5206
J . Phys. Chem. 1986, 90, 5206-5210
Solubilization Site of Naphthalene in Anionic Micelles Studied by Optically Detected Magnetic Resonance of the Excited Triplet State Sanjib Ghosh, Michael Petrin, and August H. Maki* Department of Chemistry, University of California, DaviJ, California 95616 (Received: December 12, 1985: In Final Form: May 10, 1986) Phosphorescence and optical detection of triplet-state magnetic resonance (ODMR) in zero applied magnetic field have been utilized to probe subtle changes in the microenvironment of solubilized naphthalene (N) in frozen anionic sodium n-alkyl sulfate micelles as a function of chain length and in sodium dodecyl sulfate micelles as a function of Na’ and Cs’ counterion concentration. The results are compared with those observed for N in 20%aqueous glycerol and n-decane media under similar conditions. The resolution of the phosphorescence spectrum increases and the 0,Oband shifts to the red as the chain length or the Cs’ concentration is increased, indicating a progressively less polar environment experienced by solubilized N. The decrease of the triplet-state zero field splitting (ZFS) parameter ID1 with an increase of chain length or with an increase of Cs’ concentration suggests that N is solubilized in an increasingly polarizable environment. The results indicate that the degree of hydrocarbon-water contact within the micelles decreases with an increase of chain length as well as with an increase of the Cs’ concentration in a micelle of fixed chain length.
Introduction There is considerable interest in the nature of the structural organization of multimolecular assemblies of amphiphilic molecules. In particular, knowledge of the extent of water penetration into the micellar interior is essential in developing a realistic model of the architecture of the micelle. It is axiomatic to say that an accurate picture of the local microenvironment surrounding a solubilized molecule, its local concentration, and relative orientation are of fundamental importance in understanding the nature of this solubilization phenomenon as well as in illuminating the overall structural features of the micelle. The classical model of a micelle as proposed by Hartley’ is an essentially spherical aggregation of a number of surfactant molecules surrounded by a well-defined “Stern layer”. This model requires the micellar interior to be essentially anhydrous. This two-state model has been challenged by Menger,2 whose work suggests that micelles are better described as “porous cluster” aggregates where water and counterion are capable of deep penetration into the interior region. The low average hydration numbers determined for various micellar aggregate^,^ however, mitigate against extensive water penetrability. The controversy concerning the micellar structure is mainly due to conflicting results obtained from studies to determine the location and orientation of solubilized spectroscopic probes within the local micellar microenvironment. Several experimental techniques used in the investigation of the solubilization of arenes have been summarized in ref 4. Among the arenes, the solubilization site of pyrene in micelles has been studied most extensively. Analysis of pulsed FT ‘H N M R chemical shifts5showed that pyrene is solubilized in a hydrophobic region of cationic cetyl trimethyl ammonium bromide (CTAB) micelles while the results of vibronic fluorescence intensity measurements6 suggest the opposite. Solubility data7,*also indicate that pyrene is preferentially solubilized at the surface of alkyl trimethyl ammonium bromide surfactant micelles. Recently, however, fluorescence decay deconvolution and vibronic fluorescence intensity ratios9 Hartley, G. S. Q. Reu. Chem. SOC.1948, 2, 152. (a) Menger, F. M. Arc. Chem. Res. 