Comparison of the solubilization site of naphthalene in cationic and

Oct 1, 1986 - Sanjib Ghosh, August H. Maki, Michael Petrin. J. Phys. Chem. , 1986, 90 (21), pp 5210–5215. DOI: 10.1021/j100412a064. Publication Date: ...
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5210

J . Phys. Chem. 1986, 90, 5210-5215

The observed systematic variation of the triplet-state parameters of N with a change of surfactant chain length supports the conclusion that the hydrocarbon-water contact area increases with a decrease of chain length. Our results also indicate that the presence of Cs' counterions in NaCI2S micelles decreases the water permeability within the micelle, resulting in a more polarizable environment for N. The excited-triplet-state parameters of N in frozen micelles observed in this work suggest that the Hartley model of a hydrocarbon-like interior with a definite double layer is suspect at short chain lengths but provides a better description for surfactants with longer hydrocarbon tails. A similar conclusion was drawn by Evans and Ninham2*from theoretical calculations of the free energies, enthalpies, and entropies of micellization of anionic surfactants. The observation of changing deuterium modulation depth16bwith varying alkyl chain length of the surfactant in D 2 0 micellar solutions at 4.2 K also mitigates against deep water penetration into the micellar core. Recent measurements of proton

chemical shifts25 in NaC12S, NaC,,S, and NaC16S micelles containing a Py(CH2)llCOOH(Py = pyrenyl) probe suggest that the polar head groups and the three closest methylene groups compose the aqueous micellar surface region. With increasing surfactant chain length, the extent of this region becomes less significant in comparison with the total micelle volume, and solubilized arene probes experience an increasingly less polar and more polarizable environment, as observed in our measurements of naphthalene in NaC,S micelles. Acknowledgment. We gratefully acknowledge the support of this work by the National Science Foundation. Registry No. N, 91-20-3; NaCloS, 142-87-0; NaC,,S, 151-21-3; NaC,,S, 1191-50-0;Cs, 7440-46-2. (25) Zachariasse, K. A,; Kozankiewicz, B.; Kiihnle, W. In Photochemistry and Phofobiologv;Zewail, A. H., Ed.; Harwood Academic: New York, 1983; Vol. 2, p 941.

Comparison of the Solubilization Site of Naphthalene in Cationic and Anionic Micelles as a Function of Chain Length Studied by Optical Detection of Magnetic Resonance Spectroscopy of the Excited Triplet State Sanjib Ghosh, August H. Maki,* and Michael Petrin Department of Chemistry, University of California, Davis, California 95616 (Received: May 5, 1986)

Optical detection of triplet-state magnetic resonance (ODMR) is used to study the phosphorescent state of naphthalene (N) solubilized into trimethyl-n-alkylammonium bromide (C,TABr) micelles. The results are compared with those found for N solubilized by sodium n-alkyl sulfate (NaC,S) micelles. In NaC,S the 0,O phosphorescence band of N shifts to the red and resolution increases with increasing n, indicating a progressively less polar environment. A similar trend is observed in C,TABr; for a given n, however, the N site is more polar in C,TABr. A trend in the zero-field splitting (ZFS) parameter 1 0 1 observed in different micelles suggests that the polarizability increases in the sequence NaCloS < CloTABr-- C12TABr = C14TABr= NaC12S< NaCI4S< C16TABr. The triplet lifetime decreases linearly in C,TABr micelles with decreasing n and the normally unobserved ID1 - IEI ODMR transition appears when n I14, indicating a Br- external heavy atom effect. 0 1 + (El transition is independent These effects are not seen when Br- is replaced by CI- counterion. Although the width of the 1 of n in NaC,S, it increases with decreasing n in C,TABr because of an increase in heterogeneity of the N site. This effect could result from enhanced interactions with the cationic head groups of the surfactant and/or from increased water penetration into the cationic micelles.

Introduction The study of the nature of the microenvironment in different regions of a micelle is of fundamental importance in characterizing the micellar structure. The nature of the microenvironment includes the microviscosity, polarity, polarizability, and the extent of water penetration in the surfactant aggregates.' Several methodsZ providing specific information and having inherent restrictions have been employed to investigate the solubilization site in micelles. One technique utilizes luminescence probes whose spectral characteristics depend strongly on the environment of the medium. The most widely used probe to date (1) (a) Mukerjee, P.; Cardinal, J.; Desai, N. In Micellization, Solubilization and Microemulsion; Mittal, K. L., Ed; Plenum: New York, 1977; Vol. 1, p 241. (b) Menger, F. M. Acc. Chem. Res. 1979, 12, 111. (c) Reed, W.; Politi, M. Fendler, J. J . Am. Chem. Soc. 1981,103,4591 and references cited therein. (d) Wolff, T. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 145. (e) Abu-Hamdiyyah, M.; Rahman, I. A. J . Phys. Chem. 1985, 89, 2377. (2) (a) Fendler, J. H. Fendler, E. J. In Catalysis in Micellar and Macromolecular Systems; Academic: New York, 1975, p 31. (b) Kalyanasundaram, K. Chem. SOC.Reo. 1978, 7,453. (c) Thomas, J. K. Chem. Rev. 1980, 80, 283. (d) Lindman, B.; Wennerstrom, H. In Top. Curr. Chem. 1980,87, 3 and references cited therein. (e) Ganesh, K. N.; Mitra, P.; Balasubramanian, D. J Phvs Chem. 1982, 86, 4291.

