J. Phys. Chem. 1992, 96, 2328-2334
2328
moments of inertia (I),rotational symmetry number (a), electronic ground-state statistical weight (go), and vibrational frequencies (w). Gibbs energy functions calculated with the selected molecular constants are listed in Table V; the functions are calculated for the ideal gas state at 1 atm pressure. OsO,(g). Spectroscopic constants were taken from the values summarized by McDowell et ale1, The standard enthalpy of formation listed by Kubaschewski and AlcockI5 was adopted. OsO,(g). A planar structure of D3hsymmetry was assumed, along with a 'I:electronic ground state. Following Watson et aL3
the vibration frequencies were assumed to be the same as those of W 0 3 listed in the JANAF tables." OsOz(g). The molecule was assumed to be linear symmetric, with frequencies similar to those given in the JANAF tables" for MOO, and WO,. Because of the four nonbonding 5d valence electrons, the ground electronic state was assumed to be 3Z. OsO(g). The adopted rotational constant and vibration frequency were derived from analysis of the electronic spectrum.I6 The electronic ground state was taken to be 'A, as with RuO and ~ ~ 0 . 1 7
Dynamics of Electron Attachment to AOT/H,O Reversed Micelles G. Bakale, Downloaded by UNIV OF NEBRASKA-LINCOLN on September 7, 2015 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/j100184a058
Radiology Department, Case Western Reserve University, Cleveland, Ohio 441 06
G . Beck,+ Bereich Strahlenchemie, Hahn-Meitner Institut, W-1000 Berlin 39, Germany
and J. K. Thomas* Chemistry Department, Notre Dame University, Notre Dame, Indiana 46556 (Received: March 1 , 1991)
Values of the observed rate constant, kob, of excess electrons attaching to aerosol OT (AOT) reversed micelles encapsulating varying quantities of water were measured in isooctane and tetramethylsilane (TMS) at 21 O C using a picosecond-pulseconductivity technique. Dynamic light scattering was used to determine the micellar radii in TMS from which aggregation numbers of the micelles were determined. In the lower-mobility isooctane (we = 5.3 cm2/(V s)), values of kobsapproach the diffusion-controlled attachment rate, kd, at a molar H,O/AOT ratio of 18 and appear to exceed kd at larger values of H,O/AOT. In contrast, values of kobsin TMS (we = 100 cm2/(V s)) are &fold less than kd at the maximum ratio of H,O/AOT = 27 that was studied. Several factors that could contribute to kobsappearing to exceed kd in isooctane are discussed, and the less efficient attachment of electrons to the same reversed micelles in TMS compared to attachment in isooctane is interpreted in terms of the relative attachment time, T,, and the residence time of the electron within the electron-micelle encounter radius, T,. With this model, the T , of electrons attaching to micelles at a diffusion-controlledrate is < I ps and may be identified with the solvation time of electrons in water.
Introduction Micelles, reversed micelles, and microemulsions have been intensively investigated due to their compartmentalization properties which can be exploited in applications that range from artificial photosynthesis and solar energy conversion' to enzyme encapsulation and drug deliverye2 The structural features of these species also present a model system in which unique information concerning the kinetics and dynamics of short-lived reactants such as electrons can be ~ b t a i n e d . It ~ was with the mutual objectives of elucidating the reaction mechanism of excess electrons with reversed micelles in liquids and of using excess electrons to probe the structure of water encapsulated in reverse micelles that the present study was undertaken. In a previous study, we used a picosecond-pulse-conductivity (PPC)technique to measure the rate constant, koh, of electron attachment to aerosol OT (AOT)/H,O reversed micelles in isooctane.4 The AOT/H,O/isooctane system was chosen for that study since it was one of the most thoroughly characterized reversed micellar/microemulsion systems5v6and the electron mobility, pe, in isooctane is of sufficient magnitude to yield a good signal/noise ratio. This latter criterion precluded from study two solvents that also have been well characterized with AOT/H20 reversed micelles, viz., heptane7 and cyclohexane.8 In the earlier work, we found kobs to be strongly dependent on the H,O/AOT ratio and to range from 2 X l O I 3 M-' s-I at R = 0.1 to lOI5 M-' s-l at R = 1.5; R is the ratio by weight of H,O/AOT and can 'Deceased May 24, 1988.
