J. Phys. Chem. 1989, 93, 2065-2068 shape, and location have significant consequences for the chemisorptive and catalytic properties of these catalysts, as discussed else~here.~~,*~ Acknowledgment. We acknowledge the funding of the National Science Foundation that supported this work. We are also
2065
thankful to the National Science Foundation for providing a Graduate Fellowship to one of the authors (S.A.S.). In addition, we thank the government of Japan for providing financial support to M.A. that allowed him to work at the University of Wisconsin. Registry No. Fe, 7439-89-6;CO, 630-08-0; graphite, 7782-42-5.
Replacement of Sodium Ions by Cryptate (C222) Complexed Barium Ions in Sodium Dodecyl Sulfate Micelles Studied by Electron Spin Resonance and Electron Spin-Echo Modulatlon of 5-Doxylstearlc Acid and N,N,N’,N’-Tetramethylbenzidlne Photoionization and by Viscoslty Measurements Thomas Wolff Physikalische Chemie, Universitat Siegen, 0-5900 Siegen, West Germany
and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: July 6, 1988)
Ultraviolet irradiation of solubilized N,N,N’,N’-tetramethylbenzidine (TMB) and viscosity measurements have been carried out at room temperature in mixed micellar solutions of sodium dodecyl sulfate (SDS) and barium dodecyl sulfate [Ba(DS)*]. The barium ions were complexed by the macrocyclic cryptate C222. The irradiations did not lead to constant photoionization yields in the barium/C222-containing solutions because of secondary reactions. The viscosity as a function of the Ba2+/2 mole fraction (X) changed from 1.47 mPa s at X = 0 to 1.33 mPa s at X = 1, passing through a maximum at X = 0.2. At 77 K in frozen solutions the TMB photoionization yield as measured by electron spin resonance increased by a factor of 1.8 in the range X = 0-0.2 and leveled off at larger X. Deuterium modulation depths of electron spin-echo decay curves measured at 4.2 K for TMB’ and 5-doxylstearic acid increase as a function of X in a similar way as the photoionization yields. The results indicate a higher local concentration of water molecules in the micellar surface region when the C222-complexed barium ions are present.
Introduction The size, shape, and properties of ionic micelles in aqueous solutions are known to depend on the identity of the counterions to a great extent. This dependence has been observed in a variety of colloid chemical, thermodynamic, and spectroscopic investigations’-” and is discussed in terms of the intermicellar and intramicellar electrostatic forces that are governed by micellecounterion dissociation equilibria. For instance, large and strongly binding counterions induce growing of micelles that often change in shape from spheres to rods;S*6the structure of the micellar (1) Lindman, B.; Wennerstrom, H. Top. Curr. Chem. 1980, 87, 1. (2) WennerstrBm, H.; Lindman, B. Phys. Rep. 1980, 52, 1. (3) Berr, S. S.;Coleman, M. J.; Jones, R. R.; Johnson, J. S., Jr. J . Phys.
Chem. 1986, 90,6492. (4) Almgren, M.; Swamp, S. J . Phys. Chem. 1983,87, 876. ( 5 ) Angel, M.; Hoffmann, H.; LBbl, M.; Reizlein, K.; Thurn, H.; Wunderlich, I. Prog. Colloid Polym. Sci. 1984, 69, 12. (6) Wolff, T.; Suck, T. A.; Emming, C.-S.;von Biinau, G. Prog. Colloid Polym. Sci. 1987, 73, 18. (7) Szajdinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M. J . Am. Chem. SOC.1984, 106, 4675. (8) Szajdinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M.; Coleman, M. J. J . Am. Chem. SOC.1985, 107, 784. (9) Jones, R.R.M.; Maldonado, R.; Szajdinska-Pietek, E.; Kevan, L. J . Phys. Chem. 1986, 90, 1126. (10) Wolff, T.; von BUnau, G. Eer. Bunsen-Ges. Phys. Chem. 1982, 86, 225. ( 1 1) Wolff, T.; von Biinau. G. Ber. Bumen-Ges. Phys. Chem. 1984, 88, 1098.