1979, 12, 11 1. (b) Menger, F. M.; , B. J. J . Am. Chem. SOC.1980, 102, 5936. Wennerstrdm, H.; Lindman, B. J . Phys. Chem. 1979, 83, 2931. (4) (a) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic: New York, 1975; p 31. (b) Kalyanasundaram, K. Chem. SOC.Reu. 1978, 7,453. (c) Thomas, J. K. Chem. Rev. 1980, 80, 283. (5) Gratzel, M.; Kalyanasundaram, K.; Thomas, J. K. J . Am. Cfiem.Soc. 1974, 96,7869. (6) Kalyanasundaram, K.; Thomas, J. K. J . Am. Cfiem. SOC.1977, 99, 2039. (7) Almgren, M.; Grieser, F.; Thomas, J. K. J. Am. Chem. Soc. 1979, 101, 219. (8) Kalyanasundaram, K.; Thomas, J. K. J . Phys. Chem. 1977,81, 2176. (9) Lianos, P.; Zana, R. J . Colloid Interface Sci. 1981, 84, 100
0022-3654/86/2090-5206$01.50/0
determined for cationic n-alkyl trimethyl ammonium bromide surfactants of varying chain length from n = 10-16 revealed that the probe is solubilized in the micellar palisade layer. UV absorption spectroscopy1° indicates that N and benzene are solubilized into a relatively polar environment. In contrast, however, time-correlated fluorescence studies” in anionic micelles indicate N solubilization into the micelle interior. Steady-state and time-resolved fluorescence measurernentslZhave shown anthracene to be located at the surface of the CTAB micelle. Solubilization studiesI3 of N-alkylphenothiazine in aqueous sodium, manganese, and zinc dodecyl sulfate solutions indicate that the probe preferentially occupies the palisade layer of the anionic micelle. The study of the systematic variation of selected physical properties of the probe as a function of chain length can provide valuable information concerning the solubilization site, as observed by Lianos et aL9 In a recent inve~tigation’~ of triplet-triplet energy transfer between benzophenone and N in frozen sodium dodecyl sulfate micelles, optical detection of magnetic resonance (ODMR) revealed very broad transition line widths for benzophenone while N exhibited narrow line widths. These results support a view that the environments experienced by benzophenone and N in sodium dodecyl sulfate micelles are different. The former appears to be located near the surface of the micelle while N is located in a more hydrophobic region. This study also supports the observation resulting from other investigation^^^+'^ that micelles retain their structural integrity even at very low liquid helium temperatures. This prompted us to exploit ODMR and low-temperature phosphorescence techniques to investigate the microenvironment experienced by a naphthalene probe solubilized in anionic micelles. In this article we present low-temperature phosphorescence spectra and zero-field ODMR transitions of N in a series of anionic sodium n-alkyl sulfate micelles (NaC,S, n = 10, 12, and 14). We (10) Mukherjee, P.; Cardinal, J. R.; Desai, N. R. In Micelliration, Solubilization and Microemulsions; Mittal, K. L. Ed.; Plenum: New York, 1977; Vol. 1, p 241. (1 1) Hautala, R. R.; Schore, N. E.; Turro, N. J. J . Am. Chem. SOC.1973, 95,5508. (12) Blatt, E.; Ghiggino, K. P.; Sawyer, W. H. J . Phys. Chem. 1982, 86, 446 1. (13) Moroi, Y.; Noma, H.; Matuura, R. J . Phys. Chem. 1983, 87, 872. (14) Ghosh, S.;Petrin, M.; Maki, A. H. J . Phys. Chem. 1986, 90, 1643. (15) Kutter, P.; Schmitt-Fuiman, W. W.; Bachmann, L. Presented at the Sixth European Congress on Electron Microscopy, Jerusalem, Israel, 1976; pp 119-121. (16) (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.; Harwocd Academic: New York, 1983; Vol 2, p 1071. (c) Ohta, N.; Kevan, I.. J . Phys. Chem. 1985, 89,2415. (d) Yamamoto, Y.; Murai, H.; I’Haya, Y. J. Chem. Phys. Lett. 1984, 112, 559. (e) Kevan, L.; Hiromitsu, I. Presented at the 191st National Meeting of the American Chemical Society, New York, NY, April 1986; Colloids 032.
