z. J.ElectroanaL Chem. 1973, 42, 261. (24) Kemula, W.; Krygowski, T. M. In "Encyclopediaof Electrochemistry of the Elements";Bard, A. J., Lund, H., Eds.; Marcel Dekker: New York, 1979; Vol. XIII, p 77.
2636
McIntire et al.
The Journal of Physical Chemlstty, Vol. 86, No. 14, 1882
TABLE 111: Diffusion Coefficients of NB and Corresponding Values of Various Concentrations of NBa SDSb CTAC'
F, and Fa, for SDS, CTAC, and Brij-35 Micelles at Brii-35d
0.163 1.76 0.855 0.145 0.163 2.93 0.662 0.338 0.158 1.62 0.673 0.327 0.169 1.90 0.832 0.168 0.290 2.89 0.669 0.331 0.246 1.86 0.615 0.385 0.278 2.51 0.732 0.268 0.418 2.87 0.671 0.329 0.287 2.00 0.580 0.420 0.325 2.86 0.674 0.326 0.536 2.90 0.667 0.333 0.341 2.32 0.502 0.498 0.415 2.97 0.656 0.344 0.823 2.85 0.676 0.324 0.411 2.07 0.564 0.436 0.418 2.97 0.657 0.343 1.106 2.92 0.664 0.336 0.441 2.39 0.485 0.515 0.507 3.23 0.613 0.387 0.506 1.97 0.588 0.412 0.559 3.47 0.574 0.426 0.735 2.30 0.508 0.492 0.719 3.68 0.540 0.460 0.982 2.10 0.556 0.444 0.808 3.52 0.566 0.434 0.987 3.65 0.545 0.455 All solutions contain 50 mM LiCl; 5" = 30 2 1 "C. 60 mM; viscosity of 60 mM SDS/SO mM LiC1/0.50 mM NB = 0.996 t 0.021 cSt; D, calculated to be 8.77 x lo-' cm2 s-l by using a radius of 25 A for the SDS micelle.zsa 50 mM; viscosity of 50 mM CTAC/50 mM LiCl/O.SO mM NB = 0.976 i 0.009 cSt; D, calculated to be 7.99 x lo-' cm2 s-I by using a radius of 28 A for the CTAC micelle.'$' 6% (w/v); viscosity of 6% Brij-35/50 mM LiC1/0.50 mM NB = 1.59 t 0.01 cSt; D, calculated to be 2.88 x lo-' cm2 s-l by using a radius of 48 A for the Brij-35 micelle.2sc e Total concentration of NB in system (both aqueous and micellar phases). f From eq 2 using calculated values of D, for each surfactant and cm2 s-I; DNB~Q(CTAC) = 7.11 X DNBaQ corrected for the bulk viscosity of each system, DNBaa(SDS) = 6.97 X cmz s-I. c m 2 s-I; DNBaq(Brij-35)= 4.37 x
Substrate/Micelle Interaction. There are a variety of interactions which may be operative between solubilized substrates and host micelles. Ionic micelles possess charged surfaces which can give rise to electrostatic attraction of oppositely charged specie^.^ Other substrates may associate with micelles via surface adsorption at the micellewater interface6or simply through partitioning into the relatively nonpolar interior of the micelle.' Substrates possessive of amphiphilic character have been reported to coassemble with surfactant monomers to form a mixed micelle in which the substrate is an integral member of the aggregate.*s8 Understanding the specific nature of the operative interactions between the substrate and the micelle is essential to describing the impact of the micellar environment on the functional behavior of the substrate. Measurement of the distribution of substrate between the micellar and aqueous phases at varying total concentrations of substrate can provide an indication of the nature of the interaction between that substrate and the micelle. This distribution of NB between phases has been provided by measurements of the diffusion coefficients of NB at varying concentrations of NB in the respective micelle systems. It has been assumed that the observed diffusion coefficients of NB in these systems are a linear combination of two products: DNBobsd = FaqDNBaq+ F,D, (5) where Faqand F, represent the fraction of NB in the aqueous and micellar phases, respectively, Dmaq is the diffusion coefficient of NB in a solution of supporting electrolyte alone corrected for the bulk viscosity of the micelle solution, and D , represents the diffusion coefficient of the respective micelles. The values of Dmq and DmoM were measured by single potential step chronocoulometry as described previously and are summarized in Table 111. D, values for the respective micelles were derived from the Stokes-Einstein equation (eq 3)" using literature values of micelle radii%and measured bulk viscosities (Table 111). The values of F, as well as corresponding values of binding constants and distribution coefficients (vide infra) should (25) (a) $ns = 25 A: Mazer, N. A,; Benedek, G. B.; Carey, M. C. J. Phys. Chem. 1976,80,1075. (b) rmAc = 28 A: hiss-Husson, R.; Luzzati, V. J. Phys. Chem. 1964, 68, 3504. (c) 9ru-36 = 48 A: McIntire, G. L.; Blount, H. N. unpublished data. (d) Mandal, A. B.;Ray, S.; Biswas, A. M.; Moulik, S. P. J . Phys. Chem. 1980, 84, 856.