0022-3654/89/2093-2065$01.50/0
interface is affected,’-I0 and, macroscopically, the bulk viscosity of micellar solutions may vary.6V1’J2 In several recent studies the influence of complexing cationic counterions by macrocyclic ligands such as crown ethers and cryptates has been investigated.l3-I6 It was shown for sodium dodecyl sulfate (SDS) micelles that upon crown ether complexation of sodium counterions the aggregation number decrea~es,’~ while the degree of di~sociation~~ and the photoionization yield of N,N,N’,N’-tetramethylbenzidine (TMB) in~rease.’~.’~ Simultaneously, water penetration into the micellar surface is decreased as revealed by spin-probe experiments with 5-doxylstearic acid (5-DSA).13 Similar but quantitatively distinct trends were found for lithium dodecyl sulfate (LiDS) micelle^.'^^'^ The increase of the TMB+ yield was ascribed to an average location of T M B closer to the micellar surface due to TMB-crown ether interactions. Thereby, the distance between TMB and bulk water as an electron acceptor is decreased. Since the TMB-crown ether interactions as revealed by ESR line-shape variation^'^ may interfere with the investigation of micellar properties by the TMB probe, it was desirable to find a micellar system containing a macrocyclic ligand that does not (12) (13) 467. (14) 4726. (15)
Wolff, T.; von Biinau, G. J . Photochem. 1986, 25, 239. Baglioni, P.; Kevan, L. J . Chem. Soc., Faraday Tmm. 1 1988, 84, Baglioni, P.; Rivava-Minten, E.; Kevan, L. J . Phys. Chem. 1988, 92,
Evans, D. F.; Sen, R.; Warr, G. G. J . Phys. Chem. 1986,90, 5550. (16) Evans, D. F.; Evans, J. B.;Sen, R.; Warr, G. G. J . Phys. Chem. 1988, 92, 784.
0 1989 American Chemical Society
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Wolff and Kevan
show these interactions. We therefore studied SDS micelles in which the sodium ions were gradually exchanged by barium ions complexed by 4,7,13,16,21,24-hexaoxa-l,l0-diazabicyclo[8.8.8]hexacosane (C222). Since barium dodecyl sulfate is insoluble in water and the complexing constant" of Ba2+/C222 is high ( K > 1015in water), we neither have to consider uncomplexed barium ions nor need to add a high excess of the cryptate C222 in order to ensure efficient complexing. We report changes of a macroscopic property, viscosity, in this system as well as effects on the TMB photoionization yield. Information on the structure of the micellar surface was gained from measuring the deuterium modulation depth of spin-echo decay curves in frozen D 2 0 solutions for the TMB' radical and for the 5-DSA spin probe.
Experimental Section Sodium dodecyl sulfate purchased from Eastman Kodak Co. was recrystallized three times from ethanol, washed with diethyl ether, and dried under moderate vacuum. Barium dodecyl sulfate [Ba(DS),] was precipitated from an aqueous solution of SDS upon addition of an equimolar amount of BaCl, aqueous solution. The precipitate was filtered off, washed with acetone, dried in vacuo, recrystallized from large amounts of boiling water, and dried in vacuo again. N,N,N',N'-Tetramethylbemidine (Eastman Kodak) was recrystallized from ethanol. Deuterium oxide (Aldrich, 99.8% D), 5-doxylstearic acid (Molecular Probes, Inc.), and 4,7,13,16,21,24-hexaoxa1,l0-diazabicyclo[8.8.8]hexacosane (Kryptofix 222, Merck; C222, Aldrich) were used as supplied. Surfactant solutions were prepared by weighing SDS, Ba(DS)2, and C222, adding corresponding amounts of water or deuterium oxide, and stirring these mixtures at room temperature in sealed glass vessels for several hours until the solid components were dissolved completely. The concentration of dodecyl sulfate (DS-) was 0.2 M throughout. The concentration of C222 was 1.1 times that of the barium ions. TMB was dissolved in chloroform. The desired volume was placed into a glass vessel, and the chloroform was gently removed by passing a nitrogen stream over the surface of the solvent. The TMB film left was covered by 1-5 mL of nitrogen-saturated surfactant solution and dissolved by stirring for at least 2 h at room temperature. After that, the solutions were filtered, and the T M B content was determined spectrophotometrically (between 0.1 and 0.2 