0022-3654/86/2090-5210$01.50/0

is pyrene and some of its derivative^.^^,^,^,^ The study of the vibronic fine structure of the fluorescence4 as well as fluorescence lifetime measurements in a series of cationic n-alkyltrimethylammonium bromide micelles of varying chain length and also in anionic and nonionic surfactants showed that pyrene resides in the palisade layer of the micelles. In a recent investigation5 of triplet-triplet energy transfer between benzophenone and naphthalene in frozen sodium dodecyl sulfate micelles, we showed that optical detection of magnetic resonance (ODMR) of the triplet excited state is capable of distinguishing differences in the microenvironments experienced by a benzophenone donor and the naphthalene acceptor. The former showed broad ODMR transitions while the latter exhibited narrow transition line widths, which are indicative of a fairly homogeneous environment for naphthalene. This study also indicated that micelles retain their structural integrity at very low temperatures, a conclusion supported by the results of other techniques.6 In another study,' we have shown that low-tem~~

~~~

(3) Kalyanasundaram, K.; Thomas, J. K. J . Phys. Chem. 1977.81, 2176. (4) Lianos, P. Zana, R. J . Colloid Interface Sci. 1981, 84, 100. ( 5 ) Ghosh, S.; Petrin, M.; Maki, A. H. J . Phys. Chem. 1986, 90, 1643.

0 1986 American Chemical Society

The Journal of Physical Chemistry, Vol. 90, No. 21, I986

Solubilization of Naphthalene in Ionic Micelles

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TABLE I: Parameters of the Lowest Triplet State of N in Different Micelles and Media micelle system" NaC,,S NaC12S NaC,,S CloTABr C12TABr C14TABr CI6TABr C12TACI 20% glycerol n-decane

transition polaritye ~(0,0),bnm 472.6 473.6 474.7 472.0 473.0 473.2 474.6 472.8 470.0 475.3

LgtC s 2.36 2.35 2.34 1.21 1.44 1.66 1.97 2.35 2.40 2.40

IEl,d GHz

IDl,d GHz 3.040 3.034 3.027 3.033 3.036 3.034 3.009 3.032 3.054 2.980

0.47 1 0.469 0.468 0.464 0.464 0.468 0.466 0.463 0.469 0.461

ID1 + IEI

PI - IEl

-

no no no - vw - vw no no

+ + ++ + + +

21EI

+ + + + +

+ + + +

"Aqueous solutions except for n-decane. *Measured at 4.2 K with f0.l-nm accuracy. CThel / e lifetime measured at 4.2 K with fS% accuracy.

"Obtained at 1.2 K with f 2 - M H z accuracy except for ClzTACl and C,TABr (n = 10, 12, and 14), where the accuracy is f 5 MHz. 'A plus sign indicates an increase in phosphorescence intensity, a minus sign indicates a decrease, "no" indicates that the transitions are not observed, and "VW" indicates that the transitions are very weak.

