J . Phys. Chem. 1985,89, 2087-2089
2087
Effect of Mlcellar Interface Modifications on the Reactivlty of Embedded Photoproduced Catlons Andrzej Plodtat and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77004 (Received: August 17, 1984; In Final Form: December 11, 1984)
Lifetime distributions of photoproduced N,N,N',N'-tetramethylbenzidinecation radicals (TMB') in micellar solutions of dodecyltrimethylammonium chloride (DTAC) and sodium dodecyl sulfate (SDS), with and without NaCl or l-butanol added to modify the micellar interface potential, were determined from inverse Laplace transforms of the decay curves adequately approximated by [TMB+]/[TMB+]o= exp[-(t/so)]". The constant a,determining the width of the TMB' lifetime distribution, was found to be insensitive to these micellar interface modifications, while the variation of the effective lifetime T~ was found to correlate with TMB'-water interactions measured by electron spin echo modulation experiments.
Introduction The self-association of amphiphiles in aqueous solution into micellar aggregates is well recognized' as a cooperative process requiring simultaneous participation by many amphiphilic molecules or ions. This process is driven by the transfer of hydrocarbon chains from a polar solvent to the less polar interior of a micelle, the hydrophobic effect, and opposed by the repulsion among the polar headgroups when they are brought into close contact a t the surface of the micelle. The population decay of transient photoproducts in micelles is also expected to be cooperative and it is the cooperativity that leads to the multiexponential first-order decay observed experimentally.2~3 This multiexponential pattern of decay can be adequately described2 by the right-hand side of relation 1 and in-
Imf(.)
exp(-t/T) d r = e~p[-(t/7~)*]
0
< a I1
(1)
terpreted by the left-hand side of relation 1 in terms of superpositions of exponential decays with a distribution f l ~ )and a distribution width determined by the value of a. The lower the numerical value of a the broader is the distribution; a = 1 corresponds to monoexponential decay. An increase in the hydrophobic interaction by substitution of D 2 0 for H 2 0 has been shown' to result in an increase in both the effective lifetime, T , and the width of the lifetime distribution of N,N,N',N'-tetramethylbenzidine (TMB) photoproduced cation radicals in sodium alkyl sulfate micelles. This was explained by tighter molecular packing inside the micelles formed in D 2 0 which makes the micellar dynamics more cooperative. In the present paper we have introduced additives to modify the micellar surface by partially neutralizing and by increased spacing of the polar headgroups and have investigated these factors on the lifetime of species embedded in micelles. Lifetime distributions of photoproduced TMB+ cation radicals in dodecyltrimethylammonium chloride (DTAC) and sodium dodecyl sulfate (SDS) micelles, with and without added salt or alcohol to modify the interface potential, were determined and compared with those calculated for trimethylammonium dodecyl sulfate (TMADS) from recently published results." No changes in the width of the lifetime distributions were found which means that the structure of the micelle interface region has little effect on the cooperativity of hydrocarbon chain motion within the core. However, changes at the micelle interface do affect the effective lifetime 70 which was found to correlate well with TMB+-water interactions as determined by electron spin echo modulation (ESEM) techniques." The greater the TMB+-water interaction, the shorter the effective lifetime of TMB' radicals in a given system. 'On leave from the Institute of Applied Radiation Chemistry, Technical University of Lodz, 93-590 Lodz, Poland.
