2106
J . Phys. Chem. 1987, 91, 2106-2109
For the liquid hexadecanol monolayer, we observe similar behavior. Inspection of Figures 2-4 shows that, as the acid concentration decreases, the water accommodation coefficient for the liquid hexadecanol monolayer decreases. As the acid concentration decreases from 95 to 73 wt % acid, the accommodation coefficient to 8.0 X Redecreases by a factor of ten from 8.0 X calling that a similar decrease in the acid concentration resulted in a factor of four decrease in the accommodation coefficient for the solid monolayer, we conclude that liquid monolayer cohesion is more sensitive to acidic subphase changes than is the solid monolayer cohesion. One possible explanation for this behavior is that, in the solid state, the ionization of the head groups only affects the head group interactions; however, in the liquid state the head group ionization also affects the tail-tail interactions. Another possibility is that, in the solid state, the head group interactions dominate the contributions to the activation energy, and ionization of the tails is only a second-order effect. However, in the liquid state, the tail-tail interactions contribute a greater share of the activation energy, and ionization effects are more evident. As one reduces the acid concentration further, the accommodation coefficient decreases at a much slower rate (see Figure 5). This is consistent with the results for the solid monolayer; however, the dramatic increase in the accommodation coefficient occurs at a lower acid concentration. This is because there are fewer hexadecanol molecules to ionize in the liquid
monolayer, so a lower acid concentration is required to achieve complete ionization of the hexadecanol molecules.
Conclusions A new technique has been developed to investigate the effect of subphase acidity on the cohesion of monolayers at the air and liquid interface. The technique, referred to as single particle electrodynamic balance, uses static electric fields to monitor mass transport from acidic solution droplets with adsorbed monolayers of hexadecanol. From the mass transport rates, water accommodation coefficients are determined for both the solid and liquid hexadecanol monolayer for subphase acid concentrations varying from 45 to 95 wt % phosphoric acid. It is demonstrated that, for the solid monolayer, the water accommodation coefficient increases and the monolayer cohesion decreases as the acid concentration increases from 45 to 95 wt % acid. A marked increase in the accommodation coefficient is seen at 73 wt % acid. Calculations at this acid concentration show that the droplets surface is populated with equal numbers of acid and hexadecanol molecules, supporting the hypothesis that complete ionization of the alcohol head groups results in the dramatic decrease in the monolayer cohesion. Similarly, cohesion in the liquid monolayer decreases as the subphase acid concentration increases; however, the dependence appears to be stronger for the liquid monolayer than for the solid monolayer.
Electron Spin Resonance and Electron Spin Echo Modulation Studies of N,N,N',N'-Tetramethylbenzidine Photoionization in Sodium Dodecyl Sulfate Micelles: Structural Effects of Alcohol Addltlon Pier0 Baglionit and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77004 (Received: October 16, 1986; In Final Form: December 12, 1986)
Electron spin echo modulation (ESEM) and electron spin resonance (ESR) spectra of the photogenerated N,N,N',N'tetramethylbenzidine cation radical (TMB') in frozen micellar solutions of sodium dodecyl suflate containing 2-propanol, 1-propanol, 1-pentanol, 1-octanol, 2-propanol-d7,and 1-octan0l-d~~ in H 2 0 and D 2 0 have been studied as a function of the alcohol concentration from 0 to 200 mM. Modulation effects due to the TMB' interactions with deuterium in D 2 0 and in 2-propanol-d, or 1- ~ c t a n o l - dgive , ~ direct evidence that 2-propanol is mainly located at the micellar interface whereas the alkyl chain of 1-octanol is located deeper into the micelle. Alcohol addition leads to an increase of water penetration into the micellar interface in the order 1-propanol < 2-propanol z 1-pentanol C I-octanol. The initial efficiency of charge separation upon photoionization of TMB as a function of alcohol concentration correlates with the degree of water penetration into the micelle, but the maximum photoionization efficiency seems more related to the degree of water organization at the micellar surface due to specific perturbing effects on the micellar structure dependent on the alcohol structure.