perature phosphorescence and O D M R can be utilized to probe changes in the solubilization site of naphthalene in anionic sodium n-alkyl sulfate (NaC,S) micelles with a variation in the chain length or as the concentration of added Cs+ ion was gradually increased in NaC,,S micelles of fixed chain length. N was chosen as the probe in these studies since it has smaller molecular dimensions than pyrene and hence should cause less perturbation of the micellar architecture. In addition, the sublevel kinetics of the N triplet state are well characterized.8 In this paper we present low-temperature phosphorescence, decay, and O D M R results in zero applied magnetic field for N in a series of cationic trimethyl-n-alkylammoniumbromide micelles (C,TABr, n = 10, 12, 14, and 16) as a function of chain length. The results are compared with those observed for N in 20% aqueous glycerol and in n-decane matrices under similar conditions. Similar measurements are performed for N in trimethyldodecylammonium chloride (C12TACl)micelles as a control in order to distinguish any heavy atom perturbation of N in the micelles containing bromide counterions. Lifetime data and the appearance of the normally unobserved ID1 - IEl ODMR signal indicate a relatively minor perturbation of the N triplet state by an external heavy atom effect in cationic C,TABr micelles, when n I 14. We also compare the present results with those observed for N in a series of anionic sodium n-alkyl sulfate (NaC,S, n = 10, 12, and 14) micelles in our earlier work.' It has been demonstrated that low-temperature phosphorescence and ODMR can be exploited to study differences in the microenvironment experienced by N in cationic C,TABr and in anionic NaC,S micelles having the same alkyl chain length. This study also provides further evidence for the presence of weak specific interactionsgJO of the trimethylammonium head groups with the solubilized arene probe N in C,TABr micelles. Experimental Section C16TABr(Aldrich), CloTABr,CI2TABr,C14TABr,and CI2TACl (Eastman Kodak) were purified by recrystallization from methanol. Naphthalene was vacuum sublimed several times, and glycerol and n-decane were of spectral grade (Aldrich). Solutions (6) (a) Narayana, P. A.; Li, A. S . W.; Kevan, L. J . Am. Chem. Soc. 1981, 103, 3603. (b) Kevan, L.; Li, A. S. W.; Narayana, P. A. In Photochemistry and Photobiology; Zewail, A. H., Ed.; H a r w d Academic: New York, 1983; Vol. 2, p 1071. (c) Ohta, N.; Kevan, L. J . Phys. Chem. 1985,89, 2415. (d) Yamamoto, Y.; Murai, H.; I'Haya, Y. J. Chem. Phys. Lett. 1984, 112, 559. (e) Kutter, P.; Schmitt-Fuiman, W. W, Bachmann, L. Presented at the Sixth European Congress on Electron Microscopy, Jerusalem, Israel, 1976, p 119-121. (0 Kevan, L.; Hiromitsu, I. Presented at the 191st National Meeting of the American Chemical Society, New York, NY, 1986; Colloids 032. (7) Ghosh, S.; Petrin, M.; Maki, A. H. J . Phys. Chem., preceding paper in this issue. (8) (a) Sixl, H.; Schwoerer, M. Chem. Phys. Lett. 1970, 6, 21. (b) El Sayed, M. A.; Moomaw, W. R.; Chodak, J. B. Chem. Phys. Lett. 1973, 20, 11. (9) Lianos, P.; Viriot, M. L.; Zana, R. J . Phys. Chem. 1984, 88 1098. (IO) Almffren, M.; Grieser, F.; Thomas, J. J . Am. Chem. Soc. 1979, 101, 279.

, 151

111

1'II Ill H A V E L E N G T H

511

551

Ill

lnml

Figure 1. Phosphorescence spectra of N at 4.2 K with excitation at 300 nm using 16-nm band-pass and with emission slits of 1 nm: (A) in CloTABr (6.5 X lo-' M ) with N (6.5 X M); (B) in CI2TABr(1.6 X IO-' M ) with N (1.6 X lo-) M); (C) in C,,TABr (3.5 X M) with N (3.5 X lo4 M); (D) in CI6TABr(4 X M) with N (4 X M).

of N and surfactant were prepared with triply distilled, deionized water as solvent. All solutions were ultrasonically agitated for 30 min and subsequently degassed by bubbling oxygen-free nitrogen gas for 30 min immediately prior to measurement. The concentration of the amphiphile in each solution was chosen to be 10 times its critical micelle concentration (cmc) at room temperature. The concentration of N in all surfactant solutions was adjusted with respect to the average aggregation number at room temperature of each surfactant so as to provide on the average a single probe molecule per micelle. Some micelles will contain no N, and others will contain more than one N according to a Poisson distribution of the probes. The emission, decay, and zero-field ODMR apparatus used has been described elsewhere." All ZFS transitions were obtained by employing a solid-state microwave sweep oscillator (Hewlett-Packard Model 8350B with Model 83592A plug-in) calibrated with a frequency counter (Hewlett-Packard Model 5351A). Sweep times and microwave power levels were maintained the same for all slow passage experiments. Sample excitation was at 300 nm with 16-nm band-pass, while the emission slit width was 1 nm for phosphorescence determinations and 3 nm for microwave resonance experiments. Fast passage transient ODMR experiments were performed as described elsewhere.I2 Results Phosphorescence Spectra of N in Cationic Micelles. The phosphorescence spectra of N in C,TABr ( n = 10, 12, 14, and 16) micelles at 4.2 K are presented in Figure 1. The 0,O band is red-shifted with an increase in the chain length of the surfactant. (1 1) Ghosh, S.; Weers, J. G.; Petrin, M.; Maki, A. H. Chem. Phys. Lett. 1984, 108, 87. (12) Winscom, C. J.; Maki, A. H. Chem. Phys. Lett. 1971, Z2, 264.