0022-3654/85/2089-2087$01.50/0
Experimental Section SDS, DTAC, TMB (Eastman Kodak Co.), l-butanol (Aldrich Chemical Co.), and sodium chloride (Fisher, ACS grade) were used as received to prepare the aqueous solutions of micelles with solubilized TMB according to the procedure described previ~usly.~ The micellar solutions in 75-pL micropipets were irradiated in the cavity of a Varian E-4 electron spin resonance (ESR) spectrometer with a 900-W high-pressure mercury lamp. The lamp output passed through a No. 760 Corning band-pass filter to give an irradiation wavelength of 350 f 30 nm with an intensity of lo2 W m-*. The concentration of photoproduced TMB' was monitored by recording overmodulated ESR spectra. Results Making use of the recent findings that in the presence of anionic electron scavengers the yield of photoproduced TMB' radicals in DTAC micelles increases to the level at which its ESR spectrum can be detected a t room temperature, we were able to compare TMB+ decay in 0.1 M micellar solutions of DTAC (in the presence of 5 X lo-' M of NaN03) and of SDS. The results obtained for stored (1 week) micellar solutions are presented in Figure 1 in the coordinate system In ([TMB']/[TMB'],J vs. (t/TO)0,6where [TMB'], denotes the initial concentration of TMB' determined by extrapolation back to the end of the irradiation time from which the time t was counted. The recent data4 from this laboratory on TMB' decay in the micellar solution of TMADS are also included in Figure 1. It is seen that the experimental data fit well the relation [TMB'] /[TMB'],
= e~p[-(t/7~)*]
with a common value of CY = 0.6, 7 0 being equal to 2.0 X lo3, 1.4 X lo3, and 0.30 X lo3 s for micellar solutions of SDS, TMADS, and DTAC, respectively. Interpreting the decay pattern given by eq 2 in terms of a superposition of exponential decays, relation 1, one can obtain the TMB+ lfetime distribution,f(.r), by an inverse Laplace transform of the right side of relation 2. For large lifetimes, which are of interest in kinetic studies,f(s) was evaluated (1) Tanford, C. 'The Hydrophobic Effect: Formation of Micelles and Biological Membranes"; Wiley-Interscience: New York, 1980. (2) Plonka, A.; Kevan, L. J. Chem. Phys. 1984, 80,5023. (3) Plonka, A.;Kevan, L. J . Phys. Chem. 1984, 88, 6348. (4) Szajdzinska-Pietek, E.;Maldonado, R.; Kevan, L.; Jones, R. R. M. J . Am. Chem. Soc. 1984,106, 4675. ( 5 ) Maldonado, R.; Kevan, L.; Szajdzinska-Pietek, E.; Jones, R. R. M. J . Chem. Phys. 1984, 81, 3958. (6) Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M., unpublished results. (7) Narayana, P. A.; Li, A. S. W.; Kevan, L. J . Am. Chem. Soc. 1982,104, 6502. ( 8 ) Arce, R.; Kevan, L. Trans. Faraday SOC.I , in press.
0 1985 American Chemical Society
Plonka and Kevan
2088 The Journal of Physical Chemistry, Vol. 89, No. 10, 1985
0
0.4 0
TMB+, r, .IO+,
two
I
s
0 SDS 2.0 0 TMADS 1.4 A DTAC 0.30
h
0
r-0.5 m
F
Y
L
r/r,
m
-'O
5
I
\
F
Y-1.0 v
C
0
0.2
0.I
0.3
0.4
[NaCI], M
-1.5
I.o
0.5
1.5
(t/r,10.6 Figure 1. Decay at 306 K of photoproduced TMB' in 0.1 M micellar solutions of SDS, TMADS, and DTAC. See text for details.
Figure 3. Effect of sodium chloride on the effective lifetime, ro, at 306 K of photoproduced TMB' in freshly prepared (0)and in stored (A) (1 week) 0.1 M micellar solutions of SDS. Lifetime distribution for a given value of T~ depicted in inset. 0.4
I
I
TMB+/DTAC, TMADS, SDS
0.3b Y
c
I
IO
102
103
io4
0
Figure 2. Lifetime distributions at 306 K of photoproduced TMB' in 0.1
M micellar solutions of SDS, TMADS, and DTAC.
as (3) by a simple saddle point calculation?Vl0 Other evaluations have also been discussed."
-
AT)
( l / ~ ~ ) [ ( a ~ / 2 a-(a)) l x ( T o / CyT)z-a/(l-")] 1 i 2 eXp[-(l - (U)(CUT/TO)"/"-~'] (3)
The lifetime distributions of TMB+ radicals in micelles of SDS, TMADS, and DTAC, calculated according to relation 3, are presented in Figure 2. These distributions have the same width, as indicated by the common value of a,and the only effect of the variation of the counterion or the headgroups is to change the position on the lifetime scale due to the changes of T ~ . Qualitatively similar effects are observed upon addition of additives such as salt or alcohol which are used to modify the micellar surface potential. Neither addition of sodium chloride, shown in Figure 3, nor 1-butanol, shown in Figure 4, , change the width of the TMB' lifetime distribution in SDS micelles. Both additives only decrease the effective lifetime, T ~ In . the case of sodium chloride as shown in Figure 3 the effect of salt on the aging of the micellar solution of SDS is illustrated. It was observed2 that storage of micellar solutions at room temperature before UV irradiation causes an increase of the numerical value of so and has no effect on the numerical value of a. For SDS micelles during (9) Majumdar, C. K. Solid Srure Commun. 1971, 9, 1087. (IO) Scher, H.; Montroll, E. W. Phys. Rev. B 1975, 12, 2455. (11) Helfand, E. J . Chem. Phys. 1983, 78, 1931.
