J. Phys. Chem. 1982, 86, 4291-4293
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Solubilization Sites of Aromatic Optical Probes In Micelles K. N. Ganesh, P. Mltra, and D. Balasubramanlan’ Centre for Cellular & Molecuiar 8lology. l-&derabad 500009, India (Received: August 23, 1982)
Several extrinsic optical probes that have been used to monitor the extent of water penetration in micelles are aromatic in nature. We have studied the ring current-induced alterations in the NMR spectral signals of the various protons of the surfactant chains of micelles brought about by solubilizing some of these probes. The results indicate that such aromatic solubilizates are located near the headgroup region of the micelles. These probes would thus be expected to monitor the polarity and water content of the headgroup region and not the micellar interior.
There is considerable current interest in the structural organization of supramolecular assemblies of amphiphilic compounds and in the extent of water penetration into their interiors. The Hartley model’ of a micelle being “an oil drop with an ionic or polar coat” has gained support over the year^^-^ and requires the micellar interior to be essentially anhydrous. However, this picture of micelles has recently been criticized, notably by Menger6who has suggested that micelles are better described as “porous cluster” aggregates, with considerable amounts of water in their interiors. In this model, the distinction between the micellar surface and interior is no longer clear-cut, and water solvates the surfactant apolar tail segments in the micelles just as it would a monomer surfactant molecule. Some support for the porous cluster model has appeared in recent l i t e r a t ~ r e . ~ The implications of a given structural model are significant not only to ionic micelles in water but also to related assemblies including biomembranes, since the general principles of organization of these assemblies are thought to be common.“1° The question of which model best describes micellar structures thus becomes relevant. An important feature of many of the studies supporting the porous cluster model for micelles is their use of extrinsic optical spectroscopic (and chemical) probes which monitor the extent of water penetration. Table I lists some of these “water penetration probes”. (Besides these, other probes such as functionalized surfactant molecules and steroid enones have also been used for this purpose. These are dealt with elsewhere.”) A common feature of all these probes is that they contain polar functional groups and/or aromatic rings. There are indications in the literature that ~
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(1)G. S.Hartley, Q.Rev. Chem. Soc., 2, 152 (1948). (2)P. Mukerjee and K. J. Mysels, A.C.S. Symp. Ser., No.9,239(1975). (3)P. Mukerjee in ‘Solution Chemistry of Surfactants”,K. L. Mittal, Ed., Vol. 1, Plenum Press, New York, 1979,p 153. (4)B. Lindman and H. Wennerstrom, Top. Curr. Chem., 87,l (1980). ( 5 ) H. Wennerstrom and B. Lindman, J. Phys. Chem., 83,2931(1979). (6)(a) F. M. Menger, Acc. Chem. Res., 12, 111 (1979); (b) F. M. Menger, J. Phys. Chem., 83,893 (1979); (c) F. M. Menger and B. J. Boyer, J. Am. Chem. Soc., 102,5936(1980);(d) F. M. Menger and J. M. Bonicamp, ibid., 103,2140 (1981);(e) F. M. Menger, H. Yoshinaga, K. S. Venkatasubban, and A. R. Das, J. Org. Chem., 46,415 (1981). (7)(a) N.J. Turro, Y. Tanimoto, and G. Gabor, Photochem. Photobiol., 31,527 (1980);(b) Y. Waka, K. Hamamoto, and N. Metaga, ibid., 32,27(1980);(c) T. Wolff, Ber. Bunsenges. Phys. Chem., 85,145(1981); (d) F. M. Martens and J. W. Verhoeven, J . Phys. Chem., 85,1773(1981). (8)C. Tanford, “The Hydrophobic Effect”, 2nd ed., Wiley-Interscience, New York, 1980. (9)J. N. Israelachvili, S. Marcelja, and R. J. Horn, Q. Reu. Biophys., 13, 121 (1980). (10)K. A. Dill and P. J. Flory, Proc. Natl. Acad. Sci. U.S.A.,78,676 (1981). (11)K. N.Ganesh, P. Mitra, and D. Balasubramanian in ‘Surfactants in Solution”, K. L. Mittal, Ed., in press. 0022-3054/82/2086-4291$01.25/0
TABLE I: Some of the Extrinsic Optical Spectral Probes Used in Micellar Water Penetration Studies probe
1-methylindole fluorophores: pyrene and derivatives; related polycyclic
arenes quenchers: dimethylanilines, dicyanobenzenes, cynopyridine acridine methyl viologen (acceptor);3-methyl-
method ref fluorescence quantum 7a yield and lifetime fluorescence hetero7b excimer emission
fluorescence quantum yield charge transfer
7c 7d
complexation
indole, pyrene, 1-
