4726
J . Phys. Chem. 1988, 92,4726-4130
are too low in frequency to couple significantly. Consequently, the type of correlation shown in Figure 5 should hold for ethylene on all surfaces where the DCD coordination model is applicable. The accuracy of the correlation in Figure 5 for surface-bound ethylene depends on (1) how well the symmetric CD2 scissor frequency in CzD4Brzapproximates the surface frequency for sp3 carbons in surface-bound ethylene and (2) how well the coupling between the scissors and stretch modes was chosen. Extracting the degree of rehybridization in adsorbed ethylene from surface vibrational spectra has previously been discussed by Stuve and Madix.‘O They considered couplings between the C - C stretch and the CH2(CD2)scissor vibrations and proposed a parameter called T O as a measure of rehybridization. This parameter combines the percentage shift to lower frequency upon adsorption of both the gas-phase C-C stretch and CH2(CD2)scissor vibrations; the parameter is normalized to 0 for gas-phase ethylene and 1 for 1,2-dibromoethane. One problem with this parameter is that the deuteriated and hydrogenated versions do not always predict the same degrees of rehybridizations. These discrepancies probably occur because (1) in the hydrogenated case, couplings with other CHz bending modes which may have a substantial effect as shown in Figure 6 were not considered and (2) it is not always clear which peaks to use in calculating these parameters. We feel that the deuteriated T U parameter is the more reliable of the two parameters and is comparable with our use here of the deuteriated “C-C stretching” frequency. The advantage of using just the “C-C stretching” frequency along with the correlation in Figure 5 is that choice of which spectral peaks to use is trivial, since this mode should be the only one with a vibrational frequency between 1100 and 1550 cm-’ (Figure 7).
More than one peak in the 1100-1550-cm-’ region of the deuteriated vibrational spectra suggests there is more than one bonding geometry for molecularly adsorbed ethylene, unless one or more of the peaks are overtone or combination bands. Indeed, C2D4adsorbed on most transition-metal surfaces gives only one vibrational peak between 1100 and 1550 cm-’ as can be seen from Pd( 1 and Rh( 100) are the three exTable 11. Cu( ceptions. For Cu(lOO), the 1347-cm-’ peak is almost certainly an overtone of the very intense 672-cm-I peak. On Pd( 1lo), however, the two peaks at 1246 and 1371 cm-’ have been interpreted as C-C stretches for two different types of molecular ethylene on the surface analogous to our interpretation of the Rh( 100) results here. We should add that while multiple peaks in the 1 100-1550-cm-’ region are a good indicator for multiple ethylene bonding geometries on the surface, there may be cases where the C-C stretching vibration may be of sufficiently weak intensity as to be undetected.
Acknowledgment. The experimental work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy, under Contract No. DEAC03-76F00098. We thank Profs. J. M. White, B. E. Koel, and E. M. Stuve for insightful comments on this work, and Dr. J. Stohr for a preprint of the Cu( 100) NEXAFS results prior to publication. B.E.B. gratefully acknowledges a National Science Foundation Fellowship, C.M.M. gratefully acknowledges a scholarship from the American Vacuum Society, and A.J.S. gratefully acknowledges support from the Natural Sciences and Engineering Research Council of Canada. C.T.K. gratefully acknowledges a B.P.-America Fellowship.
Electron Spin Resonance and Electron Spin Echo Modulation of n-Doxylstearic Acid and N,N,N/,N/-Tetramethylbenzldine Photoionization in Sodium versus Lithium Dodecyl Sulfate Mlcellar Solutions: Effect of 15-Crown-5 and 18-Crown-6 Ether Addition Piero Baglioni? Elisabeth Rivara-Minten, and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77004 (Received: December 15, 1987)
Electron spin echo modulation and electron spin resonance spectra of photogenerated N,N,”,N’-tetramethylbenzidine (TMB) cation radical and n-doxylstearic acids (n-DSA) in frozen micellar solutions of sodium and lithium dodecyl sulfate containing 15-crown-5and 18-crown-6 ethers in DzO have been studied as a function of crown ether concentration. Modulation effects due to n-DSA with water deuteriums give direct evidence that both crown ethers are mainly located at the micellar interface and that this causes a decrease of the hydration of the micellar interface. Crown ether complexation constants for sodium and lithium micellar counterions are reported and show that 18-crown-6> 15-crown-5for sodium counterion and 15-crown-5 > 18-crown-6 for lithium counterion. Modulation effects from TMB+ interaction with water deuteriums indicate that the TMB molecule moves toward the micelle interfacial region when sodium or lithium cations are complexed by crown ethers. The TMB+ yield upon TMB photoionization increases by about 10%with crown ether addition for SDS and LDS micellar systems, but it is greater if the absolute values for the LDS system are compared to those for the SDS micellar system. This behavior correlates with the strength of TMB+-water interactions and suggests that the main factor in the photoionization efficiency is the photocation-water interaction.