0 1986 American Chemical Society
Solubilization of Naphthalene in Anionic Micelles
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5201
TABLE I: Phosphorescence 0,O Band at 4.2 K and ZFS Parameters of the Lowest Triplet State of N in Various Micelles and Media system X(0,O): nrn (Dl IE(,b GHz A Y ~ MHz ~ ~ ~ : 2(El,bGHz A v ~ MHz ~ ~ ~ , ID(, ~ GHz 55 3.054 3.523 52 0.938 20% glycerol in H 2 0 470.0 21 3.040 3.511 25 0.942 NaCIoS in H 2 0 472.6 26 0.939 20 3.034 3.503 NaCI2S in H 2 0 473.6 3.495 25 0.936 20 3.027 NaCI4S in H 2 0 474.7 18 2.993 3.458 26 0.930 NaCI2S in H 2 0 with 474.4 0.2 M CsCl pure n-decane 475.3 3.441 30 0.922 30 2.980
+
IEl, GHz 0.469 0.471 0.469 0.468 0.465 0.461
“Measured at 4.2 K, with f0.l-nmaccuracy. bMeasured at 1.21 K, with f2-MHz accuracy. cFull width at half-maximum of transition.
also report similar observations made in NaC12Smicelles in the presence of varying amounts of added NaCl and CsCl in the low counterion concentration regime where the micelle is thought to retain its spherical shape. These results are compared with the measurements made on N in a 20% aqueous glycerol low-temperature glass and in frozen n-decane. Interpretation of the results regarding the site of solubilization and water penetration into the micelle has been compared with those obtained by other techniques. Our findings indicate that O D M R can successfully be employed to probe the nature of the microenvironment of a micelle-solubilized molecule. Subtle differences in the polarity and polarizability of the medium surrounding N as the chain length of the surfactant is varied in increments of two methylene units can be detected, as can influences due to the nature of the specific counterion associated with the micelle.
Experimental Section NaC12S (electrophoresis grade, Bethesda Research Lab), NaCloS, and NaCI4S (both from Eastman Kodak) were used as supplied. Naphthalene was vacuum sublimed several times, and CsCl and NaCl were of analytical grade. Solutions 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 with oxygen-free nitrogen gas for 30 min immediately prior to measurement. The concentration of amphiphile in each solution was chosen to be 10 times its respective 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, following a Poisson distribution. The emission and zero-field ODMR apparatus has been described elsewhere.” All ODMR transitions were obtained by employing a solid-state microwave sweep oscillator (HewlettPackard Model 8350B) calibrated with a frequency counter (Hewlett-Packard Model 535 1A). Sweep times and microwave power levels were maintained the same for all slow passage experiments. Sample excitation was at 300 nm with a 16-nm band-pass, while the emission slit width was 1 nm for phosphorescence determinations and 3 nm for microwave resonance experiments. Results Phosphorescence Spectra of N in Frozen Micelles as a Function of Alkyl Chain Length. The phosphorescence spectra of N in anionic NaC,S micelles of differing chain length at 4.2 K are shown in Figure 1. This figure also presents the phosphorescence spectra of N in 20% aqueous glycerol solution and in pure n-decane at 4.2 K. The results presented in Figure 1 show that the 0,O band of N gradually shifts to the red as the chain length of the micelle increases (Table I and Figure 2 ) . The 0,O band energies of N in the micelles studied fall within a range bracketed by that observed for N in 20% glycerol and that obtained in n-decane, where the 0,O band is the most red-shifted (Table I). Figure 1 also demonstrates that the phosphorescence emission spectrum of N becomes progressively more resolved and structured as N (17) Ghosh, S.; Weers, J. W.; Petrin, M.; Maki, A. H. Chem. Phys. Lett. 1984, 108, 87.
151.
(IS.
(11.
155.
111.
111.
111.
W A V E L E N G T H
551.
111.