be regarded as upper limits inasmuch as minimum estimates of micelle radii were used in their calculation. In addition, it was assumed that neither the cmc nor the aggregation number (N) changes with NB concentration a t these low concentrations. Substrate inclusion within micelles would be expected to decrease the cmc.' Such changes in cmc values would have little impact on the overall concentration of micelles calculated as [MI = (C,- cmc)/N where C, represents the total surfactant concentration.' Alteration of N would have a much greater effect upon [MI. Yet at the relatively low concentrations of NB employed in this work, the assumption of constant N with NB concentration would appear to be valid. The derived values of F, and Faqrepresent the relative distribution of NB between the mcellar and aqueous phases, respectively, and have been employed to provide an indication of the nature of the interactions between NB and the micelle systems examined in this work. Several types of micelle/substrate interactions have been considered with respect to NB. The first interaction examined involves substrate binding to discrete micellar ligands. The second interaction studied involves surface binding of substrate to the micelle exterior.6 Finally, partitioning of the substrate between the aqueous and micellar phasm by analogy with solvent extraction has been examined with respect to NB/micelle interaction^.^ Each respective interaction has been modeled and applied to the NB/micelle systems examined in this work. The binding of NB to discrete micellar ligands can be written as
&
NB + M M(NB) (6) where one NB molecule binds to one micelle for which KB is the equilibrium or binding constant. The concentration of NB was controlled at values less than the concentration of micelles so that the assumed stoichiometry of one is reasonable under conditions employed in this To be a valid description of the interaction in question, this model requires that KB remain invariant with alterations in NB concentration. In terms of Fa, and F,, KB can be written as (7) KB = F,[NBlt/(Faq[NBlt([Ml - Fm[NBlt)J
The Journal of Physical Chemistry, Vol. 86, No. 14, 1982 2637
Electrochemistry in Ordered Systems
20
0
40
l/(Faq[NBlt) x
05
0
Flgure 5. Surface binding of NB to SDS micelles as derived from eq 13. Regression parameters: (A) slope = (8.50 f 0.80) X intercept = (3.80f 0.14)X lo3,r = 0.987;(B) slope = (4.98f 0.64) X lO-I, intercept = (7.65 f 0.73)X lo2,r = 0.970.
10
[NBlx l o 3
Flgure 4. NB binding constants (KB)with SDS (A),CTAC (m), and Brij-35 (0)micelles as derived from eq 7 for various NB concentrations.
where [MI is the total concentration of micelles (occupied and u n o c ~ u p i e dand ) ~ ~[NB], is the analytical concentration of NB in the system. The data have been treated according to eq 7 and the results are shown in Figure 4. In nonionic Brij-35 micelles, the derived value of KB ((1.35 f 0.32) X lo3 M-9 is approximately constant with varying NB concentration. Both of the ionic micelles deviate markedly from this type of behavior and do so in opposite directions. Although this model may afford a tenable description of the binding of NB to Brij-35 micelles, it fails to account for the binding of NB to ionic micelles. Adsorption of substrate to the micelle surface has been previously reported.6 In terms of NB, this type of interaction may be written as NB
+ S,
2S,NB
(8)
where S, represents unoccupied binding sites on the surface of the micelle and S,NB represents those surface sites occupied by NB. By analogy with Langmuir surface KB can be written as KB = 6/{(1 - e)[NB11 (9) where 6 is the mole fraction of sites occupied by NB and, correspondingly, 1 - 6 is the mole fraction of sites unoccupied. The amount of NB bound to the micelle surface can be related to 6 by a proportionality constant, b, as [S,NB] = b6 (10) Rearranging and solving for [S,NB] in terms of KBand [NB] yields [S,NB] = ~ K B [ N B ] / ( ~KB[NB]) (11)
+
Inversion of eq 11 affords a mathematically convenient expression for this surface process: l/[S,NB] = ~ / ( ~ K B [ N B ] )l / b (12)
+
Equation 12 predicts that a plot of l/[S,NB] vs. 1/[NB] should provide a linear relationship for surface binding processes. The intercept of the regression reflects the value of b and in concert with the slope of the regression yields an estimate of KB, the binding constant. Qualitatively, larger values of the intercept will yield larger values of KB (26) Castellan, G. W. "Physical Chemistry", 2nd ed.;Addieon-Wealey, Reading, MA, 1971; p 435.