Introduction It is well-known that the structure and the properties of &&s are dependent on an equilibrium among van der Waals and electrostatic forces.’ The identity of thecounterion can have a large influence on the micellar properties of the ionic surfactants.2 Recently it has been suggested that the identity of the counterion can affect the “structure” of the micellar i n t e r f a ~ e . ~ It , ~ was demonstrated that the substitution of sodium by lithium counterion in dodecyl sulfate micelles opens up the headgroup structure with Present address: Department of Chemistry, University of Florence, Florence, Italy.
0022-3654/88/2092-4726$01.50/0
significant water penetration into the Stern r e g i ~ n . Small-angle ~ neutron Scattering (SANS) experiments have indicated that there (1) Israelachvili, J. N. Intermolecular and Surface Forces; Academic: New York, 1985. Mitchell, D. J.; Ninham, B. W. J . Chem. SOC.,Faraday Trans. 2 1981, 77, 601. Israelachvili, J. N.; Mitchell, D.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1916,72, 1525. Tanford, C. The Hydrophobic Effecf; Wiley: New York, 1980. (2) Berr, S. S.; Coleman, M. J.; Jones, R. R.; Johnson, J. S., Jr. J . Phys. Chem. 1986, 90, 6492. (3) Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M.; Coleman, M. J. J. Am. Chem. SOC.1985, 107,184. (4) Jones, R. R. M.; Maldonado, R.; Szajdzinska-Pietek, E.; Kevan, L. J . Phys. Chem. 1986, 90, 1126.
0 1988 American Chemical Society
n-DSA and T M B Photoionization in Micellar Solutions
The Journal of Physical Chemistry, Vol. 92, No. 16, 1988 4727 nDSA/SDS
LDS/5DSA/TMB
IA
SDS/IS.crown-5
SDS/I8-crown-B 5
I
I
7
IO
1
1
12 DOXYL POSITION, n
16
Figure 2. Variation of the normalized deuterium modulation depth as a function of the n-doxy1 nitroxide group position containing 15-crown-5 (100 mM) or 18-crown-6 (100 mM) ether in frozen n-DSA/SDS/D20
micellar solutions. r
0
0.5
1.0 1.5 r(pd
nDSA/LDS
I
2.0
Figure 1. Two-pulse electron spin echo decay envelopes of 5-DSA and TMB+ probes in LDS/D20 frozen micellar solutions with 15-crown-5
ether (50 mM) and 18-crown-6ether (50 mM). The spectral base lines have been offset vertically to avoid overlap. is greater roughness for the sodium dodecyl sulfate (SDS) micelle surface compared to the lithium dodecyl sulfate (SDS) micelle surface due to the protrusion of methylene groups into the water phase at the i n t e r f a ~ e . ~ . ~ In a previous study we investigated the effect of sodium ion complexation at the SDS micellar interface by 18-crown-6 and 15-crown-5 ethers.' We found that crown ether complexation of sodium cation does increase the photoionization yield of N , N,N',N'-tetramethylbenzidine cation (TMB+) and that this increase is correlated to increased TMB+-water interactions. However, associated experiments with a 5-doxylstearic acid (5DSA) probe indicated that the sodium ion complexation by crown ethers decreased the water penetration into the micellar interface. It is currently postulated that the observed increase in photoionization efficiency is due to movement of the neutral TMB molecule toward the micellar surface, due to decreased local ionic strength at the micellar surface by cation complexation and/or due to direct interaction of TMB with crown ether molecules by analogy to results obtained for pyrene with crown ether surfactants.* In this study we report comparative experiments on sodium dodecyl sulfate versus lithium dodecyl sulfate micelles with ndoxylstearic acids and N,N,N',N'-tetramethylbenzidine since the surface structure of these two micellar systems is different and the amount of water penetration a t the micelle surface is much greater with the lithium counterion.