Introduction During the past decade aqueous micellar solutions have been extensively used as media for the study of the mechanism of photochemical charge separation reactions,I4 principally because the "incorporation" of donor and/or acceptor solutes into micelles can drastically increase the solvated electron yield or ion pair lifetime by enhancing electron escape from the micelle and by inhibiting the charge neutralization back-reaction due to the electrostatic barrier at the micelle-water interface. Kevan and co-workers%" have shown that the photoionization efficiency of N,N,N',N'-tetramethylbenzidine (TMB) to produce TMB' in frozen micellar solution depends on various factors including the micelle size and charge.%' the nature of the counterion: and ionic salt a d d i t i ~ n . ~Also, J ~ the addition of I-butanol to sodium dodecyl 'Permanent address: Department of Chemistry, University of Florence, Florence, Italy.
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sulfate (SDS) micellar solution containing TMB produces an increase in the TMB' cation yield at low 1-butanol concentration followed by a decrease at higher 1-butanol concentration." This result was interpreted in terms of the effect of 1-butanol on the (1) Gratzel, M.; Thomas, J. K. J. Phys. Chem. 1974, 78,2248. (2)Fender, J. H.Acc. Chem. Res. 1980,13, 7. (3) Calvin, M. Photochem. Photobiol. 1983, 37, 349. (4)Narayana, P. A.; Li, A. S. W.; Kevan, L. J. Am. Chem. SOC.1981,103, 3603. (5)Narayana, P. A.; Li, A. S . W.; Kevan, L. J. Am. Chem. SOC.1982,104, 6502. (6)Arce, R.;Kevan, L. J. Chem. SOC.Faraday Trans. 1 1985,81,1025. (7) Arce, R.;Kevan, L. J. Chem. SOC.,Faraday Trans. 1 1985.81, 1669. (8) Szajdzinska-Pietek, E.;Maldonado, R.; Kevan, L.; Jones, R. R. M. J. Am. Chem. SOC.1984, 106,4615. (9)Maldonado, R.;Kevan, L.; Szajdzinska-Pietek, E.; Jones, R. R. M. J. Chem. Phys. 1984,81, 3958. (10) Hiromitsu, I.; Kevan, L. J. Phys. Chem. 1986, 90, 3088. (11) Szajdzinska-Pietek, E.;Maldonado, R.; Kevan, L.; Jones, R. R. M. J . Am. Chem. SOC.1985. 107, 6467.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 8, 1987 2107
ESEM Studies of T M B in SDS Micelles
r-,
I-PROPANOL
0
5
0.10
0.20
2-PROPANOL
0.10
5
P
L
I
I
50
0
I
100
I
200 CA(fllM)
I50
Figure 2. Dependence of normalized deuterium modulation depths as a function of 1-propanol and 2-propanol concentration in frozen TMB+/ SDS/DzO micellar solutions.
'1
0
,
,
0.5
1.0
:I 0.40
H20
1.5
2.0
T(&
z
Figure 1. Two-pulse electron spin echo decay envelopes at 4.2 K for
T
f
TMB+ in frozen SDS micellar solutions containing 75 mM 1 - c ~ t a n o l - d ~ ~ and 2-propanol-d7 prepared in both H z O and DzO. The spectral base lines have been offset vertically to avoid overlap.
micelle surface charge and on the enhancement of TMB-water interactions due to intercalation of 1-butanol between the micellar headgroups." Similar results have been obtained on the photoionization of perylene by Bernas and ~ o - w o r k e r s . ~ These ~-~~ findings show that controlled modification of the micellar structure can alter the photoionization yield. These studies are relevant not only for the optimization of net charge separation as a means to store light energy but also for controlled modification of micellar structure. A change in the photoionization efficiency or other behavior of a probe solute within the micelle reflects environmental changes of the local micellar structure. In the present study we use the N,N,N',N'-tetramethylbenzidine cation in SDS micellar solution with the double aim of clarifying the solubilities process, in terms of solubilization site, of a series of aliphatic alcohols in SDS micelles and of determining how the perturbation of the micellar structure due to alcohol addition affects the photoionization yield. The results obtained strengthen those reported in a previous study15 with a 5-doxylstearic acid probe where a strong increase in water penetration into the micellar interface was found that depended on the alkyl chain length and branching of the aliphatic alcohol. Similarly, the photoefficiency yield for TMB+ as a function of alcohol chain length and concentration indicates that the photoionization yield is correlated with the organization of alcohol solutes within the micellar structure and with the degree of water interaction of the photoproduced cation.