0022-3654/92/2096-2328$03.00/0
be converted to w , the molar ratio of H,O/AOT, by the conversion factor of 24.7 at 293 K. Further, we concluded that the electron-attachment process was diffusion-controlled only if free, nonbonded water were present in the micellar pool. In the present study, we have made additional measurements of kobs in the region of R where the transition from bonded to nonbonded water in the micellar pools occurs. Also, we have used Maitra's detailed characterization of the AOT/H,O/isooctane system to obtain more accurate evaluations of kobe5 In addition, electron attachment to the same reversed micelles was extended to another solvent, tetramethylsilane (TMS), in which pe is 20-fold greater than in isooctane, Le., 100, cf. 5.3 cmZ/(V s) a t 21 0C.9 From the magnitude and the field dependence of pelo
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( I ) (a) Gratzel, M. Acc. Chem. Res. 1981, 1 4 , 376. (b) Calvin, M. Photochem. Photobiol. 1983, 37, 349. (2) (a) Luisi, P.; Wolf, R. Solution Behavior of Surfactants; Mittal, K . L., Fendler, E. J., Eds.; Plenum Press: New York, 1982; Vol. 2, pp 887-905. (b) Luisi, P. L. Biological and Technical Relevance of Reversed Micelles; Plenum Press: New York, 1983. (3) Fendler, J. H. Annu. Rev. Phys. Chem. 1984, 35, 137. (4) Bakale, G.; Beck, G.; Thomas, J . K. J . Phys. Chem. 1981.85, 1062. ( 5 ) Maitra, A. J . Phys. Chem. 1984, 88, 5122. (6) (a) Eicke, H.-F.; Rehak, J. Hela Chim. Acra 1976, 50, 2883. (b) Zulauf, M.; Eicke, H.-F. J . Phys. Chem. 1979, 83, 480. (7) Robinson, B. H.; Steytler, D. C.; Tack, R. D. J . Chem. SOC.,Faraday Trans. 1 1979, 75, 481. ( 8 ) Day, R. A.; Robinson, B. H.;Clarke, J. H.R.; Doherty, J. V. J. Chem. SOC.,Faraday Trans. I 1979, 75, 132. (9) Allen, A. 0.;Gangwer, T. E.; Holroyd, R. A. J . Phys. Chem. 1975, 79, 25.
0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2329
Electron Attachment to AOT/HzO
TABLE I: Dewndeace of Structural and Kinetic Parameters of AOT/&O Reversed Micelles on R and w in Isooctane and TMS at 21 O c a Isooctane 0 0.10 0.20 0.30
0.50 0.75 1.o 1.5
0 2.5 4.9 7.4 12.3 18.5 24.7 37.1
19 (23) 26 30 38 43 51 84
(16) (26) 41 65 136 267 440 875
3.7 4.9 7.4 12.3 18.5 27.2
(21) 27 43 57 61 (63)
(17) 49 400 830 860 (900)
0.067 0.17 0.50 0.94 2.6 6.5 11 18
2.8 3.6 3.9 4.4 5.0 5.6 6.4 9.6
0.32 0.52 0.61 0.75 1.o 1.3 1.6 1.7
13 10 4.1 2.8 0.92
0.024 0.033 0.065 0.10 0.11 0.12
51 2.0 3.0 1.4 0.85 0.60
TMS 0.15
0.20 0.30 0.50 0.75 1.1
0.03 1.2 2.2 9.0 16 29
64 74 105 131 140 144
Downloaded by UNIV OF NEBRASKA-LINCOLN on September 7, 2015 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/j100184a058
'Values in parentheses are extrapolated. bValues for isooctane from ref 5 except as noted for TMS from Table 11. 'Calculated with eq 3' and re = 13.8 'A. dCalculated with eq 7. 'Calculated with eq 6. /From ref 6b. in TMS, it was commonly inferred that excess electrons in this liquid are delocalized or quasifree;' more recent measurements of the Hall mobilityl2 and the pressure dependence of pel3in TMS are consistent with this inference.I4 In contrast, electron transport in isooctane appears to occur via a hopping or trapping mechanism.14 To evaluate kobsof AOT/HzO reversed micelles in TMS, diffusion measurements, were conducted to obtain the micellar radius, rm, and aggregation number, W, in this solvent.