Experimental Section SDS, LDS and, TMB were purchased from Eastman Kodak Co. SDS and LDS were recrystallized 3 times from ethanol, washed with diethyl ether, and dried under moderate vacuum. n-Doxylstearic acid (n-DSA) spin probes ( n = 5, 7, 10, 12, 16) were purchased from Molecular Probes, Inc. and used as received. The crown ethers, 15-crown-5and 18-crown-6 (purity >99%) were obtained from Aldrich Chemical and used without further purification. (5) Hayter, J. B.; Penfold, J. J . Chem. Soc., Faraday Trans. 1 1981, 77, 1851. Hayter, J. B. In Physics of Amphiphiles: Micelles, Vesicles and
Microemulsions; DeGiorgio, V., Corti, M., Eds.; North-Holland: Amsterdam, 1985; pp 59-93. (6) Chen, S.H. In Physics of Amphiphiles: Micelles, Vesicles and Microemulsions; DeGiorgio, V., Corti, M., Eds.; North-Holland: Amsterdam, 1985; pp 281-302. Bendedouch, D.; Chen, S. H.; Koehler, W. C. J . Phys. Chem. 1983,87, 153, 2621. Bendedouch, D.; Chen, S. H. J . Phys. Chem. 1983, 87, 1473, 1653. (7) Baglioni, P.; Kevan, L. J . Chem. Soc., Faraday Trans. 1 1988,84,467. (8) Turro, N. J.; Kuo, P. J . Phys. Chem. 1987, 91, 3321.
"
I
1
I
I
I
1
5
7
IO
12
16
DOXYL POSITION, n
Figure 3. Variation of the normalized deuterium modulation depth as a function of the n-doxy1 nitroxide group position containing 15-crown-5 (100 mM) or 18-crown-6 (100 mM) ether in frozen n-DSA/LDS/D20
micellar solutions. Stock solutions of 0.1 M LDS and SDS were prepared in triply distilled and deoxygenated water and in deuteriated water (Aldrich). TMB was dissolved in chloroform. TMB/LDS and TMB/SDS solutions were prepared by adding the surfactant solution to a film of TMB generated by evaporating the chloroform and then by sonicating for 30 min and heating the mixture for several hours at 50 OC in a nitrogen atmosphere. Stock solutions of n-DSA and 18-crown-6 were prepared in chloroform. The samples were prepared in a nitrogen atmosphere by adding the surfactant solution to a film of the spin probe and the 18-crown-6 or the TMB/surfactant solution to a film of 18-crown-6 generated by evaporating the chloroform. The 15-crown-5was directly added to n-DSA/surfactant or TMB/surfactant solutions. The final concentrations were 0.1 M surfactant, 0.1 mM n-DSA or TMB, and from 0 to 200 mM crown ether. The samples were sealed in 3-mm-0.d. by 2-mm-i.d. Suprasil quartz tubes and frozen rapidly by plunging into liquid nitrogen. Photoirradiation of TMB was carried out at 77 K by using a Cermax 150-W xenon lamp, filtered with a 10-cm water filter and a Corning no. 7-51 filter to give a band centered at 370 nm with 80% transmittance. Electron spin resonance (ESR) spectra were recorded on a Bruker ESP 300 ESR spectrometer. Two-pulse electron spin echo signals were recorded at 4.2 K on a home-built spectrometer with 40-ns exciting pulses. Results Figure 1 shows two-pulse electron spin echo modulation (ESEM) spectra obtained for 5-DSA and TMB' probes in LDS micelles in DzO at 4.2 K with 15-crown-5 and 18-crown-6 ethers. Modulations with periods of 0.5 and 0.08 1.1s corresponding respectively to electrondeuterium and electron-protium interactions are observed on the echo decay curves. Figures 2 and 3 show the variation of the normalized deuterium modulation depth3 as a function of the n-doxy1 nitroxide group position for SDS and LDS frozen micellar solutions containing 100 mM 15-crown-5 or 100 mM 18-crown-6 ethers in DzO.Figure 4 illustrates the variation of the normalized deuterium modulation depth as a function of the crown ether concentration in 5-DSA/LDS/D20 frozen SOlution. Figure 5 reports the variation of the normalized modulation depth of TMB+/LDS/DzO frozen solutions as a function of crown ether concentration, and Figure 6 shows the TMB' yield obtained by double integration of the ESR spectra as a function of crown ether concentration.