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The 0,O band energies of N in all the micelles studied lie within a range bracketed by those observed for N in 20% aqueous glycerol and in n-decane, which exhibits the most red-shifted 0,O band (Table I). Figure 1 also shows that the phosphorescence emission of N becomes progressively more resolved and structured as the chain length in the cationic micelles increases. The phosphorescence spectrum of N in C,,TACl at 4.2 K is essentially the same as that observed in CI2TABr,and in both cases the 0,O band is blue-shifted compared with that observed in NaC,,S. Although quenching of N fluorescence was not investigated in C,TABr micelles, it was observed that the phosphorescence intensity is higher in C,TABr compared with that observed in NaC,S and C12TAClmicelles and that it increases as the chain length n decreases in a series of C,TABr micelles. The consistent variation of the phosphorescence 0,O band and the degree of the resolution of the phosphorescence as a function of chain length ( n ) clearly demonstrates that the observed phosphorescence is from N incorporated within the micelles and that the micellar structure is retained at 4.2 K under the experimental conditions employed here. Lifetime of N in Micelles. The decay of the triplet state of N as measured in NaC,S with varying n, in C12TAClmicelles, in 20% aqueous glycerol, and in n-decane at 4.2 K, was found to be a single exponential with a lifetime of ca. 2.35 s (Table I). The more rapid decays observed for the C,TABr micelles at 4.2 K, however, are multiexponential and the values presented in Table I are the average ( 1 / e ) lifetimes. Slow Passage Zero-Field ODMR Transitions and the Triplet-Sitate ZFS Parameters of N in Micelles. Slow passage ODMR transitions of the lowest triplet state of N were observed in all C,TABr and C,,TACl micellar solns. by monitoring the peak of the N 0,O phosphorescence band with 3-nm emission slit widths at 1.2 K. The microwave sweep rate was maintained constant in each case. For CI6TABr and Cl,TAC1 micelles only the ID1 14 transition, which occurs between the long in-plane and the out-of-plane axis sublevels, and the 21EI transition, which occurs between the two in-plane sublevels, appear strongly. In the other cationic micelles studied (C,TABr, n = 14, 12, and lo), on the other hand, all ODMR transitions including the normally unobserved ID1 - IEl transition are observed. The signal-to-noise ratio of the ID1 - IEI transition increases with a decrease in the chain length in the C,TABr series for n I14. The ZFS parameters and the polarities of all ODMR transitions are listed in Table I. This table also includes the ZFS values and transition polarities observed in the case of anionic micelles and the reference solvents 20% aqueous glycerol and n-decane from earlier measurements7 for comparison.

Ghosh et al.

i

21300

in H20

20% glycerol

-

0 C ,,TABr 0 NaC,S

2

t" n -decane

21000

12

10

14

16

C H A I N LENGTH Figure 2. Plot of the 0,O phosphorescence band energy of N in cationic and anionic micelles studied at 4.2 K as a function of chain length. The 0,O band energies observed in 20% glycerol and n-decane are indicated by horizontal lines for comparison.

20%glycerol in H,O3

=

260

*

2204

0

C ,TABr NaC,S

\

+

Discussion Effect of Environment on Probe Phosphorescence Emission. The blue-shift of the 0,O phosphorescence band of N and the accompanying loss of resolution in the vibrational structure in going from the nonpolar solvent n-decane to a polar 20% aqueous glycerol medium may be attributed to the lower degree of stabilization of the N triplet state by a rigid solvation geometry organized to stabilized the ground state. The blue-shift increases with solvent polarity. The 0,O band energies of the N triplet in the cationic C,TABr and anionic NaC,S micelles studied are plotted against chain length in Figure 2. The 0,O band energies of the N triplet state in 20% aqueous glycerol and in n-decane are indicated by horizontal lines. The red-shift of the 0,O band and the appearance of a more resolved phosphorescence emission, which accompany an increase in n for the cationic C,TABr surfactants, indicate a progressively less polar environment surrounding N as the chain length in the micelle is increased. In Figure 3 the width of the N phosphorescence 0,Oband in different micelles is plotted against the chain length. The gradual increase in width accompanying a decrease in chain length demonstrates that N experiences a progressively less homogeneous environment as surfactant length in the micelles is decreased. It is found that changes in the 0,O band energies and in the vibrational features of the phosphorescence spectra are relatively

10

12

14

16

CHAIN LENGTH InJ Figure 3. Plot of the 0,O phosphorescence band width (full width at half-rnaximum, fwhm) observed at 4.2 K for N in cationic and anionic micelles as a function of chain length. The 0,O band width observed in 20% glycerol and in n-decane are shown by horizontal lines for comparison.