Experimental Section SDS and TMB were purchased from Eastman Kodak Co. SDS was recrystallized three times from ethanol, washed with diethyl ether, and dried under moderate vacuum. 1-Octanol, 1-pentanol, 1-propanol, and 2-propanol (HPLC products, purity >99.9%) were obtained from Aldrich Chemical and used without further purification. 2-Propanol-d7-OH and 1-octanol-d17-OH were obtained from Merck Isotopes and used as received. Stock solutions of 0.1 M SDS were prepared in triply distilled and deoxygenated water and in deuterated water (Aldrich). TMB was dissolved in chloroform. The chloroform was then gently evaporated by passing a stream of dry nitrogen over the solvent surface, leaving a film of TMB on the wall of the flask. This film was solubilized in 0.1 (12) Bernas, A.; Grand, D.; Hautecloque, S.;Chambaudet, A. J . Phys. Chem. 1981,85, 3684. (13) Grand, D.; Hautecloque, S.; Bernas, A,; Petit, A. J . Phys. Chem. 1983,87, 5236. (14) Hautecloque, S . ; Grand, D.; Bernas, A. J . Phys. Chem. 1985, 89, 2705. (15) Baglioni, P.; Kevan, L., submitted for publication in J. Phys. Chem.
I
I
4
I-OCTANOL
$
I-PENTANOL
J 0.10 a
z
0
z
I
o
I
I
50
100
I
150
200 CA(mM)
Figure 3. Dependence of normalized deuterium modulation depths as a function of 1-octanol and 1-pentanol concentration in frozen TMB+/ SDS/DzO micellar solutions. I
TMB/SDS/2-PROPANOL-d7
n
0
--
L
5'0
A
I
I
100
150
I
200 CA(mM)
Figure 4. Dependence of normalized deuterium modulation depths as a function of 2-propanol-d, concentration in TMB+/SDS/DzO and TMB+/SDS/HzO frozen micellar solutions.
M SDS solution by sonicating for 30 min and by stirring the mixture for several hours at 50 O C in a nitrogen atmosphere. The concentrations of the sample studied were 0.1 M SDS, 1 X lo4 M TMB in H 2 0 , 8 X M TMB in D20, and 0-200 mM alcohol. The samples were sealed in 2-mm Suprasil quartz tubes and frozen rapidly by plunging into liquid nitrogen. Photoirradiation was carried out at 77 K using a Cermax 150-W xenon lamp filtered with a 10-cm filter of water and a Corning No. 7-51 filter. This allows radiation centered at 370 nm to pass with 80% transmittance. The integrated light flux at the samples was 9 X lo2 W m-2. Each sample was irradiated for 12 min. ESR spectra were recorded on a Varian E-4 spectrometer. Two-pulse electron spin echo signals were recorded at 4.2 K o n a home-built spectrometer16 with 50-11s exciting pulses. Results Figure 1 shows two-pulse ESEM spectra obtained for TMB' in SDS micelles at 4.2 K for deuteriated 2-propanol and 1-octanol in D 2 0 and H 2 0 at an alcohol concentration of 0.075 M. The (16) Ichikawa, T.; Kevan, L.; Narayana, P. A. J. Phys. Chem. 1979.83,
3378.
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The Journal of Physical Chemistry, Vol. 91, No. 8, 1987
n
8 0.20 TMB/SDS/ I-OCTANOL-dn 0
100
50
150
200 C,&mM)
Figure 5. Dependence of normalized deuterium modulation depths as a function of 1-octanol-d,, concentration in TMB'/SDS/D20 and
TMB'/SDS/H20 frozen micellar solutions.
1.4 -A I-PROPANOL I-PENTANOL
+m L
O
p
P
E 2-PROPANOL I-OCTANOL
0
50
100
150
200 Cn(mM)
Figure 6. TMB+yields measured by ESR as a function of the indicated alcohol concentrations in frozen SDS micellar solutions.
echo decay curves clearly show detectable modulation with periods of 0.5 and 0.08 ps corresponding respectively to electron-deuteron and electron-proton interactions. Figures 2 and 3 show the variation of the normalized deuterium modulation depths as a function of the alcohol concentration and of the alkyl chain length of the alcohol. The normalized deuterium modulation depths were 4 and 5 show the computed as described p r e v i o ~ s l y . Figures ~ variation of the normalized deuterium modulation depth of TMB' in SDS micelles in HzO and D 2 0 as a function of 2-propanol-d7 and 1-0ctano1-d17 concentration. Figure 6 reports the relative yields of TMB cation as a function of the alcohol concentration and of the alkyl chain length of the alcohol. The concentration of TMB' was measured from the ESR signal intensity.