4728
The Journal of Physical Chemistry, Vol. 92, No. 16, 1988 LDS/5DSA L
.60 18-CROWN-6
.20 L
A
I
15-CROWN-5
0
50 100 150 200 [CROWN ETHER], mM
Figure 4. Variation of the normalized deuterium modulation depth of 5-DSA in LDS/D20 frozen micellar solutions as a function of the crown ether concentration.
Discussion The results with n-doxylstearic acid probes and with TMB are discussed separately. 1. n-Doxylstearic Acid Spin Probes. Figures 2 and 3 show the variation of the normalized deuterium modulation depth for SDS and LDS frozen micellar solutions in the presence of 0.1 M 15-crown-5 and 18-crown-6 ethers with doxylstearic acid spin probes with the nitroxide in the doxyl group located at different positions relative to the stearic acid headgroup. The deuterium modulation depth depends on the number of interacting deuteriums and on their distance from the spin probe. The doxylstearic acid spin probes are comicellized with the surfactant molecules with the acidic headgroup at micellar s ~ r f a c e . ~A~decrease ~ ~ ~ ~in' ~the deuterium modulation depth reflects a decrease in the number of water molecules present at the micellar surface and/or a doxyl group further from the micellar surface. The data reported in Figures 2 and 3 show, for SDS/D20 and L D S / D 2 0 micellar solutions without crown ethers added, that the interactions with water are relatively strong for the 5-DSA probe, decrease for 7-DSA, reach a minimum for 10-DSA/12DSA, and increase again for 16-DSA. The main differences between the SDS and LDS behavior are represented by the absolute values of the normalized modulation depth and by the stronger increase of the modulation for the 16-DSA probe in LDS versus SDS. This can be correlated with the different interfacial structures of these two micellar These differences are attributed to hydrated lithium counterion acting as a spacer between the micellar headgroups4 to increase the local water concentration at the micellar surface. This is in agreement wifh small angle neutron scattering (SANS) experiments, where the SDS micelles, unlike LDS micelles, are found to present a rough surface with methylene groups present at the micellar s ~ r f a c e . ~This ,~ is also confirmed by the area per polar headgroup that is greater for LDS (65.2 A2)6 than for SDS (62.0 AZ)'suggesting the possibility of a greater water penetration into the headgroup region of LDS micelles. The crown ether addition generally produces a decrease in the normalized deuterium modulation. This effect is qualitatively different for LDS and SDS and for the two crown ethers. For SDS the deuterium modulation decreases in the order 18-crown-6 > 15-crown-5, while for LDS the order is 15-crown-5 > 18-crown-6. If we consider the 5-DSA p r o b e as an example, the decrease for SDS is about 20% for 15-crown-5 and about 30% for 18-crown-6 while the decrease for LDS is about 65% for 15-crown-5 and about 50% for 18-crown-6. This suggests that the crown ether interaction with SDS and LDS occurs with expulsion of water molecules present at the micellar surface. This effect is greater for LDS than for SDS probably because the LDS surface is more hydrated and the headgroup spatial distribution is more ordered in LDS than in the SDS micelles. which is more
Baglioni et al. favorable for a crown ether-micellar headgroup interaction. The different efficiencies for the two crown ethers for a decrease of deuterium modulation seems to suggest that 18-crown-6 is more partitioned at the SDS micellar surface than is 15-crown-5 ether, but that the reverse holds for LDS. Figure 4 reports the variation of deuterium modulation for 5-DSA in LDS as a function of crown ether concentration. The different plateau concentrations reflect different degrees of perturbation of the micellar surface dependent on the crown ether. This can be interpreted as different degrees of crown ether solubilization or association at the micellar surface. If the binding constant is known for a particular crown ether bound at the micellar surface, it is possible to compute the binding constant for another crown ether or for a different surfactant. Stilbs reports" binding constants of 101 f 9 and 148 f 18 for 15crown-5 and 18-crown-6 ethers, respectively, that correspond to 86% and 91% of the crown ether immobilized at the SDS micellar surface. If we take the 18-crown-6 value of 91% as a reference, we compute from the deuterium modulation plateau values that about 80% of the 15-crown-5ether is bound at the micellar surface of SDS. This is in agreement with Stilbs' results." This agreement further corroborates that the micellar structure is retained in the frozen solution. For lithium dodecyl sulfate micelles we find values of about 25% and 30% for 18-crown-6 and 15-crown-5 ethers, respectively. From the fraction @) of crown molecules associated with or solubilized in the micelle (0 Ip I1) and by using a pseudo two-phase model for the water-micelle system, it is possible to calculate an equilibrium constant for the solubilization and/or association process."-'4 This equilibrium constant is given by K, in terms of the micellar and aqueous phase concentrations (Cd and Caq) and volumes by
'
(9) Ramachandran, C.; Pyter, R. A.; Mukerjee, P. J. J . Phys. Chem. 1982, 86, 3198.