less prominent in going from C12TABr to C14TABr micelles, compared with those observed in going from CloTABr to CI2TABr or from CI4TABrto C16TABr. This trend parallels the percentage change in the aggregation number of the C,TABr micelles as n increases by two methylene units. In the concentration range of the micelles used in this study the aggregation numbers are 40, 5 5 , 65, and 90 for n = 10, 12, 14, and 16, re~pectively.~The change in aggregation number in going from Cloto C12is ca. 38%, C l z to C14 is ca. 18%, and C14 to c16 is ca. 38%. The intensity ratio of the first to third fluorescence vibronic bands of pyrene solubilized in C,TABr micelles was found4 to decrease to a relatively large extent in going from C,, to CI6. The 0,O phosphorescence band energy and the spectral resolution are essentially the same for N solubilized into C12TABr and Cl,TAC1 micelles. This indicates that the microenvironment of N in cationic micelles having the same head group and chain length is independent of the nature of the counterion. This insensitivity to counterion has been observed for pyrene solubilized in CI6TABr, C16TAC1, and C16TAOH micelle^,^ where it was found that the ratio of the intensities of the first to third fluorescence vibronic bands remained invariant to a change in counterion. Comparison of Figures 2 and 3 demonstrates that for a given alkyl chain length cationic and anionic micelles exhibit a difference

Solubilization of Naphthalene in Ionic Micelles

The Journal of Physical Chemistry, Vola90, No. 21, 1986 5213 1

I

A 10

12

14

t

-

ID1 IEl

2lEl

16

C H A I N LENGTH i n ) Figure 4. Plot of the average triplet lifetime of N obsd. at 4.2 K in cationic C,TABr micelles as a function of chain length.

in the polarity and in the inhomogeneity of the microenvironment of the micelle-solubilized N probe. The greater polarity and more inhomogeneous environment experienced by N in the cationic C,TABr micelle compared with those in the NaC$ anionic micelle having the same alkyl chain length may be explained as follows: (i) In the case of the anionic micelles having sulfate head groups, electrical charge is distributed over three oxygen atoms, while the fourth is involved in binding the alkyl chain to the sulfate group. Since the ester oxygen is not particularly hydrophilic, it may be included in the hydrophobic core,13with the result that the effective chain length of a NaC,S surfactant molecule is greater than n. Head-group repulsion is also more prominent in the quaternary ammonium cationic micelles due to steric factors. Thus NaC,S micelles may have more compact and less water-permeated structures than the corresponding C,TABr micelles. (ii) The measurement of the polarity of the microenvironment, the fluorescence lifetime, the efficiency of excimer formation by pyrene and dipyrenyl propane, the time required for solubilizing pyrene into various micellar solution^,^ and a solubility studylo suggest the existence of a weak interaction between neutral arenes and the quaternary ammonium head groups of cationic micelles of type C,TAX (X = CI, Br, OH). This interaction, which is presumably due to the attraction of the a-electron cloud of the arene and the positively charged head group^,^ draws N closer to the surface than is the case in a NaC,S micelle, where the negatively charged sulfate groups will repel the a-electron cloud of the solubilized probe. This weak interaction should be more prominent for surfactants of shorter chain length, which have correspondingly smaller micellar dimensions. This trend is clearly demonstrated by the ODMR results discussed in a later section. Effect of Counterions on the Triplet Lifetime. Comparison of lifetime data for N in different micelles (Figure 4 and Table I) clearly indicates the perturbation of the N triplet state by the heavy atom Br- counterions of C,TABr micelles. The external heavy atom effect of Br- counterions on some arenes including anthracene, its derivatives, and fluorene in C16TABrmicelles at room temperature has been observed by Wolff,14 who noticed a decrease in the fluorescence quantum yield and an increase in the triplet quantum yield of the probe while changing the surfactant from C,,TACI to CI6TABr. The multiexponential decays that we observe for C,TABr micelles are explained by the fact that N experiences a variable distribution of counterions from micelle to micelle after freezing. The consistent linear decrease in the average lifetime of N in the series of C,TABr micelles suggests that all the N are solubilized within the micelles, and N does not occupy a region very close to the polar region of the micelle surface, and it does not reside in the interior core of the micelle. If either situation prevailed, the lifetime would be insensitive to the variation of chain length since the ratio of the probe to micelle concentration is kept invariant throughout the series of micelles investigated. (13) Bondi, A. J . Phys. Chem. 1964, 68, 441.