0.I
0.2
0.3
[BuOH], M
T, S
Figure 4. Effect of 1-butanol on the effective lifetime, r0, at 306 K of photoproduced TMB' in 0.1 M micellar solutions of SDS stored for 1 week before addition of alcohol (A) and with alcohol added (A). Lifetime distribution for a given value of r0 depicted in the inset.
storage for 1 week the numerical value of 70 increased from about 0.2 X lo3 to 2.0 X lo3 s and no changes were observed upon further storage. It looks like the presence of salt promotes the aging process since there is no difference in 70 for freshly prepared and stored SDS solutions containing NaC1. The effect of 1-butanol on the aging of micellar solutions of SDS was much less evident. It is interesting to note that there is no difference in T~ for micellar solutions stored before addition of 1-butanol and solutions stored with added alcohol as shown in Figure 4. For micellar solutions of DTAC the numerical value of T~ increased from about 0.07 X lo3 to 0.30 X lo3 s during storage for 1 week. The micellar solutions stored for 1 week are denoted as aged and all results to be discussed below are for aged micelles. Aging effects in micelles have been observed before and at a time after which further aging effects are not seen it is assumed that the surfactant molecule alignment in the micelles has attained some kind of equilibrium.I2 Discussion The use of the fmt-order kinetic equation with a time-dependent rate constant, eq 2, to describe the multiexponential decay of photoproduced TMB+ radicals in micelles was rationalized pre~~
(12) Gritzel, M.; Thomas, J. K. J . Am. Chem. SOC.1973, 95, 6885.
Effect of Micellar Interface Modifications viously2and the microscopic origin of the multiexponential decay was discussed in terms of cooperative processe~.~-~ A picture was proposed in which the movement of one surfactant molecule needed to allow reaction of an embedded species depends on the configuration and dynamics of other surfactant molecules forming the micelle. In this picture it is primarily the cooperative motion of environmental molecules rather than the properties of the isolated embedded molecule which is reacting that lead to multiexponential decay. In agreement with this picture it was found3 that the differences between the distributions of TMB+ lifetimes, as given by eq 3, in micelles of sodium decyl sulfate and sodium dodecyl sulfate, either in normal or heavy water, are small compared with that caused by substitution of D 2 0 for H 2 0 which affects the environment. In a given medium the increase of alkyl chain length increases only the effective lifetime of TMB' cation radicals in micelles. As the alkyl chain length increases the TMB+ cation radicals have weaker water interactions a t the micellar interface' and decay more slowly, but the cooperative processes in the micelle interior are not changed and hence the lifetime distribution width of TMB' cation radicals is not changed, Substitution of D 2 0 for H 2 0 not only increased the effective lifetime of TMB+ but also caused a significant broadening of the TMB' lifetime distribution. The lifetime broadening effect, similar for both kinds of micelles, was explained by tighter molecular packing inside the micelles in D 2 0 than in H 2 0 due to stronger hydrophobic interaction in D20. Tighter molecular packing makes the internal micelle dynamics more cooperative and an increase of cooperativity correlates with a wider distribution of lifetimes.13 From recent ESR and ESEM studies of TMB photoionization in anionic micelles it was found4 that substitution of tetramethylammonium counterion for sodium counterion significantly influences the structure of dodecyl sulfate micellar solutions which in turn affects both the photoionization efficiency of a micellar solubilizate and the stability of the photoproduced cations. The explanation given was that sodium counterions are located in the outer part of the headgroup region so that a relatively compact headgroup structure is maintained in SDS micelles. The more hydrophobic tetramethylammonium cations are located in the inner part of the headgroup region acting as spacers between the headgroups and create a less compact headgroup structure with a higher local concentration of water. The stronger TMB+ radical-water interaction in TMADS micelles compared to SDS micelles explains fairly well both the higher photoionization efficiency of TMB in TMADS, since deeper water penetration assists in solvation of photoejected electrons which promotes charge separation, and the lower stability of photoproduced TMB' radicals due to less efficient embedding. The interesting point shown in Figure 2 is that the width of the TMB' lifetime distributions is not changed by the change in counterion. The lifetime distribution width is also the same in anionic and cationic micelles even though the effective lifetime is much less in cationic micelles. There seems to be a simple rationalization of the width invariance of TMB+ radical lifetime distributions for micelles prepared in H 2 0 . Variation of the headgroup or counterion has no effect on the hydrophobic properties of the alkyl chain on which the constrained concerted motion of hydrocarbon chains depends. This assumption is implied in Stigter's treatment of micelle formation by ionic surfactants14and by I3C spin-lattice relaxation (13) Skinner, J. L. J . Chem. Phys. 1983, 79, 1955.