(10) Baglioni, P. In Surfactants in Solution; Mittal, K. L., Bothorel, P., Eds.; Plenum: New York, 1987; Vol. 4, p 393. Baglioni, P.; Ottaviani, M. F.; Martini, G. J . Phys. Chem. 1986, 90, 5878.
where Vaqis the volume of the aqueous phase and is known from the composition of the solution and Vmicrepresents the volume of the micellar phase and is related to the apparent partial molar volume of the surfactant in the micellar phase. As a first approximation, we took the apparent partial molar volume of SDSIS and LDS molecules in micellar solution equal to 250 cm3 mol-'. The K, values for 15-crown-5 and 18-crown-6 are respectively 65 and 148 for SDS and 7 and 5 for LDS. These values must be regarded as approximate since the p value has an average error of about f 5 % . The results above indicate that the sodium ion is more strongly complexed than lithium ion and that 18-crown-6 ether is a better sodium ion complexing agent than is 15-crown-5 ether in SDS micellar solutions, while the two crown ethers have similar complexing abilities in LDS micellar solutions. It is instructive to compare these results with those obtained in water solutions. The formation constants for metal cationcrown ether complexes are known to depend on the nature of the metal cation, the crown ether structure, and the solvent. In water, complexation constants of 6.3 and 4.6 are reported for sodium ion complexation by 18-crown-6 and 15-crown-5 ether, respectively,l6-I9whereas a complexation constant of 1.3 is reported for lithium ion complexation by 18-crown-6 ether.I6 The cation complexation order in the micellar solutions by the crown ethers h a s t h e same trend a n d ion selectivity as in water solution for both (1 1 ) Stilbs, P. J . Colloid Interface Sci. 1982, 87, 385. (12) Hall, D. G. Trans. Furaduy SOC.1970,66, 1356. (13) Mukerjee, P. J . Pharm. Sci. 1971, 60, 1528, 1531. (14) Wennerstrom, H.; Lindman, B . Phys. Rep. 1979, 52, 1. (15) Brun, T. S.; Hoiland, H.; Vikingstad, E. J . Colloid Interface Sci. 1978, 63, 89. (16) Liesegang, G. W.; Farrow, M. M.; Vasquez, F. A,;Purdie, N.; Eyring, E. M. J . Am. Chem. SOC.1977, 99, 3240. (17) Frensdorff, H. K. J . Am. Chem. SOC.1971, 93, 600. (18) Izatt, R. M.; Terry, R. E.; Haymore, B. L.; Hansen, L. D.; Dalley, N. K.; Avondet, A. G.; Christensen, J. J. J . Am. Chem. SOC.1976,98,7620. Izatt, R. M.; Terry, R. E.; Nelson, D. P.; Chan, Y.; Eatough, D. J.; Brasdraw, J. S.; Hansen, L. D.; Christensen, J. J. J . Am. Chem. SOC.1976, 98, 7626. (19) Rodriguez, L. J.; Liesegang, G. W.; White, R. D.; Farrow, M. M.; Purdie, N.; Eyring, E. M. J . Phys. Chem. 1977, 81, 2118.
n-DSA and TMB Photoionization in Micellar Solutions
I
LDS/TMB
1
.50
15-CROWN-5
-
.IOo
100 150 200 [CROWN ETHER], mM
50
Figure 5. Normalized deuterium modulation depth of TMB+ in LDS/ D 2 0 frozen micellar solutions as a function of crown ether concentration.