Y

28

019

.,

.

1:O

2145

2.55

2.65

F R E Q U E N C Y

3.4

3.5

'

, 3.6

IGHZI

Figure 5. Slow passage ODMR transitions of N in C,,TACI, C,,TABr, and C,,TABr micelles at 1.2 K. The sweep rate for the transitions is ca. 7 MHz s-I, and the microwave power level is ca. 20 mW.

The observed shortenings of the probe lifetime shows that the average distance between N and' the micelle counterions decreases as the chain length decreases, resulting in an increasing heavy atom perturbation in the shorter chain surfactants. Effect of Environment on ODMR Zero-Field Transitions. ODMR Transitions and Counterion External Heavy Atom Effect. Although only two transitions, ID1 IEl and 21E1, are readily observed in C12TACI,CI6TABr, and anionic NaC,S micelles, in 20% aqueous glycerol and in n-decane matrices, all three transitions appear with comparable intensity in the C,TABr micelles when the value of n is 5 14. The observation of the normally unobserved ID( - IEI transition combined with the fact that the signal-to-noise ratio of the signal increases with a decrease in n definitely can be associated with an external heavy atom perturbation of the N triplet state in C,TABr micelles induced by Br- counterions, since the probe-to-micelle concentration ratio was maintained the same throughout the series. The ID( + IEI transitions observed for N in C,TABr micelles of n I14 appear with a decrease in the relative phosphorescence intensity (negative polarity), in contrast with the positive polarity observed for this signal in C,,TABr, C12TACI,and NaC,S ( n = 10, 12, and 14) micelles. The polarity of the 214 signal is found to be positive for all micelles studied. The ID1 - IEI transition in C,TABr micelles ( n I14) appears with negative polarity. The Occurrence of the (DI - (El transition in C,TABr ( n I14) micelles indicates either a change in the radiative character or a change in the steady-state populations of the sublevels, or both, since the change in phosphorescence intensity accompanying a microwave slow passage through any two sublevels i and j is given by

+

where Q,= k,'/k, is the radiative quantum yield and N,O represents the steady-state population of sublevel i. The steady-state populations of the triplet sublevels are governed by the intersystem crossing (ISC) rates @,) to the individual sublevels, the total decay rate constants (k,)of the individual sublevels, and the spin-lattice relaxation (SL.R) rate constants ( W , ) between sublevels i and j . These constants may be obtained by solving the set of three simultaneous equations of the type PI = ( k ,

TI

W,k)Nln- Wl,N:

- wk,Nko

(2)

For the N triplet it is well-known that Q, >> Qy > Qz,8where x and y are the in-plane long and short axes in N, respectively,

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The Journal of Physical Chemistry, Vol. 90, No. 21, 1986

and z is the out-of-plane sublevel. Considerations of the polarities of the ID1 IEl and 21EI transitions observed in N a C 3 , C16TABr, and C12TAC1micelles (Table I) indicate that the steady-state population pattern in these systems should be in the order N> > N: and Njo > N: (from eq 1). Further, N: 2 N: > N:, since the ID1 - IEI signal either is not observed (NaC,S micelles) or is observed with very weak intensity in CI6TABr and Cl2TAC1 micelles with a negative polarity. If it is assumed that the radiative quantum yields of the N triplet sublevels are ordered in the same manner in C,TABr ( n I14) micelles as they are for unperturbed N, the polarities of the ID1 + IEl and the 214 transitions observed in C,TABr ( n I14) suggest that the steady-state population pattern is N 2 > N: and N: > N 2 (from eq l ) , or N: > N> > N:. This predicts that the ID1 - IE1 transition should be of negative polarity in C,TABr ( n I14) micelles, which is exactly what is observed. However, if it is assumed that the steady-state population pattern in the perturbed N (in C,TABr micelles, n I 14) and in the unperturbed N (in other micelles) remains unchanged in the order N,,” 2 N: > N2, then the polarity of the 1 0 1 1 4 transition observed in C,TABr (n I14) micelles suggests that Q, > Q,,whereas the polarity of the 2(EI transitions predicts that Qx > Qy. This particular pattern of the radiative quantum would predict a positive polarity for yields, viz. Q, > Qx > the ID/ - IEI transition, which is opposite to that observed. Thus, consideration of the polarities of ODMR transitions leads to the following inferences: (a) the pattern of the radiative quantum yields of the unperturbed sublevels, viz. Q, >> Qy > Qz,remains unaltered in C,TABr ( n I14) micelles and (b) the pattern of the steady-state populations is altered from that observed in unperturbed triplet N and is found to be in the order of N,,” > N 2 > N:. This interpretation is corroborated by the results of fast passage microwave transient experimentsI2 performed on these systems; the shape of the response suggests qualitatively that there is no significant alteration of the normal relative radiative character of the sublevels of the lowest triplet state of N. We are not able to give a quantitative measure of the relative radiative or overall rate constants of the sublevels since the decays observed are multiexponential even at 4.2 K as mentioned earlier. Although the S L R rates between the triplet sublevels of N are sensitive to host structure,” they may be assumed to be negligible compared with the kiat 1.2 K in the cationic micelle series studied. Thus the change in the pattern