The Journal of Physical Chemistry, Vol. 89, No. 10, 1985 2089 time measurements for micelles of SDSI5 and dodecylammonium chloride.16 Hence the cooperativity of micellar dynamics is not changed and the width of the TMB+ radical lifetime distribution is not affected. Variations of headgroup or counterion do affect, however, the TMB' radical-water interaction which appears to be one of the main factors determining the effective lifetime. The above conclusions are supported by the results obtained for SDS micellar solutions with salt or alcohol added to modify the micellar surface potentia1.l' The presence of an electrolyte in an aqueous SDS solution neutralizes the surface charge and decreases the electrostatic repulsion between the headgroups. For high salt concentration it is expected that SDS micelles would behave like nonionic ones. Upon NaCl the critical micelle concentration decreases and the aggregation number increases yet the TMB' radical lifetime distribution retains the same width and the effective lifetime decreases. The decrease of TMB' effective lifetimes correlates fairly well with the increased TMB+-water interaction as deduced from ESEM experiments. It was found5 that TMB+-water interactions increase with salt addition to a maximum a t about 0.2 M and then decrease somewhat. For SDS micelles with added NaCl measurements have been made of the 13Cnuclear magnetic resonance longitudinal relaxation time of the surfactant molecule and it was found15that up to about 0.3 M NaCl the molecular motions inside the micelle did not change. This seems compatible with an unchanged distribution width of TMB' lifetimes. Alcohol molecules intercalate between surfactant ions23-26and increase the average distance between ionic headgroups which decreases the surface charge density and consequently increases the degree of micellar dissociation. The critical micelle concentration and the aggregation number decrease yet the width of the TMB+ radical lifetime distribution remains unchanged and the effective lifetime decreases. In micellar solutions of SDS, TMB+-water interactions increase upon addition of 1-butanol up to 0.1 M and remain constant with further increase of alcohol content6 Once more the effective lifetime of TMB' radicals correlates fairly well with TMB+-water interactions as deduced from ESEM experiments. Thus in a given micellar system cation radical-water interactions appear to be the main factor determining the effective lifetime of the embedded TMB' radicals. Acknowledgment. This research was supported by the Department of Energy under contract DE-AS05-80ER10745. Registry No. TMB, 366-29-0; TMB', 21296-82-2; SDS, 151-21-3; DTAC, 112-00-5; NaCI, 7647-14-5; 1-butanol, 71-36-3. (14) Stigter, D. J. Phys. Chem. 1974, 78, 2480. (15) Roberts, R. T.; Chachaty, C. Chem. Phys. Lett. 1973, 22, 348. (16) Kalyanasundaram, K.; Gratzel, M.; Thomas, J. K. J . Am. Chem. Soc. 1975, 97, 3915. (17) Grand, D.; Hantecloque, S.; Bemas, A.; Petit, A. J . Phys. Chem. 1983,87, 5236. (18) Stiger, D. J . Colloid Interface Sci. 1974, 47, 473. (19) Lianos, P.; Zana, R. J . Phys. Chem. 1980,84, 3339. (20) Hayashi, S.;Ikeda, S.J . Phys. Chem. 1980, 84, 744. (21) Almgren, M.; LBfroth, J.-E. J. Colloid Interface Sci. 1981, 81 486. (22) Croonen, Y.; Gelade, E.; van der Zegel, M.; van der Auweraer, M.; Vandendriessche, H.; De Schryve, F. C.; Almgren, M. J . Phys. Chem. 1983, 87, 1426. (23) Larsen, J. W.; Tepley, L. B. J . Colloid Interface Sci. 1974, 49, 113. (24) Zana, R.; Yiv, S. Strazielle, C.; Lianos, P. J . Colloid Interface Sci. 1981, 80, 208. (25) Jain, A. K.; Singh, R. R. B.J. Colloid Interface Sci. 1981,81, 536. (26) Almgren, M.; Swarup, S. J . Colloid Interface Sci. 1983, 91, 256.