I
+
W
LDWTMB
I
18-CROWN-6
a 0.91 0
50
100 150 200 [CROWN ETHER], mM
Figure 6. TMB+yield obtained by double integration of the ESR spectra as a function of crown ether concentration in frozen LDS micellar solutions.
the counterions studied. It is interesting to note that the complexation constants in micellar systems are greater than in water solutions. It is known that the K, constant is solvent dependent and increases in passing from water to a less polar solvent like methanol.20 The micellar interface has a disorganized water structure with a lower dielectric constant than that of bulk water. This is consistent with the larger complexation constants found here in micellar systems. In summary, crown ethers complex the counterions in SDS and LDS micellar systems. The complexation constants depend on the nature of the micellar counterion and of the crown ether. The counterion complexation occurs with expulsion of water molecules probably due to crown ether intercalation between the polar headgroups of the surfactant molecules. This is also consistent with the increased area per polar headgroup found in a monolayer study with cryptand C222.21 Our results seem to contradict fluorescence results with a pyrene probe in the cryptand C222/SDS system where an increase of water penetration into the micellar Stern layer is suggested to occur.22 We consider that the more probable explanation is that the pyrene molecule moves from the micellar core toward the more hydrated interfacial region in agreement with results reported by Turro and Kuo.* We will see below that the same behavior is found with the TMB probe. 2. N,N,N‘,N’-Tetramethylbenzidine Cation Spin Probe. In contrast to the n-DSA results, an increase in the deuterium modulation depth as a function of the crown ether concentration is found with the TMB cation probe for LDS and SDS7 micellar Systems; see Figure 5 for the LDS micellar system and ref 7 for SDS. The main differences between the two micelle types are that (a) the deuterium modulation depth values are greater for LDS than SDS micellar systems for both crown ethers studied and for all concentrations of the added crown ether and (b) for the SDS system the increase in deuterium modulation is 18(20) Michaux, G.; Reisse, J. J . Am. Chem. SOC.1982, 104, 6895. (21) Evans, D. F.; Sen, R.;Warr, G. G . J . Phys. Chem. 1986, 90, 5500. (22) Quintela, P.A.; Reno, R. C. S.; Kaifer, A. E. J . Phys. Chem. 1987, 91, 3582.
The Journal of Physical Chemistry, Vol. 92, No. 16, 1988 4729 crown-6 > 15-crown-5 while the reverse occurs for the LDS micellar system. As suggested in a previous paper,’ the increase in the deuterium modulation could be due to (i) an increase in the local concentration of water molecules at the micellar interface and/or (ii) movement of the TMB molecule toward the interfacial region to a more hydrated region of the micelle interface and to probably interact with the crown ether. We consider the second hypothesis as the most probable. Several experimental results supporting this hypothesis have already been reported.7 Another proof supporting a different TMB location in LDS and SDS micelles upon addition of crown ethers is that Na+ or Li+ complexation by crown ethers leads to a decrease in the local ionic strength in the micellar interface region. This makes the interfacial region more energetically favorable for TMB. This behavior is complementary to the movement of TMB away from the interfacial region when its local ionic strength is increased by high added salt c ~ n c e n t r a t i o n . ~Turro ~ and Kuo have studied pyrene excimer emission from pyrene solubilized in aqueous solutions of crown ether surfactants and find that the “hydrophobic” pyrene can move from the micellar interior to the more “hydrophilic” interface in the presence of crown ether headgroups.8 This is analogous to our conclusion about the movement of TMB toward the interfacial region in the presence of added crown ethers. Figure 6 reports the relative TMB+ yield for LDS as a function of crown ether concentration. The relative TMB’ yield for LDS is about double that for SDS and increases, as for SDS’, by only 10-15% with crown ether addition. This increase is near the limit of experimental error but does appear to indicate a real trend. This increase does correlate with increased TMB+-water interactions. We expect, on the basis of previous r e s ~ l t s , ’ , ~that ~-~~ an increase of the TMB+-water interactions corresponds to an increase in the TMB+ yield. In fact, for the LDS micellar system the increase in deuterium modulation depth corresponds to an increase in the relative TMB+ yield. In particular we find that the increase is 15-crown-5 > 18-crown-6 for LDS and the reverse for SDS. This trend is also consistent with the crown ether complexation constants for the micellar counterions. However, the difference in the modulation depth values between LDS and SDS does not totally account for the higher photoionization yield found in LDS micelles. Possible explanations may be related to the different interfacial structure, different local charge density, and different interfacial water structure of these two systems.26 Conclusions