of the steady-state populations of the N triplet in C,TABr (n I14) micelles can be ascribed to a change of the populating rates of the sublevels and the total decay rates of the sublevels. Since the triplet lifetime of N in C,TABr ( n I 14) micelles changes by less than a factor of 2 from the normal unperturbed lifetime, it may be concluded that SI T, ( n = x, y , z) ISC rates are affected to the greater extent by the external heavy atom effect induced by Br- counterion. The enhancement of S , TI ISC rates by the heavy atom has been suggested by Wolff14 in a comprehensive study of the luminescence quantum yields of arene triplet probes solubilized into C,,TABr and c16TACl micelles. Variation of Triplet-State ZFS Parameters of N in Micelles. The ZFS parameters of the N triplet state in different cationic micelles are compared in Table I with those observed in 20% aqueous glycerol, in n-decane, and in NaC,S anionic micelles. Values for the IEI parameter do not show a measurable change between cationic micelles of differing alkyl chain length. This is also the case for anionic micelles. For NaC$ micelles, the ID1 values show a consistent linear decrease with an increase of chain length but cationic micelles show a different trend. The value of ID1 is the highest in 20% glycerol medium and the lowest in the nonpolar medium n-decane (Table I). Destabilization of the N triplet state by the local environment is expected to perturb the a-electron distribution of the molecular wavefunction and thereby affect the ZFS parameters ID1 and IEI. These are determined by the symmetry and magnitude of the magnetic dipolar interactions between the unpaired electrons of the triplet state and may be expressed asI5

Ghosh et al.

+

+

e,,

-

-

(14) Wolff, T.Ber. Bunsen-Ges Phys. Chem. 1982, 86, 1132

(3) av

where x, y , and z are the projections of the electron-electron separation vector r12onto the principal molecular axes, with the coordinate functions averaged over the triplet spacial wave function. the z direction is perpendicular to the plane of naphthalene, and the x and y axes are located in the aromatic plane. An increase of the local polarizability has the effect of reducing the Coulombic attraction of the a-electrons with a resulting expansion of the molecular wavefunction and concomitant increase in the average separation r12.The decrease observed in the value of ID1 for n-decane compared with 20% aqueous glycerol may be attributed to this effect since the polarizability, which depends on the refractive index of the medium as (n2 - 1)/(2n2 l), decreases from 0.197 in n-decane to 0.178 in 20%aqueous glycerol. If the triplet electron distribution is assumed to expand isotropically, one obtains from eq 3

+

(4)

Equation 4 shows that both ID1 and IEI should decrease in a more polarizable environment, the magnitude of the decrease 4does not decrease depending on their absolute values. Although 1 measurably due to its small magnitude, 1DI decreases linearly as the surfactant chain length is increased in the NaC,S micelles. The value of dD/dE obtained experimentally (24) shows that the increase in the polarizability experienced by N in the longer chain NaC,,S micelles can be interpreted to lead to an isotropic expansion of the molecular electronic distribution. Whatever differences in polarizability exist at the N solubilization site as a function of chain length in C,TABr micelles (for n = 10, 12, and 14) is probably masked by either the heavy atom perturbation by Br- or by specific interactions of N with the cationic head groups in C,TAX (X = Cl, Br) micelles. Similar values of ID1 observed in C,TABr ( n 2 14) and in CI2TAC1 micelles support the presence of specific interactions rather than a heavy atom effect. A significant decrease in 101, however, occurs in C16TABr,where specific interactions are smaller, indicating that N resides in a relatively polarizable environment in C,,TABr micelles. Line Width of ODMR Transitions of N in Micelles. Narrower ODMR line widths are observed for N in surfactants of all chain lengths compared with those found in 20% aqueous glycerol solution, indicating that N experiences a more homogeneous environment in micelles. The ODMR line widths in micelles are greater than those observed for N in an n-pentane polycrystalline Sphol’skii matrix, however, where N exhibits phosphorescence from a single site with narrow ODMR transitions frequencies of a.5-MHz width at 1.2 K.” It is possible that some heterogeneity may result from multiple occupancy of micelles by N in the Poisson distribution. The widths of the ODMR transitions for NaC,S micelles studied are found to be independent of n. The widths of the ID1 14 transitions in the case of C,TABr micelles, however, increase consistently with a decrease of n as shown in Figure 6. Although the triplet state of N senses a somewhat different average environment in going from NaCloS to NaC14S,the inhomogeneous broadening originating from the spacial and orientational distribution of N within the micellar ensemble results in the same widths for the transitions in these surfactants. The line widths observed in these N a C 3 micelles are comparable to those observed for N in 3-meth~1pentane.l~The line width variation of the ID1

+

(15) McGlynn, S. P.; Azumi, T.;Kinoshita, M., The Triple1 State; Prentice Hall: Englewood Cliff, NJ, 1963. (16) (a) Gradl, G.; Friedrich, J. Chem. Phys. Left. 1985, 114, 543. (b) Gradl, G.; Friedrich, J.; Kohler, B. E.J . Chem. Phys. 1986, 84, 2079.