benzyl-1,4-dihydronicotinamide (donors) benzene and higher arenes are mildly surface-active and are located at the interface of ionic micelles near the headgroups.12 Mukerjee12diehas pointed out that “even mild surface activity on the part of a species dissolved (or solubilized) in a hydrocarbon-like or lipidlike system in contact with aqueous media gets amplified to produce serious consequences when the systems are small”-a point that has been reinforced by Almgren et a1.12‘ as well. In light of this, it is important to ascertain the location of such “water penetration probes” when they are solubilized in micelles. A convenient method to study the location of such probes, if they are aromatic, is to monitor the ring current induced alterations in the NMR chemical shifts of various protons (and carbons) of the surfactant molecules in the micelles brought about by solubilizates.12” We have used this method to locate the site of solubilization of two representative probes, a~ridine‘~ and l-methylindole,le which have been used to monitor water penetration in micelles. Benzophenone, which has been shown to be located near the headgroups in micelles,13 was also studied for comparison. (12)(a) J. C. Eriksson and G. Gillberg, Acta Chem. Scand., 20,2019 (1966);(b) J. H.Fendler and L. K. Patterson, J. Phys. Chem., 75,3907 (1971);(c) J. Ulmius, B. Lindman, G . Lindblom, and T. Drakenberg, J. Colloid Interface Sci., 65,88(1978);(d) P. Mukerjee, J. R. Cardinal, and N. R. Desai in “Micellization,Solubilization and Microemulsions”,K. L. Mittal, Ed., Vol. I, Plenum, New York, 1977,p 241; (e) P. Mukerjee and J. R. Cardinal, J. Phys. Chem., 82, 1620 (1978); (f) M. Almgren, G. Grieser, and J. K. Thomas, J. Am. Chem. SOC., 101,279 (1979);(g) K. A. Zachariasse, N. V. Phuc, and B. Kozanklewicz, J. Phys. Chem., 85, 2676 (1981);(h) D. J. Jobe, V. C. Reinsborough, and P. J. White, Can. J. Chem., 60,279 (1982); (i) R. E. Stark, M. L. Kasakwich, and J. W. Granger, J . Phys. Chem., 86,335 (1982). (13)J. H.Fendler and E. J. Fendler, ’Catalysis in Micellar and Macromolecular Systems”, Academic Press, New York, 1975,p 31.
0 1982 American Chemlcal Society
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The Journal of Physical Chemistry, Vol. 86,No. 22, 1982
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B R I I 58
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Flgure 1. Probainduced changes in the NMR chemical shifts, A6, of the various protons in the surfactant chains of CTAB, SDS, DTAB, and Brij 58 micelles. Probe:detergent concentration in every case is 1:8: A, acridine; B, benzophenone: MI, 1-methylindole. Positive values mean upfield shift and negative values mean downfield shift.
Experimental Section The surfactants cetyltrimethylammonium bromide (CTAB), dodecyltrimethylammonium bromide (DTAB), sodium dodecyl sulfate (SDS), polyethylene glycol (20) cetyl ether (Brij 58), and octyl glucoside (OG)were obtained from Sigma Chemicals. Acridine (Baker) was purified by sublimation under reduced pressure. 1Methylindole was prepared from indole by methylation and purified by chromatography,14 and benzophenone (Merck) was purified by recrystallization. The probes were solubilized at various concentrations (6,9,12, and 15 mM) in micelles of the detergents (concentration 50 mM) in DzO and the proton NMR spectra recorded on the Bruker WH 270 NMR spectrometer at the Indian Institute of Science, Bangalore, a t ambient probe temperature (- 30 "C). Results The signals due to the a-CH2,/3-CH2,and w-CH3 (and N-CH3) protons in the various surfactants are easily assigned. Except for OG, further resolution of the y-CHz (assignment following Zacharia~se'~) and two other CH2 groups down the chain (asyet unassigned) has also been possible. The interior o-CH2 protons resolve as a composite sharp manifold. Figure 1 summarizes the probeinduced changes in the chemical shifts of these specific segments of the surfactants in four different micelles. Such changes are attributable to the ring current-induced shielding (and deshielding) effects of the aromatic solubilizates on the surfactant protons. With acridine and benzophenone, such an effect is maximum for the CH2 protons that are y to the headgroup in the ionic micelles. The effect follows the order y-CH2 > P-CH2 > a-CH2 L w-CH2. With 1-methylindole as the probe, however, the maximal shielding is experienced by the P-CH2groups in the same micelles. For the nonionic micelle Brij 58, maximal perturbations were seen on the signals of the a-CH2 group of the cetyl chain that is directly attached to the polyoxyethylene segment. (Analysis and interpretation of similar results that we have obtained with these probes dissolved in the other nonionic micelle of OG have to await definite assignments of the signals to the various protons.) ~~
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(14) Y. Kikugawa and Y. Miyake, Synthesis,461 (1961). (15) K. A. Zachariasse, ref 12g, and personal communication.
Flgure 2. Variations in the A6 values with increase in the probe:detergent mole ratio: ACR, acridine: MI, 1-methylindole.