0
10 l/(Faq[NBlt) x
2c
Flgure 8. Surface binding of NB to Brij-35 micelles as derived from eq 13. Regression parameters: (A) slope = (3.17f 0.38)X lo-', interce t = (3.28f 0.21)X lo3,r = 0.968 (B) slope = (8.76f 2.1 1) X lo-, intercept = (3 f 31) X lo', r = 0.885.
P
and correspond to stronger surface interactions. Conversely, as the value of the intercept approaches zero, the corresponding value of KB becomes negligible, indicating that the system in question does not interact via a surface process. Rewriting eq 12 in terms of Faqand F , (eq 13) 1/(F,[NBlJ
= l/(bK~Faq[NB]t)+ l / b
(13)
permits an examination of NB/micelle interactions with respect to a surface process. The data were treated according to eq 13, and the results shown in Figures 5 and 6 suggest that the NB binding with SDS and Brij-35 micelles initially proceeds through a relatively strong surface process. In SDS micelles, a plot of l/(F,[NB],) vs. 1/ (Faq[NB],)indicates that, at lower concentrations of NB, a strong surface interaction exists (KBSDS = (4.45 f 0.52) X lo4M-l) while, at higher concentrations of NB,the value of KBis significantly decreased = (1.54 f 0.25) X 103M-l) (Figure 5). The results of this analysis for Brij-35 micelles are similar to those observed in SDS at lower NE4 concentrations (KBBd-= = (1.03 f 0.14) X lo4M-l) (Figure 6). At higher concentrations of NB in this system, the regression affords a near-zero intercept suggesting that the interaction between Brij-35 micelles and NB at concentrations of NB 5 0.25 mM does not occur primarily via a surface process. Cationic CTAC micelles do not appear to interact with NB in a surface mode as evidenced by the
2638
The Journal of Physical Chemistry, Voi. 86, No. 14, 1982
3d
c
/
1
I
McIntire et ai.
1
I
10
I
,
20
; / I F a q [ N B I t )x l o 3
Flgwe 7. Surface binding of NB to CTAC micelles as derived from eq 13. Regression parameters: slope = (5.13 f 0.06) X lo-', intercept = -(1.04 f 0.31) X lo2, r = 0.9997.
zero intercept of the regressed data (Figure 7). The pseudophase model of micellar catalysis has enjoyed considerable success with respect to explaining reaction kinetics in micelle^.'^^^^^ By analogy with this model, the distribution of NB between the aqueous and micellar phases may be written as
05
10
[NE] x 1 0 3
Figure 8. Pseudophase dlstrlbution coefficients (K,) of NB in SDS (O), CTAC (W), and Brlj-35 (V)mlcellar systems.