mmol/L) on a Perkin-Elmer Model 330 spectrophotometer. Similarly 5-DSA-containing samples were prepared from a stock chloroform solution of 5-DSA by evaporating the chloroform in a stream of nitrogen and dissolving the film thus produced by nitrogen-saturated surfactant solutions. Final 5-DSA concentrations were 0.2 mmol/L. The use of an ultrasonic bath was avoided because changes of the ultraviolet spectra of C222-containing surfactant solutions indicated sensitivity to sonication. Nitrogen-purged room-temperature samples were irradiated in cuvettes of 1-cm thickness. Frozen samples were rapidly frozen in liquid nitrogen in order to retain the micellar solution structure1*J9and were irradiated in ESR sample tubes (2-mm i.d. X 3-mm 0.d.) made from Suprasil quartz at 77 K. An ultraviolet-enriched Cermax 150-W xenon lamp combined with a 10-cm water filter and a Coming No. 7-60 filter (maximum transparency at 360 nm) was employed as an irradiation source. The Corning filter prevented excitation of the surfactants, and C222 in Ba2'/C222-containing solutions did not absorb at wavelengths >275 nm. The maximum TMB' yield at 77 K was reached after 25-35 min of irradiation and remained constant at longer irradiation times. The samples were therefore irradiated for 40 min each. TMB' yields were obtained from the heights of ESR signals measured on a Varian E-4 spectrometer at 77 K and corrected for concentration differences of TMB. The shapes of the ESR (17) Christensen, J. J.; Eatough, D. J.; Izatt, D. M. Chem. Reu. 1974, 74,
0
0.5
I .o
XBdDS12/2
Figure 1. Absolute viscosity ( 9 ) as a function of mole fraction ( X ) in aqueous solutions of sodium and barium dodecyl sulfate at 21.4 f 0.2 'C. The concentration of dodecyl sulfate is 0.2 mol/L throughout. The solutions contain C222 (4,7,13,16,21,24-hexaoxa-l,lO-diazabicyclo[8.8.8]hexacosane) at concentrations 1.1 times that of Ba2+.
dE
o
05
u10
XBa(DS)2/2
Figure 2. Relative yields of TMB photoionization at 77 K as a function of mole fraction ( X ) in frozen aqueous solutions of sodium and barium dodecyl sulfate/C222. The concentrations of dodecyl sulfate and of TMB at room temperature were 0.2 mol/L and 0.2 mmol/L, respectively. Error bars are given by a least-squares analysis.
signals did not differ significantly in the various surfactant solutions. Deuterium modulation depths of TMB' and 5-DSA in D 2 0 were determined from two-pulse electron spin-echo decay curves at 4 K. The method of determination20 and details of the apparatus21have been described previously. For each SDS-Ba(DS), mixture investigated [Ba(DS)2/2 mole fractions 0, 0.1,0.25,0.5, 0.75, 1.O] the TMB' yield determination was repeated between eight and twelve times and the deuterium modulation depth was measured between three and seven times. Viscosities were measured with Ostwald capillary viscometers at 21.4 f 0.2 OC. As the densities of the micellar solutions do not deviate significantly from that of water, measured values can be taken as absolute viscosities.
Results I . Experiments at Room Temperature. Figure 1 displays how the bulk viscosity of a 0.2 M SDS solution (asDs = 1.47 mPa s) changes when the sodium counterions are gradually exchanged by equivalent amounts of C222-complexed barium ions. The viscosity passes through a maximum at a Ba(DS)*/2 mole fraction (X)of 0.2 and decreases to a lowest value of 1.33 mPa s at X = 1. The effect is not due to the presence of C222 alone since a steady increase of viscosity is observed when increasing amounts of C222 are added to SDS solutions. Attempts to study photoionization yields of T M B at room temperature in SDS-[Ba(DS),/2] mixtures were unsuccessful. The TMB' yield as a function of irradiation time in the Ba(DS)2/C222-containing solutions passed through a maximum and vanished at longer irradiation times. This indicates secondary photochemical or thermal reactions of the TMB' radicals that were not investigated further. It is likely that TMB' reacts with the cryptate molecules. 2. Experiments in Frozen Solutions. At 77 K the photoionization yields proved constant at irradiation times exceeding 35 min. Relative yields are given in Figure 2 as a function of the
351.