Discussion T M B undergoes monophotonic photoionization to readily generate TMB' in anionic micellar solutions in which it is stable for tens of minute^.^ In homogeneous solutions TMB+ has a lifetime of only a few microseconds. It is possible to generate TMB+ in rapidly frozen solutions at 77 K where it is stable for several weeks. Evidence has been presented for the retention of the micellar structure in rapidly frozen s o l ~ t i o n s . It~ ~is ~possible ~ to generate TMB+ in frozen solutions, thaw the solution, and still observe TMB+ by electron spin resonance, thus indicating that the micellar structure was retained upon f r e e ~ i n g .Hashimoto ~ and Thomas17 have recently used luminescence quenching to measure micellar aggregation numbers and have found similar aggregation numbers for SDS micelles in liquid and in 77 K frozen ethylene glycol-water solutions. This also supports the retention of micellar structure in rapidly frozen solutions. The TMB' cation radical has been found by ESEM to interact weakly with water deuterons, suggesting an asymmetric location for it close to the micellar interface below the micellar h e a d g r ~ u p s .The ~ study of electron transfer from T M B to a series of x-doxylstearic acids (x-DSA) showed that the distance from TMB' to x-DSA increases from x = 16 to 5, indicating that the TMB' probe is located deeper inside the micelle than is the doxy1 group on the 5-DSA probe. (17) Hashimoto, S.; Thomas, J. K. J . Phys. Chem. 1984, 88, 4044
Baglioni and Kevan Figures 2 and 3 show the variation of normalized deuterium modulation depth as a function of alcohol concentration. It is seen that the addition of alcohol, by analogy to that found with the 5-doxylstearic acid probe, causes an increase of the TMB' interactions with water deuterium, indicating that the micellar surface of SDS becomes more hydrated in the presence of alcohol. The plateau values of the normalized modulation depth increased in the order 1-propanol < 2-propanol < 1-pentanol 2 1-octanol. If we express the increase in the modulation depth as a percentage variation that reflects the increase of the perturbation by the alcohol of the micellar structure, we find a weaker increase for TMB' relative to a 5-DSA probe15 for 2-propanol, 1-propanol, and 1-pentanol and the same increase for 1-octanol. This is consistent with the TMB+ probe being located deeper into the micelle than is the 5-DSA probe. When TMB' is located deeper into the micelle, it more strongly monitors perturbations by added alcohols that affect deeper regions in the micelle than the micellar interface. Thus, 1-octanol penetrates more deeply into the micelle and perturbs not only the micelle interface but also deeper into the micelle as indicated by the results indicating increased hydration. In other words, the longer chain alcohols disturb or disorder a thicker region of the micelle measured from the micellar surface. Further proof of different locations for different chain length alcohols is obtained from analysis of the trend of the deuterium modulation in H20and D 2 0 for 2-propanol-d7 (see Figure 4) and l - 0 ~ t a n o l - d(see ~ ~ Figure 5). The absence of deuterium modulation for 2-propanol-d7 in H 2 0 for an alcohol/surfactant mole ratio (0.75 shows that TMB' does not interact with this alochol, indicating that the alcohol is mainly located at the micelle interface. In contrast, an increase in modulation is found in DzO, giving direct indication that the addition of 2-propanol leads to an increase of water penetration that is not limited to the micellar interface. For 1-octanol-d,, direct interaction between TMB' and 1-octanol is indicated for low alcohol concentration demonstrating that 1-octanol penetrates below the micellar interface. The above results are relevant to the analysis of the TMB' yield vs. alcohol concentration shown in Figure 6. The relative TMB' yield as a function of alcohol concentration shows a maximum around 25 mM for 1-pentanol and 1-0ctanol and around 50 mM for 1-propanol and 2-propanol followed by a decrease at higher alcohol concentrations. Szajdzinska-Pietek et al.'l suggested that the increase in photoionization yield was primarily due to enhancement of the TMB-water interactions. Analysis of Figure 6 suggests that the initial increase in photoionization yield varies in the order 1-propanol N 2-propanol < 1-pentanol < 1-octanol, which is the same trend found for the increase in deuterium modulation. However, the