The results obtained from the electron spin echo modulation (ESEM) and electron spin resonance (ESR) spectra of n-DSA in SDS and LDS micellar solutions containing 15-crown-5 or 18-crown-6ethers show that the crown ethers can strongly complex the micellar counterions. The cation complexation occurs with expulsion of water molectlles probably due to the crown ether intercalation among the micellar polar headgroups. The complexation constants depend on the nature of the micellar counterion and on the crown ether structure. They are in the order 18-crown-6 > 15-crown-5 for SDS and 15-crown-5 1 18-crown-6 for LDS and are greater for sodium ion regardless of the crown ether. This trend is strictly correlated with that found in water solutions even accounting for higher complexation constants in micellar systems. Modulation effects due to the TMB+ radical indicate that the TMB molecule moves toward the interfacial region with the addition of crown ethers. This suggests that the relative TMB+ yield increase does correlate with an increase of TMB+-water interaction that in turn seems to be related t o the crown ether complexation constants for micellar counterions. (23) Maldonado, R.; Kevan, L.; Szajdinska-Pietek, E.; Jones, R. R. M. J . Chem. Phys. 1984, 81, 3958. (24) Kevan, L. In Photoinduced Electron Transfer; Fox, M. A,, Chanon, M., Eds.; Elsevier: Amsterdam, 1988; and references therein. (25) Rivara-Minten, E.; Baglioni, P.; Kevan, L., J . Phys. Chem., in press. (26) Baglioni, P.; Kevan, L. J . Phys. Chem. 1987, 91, 2106.
J . Phys. Chem. 1988, 92, 4730-4733
4730
Acknowledgment. This research was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, US.Department of Energy. Registry No. TMB, 21296-82-2; TMB', 366-29-0; LDS, 2044-56-6;
SDS, 15 1-21-3; 5-DSA, 96041-46-2; 7-DSA, 115095-62-0; 10-DSA, 115095-63-1; 12-DSA, 115095-64-2; 16-DSA, 115095-65-3; 15-crown-5, 33100-27-5; 18-crown-6, 17455-1 3-9; sodium 18-crown-6, 31270-12-9; sodium 15-crown-5, 59890-71-0;lithium 15-crown-5,74060-72-3; lithium 18-crown-6, 68129-68-0.
Activation of Encapsulation System MoO,/SnO, for Olefin Metathesis by SnMe, Katsumi Tanaka,* Masaki Sasaki, and Isamu Toyoshima Research Institute for Catalysis, Hokkaido University, Kika- ku, Sapporo 060, Japan (Received: December 16, 1987; In Final Form: February 18, 1988)
Inactive Mo03/Sn02was changed to a metathesis catalyst by treating the surface with tetramethyltin (SnMe,) at room temperature. On the catalyst olefin metathesis proceeded selectively without accompanying side reactions such as hydrogen scrambling. During the treatment of Mo03/Sn02with SnMe,, a small amount of methane formed concurrent with decomposition of %Me4. Decomposition of tetraethyltin (SnEt4) also occurred on Mo03/Sn02;however, the surface showed no metathesis activity. From these results, it was concluded that the cause for activation of Mo03/Sn02by SnMe4 is attributed not to reduction of Mo species or the presence of Sn but to the formation of CH2 species pivotal to the catalytic olefin metathesis cycle. When Mo03/Sn02was reduced with H2 or CO at 500 O C , Mo species migrated into reduced SnO, and no metathesis activity was observed even after treating the surface with SnMe4. However, when the reduced sample was reoxidized with O2 at 500 OC for 1 h followed by being treated with SnMe,, the surface was changed to metathesis active.
Introduction It is assumed that olefin metathesis on solid catalysts proceeds through metal alkylidene and metallacyclobutane intermediates, which are accepted species in homogeneous catalytic systems.' In fact, the stereoselectivities in olefin metathesis observed on SnMe4 treated molybdena-titania catalysts2 are quite similar to those in homogeneous system^,^ which supports a validity for the reaction mechanism on solid metathesis catalysts. In this sense, whether or not a surface shows metathesis activity may depend on the initiation step, a metal alkylidene formation process by a contact with olefin, and if a metal alkylidene species is formed on the surface, it may be able to participate in the propagation step, the chain carrying process involving metallacyclobutane
intermediate^:^ initiation step of propene metathesis: Mo cation
+ propene
-
Mo=CH2 or Mo=CHCH3
propagation step c,
CH2 =CHCH3 Mo=CH CH3CH=CHCH3
K
CH2=CH2
>
4
Mo=CHCH3 CH2=CHCH3
MO?