where V, is the volume of the micellar phase and VS is the total system volume. V, values for the ionic micelles were calculated from literature values of micelle radii assuming spherical geometries.% The volume of the Brij-35 micelles was calculated from the approximate radius of the hydrocarbon core (17 Use of this value is consistent with reports that molecules analogous to NB are solubilized at the interface between the hydrocarbon core and the polar poly(oxyethy1ene) sheath of the micelle.21a Additionally, reports concerning the contribution of the poly(oxyethylene) sheath to the radius of the micelle do not afford a consistent dimension from which to estimate the contribution of that portion of the micelle to the overall radius.27 KD values calculated at each NB concentration in each surfactant are shown in Figure 8. The invariance of the derived KD values for NB in CTAC micelles with NB Concentration indicates that this model provides a tenable description of the interaction between NB and CTAC micelles (KD = 60 f 1). The interaction between NB and either SDS or Brij-35 micelles is not consistent with this model as evidenced by the variation of the values of KOwith NB concentration. As the concentration of NB is increased beyond 0.50 mM in the SDS system, KD values
approach -30 (Figure 8). Independent extraction studies, however, indicate that KD of NB between 1-dodecanoland H20 is 15 f 3 and further argue against the applicability of the pseudophase model to the NB/SDS system. The models of substrate/micelle interactions presented above illustrate the differences between the interaction of NB with SDS, CTAC, and Brij-35 micelles. Nitrobenzene interacts with SDS micelles through a surface mechanism indicating intimate association of NB with the Stern layer of the SDS micelle. In nonionic micelles, the interaction of NB with the micellar phase is best described as a surface process. The "surface" may well be the interface between the hydrocarbon core of the micelle and the polar poly(oxyethylene) sheath.21a Although the nature of the interaction between NB and Brij-35 micelles at NB concentrations greater than approximately 0.25 mM has not been determined, the qualitative behavior of NB in this system is consistent with a previous report concerning NB/micelle binding in that the mode of binding changes as the concentration of NB is increased.6 Changes in the nature of the interaction between NB and the micelle with addition of substrate (NB) indicate that NB solubilization within the micelle may be dependent upon occupancy of the micelle by NB and would therefore preclude the use of a Poisson distribution to describe the distribution of NB among the micelles.28 Cationic CTAC micelles provide a pseudophase for the partitioning of NB between the aqueous and micellar phases. These differences in the nature of the interactions between NB and the respective micelles are responsible for the observed differences in the voltammetric behavior of NB in these systems (vide infra). Counterions. The influence of the Stern layer of ionic micelles on the electrochemical behavior of incorporated substrates can include electrostatic interaction with the charged head groups of the micelle as well as electrostatic interaction with the associated counterions. Protons and hydroxide ions form an important subgroup of counterions and may directly influence the electrochemical properties of micelle-incorporated substrates. It has been reported that interaction between the cation radical of MPTH and the negatively charged head groups of SDS micelles causes the stability of the radical ion to be greatly enhanced. The relatively strong interaction between this oxidized form of MPTH and the Stern layer of the SDS micelle results
(27) The effective length of the poly(oxyethy1ene) chain has been reported to vary both with temperature and ionic strength (ref 25d).
101, 279.
NB,,
2NBm
(14)
where NB, and NB, represent NB in the aqueous and micellar phases, respectively. KD, the distribution coefficient, can be written as
KD = [NBlm/[NBlaq
(15)
As in the first interaction, this model predicts an invariant value of KD with altered NB concentration. This holds true as long as neither phase is saturated with NB. In terms of Fmand Fa,, KD can be written as
KD =
( F m [NBItvd / Vm (Faq[NBItVs)/(Vs - Vm)
(16)
(28) Almgren, M.; Grieser, F.; Thomas, J. K. J.Am. Chem. SOC.1979,
The Journal of phvsical Chemlshy, Vol. 86, No. 14, 1982 2630
Electrochemistry in Ordered Systems
TABLE IV: Effect of Counterions on Voltammetric Peak Potentials for the Reduction of Nitrobenzene in the Presence of Dodecvl Sulfate Micellesa counterion EPcf,,,b mV EPcfI,,bmV -779 f 3 -1155 i 3 Li Na -7662 2 -1157 i 5 K+ -755 6 -1304i 6 TMA -774i 2 -1136 i 2 -490 t 10 d NH, -325 t 5e d NH,+ a 60 mM DS-, 0.50 mM NB; pH of bulk solution adjusted t o 7.1 2 0.1 with corresponding hydroxide; peak potentials are means of five measurements; scan rate = 50 mV s - l ; HMDE radius = 0.046 cm, T = 25 i 0.2 "C unless otherwise noted. tstandard deviation. 40 "C. Second wave not observed (see text). e pH 5.10. +
+
+
+
to less extreme values.16 Assuming the same sequence of reactions to be operative in the presence of SDS micelles, it is reasonable to expect that eq 19 is the rate-determining step in the overall voltammetric process as reaction 20 is rapid.19 Alteration of the effective hydrogen ion concentration in the region of the Stern layer would affect the rate of that reaction and thus the observed peak potential of the second wave.16 The anion radical of NB has been reported to undergo rapid protonation in aqueous media. The protonated form undergoes rapid disproportionation to yield NB and the doubly protonated dianion (I) which then loses water and completes the overall reduction.30 NE NE-.