maximum photoionization yields as a function of alcohol concentration vary in the reverse order, 1-octanol < 1-pentanol < 2-propanol N 1-propanol. These two differing trends indicate the presence of two opposing factors that affect the photoionization efficiency. One factor is probably the disorder of the surfactant molecules at the micelle surface which has been suggested to correlate with enhanced TMB-water interactions. The other factor is likely associated with the disorder of the water at the micellar surface. This latter factor suggests that a specific geometric conformation between TMB and water molecules at the micellar surface also affects the TMB photoionization probability.I7 The present data support the hypothesis that water molecules in the micellar interface act as a collective electron acceptor for TMB photoionization5 and suggest that hydrophobic alcohols can perturb the micellar interface structure to enhance solute photoionization. Alcohols at low concentration assist water penetration into the micellar interface, but at higher concentrations alcohols seem to also affect the water organization at the micellar surface due to specific perturbing effects on the micellar structure dependent on the alcohol structure. Conclusions The results obtained from the analysis of electron spin echo and electron spin resonance spectra of photogenerated TMB' in
J . Phys. Chem. 1987, 91, 2109-2117
SDS micellar solution containing 1-propanol, 2-propanol, 1pentanol, or 1-octanol show that the alcohol increases water penetration into the micellar interface structure. The degree of water penetration depends on the alkyl chain of the alcohol and 2-pentanol < varies in the order 1-propanol < 2-propanol 1-octanol. Analysis of the trend of deuterium modulation for deuteriated alcohols (2-propanol-d7 and l-octanol-d17in D20 and in H 2 0 ) gives direct evidence that 2-propanol, for an alcohol/ surfactant mole ratio less than 0.75, is located a t the micellar interface whereas 1-octanol is located deeper into the micelle. The relative TMB+ yield enhancement by alcohol increases to a
2109
maximum and then decreases. The initial yield enhancement correlates with the degree of water penetration into the micelle, but the maximum yield enhancement is suggested to be related to the degree of water organization at the micellar surface due to specific perturbing effects on the micellar structure dependent upon the alcohol structure.
Acknowledgment. This research was supported by the Department of Energy, Office of Basic Energy Sciences. P.B. thanks the Italian Ministry of Public Instruction for partial financial support.
Cobalt( I I) Tetrasuifophthalocyanine on Titanium Dioxide: A New Efficient Electron Relay for the Photocatalytic Formation and Depletion of Hydrogen Peroxide in Aqueous Suspensions Andrew P. Hong, Detlef W. Bahnemann; and Michael R. Hoffmann* W. M.Keck Laboratories, California Institute of Technology, Pasadena, California 91 125 (Received: November 6, 1986)
A novel synthesis for the covalent linkage of cobalt(I1) tetrasulfophthalocyanine (CoI'TSP) to the surface of titanium dioxide (Ti02) particles (d d 0.5 pm) is described. Upon irradiation with light that exceeds the bandgap energy (E,) of Ti02 (Le., X 5 380 nm), Col*TSPis reduced to Co'TSP under anoxic conditions both as a dry powder and in aqueous suspension. The photochemical reduction is shown to be fully reversible in the presence of molecular oxygen (02).Hydrogen peroxide (H202) is produced upon irradiation of an aerated aqueous suspension of the 'hybrid" catalyst, Ti02-Co1'TSP. Formation kinetics are followed in situ with a polarographic detector (detection limit %lo-' M H202);quantum yields, I # J ~ between ~ ~ ~ , 0.16 and 0.49 have been determined. The reactive photocatalytic center appears to be generated by the attachment of molecular oxygen in the open apical coordination site of the hybrid CoI'TSP complex. Formation of Co1''TSP-02'- is enhanced by the binding of TiO- surface groups in the opposite apical position. Hydrogen peroxide is produced in a two-step electron transfer from the conduction band via the Co(II1) center. The involvement of free radical intermediates in this mechanism appears to be highly unlikely. On the basis of its observed chemical and photochemical stability and the high quantum yields for O2reduction, the newly developed hybrid material is proposed to be applicable as a potent and stable oxidation catalyst.