Solubilization of Naphthalene in Ionic Micelles

6oL

209:glyeerol in H,O

10

12

14

-

16

C H A I N LENGTH i n ) Figure 6. Plot of the line width (fwhm) of the ID1 + IEI zero-field transition of N observed at 1.2 K in cationic and anionic micelles as a function of chain length. The widths observed at 1.2 K in 20% glycerol and n-decane are indicated by horizontal lines.

+ IEI transitions shown in Figure 6 definitely suggests that the C,TABr cationic micelles have a more open structure than the corresponding NaC,S anionic micelles of the same alkyl chain length. This factor, allowing solvent intrusion, and also the presence of a weak specific interaction between N and the charged head groups probably are the main contributions to the inhomogeneous broadening. The increase in line width with decreasing chain length in C,TABr micelles also indicates that these broadening factors become more significant as the chain length of surfactant decreases. Solubilization Site of N and Micellar Structure. The consistent variation of triplet-state properties, including the energy, 0,O phosphorescence line width, ZFS parameter ID!,and ODMR line widths of N in the micellar systems studied, supports the conclusion proposed previously7that N is located preferentially in the palisade layer of micelles where the probe is sensitive to slight variations in the degree of hydrocarbon-water contact resulting from a change in the surfactant chain length. The structure of equilibrated micelles is governed by four energetic contribution^:'^ surface energy (hydrocarbon-water in-

The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5215

terface), head-group repulsion (steric and electrostatic), conformation energy (trans/gauche isomerism), and packing increments (enthalpic and entropic). Our earlier work’ on the triplet state of N in frozen NaC# micelles suggested that the Hartley model,’8 which proposes a hydrocarbon-like interior with a definite double layer, is suspect a t short chain lengths but provides a better description for surfactants with longer hydrocarbon chains, as suggested by Evans et al.19 and demonstrated experimentally by several others.le,6b-20.21The present work employing a N triplet probe solubilized into C,TABr and C12TAClmicelles supports this line of thinking. A comparative study between cationic and anionic surfactants suggests a more open structure for the C,TABr micelles that results in a greater probability of water and counterion penetration into micelles of shorter chain length. This is accompanied by a small heavy atom perturbation of the N triplet state in the presence of Br- counterion.

Conclusion The experimental results presented in this paper reveal an external heavy atom effect produced by Br- counterions on the probe N solubilized into C,TABr micelles, the degree of the perturbation increasing progressively with a decrease in the chain length of the surfactant. Our findings suggest a more open structure for the C,TAX micelles compared with NaC,S micelles for a given n and also add support to the existence of a specific interaction between the arene probe and the quaternary ammonium head groups of the C,TAX surfactant molecules. Acknowledgment. We gratefully acknowledge the support of this work by the National Science Foundation. Registry No. N , 91-20-3; NaC& 142-87-0; NaC12S, 151-21-3; NaCI4S,1191-50-0; CloTABr, 2082-84-0; C12TABr,1 1 19-94-4; CI,TABr, 1 1 19-97-7; CI6TABr,57-09-0; C12TAC1,112-00-5. (17) Poland, D. C.; Scheraga, H. A. J . Colloid Interface Sci. 1966, 21, 273. (18) Hartley, G. S. Q.Rev. Chem. SOC.1948, 2, 152. (19) Evans, D.; Ninham, B. W. J . Phys. Chem. 1983, 87, 5025. (20) Ottaviani, M. F.; Baglioni, P.; Martini, G. J . Phys. Chem. 1983, 87, 3 146. (21) Zachariasse, K. A.; Kozankiewicz,B.; Kiihnle, W. In Photochemistry and Photobiology, Zewail, A. H., Ed.;Harwood Academic: New York, 1983; Vol. 2, p 941.