+
e-
+
2NEH.G
in significantly altered MPTH electrochemical behavior? Similarly, the reduction of phthalonitrile and fluorenone in cationic CTAB micelles has been reported to occur at less negative potentials than in isotropic media, indicating an interaction between the products of the reductions, i.e., the anion radicals, and the cationic head groups of the micelle.ls Finally, the enhanced concentration of protons in the Stern layer of anionic micellesmand hydroxide ions in the Stem layer of cationic micelles5have been reported to be responsible for micellar catalysis of various reactions. The nature of the interaction of NB and NB-. with the Stem layer of SDS micelles has been probed by variations in both the nature and concentration of counterions. The examination of the effects of micelle solubilization on the observed formal potentials of incorporated substrates must consider the effect of the micelle environment on the products of the electron transfers in terms of the nature of the species generated and the corresponding kinetic demeanor of those species.16 In the case of NB, both the anion radical (first wave) and the dianion (second wave) are susceptible to consumptive homogeneous chemical reactions. In nonaqueous media, the second voltammetric wave has been reported to be a combination of electron transfers and chemical reactions: lo first wave
NE t e-
(17 )
NE-.
second wave NE-.
+
(18)
NE'-
e-
-
0\,/OH
0
1
Y
H
\N/-
OH
I
For a kinetically controlled voltammetric process, an increase in the rate of the reaction consuming the product of the electron transfer (the dianion) should shift the observed peak potential for the anion radical/dianion couple (29) Kurz, J. L. J. Phys.
Chem. 1962,66, 2239.
6 NE-.
H+ NE
+
NBH.
(22)
+ HG\,PH
I
(241
G II
H\N/OH
I
Thus, both NB-• and the dianion are susceptible to kinetic processes which may give rise to the observed electrochemical and spectroscopic behavior of NB in these systems. Additionally, both the anion radical and the dianion of NB have been reported to undergo ion pairing with the cation of the supporting electrolyte in nonaqueous media, suggesting the likelihood of ion pairing between these species and micelle counterions in the Stem region of SDS micelles. These interactions between the reduced forms of NB and the micelle counterions must be considered in explaining the observed voltammetry of NB in this meChanging the nature of the counterion exerts a negligible effect upon the observed peak potential for the formation of NB-..The peak potential for the second wave was found to be much more dependent upon the nature of the counterion (Table IV). In the presence of LDS, KDS, and TMADS, the general voltammetric behavior of NB is similar to that observed in the presence of SDS in that two well-resolved reductive processes are present. In KDS, the second wave is shifted to more negative potentials, while the reduction of NB in ADS occurs via a single fourelectron process at a more positive potential than in either isotropic aqueous solution or cationic CTAB and CTAC micelles. In studies of ion pairing of NB-a in nonaqueous media, the extent of association has been reported t o be Li+ > Na+ > K+> The behavior of the voltammetric wave corresponding to the reduction of NB to NB-in these systems does not reflect a significant influence of counterion type. The pronounced positive shift in the observed reduction potential of NB, as well as coalescence of the separate reductive processes evident in LDS, SDS, (30) Asmus, K.-D.; Wigger, A.; Henglein, A. Eer. Bunsenges. Phys. Chem. 1966, 70, 862.
2640
The Journal of Physical Chemistry, Vol. 86, No. 14, 1982
TABLE V: Linear Sweep Voltammetric Peak Potentials for the Reduction of Nitrobenzene in 60 mM SDS with Added NaCl"
McIntire et ai.
KDS, and TMADS into a single overall reduction in the presence of ADS, clearly points to the kinetic effect of the acidic NH4+counterions in the protonation of the reduced forms of NB in this system.16 When the concentration of sodium ions was increased to 0.7 M by addition of NaCl to the SDS micelle solution, the observed peak potential for the reduction of NB to NB-. was seen to shift to more positive values by approximately 8 mV (Table V). In contrast to this small variation in potential with added sodium chloride, the peak potential for the second wave was shifted more than 100 mV in the same direction (Table V). These results are consistent either with ion pairing between the reduced forms of NB and the sodium counterions or with more rapid consumptive processes of NB-. and the dianion with elevation in sodium ion concentration. The minor alterations of the peak potential for the reduction of NB to NB-. both in the presence of increased concentrations of counterions (Na+) and in the presence of various counterions argues against ion pairing between NB-- and dodecyl sulfate counterions. Further reduction of NB-. to the dianion (second wave) gives rise to phenylhydroxylamine in all cases and indicates that protonation of the dianion occurs rapidly in dodecyl sulfate micelles. The availability of protons in the Stern layer of these micelles is impacted by the relative strength of counterion binding which parallels the Krafft points of the various dodecyl Those ions more tightly bound to the micelle will replace less strongly held ions at the micelle surface when introduced into the s ~ l u t i o n .The ~ Krafft points of KDS (38 0C),32SDS (23 oC),32aand LDS (12 0C)32are reflective of the relative strengths of binding of these ions to the micelle surface. Bunton et al.34have reported that SDS micelles do not discriminate between Na+ and H+ with respect to counterion binding. Potassium ion will therefore be more strongly bound to the micelle surface than H+,and the effective H+concentration in the Stern region of KDS micelles would be expected to be less than that in LDS and SDS micelles. The observed peak potentials for the reduction of NB-. to the dianion in these systems (Table IV) are consistent with the effects of the nature of the counterion on the availability of protons in the Stern layer of these systems.