(18) Narayana, P. A.; Li, A. S. W.; Kevan, L. J . Am. Chem. SOC.1981, 103, 3603. (19) Bachmann, L.;Dasch, W.; Kutter, P. Eer. Bunsen-Ges. Phys. Chem. 1981, 85, 883.
(20) Maldonado, R.; Szajdinska-Pietek, E.: Kevan, L. Faraday Discuss. Chem. SOC.1984, 78, 165. (21) Ichikawa, T.; Kevan, L.; Narayana, P. A. J. Phys. Chem. 1979, 83, 3378.
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2067
Replacement of Na+ by Cryptate-Complexed Ba2+ I
t
I
0.5
0
I .0
XBa(DS)2/2
Figure 3. Normalized deuterium modulation depth of TMB' electron spin-echo decay at 4.2 K in frozen micellar solutions of sodium and barium dodecyl sulfate/C222 in D20 as a function of mole fraction (A').
The concentrations of dodecyl sulfate and TMB at rcam temperature were 0.2 mol/L and 0.2 mmol/L, respectively. Error bars represent the scatter extremes of the values. i
ni N WI
0
0.5
IO
*B~(DS)~/Z
Figure 4. Normalized deuterium modulation depth of 5-doxylstearicacid (5-DSA) echo decay at 4.2 K in frozen micellar solutions of sodium and barium dodecyl sulfate/C222 in D20as a function of mole fraction The concentrations of dodecyl sulfate and 5-DSA at rcam temperature were 0.2 mol/L and 0.2 mmol/L, respectively. Error bars represent the scatter extremes of the values.
(a.
Ba(DS)2/2 mole fraction. It can be seen that the photoionization yield increases by about 80% as compared to pure SDS solutions. This increase takes place within the range X = 0-0.2 and levels off at higher mole fractions of Ba(DS),/2. In contrast to SDS micelles with crown ether complexed sodium counter ion^,'^ no cryptate-TMB interactions affected the ESR line shape. A similarly shaped curve is obtained when the deuterium modulation depth of the TMB+ spin-echo decay in deuterium oxide solutions is plotted as a function of the mole fraction of Ba(DS)2/2 (see Figure 3). A small minimum at X = 0.75 appears in both Figures 2 and 3 that may be an accidental feature when error limits are considered. Figure 4 shows the deuterium modulation depth of the 5-DSA electron spin-echo decay that again increases when sodium is exchanged by barium/C222. In contrast to the TMB+ results, this result is different from those of previous experiments of crown ether complexation of sodium ions that showed a decrease in the deuterium modulation depth of 5-DSA with increasing crown ether con~entration.'~J~
Discussion Both the higher photoionization yield and deuterium modulation depth of TMB+ indicate a more efficient interaction of TMB+ with HzO or D 2 0 molecules in the barium/C222-containing systems as compared to SDS. A more efficient TMB-water interaction can be brought about either by a shorter average distance between TMB and the micellar surface or by an increase in the local concentration of water in the micellar interface region. The second interpretation seems preferable since the 5-DSA experiments resulted in a similarly shaped curve (compare Figure 4 with Figures 3 and 2). The 5-DSA spin probe has been shown13J4-22 to probe the interfacial region rather than the micellar interior. This interpretation is supported further by geometrical considerations. According to recent data published by Evans et a1.,I6 the area per dodec 1 molecule at an air-water interface is for Ba(DS)z/C222, Le., very similar 65 Az for SDS and 57 values. However, in the system containing the divalent BaZ+ions two dcdecyl sulfate moieties (area 114 A) are associated with one
i2
(22) Baglioni, P.;Carla, M.; Dei, L.; Martini, E. J. Phys. Chem. 1987, 91, 1460.