511
(nm)
Figure 1. Phosphorescence spectra of N at 4.2 K with excitation at 300 nm with 16-nm band-pass and with emission slit width of 1 nm: (a) in 20% aqueous glycerol with N (0.001 M); (b) in NaCloS (0.34 M) with N (0.003 M); (c) in NaCI2S (0.086 M) with N (0.0008 M); (d) in NaC14S(0.024 M) with N (0.0002 M); (e) in NaC12S(0.086 M) with N (0.001 M) and added CsCl (0.2 M); (f) in pure n-decane with N (0.001 M).
moves in order from an aqueous 20%glycerol medium, through the NaCloS, NaCI2S, and NaCl,S micelles, and finally to a n-decane matrix. The consistent variation of the 0,O band of the phosphorescence and the degree of 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 used. Micellar structure present at room temperature has been found recently to be retained upon freezing to 4.2 K.16e Phosphorescence Spectra of N in Frozen NaC12SMicelles with Varying Amounts of Added NaCI and CsC1. The addition of NaCl to a solution of fixed N and surfactant concentrations to a final Na+ concentration of ca. 0.2 M does not result in a noticeable change in the phosphorescence spectrum of N in NaC12S micelles compared with that observed when no NaCl is added to the micellar solution. However, the addition of CsCl under similar conditions produces a more resolved phosphorescence spectrum
5208 The Journal of Physical Chemistry, Vol. 90, No. 21, 1986
t
Ghosh et al.
21300
+.
- - - - -.-..... 20% glycerol in H20
-...-. -. -
t
3.00
-
___
e . . ....-. ..--n decane ........._...__ ..
21000
t 2.98
c --.-.-__ n-decane
10 12 14 C H A I N LENGTH ( n )
10
Figure 2. Plot of phosphorescence 0,O band energy of N in micelles as a function of NaC,S surfactant chain length, n = IO, 12, and 14. The phosphorescence 0,O band energies observed in 20% glycerol and n-decane are indicated by horizontal lines for comparison.
with the N 0,O band red-shifted with respect to Na+ ion. Figure l e presents a typical phosphorescence spectrum of N in NaC12S micelles in the presence of 0.2 M CsCI. Slow Passage ODMR Transitions and the Triplet-State ZFS Parameters of N in Anionic Micelles. Slow passage ODMR transitions of the lowest triplet state of N was observed in all micellar solutions, in 20% aqueous glycerol, and in pure n-decane by monitoring the center of the N phosphorescence 0,O band observed in the respective media with 3-nm emission slit widths. In each case two transitions, the ID1 + JEl (between the long in-plane and out-of-plane axis sublevels) and the 214 (between the two in-plane axis sublevels), were observed. The other transition, ID1 - 14, was obtained by doubleresonance techniques. The center frequencies of the transitions, their half-widths, and the respective ID1 and 1El values are presented in Table I. All transitions except 214 in 20% aqueous glycerol appeared as an increase in the relative phosphorescence intensity. The accuracy in the measurement of the frequencies of the transitions is better than f 2 MHz in all slow passage experiments. Values for the IEl parameter show barely significant changes between micelles 0 1 values, however, show of differing alkyl chain length. The 1 a linear decrease with an increase of chain length (Figure 3). The values of ]Dl observed are highest in a 20% glycerol medium and lowest in the nonpolar n-decane. Although the value of ID1 observed in NaC,,S micelles does not change as a function of added NaCl up to a final concentration of 0.2 M Na', it decreases with the addition of CsCl as shown in Figure 4. It is noted that although the widths of the transitions observed in different micelles do not vary significantly, they are somewhat narrower than the widths observed in n-decane, but they are much narrower than those observed in the glycerol medium. Discussion Effect of Environment on Probe Phosphorescence Spectra. The blue-shift of the phosphorescence 0,Oband 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 solution may be attributed to the lower degree of stabilization of the N triplet state by a rigid solvation geometry organized to stabilize the ground state. The linear red-shift of the 0,O band (Figure 2 ) and the appearance of a more resolved phosphorescence emission, which accompanies an increase in the chain length of the surfactant (Figure l ) , indicates a progressively less polar environment surrounding N as the chain length of the micelle is
12
CHAIN
14
LENGTH ( n )
20% glycerol in H F
\
u 10
12
14
C H A I N LENGTH
In)
Figure 3. Plot of the triplet-state ZFS parameters ID1 and [El of N for NaC,S micelles (n = IO, 12, and 14). The surfactant and N concentrations used are the same as given in Figure I . The ZFS parameters observed in 20% glycerol and in n-decane are shown by horizontal lines for comparison.