Introduction
Electron-transfer reactions following the light-induced charge separation in submicron semiconductor particles have been studied in great detail during the past decade. The original interest in these systems stems from reports that they facilitate the simultaneous formation of dihydrogen and dioxygen from water.' Since the overall reaction of two conduction band electrons, e-CB,,and four valence band holes, h+VB,is required to achieve this direct storage of solar energy in the form of chemical energy (reactions 1 and 2): laboratory studies have shown that appropriate electron 2 H 2 0 + 2e-cB H z ( t ) + 20H,, (1)
-
relays are necessary to compete with the e-/h+ recombination and thus achieve reasonable but still rather small quantum While suitable metal deposits (e.g., Pt) due to their ability to store electrons and/or adsorbed hydrogen atoms* are good catalysts for reaction 1,4-7 metallic oxides (e.g., Ru02, Rh203)seem to catalyze the four-hole water ~ x i d a t i o n . ~ - ~ Ph~todeposition~*~ usually yields highly dispersed metal clusters on the semiconductor's surface. An ohmic contact between these metal islands and various n-type semiconductors has been proposed9 to account for a potential gradient which drives the e-CB 'Permanent address: Hahn-Meitner Institut GmbH, Bcreich Strahlenchemic, Glienicker Strasse 100, DlOOO Berlin 39, Federal Republic of Germany. *To whom correspondence should be addressed.
to the metal and repells the h+vBfrom this part of the interface,I0 thus resulting in the necessary e-/h+ separation. The major drawback of these catalytic systems is the efficient thermal catalysis of undesirable back-reactions by the Pt deposit.1° In this paper we establish the effectiveness of cobalt(I1) tetrasulfophthalocyanine (Co"TSP) as an electron relay, which is chemically bound to the surface of titanium dioxide (TiOz).
(1) Recent reviews on this subject include: (a) Bockris, J. O'M.; Dandapani, B.; Cooke, D.; Ghoroghchian, J. Int. J. Hydrogen Energy 1985, 10, 179. (b) Fendler, J. H. J . Phys. Chem. 1985, 89, 2730. (c) Somorjai, G. A.; Hendewerk, M.; Turner, J. E. Catal. Rev.-Sci. Eng. 1984, 26, 683. (2) Vanden Kerchove, F.; Praet, A.; Gomes, W. P. J . Electrochem. SOC. 1985, 132, 2357. (3) Sobczynski, A.; White, J. M. J. Mol. Catal. 1985, 29, 379. (4) Magliozzo, R. S.; Krasna, A. I. Photochem. Photobiol. 1983, 38, 15. (5) (a) Blondeel, G.; Harriman, A.; Williams, D. Sol. Energy Mater. 1983, 9, 217. (b) Blondeel, G.; Harriman, A.; Porter, G.; Urwin, D.; Kiwi, J. J . Phys. Chem. 1983,87, 2629. (6) (a) Lehn, J.-M.; Sauvage, J.-P.; Ziessel, R. Noun J . Chim. 1979, 3, 423. (b) Lehn, J.-M.; Sauvage, J.-P.; Ziessel, R. Nouu.J. Chim. 1 9 8 0 , 4 3 5 5 . (c) Lehn, J.-M. In Proceedings of the 3rd International Conference on Photochemical Cowersion and Storage of Solar Energy, 1980; Connolly, J. S., Ed.; Academic: New York, 1981; p 161. (7) (a) Duonghong, D.; Borgarello, E.; Griitzel, M. J. Am. Chem. SOC. 1981, 103, 4685. (b) Yesodharan, E.; Griltzel, M. Helu. Chim. Acta 1983, 66, 2145. (8) Henglein, A,; Lindig, B.; Westerhausen, J. J . Phys. Chem. 1981,85, 1627. (9) Aspnes, D. E.; Heller, A. J. Phys. Chem. 1983, 87, 4919. (10) Gerischer, H. J . Phys. Chem. 1984, 88, 6096.
0022-3654/87/2091-2109$01.50/0 0 1987 American Chemical Society