counterion (cross section of Ba2+/C222 is about 80 A2), Therefore, every two of the surfactant chain heads interact with one counterion. In this way the compact SDS surface is transformed into a less compact surface with increased surface area and more direct water-hydrocarbon interaction. The system counteracts this entropically unfavorable situation by increasing the aggregation number from 64 (SDS) to 100 [Ba(DS)2/C222]16to increase the head-group density at the micellar surface. Also, the bulky complexed counterions may prevent close ion-pair formation of dodecyl sulfate and counterion so that intercalation of water additionally increases the local water concentration in the interfacial region. The same trends, i.e., increasing photoionization yield and deuterium modulation depth of TMB' and 5-DSA electron spin-echo decay, were found previously when sodium counterions were replaced by the smaller lithium ion^.^.'^ This effect was interpreted in terms of a more open surface structure in the LiDS micelles as compared to SDS micelles. The hydrated lithium ions are intercalated among the head groups in the micellar surface region in contrast to hydrated sodium ions that are outside the head-group surface region. This interpretation was strongly supported by small-angle neutron-scattering experiments revealing a greater "roughness" of an SDS surface compared to an LiDS s~rface.~~~~~ The Ba2+/C222 ions are also intercalated between the head groups as are hydrated lithium ions. Attached to the cations there will be positively polarized water molecules. The assembly of polarized water molecules may contribute to the increase in the photoionization yield by being a better electron acceptor than unpolarized water or water surrounding negatively charged sulfate groups. This is consistent with the analogous results in LiDS and Ba(DS)&222 micelles and is also consistent with the observation of a 2 times larger photoionization yield at 77 K in cationic alkyltrimethylammonium micelles as compared to SDSzSbecause positively polarized water surrounds the cationic head groups. It is relevant to compare the present work with analogous experiments using crown ether complexed sodium ion^.^^,'^ There, a small increase in the photoionization yield was observed accompanied by a decrease of the local concentration of water molecules in the micellar surface region as probed by 5-DSA. The crown ethers (1 5-crown-5 and 18-crown-6) primarily associate with SDS micelles in the surface region where they expel water molecules and complex sodium ions, thereby expelling also the water molecules of the hydration shell of the ions. The complexed sodium ion is formed due to high local concentrations of the crown ethers rather than due to a high complexing constant." Therefore, free crown ether molecules are also present that can interact with TMB or TMB+ as 0b~erved.I~Due to the increased space requirements of the sodium-crown ether complexes, the micellar aggregation number decreases from 64 to 40 to accommodate greater inter-head-group distances.I5J6 Therefore, the volume and radius of the hydrocarbon micellar core decrease so that solubilized TMB has an average location nearer to the micellar surface for geometrical reasons as well as due to interactions with crown ethers. These factors explain the 10% photoionization yield increase.13 The observed macroscopic viscosity differences of SDS and Ba(DS)z solutions cannot be explained on the basis of the present results alone. Nevertheless, a common feature of all the figures in this paper is that at Ba(DS)z/2 mole fractions X > 0.2 the properties of the barium dodecyl sulfate micelles prevail over those of SDS micelles. The ESR yield data and the ESE modulation data level off above this mole fraction, and, in the viscosity measurements, the eventual viscosity decrease starts. In order to rationalize the low viscosity in Ba(DS)z/C222 micellar solutions, one may consider the average micelle-micelle distance, d, which (23) Hayter, J. B.; Penfold, J. J . Chem. Soc., Furuduy Trans. 1,1981,77, 1851. (24) Bendedouch, D.; Chen, S.H.; Koehler, W. C. J . Phys. Chem. 1983, 87. 153. (25) Narayana, P. A,; Li, A. S.W.; Kevan, L. J. Am. Chem. Soc. 1982, 104, 6502.