3.04
3.03
-
1-7
3-
3.01
\
3.00t NaC12S 2.99
0
w i t h Nn' w i t h Cs*
0 0.1 CONCENTRATION
0.2 OF ADDED ION
(MI
Figure 4. Plot of the triplet state ZFS parameter ID1 for N in NaC,,S micelles containing varying amounts of added CsCl and NaCI. The concentration of surfactant is 0.086 M and N is 0.001 bl.
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5209
Solubilization of Naphthalene in Anionic Micelles increased. The red-shift of the 0,O band and the concomitant structured phosphorescence of N in NaCI2S micelles with the addition of CsCl compared to a system of similar NaCl concentration also indicate that N experiences a less polar environment in NaC$ micelles when Cs' counterions are associated with the micelle. Thus the association of Cs+ ions with the NaCI2Smicelles could act to decrease the extent of hydration of the micelle. Effect of Environment on ODMR Transitions and ZFS Parameters. The interaction of the N triplet state with the local environment can be expected to perturb the r electron distribution of the molecular wavefunction and thereby affect the zero-field 0 1 and IEI are splitting parameters. The ZFS parameters of 1 determined by the symmetry and magnitude of the magnetic dipolar interactions between the unpaired electrons of the triplet state and can be expressed asl8
greater than those observed for N in a n-pentane polycrystalline Shpol'skii matrix, where N exhibits phosphorescence from a single site with narrow ODMR transitions of ca. 5-MHz width" at 1.2 K. These transitions occur as a decrease of relative phosphorescence intensity. Although the triplet state of N senses a slightly different environment in going from NaC,,S to NaC14S or to NaCI2Smicelles containing Cs' ion, the inhomogeneous broadening originating from the spacial and orientational distribution of N within the micelles is comparable in these different surfactant structures. It is possible that some heterogeneity may result from multiple occupancy of micelles by N in the Poisson distribution. Solubilization Site of N . Although fluorescence lifetime measurements in anionic micelles" suggest that N is solubilized into an inner core of hydrophobic environment, UV absorption studied0 promote a relatively polar environment for N in NaC12S micelles. The phosphorescence of N at room temperature in NaCI2S has not been observed, but triplet emission has been detected at room temperature from micelles composed of NaC12S TIClzS and NaC$ + AgC12Ssurfactant mixture^.'^^^^ In the presence of T1' and Ag', N exhibits a drastically shortened triplet (T,) lifetime compared with that observed in organic glassy solvents. This has been attributed to enhanced spin-orbit coupling resulting from heavy atom perturbation. From the comparison of the lifetimes of the triplet states of N and pyrenc in silver dodecyl sulfate micelles at room temperature, Humphry-Baker et al.19bsuggested that N is likely to reside closer to the micelle-water interface than does pyrene. The consistent variation of both the phosphorescence and ODMR frequencies obtained in this work suggest that N is neither buried in the essentially hydrophobic core of the micelle nor adsorbed near the very polar surface region. This supports the idea that N is located preferentially in the palisade layer, where it is sensitive to slight changes in the degree of hydrocarbon-water contact as a result of changing the surfactant chain length or when the extent of hydration is affected by the presence of Cs+ ion. As the chain length or Cs+ concentration increases, N experiences a more compact and less hydrated environment surrounding its solubilization site. Similar conclusions have been found in the case of pyrene solubilized into cationic micelles of varying surfactant chain length investigated by fluorescence technique^.^ Micellar Structure and Extent of Water Penetration. The structure of the equilibrated micelle is determined by the balance between hydrophobic forces, which tend to minimize the hydrocarbon-water contact area, and repulsive forces between ionic head groups, which maximize their separation and thereby introduce some degree of water penetration into the micelle. A recent estimate of the distribution of amphiphilic additives between aqueous NaC,S (n = 10-16) micelles and the solvent in the solutions of these surfactants by conduction studiesz1showed that the distribution coefficient of amphiphilic probe decreases with increasing surfactant chain length, indicating a lower degree of water penetration in the micelles having longer surfactant tails. These authors2] also reviewed several ~ o r k s supporting ~ ~ - ~ ~their conclusions. Further support comes from electron spin-echo modulation studies'6b of N,N,N',N'-tetramethylbenzidine cation-water interactions in frozen micellar solutions at 77 K, which have found that the degree of cation-water interaction increases with decreasing alkyl chain length in anionic surfactants.