C
(1) Rooney, J. J.; Stewart, A. Catalyst; Kemball, C., Ed.; Specialist Pe-
riodical Report, The Chemical Society: London, 1977; Vol. l , Chapter 8. Grubbs, R. H. Comprehensive Organometallic Chemistry; Wilkinson, G., Ed.; Pergamon: New York, 1982; Vol. 8, Chapter 54. Ivin, K. J. olefin Metathesis; Academic: New York, 1983. Casey, C. P. Reactive Intermediates; Jones, M., Moss, R. A,, Eds.; Wiley: New York, 1985; Chapter 4. (2) Tanaka, K.; Tanaka, K.; Takeo, H.; Matsumura, C. J. Am. Chem. SOC. 1987, 109, 2422. (3) Leconte, M.; Basset, J. M. J. Am. Chem. SOC.1979, 101, 7296. Garnier, F.;Krausz, P. J. Mol. Catal. 1980, 8, 91. Ofstead, E. A.; Lawrence, J. P.; Senyek, M. L.; Calderon, N. Ibid. 1980, 8, 227. Leconte, M.; Taarit, B.; Bilhou, Y.; Basset, J. M. Ibid. 1980,8, 263. Bosma, R. H. A,; Xu,X.D.; Mol, J. C. Ibid. 1982, IS, 187. (4) Tanaka, K.; Tanaka, K. J. Chem. SOC.,Faraday Trans. I 1988, 84, 601.
0022-3654/88/2092-4730$01.50/0 , I
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Consequently, it is interesting to elucidate the behavior of grafted CH2 species on solid surfaces, So far M o o 3 supported on various kinds of metal oxide supports that are completely metathesis-inactive materials have been treated with SnMe4 to convert them to metathesis catalysts. For instance, Mo03/Ti02 treated with SnMe4 (hereafter denoted as Mo03/Ti02-SnMe4),",5 M 0 0 ~ / S i o ~ - S n M eMo03/A1203-SnMe4, ~,~ and M o 0 3 / Z r 0 2 changed into super-active olefin metathesis catalysts.' While their reduced catalysts were less active for metathesis, however, their metathesis activities were also drastically enhanced more than by a factor of lo3 by treating with SnMe4. Consequently these metathesis activities are not induced by reduction of Mo species on their surfaces but are interpreted by grafting CH2 on Mo species formed as a result of CH4 evolution during the treatment with &Me4. Contrary to these, Moo3 supported on ZnO, MgO, and G e 0 2 did not have any metathesis activity irrespective of the oxidized and reduced forms even after being treated with SnMe4. These results clearly induce an effect of support on grafting CH2 species on Mo cations formed by decomposition of &Me4. Compared to these supported molybdenum oxide catalysts, the Mo03/Sn02 system showed an unexpected behavior. That is, metathesis-inactive Mo03/Sn02 changed to an olefin metathesis-active catalyst by treating it with SnMe4; however, the H2reduced surface showed no activity following treatment with SnMe4.' In this paper, first the cause of metathesis activity generation on Mo03/Sn02 treated with SnMe4 is elucidated, and second the cause of metathesis inhibition on reduced MoO,/SnO2, treated with SnMe, is studied.
Experimental Section Molybdenum trioxide supported on S n 0 2 was obtained by immersing S n 0 2 powder into an aqueous solution of ammonium paramolybdate salt; water was then removed by heating the solution at 120 OC. The starting material was oxidized with O2 at 500 O C to obtain 6.7 wt 5% Mo03/Sn02. Here S n 0 2 was obtained by oxidation of Sn(OH), under O2flow at 700 OC for 12 h, and oxide formation was ascertained by the oxygen 1s X-ray photo(5) Tanaka, K.; Tanaka, K. J. Chem. SOC.,Chem. Commun. 1984,748. (6) Tanaka, K.; Tanaka, K. J. Chem. SOC.,Faraday Trans. I 1987, 83, 1859. (7) Tanaka, K.; Tanaka, K. Hyomen (Surface) 1987, 24, 275.
0 1988 American Chemical Society