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The Journal of Physical Chemistry, VoL 93, No. 5, 1989
increases as a consequence of the aggregation number change from 8 nm (SDS) to 10 nm [Ba(DS),/C222] as calculated with
d = (NAc,)-Il3 where N A is Avogadro’s number and c, is the concentration of micelles. The larger distance decreases micelle-micelle interactions, thus allowing a more independent diffusion of individual micelles giving rise to lower bulk viscosity. Interactions of ionic micelles are electrostatic to a great extent and depend on the degree of counterion dissociation. One may assume a smaller degree of dissociation in the Ba(DS)*/C222 micelles relative to SDS micelles because of a greater electrostatic attraction of the bivalent counterions to the micellar surface and because the enthalpy of hydration of dissociated sodium ions might be more negative than that of the bulky complexed barium ions. The smaller degree of dissociation additionally supports easier diffusion of individual micelles. This interpretation of the viscosity data is also supported by the viscosity increase with the addition of C222 to SDS solutions in the absence of barium ions. Under these conditions Evans et aI.l53l6found a decrease of the SDS aggregation number and an increase of the degree of dissociation, Le., a closer distance between micelles, both of which are consistent with the observed increase in viscosity. It is of interest to consider possible influences of the heavy barium counter ion^^^^^^ on the photochemistry of TMB. Previous mechanistic ~ t u d i e s ~in ~ -various ~’ organic solvents and in micellar solutions revealed three main deactivation routes of excited TMB: fluorescence, triplet-state formation, and photoionization. Quantum yield determinations indicated that photoionization occurs in the excited singlet state rather than in the triplet state. However, it is not entirely clear whether photoionization competes (26) Wolff, T. Be?. Bunsen-Ges. Phys. Chem. 1986, 96, 1132. (27) yon Biinau, G.; Wolff, T. Ado. Photochem. 1988, 14, 273. (28) Alkaitis, S. A.; Gratzel, M. J . Am. Chem. SOC.1976, 98, 3549. (29) Hashimoto, S.;Thomas, J. K. J . Phys. Chem. 1984, 88, 4044. (30) Das, P. K.; Muller, A. J.; Griffin, G . W.; Gould, I. R.; Tung, C.-H.; T w o , N. J. Photochem. Photobiol. 1984, 39, 281. (31) Arce, R.; Kevan, L J . Chem. Soc., Faraday Trans 1 1985,81, 1669.
Wolff and Kevan with fluorescence and intersystem crossing in the equilibrated first excited state or occurs directly prior to singlet-state relaxation. A heavy-atom effect on the quantum yield of photoionization can be expected in the former case only. Since the intersystem crossing rate would be increased by a heavy-atom effect, a decrease of the photoionization quantum yield would be expected. In our experiments this would lead to prolonged irradiation times needed to reach the maximum ionization yield. Since this was not observed, direct photoionization prior to singlet-state relaxation seems more likely. Conclusions The replacement of sodium counterions by cryptate C222 complexed barium ions increases the photoionization yield of TMB at 77 K and the deuterium modulation depth of TMB’ and 5-DSA electron spin-echo decay curves. These effects can be ascribed to an increase of the water concentration in the micellar surface region caused by the reduction of the number of counterions and by changes of the head-group arrangement upon introduction of bivalent counterions. Analogous results in lithium dodecyl sulfate micelles suggest that positively polarized water in the hydration shell of cationic counterions is a better electron acceptor than unpolarized water. The smaller viscosity values measured in Ba(DS)2/C222 micelles can be understood in terms of different intermicellar distances and degrees of micelle-counterion dissociation. Acknowledgment. T.W. thanks the Fulbright Commission, Bonn, for a travel grant, the Fordervereine der Universitat Siegen for supporting the exchange of scholars with the University of Houston, and the Universitat Siegen as well as the Minister fur Wissenschaft und Forschung des Landes Nordrhein-Westfalen for the grant of a leave. This research was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U S . Department of Energy. We thank T. Hiff and C. Sass for technical assistance. Registry No. TMB,366-29-0; SDS, 151-21-3; C222, 23978-09-8; 5-DSA, 29545-48-0; Ba(DS)2, 2351-09-9; TMB’, 21296-82-2; D20, 7789-20-0.