Experimental Section The PPC system used was essentially that described in our earlier worP in which a parallel-plate ion chamber was irradiated in 50 pulses/s by 30-ps (fwhm) fine-structure electron pulses generated within a 2-ns electron pulse by the Hahn-Meitner Institute L-band linear accelerator. The electron-current decay in the 770-ps time window between the fine-structure pulses was monitored with a Tektronix S-4 sampling head (25-ps rise time) coupled by a 50-0 transmission line terminated by the ion chamber. The overall rise and fall times (10-90%) of the system were 80 and 70 ps, respectively; additional details regarding the PPC system, data handling, and electron half-life measurements are given in ref 15. Since reversed micelles of AOT/H20 form only in solutions at concentrations exceeding a lower limit [e.g., 0.01 mM at R = 0.5 (S)], the t l l zmeasurements reported could not have been conducted in the nanosecond time regime at a 50to 100-fold lower micelle concentration. AOT (Fluka purum) was purified using the technique described by Calvo-Perez et a1.16 which consisted of dissolving 50 g of AOT in 500 mL of a 5:l (v/v) cyc1ohexane:methanol solution that was then passed through a 1-m column of charcoal (Merck) that had been heated overnight at 200 OC and washed with 2 L of the 5:l cyc1ohexane:methanol solution. The solution collected was filtered to remove traces of charcoal, and the bulk of the solvent was removed by rotary evaporation at 50 OC. The remaining solvent was removed from the AOT by heating in a vacuum oven a t 50 OC for 2 days. The purified AOT was stored in a desiccator over molecular sieve 5A in an argon atmosphere. (IO) Sowada, U.;Bakale, G.; Schmidt, W. F. High Energy Chem. (Engl. Ed.) 1977, 10, 290. (11) (a) Jortner, J.; Gaathon, A. Can. J . Chem. 1977, 55, 1801. (b) Schmidt, W. F.Ibid. 1977,55,2197. (c) Yakovlev, B. S. Russ. Chem. Rev. 1979. 48. 615. (12) Munoz, R. C.; Ascarelli, G. Chem. Phys. Lett. 1983, 94, 235. (13) (a) Munoz, R. C.; Holroyd, R. A.; Nishikawa, M. J . Phys. Chem. 1985,89,2969. (b) Munoz, R.C.; Holroyd, R.A. J . Chem. Phys. 1986,84, 5810. (14) Holroyd, R. A. In Radiation Chemistry: Principles and Applications; Farhatiziz, Rodgers, M. A., Eds.; VCH: New York, 1987; pp 201-235. (15) Beck, G. Radial. Phys. Chem. 1983, 21, 7. (16) Calvo-Perez, V.;Beddard, G. S.; Fendler, J. H. J . Phys. Chem. 1981, 85, 2316.
Isooctane and TMS were purified as described in ref 4 and 17, respectively, and AOT was added to either solvent to yield a concentrated solution to which aliquots of HzOwere subsequently added to yield reversed-micellar solutions having the desired R . These stock solutions were shaken 1-2 min prior to being injected via syringe (15-50 fiL) into 750 pL of pure solvent in the ion chamber which had been degassed and checked for purity as described in ref 17. Measurements of rl/2 were generally conducted in triplicate and agreed within f5%; total experimental error in the values of tllZ reported is estimated to be