In all these cases, the effects observed on the w-CH2and w-CH3signals were smaller than those on the other protons. Also, with acridine and 1-methylindole as solubilizates in CTAB micelles, these signals showed downfield shifts, indicating deshielding effects (Gratzel et al.16 had observed similar deshielding of these protons with pyrene solubilized in CTAB micelles). In DTAB micelles, these probes induced downfield shifts of the w-CH3signals as in CTAB, but small upfield shifts of the w-CHz protons in contrast. It was also thought desirable to study the variations in such probe-induced effects as a function of the probe/ detergent mole ratio, since some reports1Wehave suggested that, at high mole ratios, the solubilized molecules might move (translocate) from the headgroup region of the micelle to the interior. The representative results shown in Figure 2 reveal that, as this mole ratio is increased from 1:lO to 1:3, the induced effects on all the signals display a monotonic undramatic variation. In addition, however, in the particular case of the w-CHz signals, the peak profile was found to change with an increase in the probe mole ratio. The w-CH2 manifold, discernible separately as a 20-proton peak and 4-proton peak in the pure CTAB micelle, underwent a splitting of the 20-proton signal into further subgroups upon addition of the probe; such an alteration of this peak profile has been seen earlier in CTAB micelles with methylnaphthalene as the solubilizate.lZc
Discussion The results that we have obtained are consistent with the interpretation that all the probes studied here are located in the headgroup region of the micelles of CTAB, DTAB, and SDS. For Brij 58 micelles, the probes are positioned around the a-CHz group of what is referred to as the palisade region-largely at the junction between the polyoxyethylene segment and the cetyl chain. In all cases, the probes induce only a shift of the existing peaks and do not give rise to multiple signals for the same set of protons. This would suggest that the probe exchanges its location between the headgroup region and the interior at a rate faster than the NMR time scale, yet with the average residence time weighted in favor of the headgroup region, or that the surfactant monomers in the micelles exhibit differential protrusions into the bulk water phase, as suggested by Anian~son,'~ which results in differential exposure of the various surfactant segment to the ringcurrent effects of the probe. The present results cannot distinguish between these possible processes. Thus the larger shielding effects seen in the w-CHz and o-CH3 protons of SDS and Brij 58 micelles might suggest either (16) M. Gratzel, K. Kalyanasundaram, and J. K. Thomas, J. Am. Chem. Soc., 96, 7869 (1974). (17) G. E. A. Aniansson, J. Phys. Chem., 82, 2805 (1978).
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The Journal of Physical Chemistry, Vol. 86, No. 22, 1982 4293
a greater mobility of the probes within, and/or more facile protrusions of the monomers in these micelles than in those of CTAB, DTAB, or OG. The fact that these induced effects do not display any notable changes upon increasing the probe/surfactant mole ratio (from 1:lO to 1:3) suggests that there is no perceptible translocation of the solubilized species in this concentration range. Eriksson and Gillberg12"had found such probe translocations at a probe/detergent mole ratio of 1:1, and the data of Ulmius et al.lZcsuggest a possible movement of the probe 1-methylnaphthalene from the headgroup region into the interior at mole ratios around 1:l. At such high concentrations, the possibility of the added probe perturbing the structure of the surfactant micelle exists; the changes in the peak profile and the splitting of the w-CHz signals seen at high probe mole ratios has been thought to reflect probe orientation effects and the microdynamics of the aggregate;'% we have seen similar effects in the present systems at high probe mole ratios (vide supra). It thus appears safer to work with as low a probe concentration as possible in such NMR experiments in order to minimize any perturbations in the micellar structure, and we have thus restricted most of our studies to the (presumably safer) probe/surfactant mole ratio of 1%.
are preferentially positioned in the headgroup region of the various types of micelles in which they are solubilized; in Brij 58 micelles their location is at the palisade region. Hence these probes sense and monitor not the micellar interior but the headgroup-water interface. In light of these and earlier results on benzenoid aromatic solubilizates, it would appear that all such probes listed in Table I are similarly located and report on the polarity and water content of the headgroup region of the host micelles. Support for the porous cluster model for micelles, at least with such aromatic optical probes, is hence not unequivocal. This preferential location of such solubilizates is consistent with the idea that even mild surface activity of a solubilizate gets highly amplified in hosts of micellar (or membrane) dimensions. We have been able to show elsewhere" that even alicyclic enones exhibit such surface activity. It is not clear from the present results whether water penetrates up to the y-CH, of micelles such as CTAB or DTAB, or whether the topography of the ring currenteffected shielding domains of such bicyclic and tricyclic aromatic compounds is such that the maximal effects are observed for the y- and P-CH, groups even if the probe is positioned at the Stern layer.
Conclusions Based on the results described, we conclude that the "water penetration probes" acridine7aand l-methylindole7c
Acknowledgment. We thank the Bangalore NMR facility for access to their spectrometer. P. Mitra is the recipient of a CSIR research fellowship.