+
where g = 2, 0 is the Bohr magneton, and xI2, y 1 2 ,and zI2are the projections of the electron-electron separation vector r I 2onto 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 x and y are the principal axes in the aromatic plane. An increase in the local polarizability has the effect of reducing the Coulombic forces acting on the r electrons with a resulting expansion of the molecular wavefunction and concomitant increase in their average separation r I 2 . The decrease observed in the value of ID1 in n-decane compared with that in 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% glycerol. If the triplet electron distribution is assumed to expand isotropically, one obtains from eq 1
+
This shows that both ID1 and IEI should decrease in a more polarizable environment; the magnitude of the decrease is proportional to the absolute value of D or E . Although E does not change a great deal due to its small magnitude, a linear decrease of D is observed as the surfactant chain length in the micelle is increased. The value of dD/dE observed experimentally when the alkyl chain length changes from 10 to 14 is ca. 5 , whereas the ratio DIE is ca. 6. This shows that the increase in the polarizability experienced by N in the longer chain micelles is interpretable as an effectively isotropic expansion of the molecular electronic distribution. The effective expansion of r I 2is estimated to be about 0.09% in changing from a Clo to a C I 4 micelle. A similar but more pronounced decrease in ID1 is observed as shown in Figure 4 when the Cs' concentration is increased in a micelle of fixed chain length. It is noted that the effect of higher Cs' ion concentration on the ZFS parameter ID1 is relatively more pronounced than its effect on the red-shift of the 0,O band when both are compared with the changes observed when the chain length of the micelle is increased by two methylene units. This suggests that Cs' ions bound to the micelle produce a larger effect of increasing polarizability than of decreased polarity. Narrower line widths observed for N in surfactants of varying chain length compared with those observed in 20% aqueous glycerol solution indicates that N experiences a more homogeneous environment in the micelles. In fact, the environment in the micelles is apparently more homogeneous than that found in n-decane. The transition line widths in micelles, however, are (18) McGlynn, S . P.; Azumi, T.; Kinoshita, M. The Triplet Srate; Prentice Hall: Englewood Cliff, N J 1963.
(19) (a) Kalyanasundaram, K.; Grieser, F.; Thomas, J . K. Chem. Phys. Lett. 1977, 51, 501. (b) Humphry-Baker, R.; Moroi, Y . ;Gratzel, M. Chem. Phys. Lerr. 1978, 58, 207. (20) Cline Love, L. J.; Skrilec, M.; Habarta, J. G. Anal. Chem. 1980, 52, 754. (21) Abu-Hamdiyyah, M.; Rahman, I. A. J . Phys. Chem. 1985,89,2377. (22) Evans, D.; Ninham, B. W. J . Phys. Chem. 1983, 87, 5025. (23) Ottaviani, M. F.; Baglioni, P.; Martini, G. J . Phys. Chem. 1983,87, 3146. (24) (a) Almgren, M.; Greiser, F.; Thomas, J. K. J . Chem. SOC.,Faraduy Tram. I 1979, 75, 1674. (b) Aniansson, E.A. G.; Wall, S . N.; Almgren, M.; Hoffmann, H.; Kielmann, I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. J . Phys. Chem. 1976, 80, 905. (c) Donahue, D. J.; Bartell, E. F. J . Phys. Chem. 1952, 56, 480.
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. These effects are not seen when Br- is replaced by CI- counterion. Although the width of the 1 0 1 + (El transition is independent 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