The relative stability of NB-. in the presence of LDS, SDS, KDS, and TMADS compared to that in isotropic aqueous solution may result from the similarity of NB-. to an anionic surfactant monomer. The polar nitro functionality becomes even more polar as the anion radical while the phenyl moiety acts as a short hydrophobic tail. Rather than repulsion of NB-. from the anionic micelle arising from Coulombic considerations, the amphiphilicity of NB-. affords an interaction between the anion radical and the anionic micelle that may well be more robust than that between the neutral precursor (NB) and the micelle. Such coassembly of surfactant-like substrates with similarly charged micelles has been invoked to explain the catalytic properties of an asymmetric viologen molecule in a cationic micelle system toward photochemical hydrogen g e n e r a t i ~ n .Finally, ~ repulsion of NB-- from the SDS micelle would place the anion radical in the isotropic aqueous phase wherein that species has been shown to be much less stable than in the anionic micelles themselves (Figure 1A and Figure 2). This clearly points to retention of NB-. within the SDS micelles upon reduction of NB in this system. Summary. There are various types of interactions which may be operative between substrates and micelles including electrostatic interactions with micelle head groups or associated counterions, adsorption of substrates to micellar interfacial zones, and partitioning of substrates between the aqueous and micellar phases. Models of these types of interactions have been derived and applied to the interaction of NB with anionic SDS, cationic CTAC, and nonionic Brij-35 micelles. Results indicate that NB interacts with SDS in a surface process at the Stern layer of this system. Residence of the anion radical of nitrobenzene in this same region results in enhanced stability of this species with respect to consumptive homogeneous chemical reactions. This enhanced stability gives rise to the observed voltammetric behavior observed in this system. In the presence of cationic CTAC micelles, nitrobenzene partitions between the aqueous and micellar phases. Residence of NB in the Stern layer of these micelles has been indicated by UV spectrophotometry. Residence of NB-. in the Stern layer gives rise to local concentration enhancement of NB-. in this microscopic region of solution and causes its second-order decay process to proceed more rapidly.30 It is, therefore, not surprising that NB-. is not detectable (via ESR) in the cationic micellar system. At low concentrations of NB, the nonionic micelles examined in this work afford a strong surface interaction with nitrobenzene. At higher concentrations of substrate, the nature of the interaction changes from the observed surface interaction to an as yet undetermined one. In the absence of counterions and charged head groups, the stability of NB-- in this system as evidenced by ESR appears to be the result of altered kinetic parameters although the mechanism of kinetic alteration may differ significantly from that in SDS.
(31) Jain, M. K., private communcation, 1980. (32) McIntire, G. L.; Chiappardi, D. M.; Blount, H. N., unpublished data. (33) Corkill, J. M.; Goodman, J. F. Trans.Faraday SOC.1962,58,206. (34) Bunton, C. A.; Ohmenzetter, K.; Sepaveda, L. J . Phys. Chem. 1977, 81,2000. (35) Ludwig, P.; Layloff, T.; Adams, R. N. J.Am. Chem. SOC.1964, 86, 4568.
Acknowledgment. Partial support of this work by the National Science Foundation (CHE78-03312)is gratefully acknowledged, as are stimulating interactions with Lila M. Gierasch. G.L.M. acknowledges support from The Electrochemical Society, Inc., in the form of the Edward Weston Fellowship. The experimental expertise of E. E. Bancroft and T. H. Walter is also appreciated.
[NaCII, M Ob 0.047 0.107 0.314 0.700
Epcf l , , c mV -772 t -769 2 -766 t -764t -764t
1 2 4 3 3
EPcf ?,'c
mV
-1188 t 4 -1137 t 4 -1118 T 3 -1088 2 3 -1065 t 3
a NB concentration = 1 .OO mM; scan rate = 50 mV s - ' HMDE radius = 0.046 cm; peak potentials are means of
four determinations; T = 25 t 0.2 "C;pH 6.5 No NaCl added. tstandard deviation.
t
;
0.2.
